Controls on Sonic Velocity in Carbonates

8/2/2019 Controls on Sonic Velocity in Carbonates

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288 F.S. Anselmetti and G. P. Eberli PAGEOPH,

1 . I n t r o d u c t i o n

Knowledge of the relation between sonic velocity in sediments and rocklithology is one of the keys to interpreting data from seismic sections or fromacoustic logs of sedimentary sequences. Reliable correlations of rock velocity withother petrophysical parameters, such as porosity or density, are essential forcalculating impedance models for synthetic seismic sections (BIDDLE e t a l . , 1992;CAMPBELL and STAFI.EU, 1992) or identi fying the origin of reflectivity on seismiclines (SELLAMI t a l . , 1990; CHRISTENSEN and SZYMANSKI, 1991). Velocity is thusan important parameter for correlating lithological with geophysical data.

Recent studies have increased our understanding of elastic rock properties insiliciclastic or shaly sediments. The causes for variations in velocity have beeninvestigated for siliciclastic rocks (VERNIK and NuR, 1992), mixed carbonatesiliciclastic sediments (CHRISTENSEN and SZYMANSKI, 1991), synthetic sand-claymixtures (MARION e t a l . , 1992) or claystones (JAPSEN, 1993). The concepts derivedfrom these studies are however only partly applicable in pure carbonates. Carbon-ates do not have large compositional variations that are, as is the case in the othersedimentary rocks, responsible for velocity contrasts. Pure carbonates are character-ized by the lack of any clay or siliciclastic content, but are mostly produced anddeposited on the top or on the slope of isolated or detached carbonate platforms,that have no hinterland as a source of terrigeneous material (WILSON, 1975;EBERLI, 1991). They consist of over 95% of the carbonate minerals calcite (low-and high-Mg), dolomite and aragonite. These minerals have very similar physicalproperties, which excludes compositional variation as a major reason for the largevariability in velocity of carbonates.

Theories that describe sonic wave propagation in porous media (GASSMAN,1951; BLOT, 1956) are hard to apply in the complex system of pure carbonatesbecause they form a variety of unique diagenetic rock fabrics with specific elasticproperties. In order to quantify the physical properties, sonic velocity in purecarbonate samples from three different areas tha t cover a wide range of depositional

environments and lithologies have been measured. Measurements o f compressional-wave velocity ( U p ) and shear-wave velocity (Us) were performed under confiningand pore-fluid pressures, which accurately simulate i n s i t u subsurface conditions.Our study includes carbonates at all stages of diagenetic alteration and comple-ments studies on the velocity of carbonates which were limited to highly lithified,low porosity carbonate rocks (RAFAVICH e t a l . , 1984; WANG e t a l . , 1991) or topelagic, deep water carbonates (SCHLANGER and DOUGLAS, 1974; MILHOLLAND ta l . , 1980; URMOS and WILKENS, 1993).

Sonic velocity measurements were done in combination with a thorough litho-

logic and diagenetic examination of thin sections and XRD analysis. Porosity in thesamples ranges from 0 to 60% and the depositional environment varies from theprotected shallow water platform over reefal and platform-marginal sediments to


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Vol. 141, 1993 Controls on Sonic Velocity in Carbonates 289

deeper slope deposits. The correlation of the velocity measurements with thelithology and the mineralogy data enables us to assign depositional and diageneticstages to characteristic velocities. Furthermore it allows tracing of diageneticevolution and velocity development from the time of deposition through differentburial stages, recognizing that each diagenetic process alters velocity in its charac-teristic way.

2. Sample Areas

This study presents the correlation of physical rock properties with rocklithology based on velocity analyses of 210 discrete samples from three differentareas; (1) modern carbonate mud from Florida Bay, (2) two deep drill holes inGreat Bahama Bank and (3) the Maiella, an exhumed carbonate platform inCentral Italy. An understanding of the geological setting of the three areas isessential in order to relate the physical properties of the carbonates to the rocklithology.

A. Velocity Samples fr om Modern and U nconsolidated Carbonate Sediments:Artificially Compacted Carbonate M ud fr o m Clue tt Key, Florida Ba y

(South Florida)The velocities of 20 carbonate mud samples were measured at various stages of

artificial compaction in order to determine the increase of velocity caused by theporosity reduction during pure mechanical compaction. The mud was collected withpush cores of approximately 70 cm length from the interior pond of Cluett Key(Figure 1), a mangrove island in Florida Bay.

Florida Bay is a triangular shaped shallow lagoon on the southern part of theFlorida peninsula. This protected bay is subdivided by a series of mudbanks withseveral mangrove-fringed islands (ENos and PERKINS, 1979). The Holocene sedi-ments on the islands overlie Pleistocene bedrock and are up to 4 m thick. The baseof the Holocene is often marked by a peat layer which is overlain by a successionof dominantly mud to wackestones with few intercalations of shell-rich stormlayers. Unconsolidated carbonate mud of the upper part of the Holocene sectionwas used for the compaction-velocity experiments. The samples were taken fromparts of the cores in which no roots or shell fragments disturb the homogeneousmud.

Mud from the islands and the mudbanks in Florida Bay have porosities thatrange f rom 61 to 78% (ENOS and SAWATSKY, 1981). Gamma ray attenuation

measurements with cores from Cluett Key gave an average porosity for theHolocene sediments of 63% (VIDLOCK, 1983). The mineralogical composi tion,determined on carbonate mud from Jimmy Key, an adjacent island (BuRNs and


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290 F. S. Anselmetti and G. P. Eberli PAGEOPH ,

Figure 1Location map showing the positions of the deep core borings Unda and C lino along the Western seismic

line on Great Baha ma Bank and the location of Cluett Key in Florida Bay.

SWART, 1992) averages 65% arago ni te , 20 % h igh Mg-calci te and 15% low M g-cal-c it e. These va lues a re s t ab le fo r the who le Ho lo cene sec t ion an d on ly t races ( < 5%)of do lomi te a re observed . Be tween the su r face and 70 cm dep th , no d e tec tab led iagene t ic a l t e ra t ions occu r, a l though var ia t ions in po re wate r chemis t ry ind ica tetha t ea r ly d iagene t ic p rocesses such as do lomi t i za t ion have a l ready s ta r t ed (BURNS

and SWART, 1992).

B. Velocity Samples from Cores of Deep Drillholes: Pleistocene to MioceneCarbonates from Core Borings in Great Bahama Bank (Bahamas Drilling Project)

Tw o c o n t i n u o u s c o re b o r i n g s f ro m t h e B a h a ma s D r i l l i n g P ro j e c t , l o c a t e d o n amul t i -chann e l se i smic l ine on Gr ea t Bah am a Ban k (F igu re s 1 and 2 ), p rov ide anexce l len t oppor tun i ty to co r re la te the phys ica l p roper t i es o f Miocene to P le i s tocenecarbona te sed imen ts wi th the i r depos i t iona l l i tho log ies and d iagene t ic s t ages .

Eigh ty -n ine samples f rom bo th d r i l l ho les were ana lyzed . Un l ike o lder and exhumedou t c rop samples , the young age o f the d r i ll ed sed imen ts a l lows meas u rem en t o fson ic ve loci t ies o f ca rbo na tes tha t a re par t ly unco nso l ida ted and tha t a re no t in


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Vol. 141, 1993 Contro ls on Sonic Velocity in Carbonates 291

W S W C l i n o U n d a E N E

o S t r a i t s ~ ~ = _ ~ G r e a t B a h a m a B a n k

l U K m ~ ~ - - / ~ -E ~ ~

o ~ ~ ~

Figure 2Part of Western line displaying modern platform margin and drill sites Unda and Clino. The successionof inclined reflectors below the modern shallow water platform document the progradation of the

platform edge over inclined slope sediments for a distance of over 25 km.

their final stage of post-depositional alteration . The variety of diagenetic processesencountered enabled us to trace the velocity evolution of different carbonatesediments under different diagenetic conditions through burial history and time.

The two holes (Unda and Clino) penetrated to depths of 442 and 662 m belowseafloor, respectively. The continuous cores had an average recovery of over 80%.The top of the rock section in both holes is of Pleistocene age. The oldest drilledsediments are dated as Middle Miocene at the bottom of Unda, whereas the bottomof Clino reaches an age of Late Miocene (Figure 3). The retrieved lithologies rangefrom platform-interior to platform-margin and slope carbonates; there is no silici-clastic sediment on this isolated carbonate platform (KENTER e t a l . , 1991).

Hole Unda, located 10 km from the modern platform edge, is characterized bythree successions of shallow-water platform sands and reefal deposits that alternatewith fine-sand and silt-sized deeper marginal deposits. The two intervals of deeper-water sediments record periods of rapid rise of sea level and probable backsteppingof the platform and reefal units. Hole Clino, 7 km closer to the modern platformedge, penetrated a single interval of shallow platform and reefal sediments overlyinga thick succession of slope sediments. The nearly 500 m of fine-sand to silt-sizedslope sediments below 200 m have a variable amount of planktonic foraminiferasand are, except for some intervals with coarse-grained, mainly skeletal sands,remarkably poor in coarser material.

This succession of depositional environments (Figure 3) shows the progradation

of the whole platform over the underlying slope sediments. The platform rimprograded over 25 km to the west into the Straits of Flor ida (EBERI~I andGINSBURG, 1989).


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292 F .S . Anselmetti and G. P. Eberli PAG EOP H,

Figure 3Correlation of Vpand V (at 8 MP a effective pressure) with depth, deposition al environment and age ofthe drilled sediments from the two d rill holes Unda and Clino on G reat Bahama Bank. Velocityinversions are common in both holes and show that the effect of diagenetic alterations and sediment typedominate over the velocity effect of depth. Velocities of carbonates that were deposited on the shallowwater pla tform (shaded areas in grap h) have larger v ariability and higher velocities than velocities from

deeper water samples.

N o t o n l y t he d e p o s i t i o n a l l i th o l o g y, b u t a l so t h e d i a g e n e t ic o v e r p r i n t i n g c h a n g e s

s e v er a l t i m e s d o w n h o l e . T h e u p p e r p a r t s o f b o t h h o l e s ar e c h a r a c t e r i z e d b y e a r l y

m a r i n e a n d s u b s e q u e n t i n t e n s e f r e s h w a t e r d i a g e n e s i s . M a n y s a m p l e s s h o w i n t e n s e

d i s s o l u t i o n f e a t u r e s , a s w el l a s e x t e n s iv e c e m e n t a t i o n , w h i c h l e d t o s p e c if i c r o c k

f a b r i c s w i t h c h a r a c t e r i s t i c e l a s t i c p r o p e r t i e s . I n t h e l o w e r p a r t o f C l i n o , t h e

p e r i p l a t f o r m s l o p e s e d i m e n t s s h o w n o m a j o r a l t e r a t i o n s a n d o n l y t h e p l a t f o r m

d e r i v e d t u r b i d i t e la y e r s a r e m o r e c e m e n t e d . L i t t le d o l o m i t e o c c u r s b e l o w a h a r d -

g r o u n d a t 5 36 m . D o l o m i t i z a t i o n in U n d a i s c o n s i d e r a b l y m o r e p e r v a s i v e a n d f o r m s

e i t h e r a f a b r i c d e s t r u c t i v e s u c r o s i c d o l o m i t e o r a c r y s t a l l i n e m i m e t i c d o l o m i t e( D AWA N S a n d S WA RT, 1 98 8) , d e p e n d i n g o n t h e p r e c u r s o r. I n t h e l o w e s t p a r t o f

U n d a , d o l o m i t e d i s a p p e a r s a n d t h e r o c k s a g a i n s h o w i n te n s e d i s s o l u t io n f e at u r es .


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C. Velocity Samp les fr om Outcrops: Mon tagn a della Maiella (Abruzzi, Italy)

The Maiella is an uplifted and exhumed carbonate platform in Central Italy

(Figure 4) that is exposed in several valley flanks. The exposed platform and slopecarbonates range in age from the Lower Cretaceous to the Upper Miocene. Onehundred and one samples were collected and velocities were determined. Theknowledge of the physical properties in combination with the assessment of thelarge-scale geometrical pattern of the outcropping rock formations enables us tocalculate synthetic seismic sections using computer simulations in order to see theseismic response of a particular geological setting (ANSELMETTI and EBERLI, 1991).

The margin of the Maiella carbonate platform is characterized by a steepescarpment during Early Cretaceous time that became buried during the Late

Cretaceous and developed into a low-angle ramp in the Paleogene. The sedimentsof the external platform are mostly rudist biostromes and carbonate sandbodieswhereas the platform interior is mainly made of limestones deposited in a shallowsubtidal to supratidal environment, such as wackestones and fenestreal mudstones(CRESCENTI et al., 1969; SANDERS, 1994). A distinct mid-Cretaceous, karsticunconformity separates the Cretaceous platform section into an upper and a lowerunit. On the adjacent slope, several mega-breccias onlap this platform margin(EBERLI et al., 1993; VECSE~, 1991). They were deposited during sea-level lowstandsthat caused the exposure of the platform top and the erosion and downslope

Figure 4Map showing the location of the Montagna della Maiella, in the Abruzzi, Central Italy.


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transport of platform fragments. These breccias are intercalated with calcareousturbidites and pelagic carbonates that form the normal background sedimentation.In the lower Paleogene, a relative deepening of the platform resulted in a backstep-ping of the platform margin and the steep escarpment was slowly infilled. Finally,during the Oligocene, reefal units of the plat form margin prograded over the formerdeeper shelf and slope deposits and formed a wide and shallow shelf. This generalevolution of backstepping and prograding of an isolated carbonate platform hasstriking similarities with the evolution of the modern Great Bahama Bank.

Unlike the Great Bahama Bank, the Maiella platform shows almost no signs ofdolomitization. This explains why, despite their older age, most samples are betterpreserved than many dolomitized Bahamas carbonates. Some of the bioclasticsands of the Upper Cretaceous still have porosities of over 30%, whereas most ofthe platform deposits are densely cemented.

3. Methods

A. Sampling Technique

The samples used for velocity determinations are cylindrical miniplugs 2.5 or

3.8 cm in diameter. The miniplugs of unconsolidated mud from Florida Bay weresampled from short push cores 7.6 cm in diameter. So as to avoid compaction andfabric destruction during sampling, a thin-wall tube with a diameter of 3.8 cm wasused to cut the miniplugs vertically out of the cores. The 2-4 cm long cylinderswere compacted longitudinally by a hydraulic press with a uniaxial pressure of upto 170 MPa. The velocities of the mud samples were measured at variable degreesof compaction. The maximum compaction reached was approximately 50% so thatthe initial porosity of 63% was reduced to 26%.

The samples from the Bahamas deep drillings were cored horizontally, or

occasionally vertically, into the 7.6 cm diameter cores. Plugs from the Maiella werecored from hand samples collected in outcrops. All rock cores were trimmed to alength between 1.5 and 5 cm. The end surfaces were polished to make them flat andparallel in order to allow a good transmission of the acoustic signal.

B. Velocity Measurements

The velocities from all Bahamas samples and from 29 of the Maiella sampleswere measured applying a pulse transmission technique (BIRcrl, 1960) with an

apparatus shown in Figure 5. The velocity samples were water saturated andjacketed in rubber or heat shrink tubing which seals the pore fluid from theconfining oil in the pressure vessel. Confining and pore-fluid pressures are chosen


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Figure 5Schematic cross section of ultrasonic velocity meter.

independen t ly to s imula te mos t accu ra te lyin situ cond i t ions o f a bu r ied sed imen t .Pore- f lu id p ressu res as h igh as 50 M Pa can be ob ta ined bu t m os t exper im en ts were

run a t 2 MP a. Conf in ing p ressu re i s var ied be tween 3 and 100 MP a, resu l ting in aneffec tive ma x i m um pressu re (conf in ing p ressu re m inus po re - f lu id p ressu re) o f up to98 MPa. However, many samples co l l apsed and fa i l ed a t p ressu res be low 100 MPa.

With in the end caps , p iezoelectr ic crys tals create a s ignal wi th a center frequencyof 0 .6 to 1 .2 MH z. T he sam e pa i r o f t ransducers genera tes one compress io na l -w avesignal (Vp) and two orthogonal ly polar ized shear-wave s ignals (V,~, V,2) . Thet r a n s d u c e r s a r e a r r a n g e d s o t h a t t h e w a v e s p ro p a g a t e a l o n g t h e c o re a x i s . Th eelectr ical s ignal produced by the receiver crys tal i s ampli f ied , f i l tered , and fed in toa d ig i ta l osci l loscope. The osci l loscope d ig i t izes the u l t rasonic s ignals and t ransfers

t h e di g it iz e d w a v e fo rm s t o a Ma c i n t o s h Q u a d ra c o m p u t e r fo r d i s p la y a n d t i meser ies ana lys i s . A cus tomized ana lys i s so f tware package co l lec t s the da ta as afunct ion of effect ive pressure , and calculates the t r avel t imes of the s ignals as wel l


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296 F. S. Ansetmetti and G. P. Eberli PAGEO PH,

as the th ree ve loc i t i es(Vp, g s l , Vs2 a t ev e ry p r e s s u re s t ep . Th e V u s ed fo r t h ec a l c u l a t i o n o f t h eVp/Vsr a t i o i s t h e mean Vso f t he t w o m e a s u r e m e n t s .

T h e v e l o ci ti e s o f th e c o m p a c t e d m u d f r o m F l o r i d a B a y w e r e m e a s u r e d w i t ht h e sa m e s et o f t r a n s d u c e r s b u t w i t h a b e n c h t o p m e a s u r i n g s y s t e m n o t u n d e rc o n f i n in g p r es s u re . T h i s m e a s u r i n g s y s te m a l l ow s r e c o g n i t i o n o f c o m p a c t i o n d u et o t h e a x ia l p r e s s u r e o f t h e t r a n s d u c e r s d u r i n g t h e m e a s u r e m e n t . T h e t w o t r a n s d u c -e rs w e r e p r es s e d t o g e t h e r w i t h a p i s t o n a t a n a x ia l p re s s u r e o f 0 . 1 - 1 M P a . T h i sr e l a t i v e l o w p re s s u re a l l o w ed t h e t r an s mi s s i o n o f th e s i g n al f ro m t h e t r an s d u c e r si n t o t h e m u d b u t d i d n o t c o m p a c t d r a s ti c a ll y t he s ti ll d e f o r m a b l e m u d s a m p le s .S o m e m e a s u r e m e n t s w e r e p e r f o r m e d o n u n c o m p a c t e d m u d , h o w e v e r th e m i n i m a lr e q u i r e d t r a n s d u c e r p r e s s u r e r e d u c e d t h e s a m p l e l e n g t h b y a f e w p e r c e n t . C o r r e c -t i o n s f o r l e n g t h c h a n g e a r e m a d e s o a s n o t t o p r o d u c e e r r o r s i n t h e v e l o c i t yd e t e r m i n a t i o n .

A p a r t o f t h e Ma i e l l a mi n i p l u g s (7 2 s amp l e s ) w as me as u re d w i t h a s imi l a ra p p a r a t u s i n t h e p e t r o p h y s i c s l a b o r a t o r y a t t h e U n i v e r s i ty o f G e n e v a , S w i t z e rl a n d .T h e t r a n s d u c e r p a i r o f t hi s m a c h i n e o n l y c re a t e s a p - w a v e s ig n al . M e a s u r e m e n t sw e r e p e r f o r m e d d r y w i t h o u t p o r e - f l u i d p r e s s u r e a n d u n d e r c o n f i n i n g p r e s s u r e sv a r y i n g u p t o 4 0 0 M P a .

T h e p r e c i si o n o f t h e v e l o c i ty m e a s u r e m e n t s i s m a i n l y a f u n c t i o n o f t h e q u a l i t yo f t h e s amp l e . I n w e l l c em en t ed , h i g h v e l o c i t y samp l e s , t h e l o w er t r an s d u ce r r ece i v e sa c l e a r p e a k a s f ir s t b r e a k w h i c h a l lo w s m e a s u r e m e n t s o f v e l o c it y w i t h a n e r r o r o fl e s s t h a n 1 % . F r i a b l e , u n c o n s o l i d a t e d s a m p l e s t e n d t o c o m p a c t a n d r e d u c e t h e i rs a m p l e l e n gt h b y u p t o 5 % . I n a d d i t io n , t h e y s o m e t i m e s p r o d u c e o n l y a m o d e r a t ef i r st b r eak s i gn a l, e s p ec i a l ly fo r V s, s o t h a t t h e e r r o r o f v e l o c i ty d e t e r m i n a t i o n s i nt h e se d i ff ic u lt s a m p l e s p r o b a b l y a m o u n t s t o a p p r o x i m a t e l y 5 % .

C. A dd it ional Prop ert ies

In ad d i t i o n t o t h e v e l o c i t y d e t e rm i n a t i o n s , s ev e ra l o t h e r an a l y s e s w e re p e r-

f o r m e d . D r y b u l k d e n si ti e s w e r e c a l c u l a te d b y w e i g hi n g t h e o v e n d r i e d r o c k p l u g sa n d c a l c u l a ti n g t h e v o l u m e b y m e a s u r i n g d i a m e t e r a n d l e n gt h . X R D a n a l y se s w e r ep e r f o r m e d o n t h e c u t - of f s o f d ri l le d p lu g s f r o m t h e B a h a m a s s a m p le s . C a l i b r a t i o nw i t h c a r b o n a t e - s t a n d a r d s a l lo w e d d e t e r m i n a t i o n o f t h e p e r c e n t a g e o f ca l ci te ,d o l o m i t e a n d a r a g o n i t e . B e c a u s e t h e s e a l m o s t p u r e c a r b o n a t e s c o n s i s t d o m i n a n t l y( > 9 5 % ) o f th r e e c a r b o n a t e m i n e ra l s, t h e p e r c e n ta g e o f th e m i n e ra l s d e te r m i n e s at h eo re t i c a l g r a i n d en s i t y :

% c a l c i t e 9 -t- % d o l o m i t e 9 Pdolomite+ % a r a g o n i t e - P a r a g o n i t e

/)grain = 1O0

Pcalcite = 2.71 g/c m 3; PdoLomite 2. 87 g/c m 3; Paragonite= 2 .93 g/c m 3.


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Vo l. 141, 1993 Con trols on Sonic Velocity in Carbon ates 297

Th e ro ck p o ro s i ty i s c a l cu la t ed b y co m p ar in g th e ca l cu la t ed g r a in d en s i ty w i tht h e m e a s u r e d d r y o r s a t u r a t e d b u l k d e n s i t y

% p o r o s i t y P g r a i n - - P b u l k

1 O 0 P g r a i n - - P f l u i d "

T h i s e a s y w a y t o d e t e r m i n e r o c k p o r o s i t y in p u r e c a r b o n a t e s w a s c o m p a r e d w i t ht h e r e s u l t s f r o m o t h e r t e c h n i q u e s s u c h a s h e l i u m d e n s i t o m e t r y a n d A r c h i m e d e sp r in c ip le . Ou r p o ro s i ty v a lu es a re sy s t em a t i ca l ly 0 -3 % h ig h e r th an p o ro s i t i e so b ta in ed b y th e o th e r m e th o d s . Th e d i f f e r en ce i s c au sed b y th e f ac t t h a t t h e h e l iu md e n s i t o m e t r y a s w e ll as t h e A r c h i m e d e s m e t h o d a r e b a s e d o n p e n e t r a t i o n o f a p o r ef lu id o r g a s (wa te r, an d h e l iu m r e sp ec t iv e ly ) i n to th e p o re sp ace an d th e r e fo r e a r e

a fu n c t io n o f p e rm eab i l i t y. I n ad d i t io n , i so l a t ed an d c lo sed p o ro s i ty is n o t p en e -t r a t ed b y th e p o re f lu id an d i s t h e r e fo r e n o t d e t ec t ed , wh e reas o u r m e th o d b ased o nth e d en s i ty an d X- r ay an a ly ses a l so co n s id e r s t h i s c lo sed p o ro s i ty.

Cu t -o f f s o f t h e m in i -p lu g s were a l so u sed to m ak e th in sec t io n s f ro m m o s tv e lo c i ty sam p les . Th in sec t io n s were ex am in e d in o rd e r t o d e t e rm in e f ac to r s su ch a ssed im en t ty p e , co m p o s i t io n , g r a in s i ze , p o ro s i ty ty p e an d d i ag en e t i c a l t e r a t io n s .Th ese ex am in a t io n s en ab le u s to co r r e l a t e th e p h y s ica l p ro p e r t i e s w i th th e l i t h o lo g -i ca l p a r am e te r s .

4. Velocity Da ta

A . Vp and Vs

In th e fo l lo win g d esc r ip t io n s an d co r r e l a t io n s , v e lo c it ie s a t a co n f in in g p r e ssu reo f 10 M Pa an d a p o re - f lu id p r e ssu re o f 2 M Pa a r e d i scu ssed . Th e r e su l tin g e ff ec tiv ep re ssu re o f 8 M Pa (1 0 M Pa fo r d ry sam p les ) i s h ig h en o u g h to a l lo w a g o o d s ig n a lt r an sm iss io n b u t d o es n o t cau se s ig n i f ican t f r ac tu r in g in th e h ig h p o ro u s sam p les .T h e Vp m easu rem en t s o n d ry sam p les a r e a l so p r e sen ted h e r e b ecau se m a jo rd i f f e r en ces b e tween d ry an d sa tu r a t edVpo n ly h av e to b e ex p ec ted in r o ck s wi th ad o m in a n t c r ack p o ro s i ty (NU R a n d SIMMONS, 1 96 9), wh e reas th e sa tu r a t io n o fro u n d - sh ap ed p o re s , ab u n d an t i n o u r sam p les , d o es n o t i n f lu en ceVpdras t ica l ly.

Desp i t e t h e l im i t ed v a r i ab i l i t y in m in e ra lo g y, t h e m easu red ca rb o n a te s h av e anex t r ao rd in a r i ly w id e r an g e in v e lo c i t i e s .Vpv a r ie s b e tw een 1 7 0 0 an d 6 5 00 m /s , Vsbetw een 700 and 3400 m/s . Th e th ree d i ffe ren t da ta se ts have d i ffe ren t ranges inVpa n d Vs ( F i g u r e 6 ). T h e u n c o n s o l i d a t e d m u d s a m p le s f r o m F l o r i d a B a y h a v e t helo wes t v e lo c i ti e s w i th a m in im u mVpand Vs o f 1700 and 700 m/s , respec t ive ly. TheBa h am as an d th e M a ie l l a sam p les r each v e lo ci ti e s o f u p to 6 5 0 0 m /s( V p )a n d

3400 m/s (Vs) .Unlike s i l ic ic las t ic sed iments , where var ia t ions in minera logy (e .g . ,c l ay - co n ten t ) c an cau se l a rg e v e lo c ity co n t r a s t s , t h e d i f f e r en t ca rb o n a te m in e ra l s ,ca l c i t e , d o lo m i te an d a r ag o n i t e , h av e v e ry s im i l a r p h y s ica l p ro p e r t i e s so th a t


12/37

298 F .S . Anselmet ti and G. P. Eberli PAG EO PH ,

40

30

20

lO

o

e~

v

20 ,

Florida Bay samples15

1~olOOO 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 7 0 00

Vp (m/s)

Figure 6Range ofVpfor the thre e investigated areas ( at 8 M Pa effective pressure). The M aiella samples have ahigher average velocity than the Baham as samples. The artificially compacted mu d samples from F lor ida

Bay have, despite compaction of up to 5 0%, only low velocities. The large range of velocity in allsamples is remarkable for the restricted mineralogy of the pure carbonates.

d i ff e r en c e s b e t w e e n t h e m c a n n o t b e r e s p o n s i b l e f o r t h e l a r g e v a r i a b i l i t y i n v e lo c i t i e s .

C o n s e q u e n t l y , t h e w i d e r a n g e o f Vp a n d Vs i n c a r b o n a t e s h a s t o b e e x p l a i n e d w i t hd i f fe r e n t f a b r i c s a n d t e x t u r e s a n d n o t w i t h t h e d i f f e re n t m i n e r a l s o f th e r o c k s .

B. Acoustic ImpedanceT h e o b s e r v e d r a n g e i nVp a n d Vs a n d t h e r e f o r e t h e l a rg e c o n t r a s t s i n a c o u s t i c

i m p e d a n c e ( F i g u r e 7 ) c a n e x p l a i n t h e e x c e l l e n t s e i sm i c re f l ec t i vi t y o f m a n y s e i sm i c


13/37

Vo l . 1 4 1, 1 9 93 C o n t r o l s o n S o n i c Ve l o c i t y i n C a r b o n a t e s 2 9 9

25

2O

t.

z

0

0 2 4 6 8 10 12 14 16 18 20Acoustic Imp edance (10 6 kg/m2s)

F i g u r e 7R a n g e o f a c o u s t i c i m p e d a n c e o f t h e B a h a m a s a n d M a i e l l a s a m p l e s ( a t 8 M P a e f fe c ti ve p re s s u r e) .I m p e d a n c e s o f h i g h v e lo c i ty a n d d e n s e r o c k s a r e o v e r f iv e ti m e s h i g h e r t h a n i m p e d a n c e s o f l o w v e l o c it yr o c k s . T h e s e i m p e d a n c e v a r i a t i o n s i n t h e p u r e c a r b o n a t e s a r e c a u s e d b y d i ff e re n c e s i n f a b ri c a n d n o t b ydifferences in com position. The large ran ge exp lains theg o o d reflectivity observed in m an y seismic

sections of carbonates.

s e c t io n s in p u r e c a r b o n a t e s . T h e t w o d r i ll h o l es in t h e B a h a m a s , f o r in s t a n c e , h a v ea c o u s t ic i m p e d a n c e v a l u es t h a t r a n g e f r o m 3 . 8 - 1 7 . 4 . 1 0 6 k g / m 2 s. T h e o b s e r v e dg o o d r e f l e c to r s o n s e is m i c s ec t io n s , o f t e n b e l i e v e d to b e c a u s e d b y i n t e r c a l a t i o n s o fn o n c a r b o n a t e s e d i m e n t s , c a n i n f a c t b e e x p l a i n e d b y v a r i a t i o n s i n th e f a b r i c o f t h ec a r b o n a t e s .

c . v /vs

S imi l a r t o Vp an d Vs, the Vp/Vs ,w h i c h w a s o n l y m e a s u r e d u n d e r s a t u r a t e dc o n d i t i o n s , a l s o h a s a w i d e r a n g e . T h e c r o s s p l o t o fVp/Vsw i th Vp( F i g u r e 8 ) s h o w st h a t t h e r a t i o i n i n d u r a t e d r o c k s w i t h h i g hVpn o r ma l ly f a l l s be tw e e n 1 . 8 a n d 2 . A tl o w e r Vp,th e Vp/ V r a t i o c a n r e a c h s u b s t a n t i a l l y h i g h e r v a l u e s o f u p t o 2 . 6. T h e s eh i g h e r Vp/ Vsr a t i o s r e f l e c t t he f a c t t h a t i n g e n e r a l V~ i s m or e a f f e c t e d b y th e h ig h lyp o r o u s f a b r ic o f t h e l o w - v e lo c i ty c a r b o n a t e s t h a n V . I t m u s t b e t a k e n i n t o a c c o u n tt h a t s o m e r e a d i n g s o f th e s h e a r w a v e v e l o c i t y a n d t h u s t h e V p /V ~ r a t i o m i g h t h a v ea l a rge e r r o r , due t o a ba d V s - sig na l q ua l i t y, e . g ., w h e n a l ow she a r w a ve a m p l i t ud eis c o m b i n e d w i t h a h i g h b a c k g r o u n d n o is e . T h e r e f o r e so m e o f th e e x t r e m eVp/Vsv a l u e s m i g h t b e u n r e l ia b l e ; h o w e v e r , t h e s e fe w v a lu e s d o n o t c h a n g e t h e g e n e r a l

p a t t e r n o f th e Vp/V~r a n g e . T h e a r t if i ci a ll y c o m p a c t e d s a m p l e s f r o m F l o r i d a B a yh a v e a n e x t r e m e r a n g e o fVp/Vsf r o m 1 . 7 t o 2 . 8 w i t h i n a n a r r o w r a n g e o fVpf r o m1 70 0 t o 2 30 0 m / s . T h e u n c o m p a c t e d m u d - f a b r i c w i th a p o r o s i t y o f a p p r o x i m a t e l y


14/37

3 0 0 F. S . A n s e l m e t t i a n d G . P. E b e rl i PA G E O P H ,

3 i ~ i i i

[ ] Baham as samples2 .8 9 9 Maiella samples

9 Florida Bay samp les2.6 ~ []

% []2 . 4 ~ 1 ~ 1

[ ]

;~ 2.2 [ ]

1.8 ; [] ~ % ~ n n ~ [ ]

1.6 I t t I

1000 2000 3000 4000 5000 6000 7000V p (m / s )

F i g u r e 8V p / V a s a f u n c t i o n o fVp ( a t 8 M P a e f fe c ti v e p r e s s u r e ) . T h e w i d e r r a n g e o fV p / V ~t o w a r d s h i g h e r v a Iu e si n l o w v e l o c i t y r o c k s s h o w s t h a t s h e a r w a v e s a r e m o r e a f f e c t e d b y h i g h p o r o s i t y f a b r i c s t h a nc o m p r e s s i o n a l w a v e s . S o m e o f th e e x t r e m e l o w a n d h i g h v a l u e s m i g h t b e c a u s e d b y a q u e s t i o n a b l e

r e g i s t r a t io n o f t h e a r r i v i n gV s s i g n a l , i n p a r t i c u l a r i n h i g h p o r o s i t y r o c k s .

63% is not strong enough to sustain shear stresses (LAUGHTON, 1957) and inhibitstransmission of the shear-wave signal. The little compacted mud samples have highVp/V~of up to 2.8. With increasing compaction, the V p / V, of these Florida Baysamples approach more "normal" values around 2.2.

5. Factors Affecting Velocity

In many sedimentary rocks, the concept of grain and matrix supported fabric

with a critical porosity is able to explain the variations in velocity (Nup, et al., 1991)and to relate them to differential composition. The high susceptibility of carbonatestowards diagenetic changes however, causes cementation, dissolution and recrystal-lization processes that form rock fabrics unique to carbonates with velocity patternsthat do not simply reflect the compositional variations of the sediment.

Acoustic velocity in carbonates is a complex funciton of several factors. We candistinguish between rock-intrinsic and rock-extrinsic parameters. Intrinsic parame-ters, such as porosity, pore type, composition or grain size, are factors that areconnected with the lithology and thus, with the physical properties of the rock

fabric. Rock-extrinsic parameters are factors that are not physically connected tothe rock fabric, but are determined by external boundary conditions. Examples ofrock-extrinsic parameters are burial depth, confining pressure and age of the


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Vo l . 1 41 , 1 9 9 3 C o n t r o l s o n S o n i c Ve l o c i t y i n C a r b o n a t e s 3 01

sediment. It will be shown that in carbonates the rock-intrinsic factors are moreimportant than the extrinsic ones.

A. Veloc ity as a Funct ion o f Rock-ex t r ins ic Param eters

The e ffec t o f mechan ica l compact ionThe compaction experiments and the velocity measurements on modern carbon-

ate mud from Florida Bay were performed to determine the change in velocity dueto a porosity reduction from solely mechanical compaction. The samples are purecarbonate mud and have a special lithology that is rarely encountered in the othermeasured carbonate samples. However, most o f the measured samples have,

together with the coarser grain fraction, a large amount of micrite in the matrix.Therefore, we suspect that compaction in our other samples would have a similareffect on velocity as in the compacted mud.

It is known that porosity has a major influence on velocity and a porositydecrease usually produces a velocity increase. The Florida Bay samples show arelatively subtle increase in velocity with increasing compaction or decreasingporosity (Figure 9). At porosities close to 60%, poorly compacted samples have aVp of 1700 m/s and no measurable V,. The samples had to be compacted by10-15% in order to receive a Vs signal. This corresponds well with the measure-

ments of LAUGttTON (1957) who only detected shear waves in unconsol idated

2500 - -

2000

, ~ 1 5 0 0@

lOOO

I I I r r I

vp,_9 r i

[][ ] [ ]

V s [ ] [ ]

[ ][ ]

500 i i J _ i ~t I _ _25 30 35 40 45 50 55 60

P o r o s i t y ( % )

F i g u r e 9I n c r e a s e o f V p a n d V a t d e c r e a s i n g p o r o s i t i e s i n t h e d i ff e r en t ly c o m p a c t e d F l o r i d a B a y s a m p l e s . T h ev e l o ci t y i n c r e a s e is th e e f fe c t o f p u r e c o m p a c t i o n . I n i t i a l p o r o s i t y o f th e c a r b o n a t e m u d i s o n a v e r a g e

6 3 % . T h e l it tl e c o m p a c t e d s a m p l e s w i t h p o r o s i t i e s a b o v e 5 0 - 5 5 % c o u l d n o t t r a n s m i t a V s i g n a l.


16/37

3 0 2 F. S . A n s e l m e t t i a n d G . P. E b e rl i PA G E O P H ,

ocean sediments above a compacting pressure of 5 MPa. At maximum compactionwith porosities o f 29%, Vp increases to 2200 m/s and V, lies around 900-1100 m/s.The gradient of the measured increase in velocity of the compacted mud issignificantly lower than the observed gradients in the other Bahamas and Maiellasamples (Figure 10). The mud samples display, due to their low shear modulus, abehavior similar to material that has no rigidity as suggested by HAMILTON 1 9 7 1 ) .He showed that, unlike liquids, most unconsolidated marine sediments do possessrigidity (shear modulus > 0) and have a definite structure. The Wood equation(WooD, 1941), valid for mediums without shear modulus or rigidity, can be usedto compare the observed porosity-velocity relation of the artificially compactedsediments.

g p = [ ( ( I ) f l f l u i d ~ - ( 1 - - ( I ) ) f l s o l i d ) ( ( I ) p f l u i d - ~ -(1 - ( i ) ) P s o l i d ) ] - - l /2

q) = fract ional porosi ty; fl = compressibility; p = density.

The Wood equation was calculated using values for water (4.06- l0 -1~ m2/N)and for calcite (1.34" 10-11 m2/N) compressibility, because the elastic properties ofaragonite are not well-known. The comparison of the calculated with the measuredvelocities reveals that the measured velocities of the mud samples are in fact onlyslightly above the values predicted by the Wood equation, whereas all other samplesthat were altered during diagenesis show much higher velocities (Figure 10). The

70 00 ~ "'OJ ' ' ' '

6000 ~ l , |

I ~ o .,,~ .% . . . . . 94 o o o [ 9 9 "

~ " o " ' b - . . c ~ - . . " _ " " "

. . . . .o o o " : - : " "

---O --- M aiella samples

1000 . . . .E3--- Florida B ay sam ples

. . . . 9 - -- B aham as s am p les - - Wood equat ion0 I I I I I

0 10 20 30 40 50 60P oros i t y (%)

F i g u r e 1 0Ve l o c i t y a s a f u n c t i o n o f p o r o s i t y f o r t h e t h r e e d a t a s e t s . V p a n d p o r o s i t y s h o w c l e a r ly a n i n v e r s ec o r r e la t io n , b u t t h e g r a d i e n t o f i n c re a s i n g V p a t d e c r e a s i n g p o r o s i ty i s m u c h l o w e r i n t h e c o m p a c t e dF l o r i d a B a y s a m p l e s t h a n i n t h e n a t u r a l r o c k s o f t h e B a h a m a s a n d t h e M a i e ll a . Ve l oc i ti e s o f t h e m u d

s a m p l e s a r e o n l y s l ig h t ly h i g h e r t h a n v e lo c it ie s o f m e d i u m s w i t h o u t r i g id i ty, c a l c u l a te d u s i n g t h e W o o de q u a t i o n ( W o o D , 1 94 1). T h i s s h o w s t h a t p o r o s i ty r e d u c t i o n d u e t o m e c h a n i c a l c o m p a c t i o n h a s o n l y am i n o r e f f e c t o n V p , w h e r e a s p o r o s i t y r e d u c t i o n d u e t o d i a g e n e t i c p r o c e s s e s (e . g ., c e m e n t a t i o n ) c a n

i n c r e a s e r i g i d i ty w h i c h r e s u l t s in h i g h e r v e l o c it ie s .


17/37


lower gradient for the mud samples can thus be explained by the absence ofaddit ional processes tha t increase the rigidity of the rock and normally accompanythe compaction of sediments. The artificial compaction experiments happen so fastthat no diagenetic alterations are initiated. Normally, the effects of diageneticprocesses, such as recrystal lization or cementat ion, are superimposed on the effectof porosity reduction. The velocity, therefore, represents the combined effect ofthese different processes. The difference between the veloci ty-porosity correlation innatural rocks and in the artificial compacted rocks demonstrates how muchdiagenesis contributes to the observed velocity increase. In fact, our samplesdocument that diagenetic alterations are more effective in increasing velocity thancompaction, because they significantly increase the rigidity of the rock.

With a uniaxial pressure o f 170 MPa, the poros ity could not be reduced tounder 29%, indicating that the microfabric of the rock, consisting of 65% aragoniteneedles, is close to the densest packing that can be reached just by mechanicalcompaction. This experimental compaction also shows that pure mechanical com-paction only plays a minor role in carbonates. Samples with porosities between 0and 25 percent can only reach their actual porosity with the aid of diageneticclosing (cementation) of part of the pore space.

Burial depth and age o f the sediment

The measured samples taken from the two core borings of the Bahamas DrillingProject clearly show tha t in these carbonates, velocity is neither primarily a functionof the sediment age, nor of the burial depth. In contrast to the usual assumptionthat velocity increases with depth (HAMILTON, 1980; JAPSEN, 1993), the depth plotsof Vp and Vs (Figure 3) in the two drillholes display velocity inversions that makevelocity predictions, based only on depth, impossible. Both holes display a patternof high variability of velocity in the carbonates that were deposited in a shallow-wa-ter environment like sediments from the reef, platform margin or platform interior.These high velocity zones, in Clino above 220 m and in Unda between 290 and

370 m and above 120 m, overlie zones of low velocities that cause the observedvelocity inversions. The distinct jump to higher velocities at Unda 293 m, forinstance, marks the transition from a fully dolomitized carbonate sand to adolomitized reefal unit. Below the reefal unit, both Vp and Vs decrease again,resulting in a velocity inversion. Rocks of the low velocity zones are mostlycarbonates that were deposited in deeper water and underwent less diageneticalteration than the shallow-water carbonates. The inverse trend with decreasingvelocities at greater depths indicates tha t diagenetic processes other than simplecompaction substantially control the velocity evolution. In the young sediments of

shallow Clino and Unda, high velocities are attained due to intense diageneticalterations which occur much faster than compaction due to an increased over-burden.


18/37


The Maiella carbonates also demonstrate that depth, or in this case age, doesnot necessarily influence velocity. Some of the Upper Cretaceous rudist sands areamong the oldest but also have the slowest velocities of the measured samples fromthis data set. The Vp of 3300 m/s is remarkably low for Cretaceous carbonates,document ing again the insignificance of absolute age for velocity evolut ion in thesesamples. The reason for this low velocity is the preserved high porosi ty and theassociated interparticle pore type.

As a consequence, the depositional environment and the diagenetic alteration ismuch more importan t for velocity evolution than age or depth. Velocity predictionscannot be made solely with the knowledge that a carbonate sediment has a certainage and/or is at a certain depth, rather the acquisition of some additional, intrinsicrock parameters is necessary to produce a reliable velocity estimation.

Effective pressureTo investigate the pressure dependence of V; and V~, sonic velocities o f the

Bahamas and Maiella samples were measured under varying confining and pore-fluid pressures. Sonic veiocky in rocks is a funct ion of the differentia[ pressure, oreffective pressure, which is the difference between the confining and the pore-fluidpressure. Minor departures from this relation are caused by changing pore-fluidproperties at different pressures (COYNER, 1984).

At low pressures, all samples show an increase in velocity with increasingeffective pressure due to better grain contacts, changing pore shapes and closing ofmierocracks (GARDNEk et at . ,1974). This increase is large for slow, unconsolidatedsamples, whereas the velocities o f indurated, dense samples are usually less affectedby higher pressures. All velocity-pressure traces of the Bahamas samples plotted inthe same graph (Figure 11) display a systematic pattern with higher gradients forlow-velocity samples and lower gradients for fast samples. A minor part of theobserved velocity increase is an artifact because compaction at increasing pressuresreduces the sample length which is used to calculate the velocities.

A characteristic of many samples is tha t bo th Vp and V~ reach a maximumduring increasing pressure and suddenly begin to decrease above a critical pressure.This velocity decrease is caused by a continuous disintegration and collapse of thesample in the pressure vessel, which progressively destroys the partly cementedgrain contacts that supported the transmission of the acoustic signal. Eventually,velocity can increase again (e.g., sample Unda 141 m, Figure 12d) because thenewly formed fractures that reduced the velocity can close by a further increase inconfining pressure. In these samples, velocities tha t are measured under decreasingpressure at the end of a hysteresis loop (Figures 12d,e) are much lower than at thesame pressures in the first part of the loop, because the fractured plugs completely

fall apart and form an unconsolidated fabric with loose fragments. V p / V,increasesdramatically above the critical pressure (Figure 12f) and continues to increase withdecreasing pressure in the hysteresis loop. Shear-wave velocity is extremely affected


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Vo l . I 4 I , 1 9 9 3 C o n t r o l s o n S o n i c Ve l o c i t y i n C a r b o n a t e s 305

5500

4500

3500

2500

i i i I6500

, . , : . ' , , , , , , , , . . . . . .

1500 I i . . . . . . i ,20 40 60 80 100

E ff e c t i v e p r e s s u r e(MPa)

F i g u r e 1 lVe l o c i t y e v o l u t i o n o f t h e B a h a m a s s a m p l e s a t i n c r e a s i n g e ff e c ti v e p r e s s u r e . E a c h t r a c e r e p r e s e n t s t h ev e l o c it i es f o r o n e s a m p l e a t d i f f e r en t p r e s s u r e s .The g r a d i e n t o f lo w - v e l o c i t y s a m p l e s i s h i g h e r t h a n t h einc reas e o f ve loc i t i e s in s ample s w i th h igh ve loc i t i e s . D e creas in g ve loc i t ie s a t h igh er p res s u res m ark ac r i ti c a l p r e s s u r e a t w h i c h t h e s a m p l e s a r e i n t e n s el y f r a c t u r e d a n d c o l l ap s e . A m i n o r p a r t o f t h e v e l o c i ty

i n c r e a s e i s a n a r t i f a c t c a u s e d b y c o m p a c t e d s a m p I e l e n g t h , t h a t i s u s e d f o r v e l o c i t ycalculation.

by the destroyed fabric that results in a nonelastic behavior. A similar behaviorwith decreasing velocity during increasing pressure was also observed in someCretaceous samples from the Maiella, indicating that age or burial depth is not aguarantee for consolidation and lithification of a carbonate sediment.

In contrast to the nonelastic behavior, the hysteresis loops of fast, more lithifiedsamples (e.g., sample, Clino 657 m (Figures 12a-c) show a gentle increase invelocities with increasing pressure. Vp, Vs and also Vp/V s eventually reach a plateauat high pressures and they nearly reach the former velocities at decreasing pressures.In these cases, the plugs are perfectly intact when they are removed from thepressure vessel, documenting the elastic behavior of the high-velocity rocks.

The critical pressure at which the first velocity decrease occurs varies with thedifferent lithologies. Dense, indurated rocks display no evidence of fabric destruc-tion up to the highest measured pressures of 100 MPa, whereas soft, unconsol idatedsamples show signs of velocity decrease already at 5 MPa. These low critical

pressures demonstrate that some carbonates, especially most slope deposits orsucrosic dolomites, became buried without being progressively indurated. Underhydrostatic conditions, an effective pressure of 5 MPa is equivalent to a burial o f


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3 06 F. S . A n s e l m e tt i a n d G . P. E b e rl i PA G E O P H ,

4 6 0 0

4400

( m / s )

4200

4000

2800

2 6 0 0

( m / s )

2400

S a m p l e C l i n o 6 5 7 m

V p

(a )i I P i

J i

V s l + V s 2

., r

(b )

S a m p l e U n d a 1 4 2 rn

9 /(d )

V s l + V s2

(e )i i i i

4000v p

3800

( m / s )

3 6 0 0

3400

2200

2000

( m / s )

1800

2200 - - , , - - , , - - 1600

1.8 2.1

1 ,7

VpN~ V p / v s

(f )i i i F

20 40 60 80C o n f i n in g p r e s s u r e ( M P a )

1 .9

(c )i i i i _ _

20 40 60 80 100C o n f i n i n g p r e s s u r e ( M P a )

1 .8100

F i g u r e 1 2Tw o e x a m p l e s f o r e la s t ic a n d n o n e l a s t i c b e h a v i o r : S a m p l e C l i n o 6 5 7 m s h o w s a s t e a d y i n c r e a s e i n V p ( a )

a n d V s ( b ) ; t h i s in c r e a s e is m a i n l y c a u s e d b y t h e c l o s i n g o f m i c r o c r a c k s a t e l e v a t e d p r e s s u r e s . T h ei n c r e a s i n g V p / V,( c) s h o w s t h a tVp i n c re a s e s m o r e t h a n V . T h e v a l u e s re a c h a p l a t e a u a t h i g h p r e s s u r e sa n d a p p r o a c h s t a r ti n g c o n d i t i o n s a t t h e e n d o f th e h y s t e re s i s -l o o p . A f t e r t h e e x p e r i m e n t , t h e p l u g s h o w sn o s i g n s o f d a m a g e . S a m p l e U n d a 1 4 2 m s h o w s a s i m i l a r i n c r e a s e in V p a t l o w p r e s s u r e s b u t a t a c r i ti c a lp r e s s u r e o f 4 0 M P a , V p a n d V, s t a r t t o d e c r e a s e ( d a n d e ) . T h i s d e c r e a s e is t h e r e s u l t o f f a b r i c d e s t r u c t i o ni n t h e p r e s s u r e v e s s e l. T h e p l u g i s fr a c t u r e d a n d c e m e n t e d g r a i n c o n t a c t s a r e d e s t r o y e d s o t h a t v e l o c i t ie sd e c r ea s e . A b o v e 6 0 M P a , V p s t a r t s to i n c r ea s e a g a i n ( d ) b e c a u s e t h e n e w l y f o r m e d f r a c tu r e s a r ep r o g r e s s i v e l y c l o s e d . Wi t h t h e r e l e a se o f p r e s s u r e , V p a n d V d e c r e a s e d r a m a t i c a l l y b e c a u s e t h e f r a c t u r e dp l u g d i s i n t e g r a t e s . T h eV p / Vs i n c re a s e s r e m a r k a b l y a b o v e t h e c r i ti c al p r e s s u r e (4 0 M P a ) a n d b e c o m e s

e v e n h i g h e r a t t h e e n d o f t h e h y s t e r e s i s - l o o p (f ) . P o r e - f l u i d p r e s s u r e e q u a l s 2 M P a f o r a ll s a m p l e s .


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Vol. 14 l, 1993 Controls on Sonic Velocity in Carbonates 307

l es s t h a n 5 0 0 me t e r s . T h e re fo re , so m e p a r t s o f t h e d r i ll ed co re s m u s t h a v ein si tuc o n d i t i o n s t h a t c a u s e d e v e l o p m e n t o f c r a c k s a n d f r a c tu r e s . T h i s o b s e r v a t i o nco i n c i d e s w e l l w i t h o p en o r p a r t l y cem en t ed f r ac t u r e s t h a t a r e v i s ib l e i n p a r t o f t h ec o r e s. A l s o r e m a r k a b l e is t h a t m a n y s a m p l e s s h o w n o s ig n s o f f a b r ic d e s t r u c t i o n u pt o h i g h p r e s s u r e s , d e m o n s t r a t i n g t h a t p o r o s i t y w i t h i n p a r t l y c e m e n t e d r o c k s c a n b ep r e s e rv e d , e v e n a t p r e s su r e s o f 1 00 M P a o r a t d e p t h o f a p p r o x i m a t e l y 5 k m .

B . Ve l o c i t y a s a F u n c t i o n o f R o c k - i n t r i n s i c P a ra m e t e r s

D e p o s i t i o n a l l i t h o l o g y

T h e l i t h o lo g y o f a c a r b o n a t e s e d i m e n t a t t h e t i m e o f d e p o s it i o n h a s s t r o n g

i n f l u en ce o n t h e ev o l u t i o n o f v e l o c it y, i n t h a t i t co n t ro l s f u t u r e a l t e r a t i o n s o f t h ero ck . A t t h e t i me o f d ep o s i t i o n a l l u n c o n s o l i d a t ed s ed i men t s h av e s i mi l a r v e l o ci t ie sb e t w een 1 55 0 an d 1 80 0 m/ s . T h e d i f f e r en t li t h o l o g ie s h av e , d e s p i t e t h e i r s i m i l a rv e l o c i t i e s , d i f f e r en t s u s cep t i b i l i t i e s t o d i ag en e t i c a l t e r a t i o n t h a t w i l l ch an g e t h ep h y s i ca l p ro p e r t i e s a n d t h u s t h e v e l o c it i es . Th e d i ag en e t i c s u s cep t i b i l it y o f s p ec ia ls ed i m en t t y p es cau s e s f a st o r s l o w a l t e r a t i o n s o f t h e ro ck f ab r i c , d ep en d i n g o n t h ed i a g e n e ti c p o t e n t i a l o f t h e s e d i m e n t(SCHLANGER a n d DOUGLAS, 1 9 7 4 ) an d o n t h ed i ag en e t i c r eg i me .

Th e d i ag en e t i c p o t en t i a l i n c a rb o n a t e s i s ma i n l y a f ac t o r o f t h e g r a i n s i ze an d

t h e a m o u n t o f m e t a s t a b l e m i n e ra l s. A h i g h c o n t e n t o f f i n e- g r a in e d m i c r it i c m a t e r i a l ,a s f o u n d i n mu d o r w ack es t o n es , r e s u l t s i n a l o w p e rmeab i l i t y. Th e r e s u l t i n g l o wf l ui d f l o w in h i b i t s o r s l o w s d i ag en e t i c a l t e r a t i o n s t h a t r e l y o n t r a n s p o r t o f ch emi ca lc o m p o n e n t s i n t h e w a t e r. I n c o n t r a s t t o f i n e - g r a i n e d r o c k s , s e d i m e n t s w i t h ag r a i n - s u p p o r t e d f a b r i c a n d a l o w c o n t e n t o f m i c ri t e ( g r ai n - t o p a c k s t o n e ) h a v e ah i g h e r p e rmeab i l i t y an d , a s a co n s eq u en ce , a h i g h e r f l u i d f l o w. Th i s a cce l e r a t e sd i ag en e t i c p ro ces s e s an d t h e s ed i men t i s q u i ck l y a l t e r ed an d co n s o l i d a t ed . Th u s ,o r i g i n a l co a r s e -g ra i n ed ro ck s can r each h i g h e r v el o c i ti e s a f t e r a s h o r t t i m e o f b u r i a l ,w h e rea s f i n e -g ra i n ed ro ck s t en d t o p r e s e rv e t h e i r u n a l t e r ed f ab r i c an d t h e i r s l o w

v e l o c i ty f o r l o n g e r b u r i a l d u r a t i o n s .I n a d d i t i o n t o g r a i n s iz e, t he a m o u n t o f m e t a s t a b l e m i n e r a ls , s u c h a s a r a g o n i t e

o r h i g h - m a g n e s i u m c a l c i t e c o n t r o l s t h e d i a g e n e t i c p o t e n t i a l(SCHLANGER a n dD O U G LA S, 1 9 7 4 ) . A h i g h amo u n t i n me t a s t ab l e co mp o n en t s c au s e s a h i g h d i ag e -n e t i c p o t en t i a l an d l e ad s to a r ap i d d i s s o l u t i o n o r r ec ry s t a l l i z a t i o n o f t h e s ed i men t .Th i s a l t e r a t i o n can en h an ce t h e e l a s t i c p ro p e r t i e s an d i n c r ea s e p e rmeab i l i t y, r e s u l t -i n g i n acce l e r a t ed l i t h i f i c a t i o n .

T h e c o m p a r i s o n o f t h e v e l o ci t y r a n g e w i t h t h e d e p o s i t io n a l e n v i r o n m e n t c o n -f i rms t h e s e r e l a t i o n s h i p s . F o r ex amp l e , i f t h e Ma i e l l a s amp l e s a r e g ro u p ed i n t o t w o

c a t e g o r ie s o f d e p o s i ti o n a l e n v i r o n m e n t , ( 1 ) p l a t f o r m d e p o s i ts a n d ( 2 ) ba s in , s lo p ean d d eep e r s h e l f d ep o s i t s (F i g u re 1 3), th e v e l o c i t y r an g e o f t h e t w o ca t eg o r i e s fo r mt w o d i f f e r en t c l u s t e rs t h a t o n l y o v e r l ap i n t h e h i g h v e l o c i t y a r ea . Th e p l a t fo rm


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3 08 F. S . A n s e l m e t t i a n d G . P. E b e rl i PA G E O P H ,

7000

I

6000

2 5 0 0 0g

4000

3000

2000 i __

0 40 50

I I I

dopo its

Depo sits from deepershelf to slope and b asin

I ] I

10 20 30P o r o s i t y ( % )

F igu re 13Ve l o c i ty v s. p o r o s i t y ( a t 8 M P a e f fe c t iv e p r e s s u r e ) c o m p a r e d w i t h d e p o s i t i o n a l e n v i r o n m e n t s f r o m t h eM a i e l l a s a m p l e s . C a r b o n a t e s d e p o s i t e d o n t h e s h a l l o w - w a t e r p l a t f o r m h a v e a n a r r o w r a n g e w i t h h i g hVpa n d l o w p o r o s i t i e s . S e d i m e n t s f r o m t h e d e e p e r s h e l f , s l o p e o r b a s i n s h o w a h i g h e r v a r i a b i l i t y t o w a r d s

l o w e r Vp,b u t m a x i m a l V p a r e t h e s a m e a s in t h e p l a t f o r m r o c k s .

carbonates have a s ignif icant ly h igher average veloci ty than the bas in and s lopecarbon ates . In the Bah am as cores , the re la t ion betw een shal low water depos i ts anddeeper water depos i ts i s s l ight ly d i fferent wi th an over lap of veloci t ies of the twocategor ies in the low veloci ty area (F igure 3). The m ajor i ty o f the Bah am ian s lopecarbona tes a re unconso l ida ted and thus s low, whereas the p la t fo rm ca rbona tesshow a larger range towards h igher veloci t ies , but have s imilar minimal veloci t ies .

The ex plana t ion for these observed veloci ty pat terns is that the p la t formcarbonates are usual ly h igh in coarse-skele ta l gra ins or non-skele ta l gra ins (ooids ,pelo ids) . They cons is t pred om inan t ly of aragoni te w hich is me tas table in sea water.

S lope or deeper water depos i ts are normal ly character ized by a h igh rn icr i t ic gra inf ract ion and by a h igher con tent in pelagic calcareous org anism ( foraminiferas ,coccol i thes) and cons is t mainly o f mo re s table low -M g calc ite shel ls . T herefore ,shal low-water carbonates fu l f i l l both condi t ions for fas t d iagenet ic a l tera t ions :coarse gra in s ize and a h igh am ou nt of me tas table minerals . S om e turbid i tesdepos i ted on the s lopes , that conta in many skele ta l and aragoni t ic f ragments f romthe p la t fo rm top , are s imilar to p la t form depo s i ts (EBERL~, 1988), and thus d i fferentf rom the norm al bac kg rou nd s lop e depos i ts . The d i fferent ia l depo s i t ional l ithologiesexpla in the d i fferent ranges o f the veloci ty me asurem ents in the d i fferent sediment

types . In the o lde r Maie ll a s amples , mos t o f the p la t fo rm ca rbona tes had enoug htime for diagenetic al terations to reach their high, f inal velocit ies , and as aconse quen ce c lus ter a t h igher values than the s lope sediments (F igure 13) . In the


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Vol. [4i, 1993 Controls on Sonic Velocity in Carbonates 309

Bahamas cores, which are younger than the Maiella samples, not all the platformdeposits reach high velocities, but both the average velocity and overall velocityrange are much higher than in most of the slope samples. Only the few turbiditesin the slope section containing platform derived material have high velocitiesresulting in some velocity variability (Figure 3).

The data strongly suggest that the depositional environment of a carbonatesediment affects the starting conditions under which a sediment undergoes diage-netic alterations. This indirect influence controls direct, rock-intrinsic parameterssuch as porosity, pore type, density and mineralogy.

Mine ra logyIn siliciclastic rocks, the physical variety of minerals is large (e.g., quartz and

clay), and mineralogy has more influence on sonic velocity than in carbonates(CHRISTENSENand SZYMANSKI,1991). The minimal influence of mineralogy onvelocity in carbonates can be partially explained by the small velocity contrasts ofthe two dominant carbonate minerals calcite (6500 m/s) and dolomite (6900 m/s).Pure carbonates have little initial velocity differences due to mineralogy. Themeasured samples from the Bahamas cores are comprised >95% of mineralscalcite, dolomite and aragonite and the Maiella carbonates consist almost purely ofcalcite.

Our data suggest that changes of this mineralogical composition have no major

influence on velocity. A plot of the dolomite content versus velocity of the Bahamassamples clearly shows that there is no correlation between dolomite content andvelocity (Figure 14). This lack of correlation is also shown by two measured plugsof Unda, only 7 m apart and both made of 100% dolomite: sample Unda 286 m hasa Vp of 2697 m/s and a Vs of 1052 m/s, whereas sample Unda 293 m has a Vp andV, of 5953 m/s and 3187 m/s, respectively. The high-velocity sample is a "reefa l"-dolomite with a fabric preserving dolomitic cementation resulting in a total porosityof 14%, whereas the low-velocity sample is a sucrosic dolomite with high interpar-ticle porosity of 49%. This example demonstrates that velocity depends on the typeof dolomite and thus the associated porosity and pore type, and that mineralogyalone is not a characteristic parameter for determination of velocity in carbonates.

While the mineralogical composition in carbonates has little influence onvelocity, the processes that al ter mineralogy, such as sucrosic dolomit izat ion ordolomitic cementat ion, have a strong influence on velocity. These processes alsoalter, in concert with changing mineralogy, porosity and porosity type. For exam-ple, fabric destructive dolomitization also destroys most of the earlier cementation,creating an undercemented and loose dolostone with petrophysical characteristicssimilar to a semi-lithified carbonate sand.

Po ros i t y and po re t ypesVelocity is strongly dependent on the rock porosity (WANG e t aL , 1991;

RAFAVICH et al., 1984). A plot of porosity versus velocity displays a clear inverse


24/37

3 10 F. S . A n s e l m e t ti a n d G . P. E b er li PA G E O P H ,

7000

6000

5000

~,~4000eL

3 0 0 0

2000

1000

I

iO

. o , t . s . . . . .

I I I I - -

0 20 40 60 80 100D o l o m i t e c o n t en t (% )

F i g u r e 1 4Ve l o c i ty a t 8 M P a e f fe c ti v e p r e s s u r e a s a f u n c t i o n o f d o l o m i t e c o n t e n t i n t h e B a h a m a s s a m p l e s . T h e r ei s n o c o r r e l a t i o n b e t w e e n t h e s e t w o f a c t o r s . D i f f e r e n t d o l o m i t e t y p e s , s u c h a s s u c r o s i c d o l o m i t e o rd o l o m i t i c c e m e n t h a v e t o t a l l y d i ff e r e n t e f fe c t s o n v e l o c i ty, t h e r e f o r e d o l o m i t e c o n t e n t a l o n e c a n n o t b e

u s e d a s a n i n d i c a t o r f o r v e l o c i t y.

trend; an increase in poros ity produces a decrease in velocity (Figure 15). Thegeneral trend of the Bahamas and Maiella samples has correlation coefficients of0.94 for Vp and 0,92 for Vs. Nevertheless, the measured values display a largescatter around this inverse correlation in the velocity-porosity diagram. Velocitydifferences a t equal porosit ies can be over 2500 m/s, in par ticular at higherporosities. For example, rocks with porosities of 40% can have velocities between2100 m/s and 5000 m/s, which is an extraordinary range for rocks with the samechemical composition and the same amount of porosity. This discrepancy is causedby the ability of carbonates to form cements and special fabrics with pore types thatcan enhance the elastic properties of the rock without filling all the pore space. The

high elastic moduli result in velocities that are higher than velocities predicted bytheoretical equations, such as the time average equation ( W Y L L I E e t al . , 1956), asshown in Figure 15.

In other data sets, such as synthetic sand-clay mixtures (MARION e t a l . , 1992)or siliciclastic sediments (VERNZK and NUR, 1992), a similar scattering in thevelocity-porosity diagram is observed. But unlike carbonates, the scattering in theserocks can be explained by compositional variations, in particular by changes in claycontent. WILKENS e t a l . (1991) noticed that velocities of low-porous basalts arevery dependent on the pore shapes. Samples containing pores with low aspect ratios

(cracks) are associated with lower velocities, compared to samples with round poresor high aspect ratios. As a result, high velocity contrasts are observed betweenrocks without large variations in total porosity. The pores in our high porosity


25/37

Vo l . 1 41 , 1 9 93 C o n t r o l s o n S o n i c Ve l o c i t y i n C a r b o n a t e s 3 I 1

7000 [-- ~ , -~ ,.. . - - 9 - " Vp [= 6503* e ( - 0 "01 87 P o t ) ]

6000 ~ q ~ [~ . ~ . . . . . Us [= 35 61 , e (-Om l 4 P ot )]

5 o o o ~ , ~ a ~ . , , , , ,

r ~ - ~ " ' , . ' , " " " 4 o o o , . .

. ~ n ~ E m " .

3 o o o o

[3 "~ - o o o g o

~ooo - " - - ~ - - ~ - ~ o - ' ~ - ' ~ " 9 -

1000 Time-average t ~ % - ~ .~_equationfor Vp

0 t I I I I

0 10 20 30 40 50 60P o r o s i t y ( % )

F i g u r e 1 5V p a n d V f r o m t h e B a h a m a s a n d t h e M a i e l la s a m p l e s a t 8 M P a e ff ec ti v e p r e s s u r e as a f u n c t i o n o fp o r o s i t y w i t h e x p o n e n t i a l b e st fi t e q u a t i o n s ( d a s h e d l in e s) . B o t h V p a n d V d e m o n s t r a t e t h e t r e n d o fd e c r e a s i n g v e l o ci t ie s w i t h i n c r e a s i n g p o r o s i t i e s , b u t s c a t t e r, e s p e c ia l ly a t h i g h e r p o r o s i t ie s , a r o u n d t h ed o t t e d b e s t f it li n e s. T h e s c a t t e r i n g is a r e s u lt o f s p e c ia l f a b r i c s a n d p o r e t y p e s t h a t e n h a n c e t h e e l a s ti cm o d u l i o f th e r o c k w i t h o u t f il li ng al l th e p o r e s p a c e. A s a c o n s e q u e n c e , m e a s u r e dVpa r e h i g h e r t h a nVp

p r e d i ct e d b y t h e o r e ti c a l e q u a t i o n s s u c h a s t i m e - a v e r a g e - e q u a t i o n ( W v L L I Eet al. , 1956) .

ca rbo na t es genera l l y have h igh aspec t r a ti o s . In t h i s case, t he h igh v e loc i ty con t ras t sbe tween roc ks w i th s im i l a r t o t a l po ros i t y can be r e l a t ed t o spec if ic po re t ypesresu l t ing i n cha rac t e r i s t i c and ve ry d i f f e ren t e l a s ti c p roper t i e s . B ased on t h in s ec t i ono b s e r v a t io n s , t h e B a h a m a s s a m p l e s c a n b e g r o u p e d i n t o f iv e c a t e g o ri e s o f p r e d o m -inan t po re t ypes wh ich a l l have cha rac t e r i s t i c c l u s t e r s i n t he ve loc i t y -po ros i t yd i a g r a m ( F i g u r e 1 6 ) . T h e f iv e d o m i n a n t p o r e t y p e s t h a t c a n b e d i s ti n g u i s h e d ar e:

(1) Interpart ic le and intercrysta l line po rosi ty (Figures 1 7a,b ) :T h e p o r o s i t y b e -tween t he componen t s o f a s ed imen t i s t he i n t e rpa r t i c l e po ros i t y. Th i s po ros i t y

p r e d o m i n a t e s a f te r d e p o s i t i o n o f a s e d i m e n t w h e n g r a i n s f o r m a l o o s e p a c k a g e w i t hl i t t l e cemen ta t i on . In t e rc rys t a l l i ne po ros i t y deve lops a t a l a t e r s t age du r ing d i agen -e si s, w h e n n e w l y c r y s ta l li z e d m i n e r a l s s u c h a s d o l o m i t e r h o m b o h e d r a f o r m a l o o s eaggrega t e . I t has a s im i l a r pe t rophys i ca l behav io r a s i n t e rpa r t i c l e po ros i t y. Thea c c u m u l a t i o n o f u n c o n n e c t e d g r a i n s w i t h o u t c e m e n t o r m a t r i x r es u lt s in a l o wve loc i t y because t he ro ck h as l ow e l as t ic m odu l i due t o t he l ack o f a r i g idf r a m e w o r k . M o s t o f t h e se s a m p l e s th e r e f o r e s h o w a n e g a t iv e d e p a r t u r e f r o m t h eaverage ve loc i t y -po ros i t y co r re l a t i on (F ig u re 17c).

(2) Micro -po ros i ty :M i c r o - p o re s ( < 1 0 # m ) a r e a b u n d a n t i n c a r b o n a t e m u d ,

e i t he r in a m ic r it ic g ra in o r i n t he m ic r it i c ma t r i x . H igh m ic ro -po ros i t y is t huse x p e c te d i n c a r b o n a t e s w i t h a h i g h m i c r it ic c o n t e n t . D u e t o t h e l a c k o f c e m e n t a t i o ntha t r e su l t s in an un con nec t ed g ra in f ab r i c , m ic ro -p o ros i t y has a s im i l a r e f f ec t on


26/37

312 F. S . Anse lmet ti and G. P. Eber li PAG EO PH,

7 0 0 0 ~ - ~ ,

6 ~ 1 7 6 1 7 6o ~ 8o

5 0 0 0 ~

~ _ ,4 0 0 0

3000

2000

I 0 0 0

D o m i n a n t p o r e t y p es :

9 i n t e r p a r t i c l e a n di n t e r c r y s t a l l i n e p o r.

[ ] i n t r a p a r t i c l e p o r.

[ ] m o l d i c p o r. ( > 3 0 0 g in )

g m o l d i c p o r. ( < 3 0 0 g m )

9 m i c r o p o r o s i t y

,0 , ! o w ~ o r o s i t y, _ ! n te n s e l y c e m e n t e d

1 0 2 0 3 0P o r o s i t y % )

Figure 16

oO

[] F21[] []

[ ] [ 3 [ ] [ ]9 [ ] [3 [3 0

~ *

l ~ Elm

L . . . . I . . . .4 0 5 0 6 0

Velocity vs. porosity diagram from the Bahamas samples, with observed categories of different poretypes. T he large scattering, e.g. velocities from 2200 to 500 0 m /s at porosities of 40% , are a result ofdifferent p redo m inan t pore types in the analyzed samples. Rocks w ith moldic or intrapart icle porosi tyhav e positive depa rtures from the general trend whereas ro cks with interparticle, intercrystalline ormic ro-po rosity have relatively low velocities and th us show nega tive depa rtures. Velocities are taken at

an effective pressure of 8 MP a.

v e l o c i ty a s f i n e - g r a i n e d , i n t e r p a r t i c le p o r o s i t y a n d a l so s h o w s a n e g a t i v e d e p a r t u r e

f r o m t h e a v e r ag e v e l o c i t y -p o r o s i t y t re n d .

( 3 ) M oIdic poro sity (Figures 17d,e): M o l d i c p o r o s i t y d e v e l o p s b y d i s s o l u t io n o fg r a i n s w i th a m e t a s t a b l e m i n e r a l o g y ( e . g. , g r a in s o f a r a g o n i t e a n d h i g h M g - c a l c it e ) .

S e l e ct iv e d i s s o l u t i o n c a n o c c u r b e f o r e, a f t e r o r d u r i n g c e m e n t a t i o n o f t h e i n t e r p a r -

t ic l e p o r e s p a c e. A f t e r d i s s o l u t i o n , t h e r o c k c o n s i s t s m a i n l y o f m o l d s a n d t h e

p a r t i a l l y c e m e n t e d f o r m e r i n t e r p a r t i c l e p o r e s p a c e w h i c h is a f a b r ic t y p e w i t h h i g h

e la s ti c p r o p e r t i es . S a m p l e s i n w h i c h m o l d i c p o r o s i t y p r e d o m i n a t e s , h a v e h i g h e r

. . . . ,)

Figure 17Tw o examples for pore types with characteristic clusters in the V p-porosity diagram: a) Photo m icrog raphof sample U nd a 286 m as an examp le for a rock w ith intercrystal l inc porosi ty. The rock consistscompletely of micro-sucrosic dolomite . Plug-porosi ty is 49%. b) Computer scan of photomicrographabov e (porosity black, particles white) with characteristic patte rn of loose particles (d olom ite rho m bo he-dra) surrounded by connected pore space, c) Vp-porosi ty diagram of al l samples with dominantinterparticle and intercrystalline porosity. Vp are in general below average tre nd d ue to the tack o fconn ect ions between the grains, d) Photom icrograph o f sample U nd a 65 m as an example for a rock withcoarse m oldic porosity. All grains were dissolved after ce m entatio n of the in terparticle pore space.Plug-porosi ty is 37%. e) Com puter scan o f photom icrograph above (poro si ty black, part ic les white). T henon conn ected mo lds are integrated in a framework of sparry cement, f ) Vp-porosity diagram of al lsamples with do m ina nt coarse m oldic porosity. Vp are significantly ab ove average Vp-porosity trend du e

to the rigid framework of the rocks.


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314 F. S. Anselmetti and G. P. Eberli PAGEOPH ,

ad d i t io n , v e lo c i ty i s d ep e n d en t o n th e d i am e te r o f th e m o ld s . Ve loc i ti e s a r e h ig h e rin coarse mold ic rocks , whereas f ine-mold ic samples a re re la t ive ly s lower.

(4) Intraparticle porosity: F r a m e s t o n e s a n d b o u n d s t o n e s , f o r m e d b y o rg a n i s m ssu ch a s co r a l s o r b ry o zo an s , co n s i s t o f a co n s t ru c t io n a l f r am e w o rk wi th a p o ro s i tyth a t i s em b ed d ed in th e so l id f r am e . Th e re fo r e , t h e se sam p les sh o w a s im i l a rv e lo c i ty -p o ro s i ty p a t t e rn to r o ck s wi th co a r se m o ld ic p o ro s i ty th a t a l so h av e af ramework wi th h igh e las t ic r ig id i ty, resu l t ing in h igh ve loc i t ies . The samples wi thp red o m in an t i n t r ap a r t i c l e p o ro s i ty a l l sh o w p o s i t i v e d ep a r tu r e s f r o m th e g en e ra lt r en d in th e v e lo c i ty -p o ro s i ty d i ag ram .

(5) Low porosity sam ples w ith dens e cementation:Th ese sam p les sh o w anex ten s iv e , b lo ck y cem en ta t io n w i th p o ro s i ti e s o f 2 0 % o r l es s. T h ey a r e c lo se to th ef ina l s tage o f d iagene t ic evo lu t ion . Veloc i t ies a re h igh an d c lose to the in t r ins icve loc i t ies o f the minera ls ca lc i te (6500 m/s) and do lomite (6900 m/s) . These samplesfo rm th e u p p e r p a r t o f t h e v e lo c i ty -p o ro s ity co r r e l a t io n l in e .

As d i scu ssed ab o v e fo r t h e ca se o f th e Ba h am as sam p les , t h e sp ec if ic e ff ec ts o fth e v a r io u s p o re ty p es o n e l a s t i c p ro p e r t i e s o f r o ck s ex p la in wh y ro ck s wi th th esam e p o ro s i ty can h av e ex t r em e ly d i f fe r en t v e lo c it ie s . Th e m o s t s ig n if i can t v e lo c i tyco n t r a s t s a t eq u a l p o ro s i t i e s a r e m easu red b e tween co a r se m o ld ic r o ck s an d ro ck sin wh ich in t e rp a r t ic l e p o ro s i ty p r ed o m in a te s (F ig u re s 17 an d 1 9 ). M o ld ic r o ck s w i th4 0 -5 0 % p o ro s i ty can h av e v e loc i ti e s u p to 5 00 0 m /s , wh e reas r o ck s w i th in t e rp a r t i-c le o r in te rcry s ta l l ine poro s i ty can h ave ve loc i t ies tha t a re up to 2500 m /s o r 50%lo wer fo r t h e sam e p o ro s i t ie s . Th i s r e l a t io n sh ip b e tween p o re ty p e an d v e lo c i ty cana l so b e seen in th e sam p les m easu red f ro m th e Ma ie l l a . Th e Cre taceo u s ru d i s tsan d s , co n s i s t in g o f i n d iv id u a l , n o t co n n ec ted ru d i s t f r ag m en t s w i th o n ly l i t t l ecem en ta t io n , h av e a p r ed o m in an t i n t e rp a r t i c l e p o ro s i ty an d h av e th e r e fo r e v e ry lo wveloc i t ies a round 3000 m/s .

As a co n seq u en ce , v e lo c ity e s t im a t io n fo r a g iv en ca rb o n a te sam p le sh o u ld n o tb e p e r fo rm ed u s in g o n ly th e p o ro s i ty v a lu es , b u t i n co m b in a t io n wi th an a sse ssm en to f th e p o re ty p e . Th e o b se rv ed co m p l i ca t ed v e lo c i ty -p o ro s i ty p a t t e rn , wh ich cau sesa s im i la r im p e d an c e -p o ro s i ty p a t t e rn , im p l i e s th a t an im p e d an ce co n t r a s t b e tween

two l ay e rs can o ccu r ev en w i th o u t a p o ro s i ty ch an g e , d u e o n ly to d i f f e ren t p o retypes .

DensitySe i sm ic re f l ec tio n p a t t e rn s a r e a f u n c t io n o f aco u s ti c im p ed a n ce a n d th e r e fo r e

th e co m b in ed p ro d u c t s o f v e lo c i ty an d b u lk d en s i ty. I n m an y ca se s tu d ie s, o n ly o n ep a ram e te r, e i th e r v e lo c i ty o r d en s i ty, i s k n o wn an d th e o th e r f ac to r h a s to b ees t im a ted w i th em p i r ica l co r r e l a t io n s . B ecau se d en s i ty is c lo se ly r e l a t ed to p o ro s i ty,v e lo c i ty sh o ws a g o o d co r r e l a t io n w i th d en s i ty (F ig u re 1 8 ) . Desp i t e t h e g o o d

cor re la t ion coeff ic ien ts o f 0 .94 fo rVp an d 0 .93 fo r Vs, the da ta in a p lo t o f ve loc i tyvs . densi ty sca t te r s a round a bes t - f i t cu rve which a lso re f lec ts the carbonate spec i f icp o re ty p es . Us in g th e g en e ra l v e lo c ity -d en s i ty t r en d in o u r d a t a se t, we can im p ro v e


29/37

Vo l . 1 41 , 1 9 93 C o n t r o l s o n S o n i c Ve l o c i t y i n C a r b o n a t e s 3 1 5

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F i g u r e 1 8Ve l o c it y a s a f u n c t i o n o f d e n s i t y in t h e B a h a m a s a n d M a i e t l a s a m p l e s a t a n e ff e ct iv e p r e s s u r e o f 8 M P a .T h e s o l i d li n e r e p r e s e n t s V p c a l c u la t e d b y " G a r d n e r ' s L a w " [ d e n s i t y ( g / c m3) = 0 .2 3- Vp(f t / sec) i /4] , a ne m p i r i c a l f o r m u l a f o r a ll s e d i m e n t a r y r o c k s , w h i c h i s o f t e n u s e d t o c a l c u l a t e i m p e d a n c e v a l u e s o n l y f r o mv e l o c i t y o r d e n s i t y d a t a ( G A R D N E Re t a l . , 1 9 7 4) . T h e v e l o c it ie s o f th e m e a s u r e d c a r b o n a t e s a r e a l l h i g h e rt h a n t h e G a r d n e r v e l o c it ie s . T h e e q u a t i o n h a s t h u s t o b e m o d i f i e d to w a r d s h i g h e r v e l o ci t ie s i n o r d e r t o

p r o d u c e m o r e r e l i a b le v e l o c i t y - d e n s i t y p a i r s i n c a r b o n a t e s ( f o r s u g g e s t e d e q u a t i o n s s e e t e x t) .

t h e v e l o c i ty - d e n s i ty c o r r e l a t i o n s f o r p u r e c a r b o n a t e r o c k s b e c a u s e m o s t e m p i r ic a lf o r m u l a s , s u c h a s G a r d n e r ' s L a w ( G A R D N E Ret al . , 1974) , a re main ly va l id ins i l i c i c l a s t i c r o ck s . G a rd n e r ' s Law i s an emp i r i ca l eq u a t i o n fo r s ed i men t a ry ro ck sr e l a t i n g Vpt o d en s i t y.

den s i ty (g /c m 3) = 0 .23 9[Vp(ft/sec)]i/4 or V p ( m / s )= 108 .9 . [dens i ty (g /cm3)] 4.

Th i s fo rmu l a i s ma i n l y u s ed t o ca l cu l a t e i mp ed an ce v a l u e s f ro m e i t h e r d en s i t y o rv e l o c i t y d a t a s o as t o m ak e i m p ed an ce e s t i ma t i o n s f o r s e is mi c m o d e l s i n sed i men -t a r y s e q u e n ce s . H o w e v e r , a ll o u r m e a s u r e d v e lo c it ie s a r e h i g h e r t h a n t h e G a r d n e re q u a t i o n p r e d i c t s . T h i s i m p l i e s t h a t G a r d n e r ' s e q u a t i o n , w h i c h i s a n a v e r a g ef o r m u l a f o r a l l s e d i m e n t a r y r o c k s , r e q ui r e s a m o d i f i c a t i o n f o r c a r b o n a t e s t o w a r d sh i g h e r v e l o c i t ie s t o p r ed i c t r e l i ab le v e l o c i t y -d en s i t y p a i r s. Bas ed o n t h e d a t a f ro mt h e Bah amas an d t h e Ma i e l l a s amp l e s , w e s u g g es t t h e s e emp i r i ca l co r r e l a t i o n s

V p ( m / s )= 52 4 . [dens i ty (g /cm3 )] 2 -48V~, (m/ s )= 199 . [dens i ty (g /cm 3)] zs4 .

Th es e co r r e l a t i o n s , s p ec if i c f o r c a rb o n a t e s , s h o w a b e t t e r f i t an d d e s c r i b e mo reaccu ra t e l y t h e v e l o c i t y -d en s i t y r e l a t i o n , b u t s h o u l d n o t b e ap p l i ed i n s i l ic i cl a st ic o rmi x ed ca rb o n a t e - s i l i c i c l a s t i c r o ck s .


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3 16 F. S . A n s e l m e t ti a n d G . P. E b e r li PA G E O P H ,

6. Ve locity Evolution during Diagenesis

Diagenesis is a very imp ortan t p rocess in carbonates tha t can t ransform al i thified sediment into a rock with completely different physical propert ies. Thesepost -deposi t iona l p rocesses can a l te r poros i ty and cause t ransformat ions in tod i ffe ren t pore types tha t p rodu ce charac ter i s tic pa t te rns in ve loc ity evolu t ion .

The f i rs t p rocess tha t a l te rs ve loc i ty and poros i ty is early com pact io n of thesed iment : in i t i a l conso l ida t ion , dewater ing and gra in rearrangement wi th no crack-ing o r b reak ing o f the componen t s . In it ia l va lues o f app rox ima te ly 50 -60 % fo rpor osi ty an d 1600 m /s for Vp, chang e during this f i rst con sol ida t ion stage to valuesc lose to 40 -5 0% and 2000 m/s respec tive ly (Figure 20a). At th is ear ly s tage , thesed iments a re charac ter ized by an in te rpar t ic le poros i ty (gra ins tones) o r a h ighmic ro -po ros i t y (mud to packs tones ) .

Different diagenet ic proce sses wil l a lso affect the future e volu t ion o f poro si ty.The ve loc i ty e ffec t o f thi s evolu t ion , in par t icu lar the e ffec t o f the t ransfo rma t ion ofpore types , can be descr ibed by a ve loc i ty -poros i ty pa th (Figures 19 and 20) .During i ts burial history, every sediment undergoes such a specific veloci ty-porosi typa th , which s ta r t s a t deposi t ion and ends a t the measured ve loc i ty -poros i ty va luesof the last diagenet ic stage. This path is not necessari ly a st raight l ine becausedi ffe ren t pore types a re c rea ted and eventua l ly des t royed dur ing d iagenesis. To passthe different clusters in the velo ci ty-po rosi ty diag ram cause d by the specific po retypes , the shape of the pa th i s ra ther a curved l ine which depends on the t iming ofthe diagenet ic events.

A g ood exam ple for a loop dur ing the ve loc i ty -poros i ty pa th i s the fabr icinvers ion o f a g ra ins tone to a coa rse mold ic rock . A c lean unc onsol ida ted o o id sandfrom Cat Cay (Bahamas) , tha t has a deposi t iona l , main ly in te rpar t ic le poros i ty of40 -5 0 % (ENOS and SAWATSKY, 1981), has a Vp of 1779 m /s a t 8 M P a effect ivepressure (Figure 19a). The " sam e" roc k (sample U nda 65 m), bu t a f te r cem enta t ionof the in te rpar tic le p ore space and a f te r the d isso lu t ion o f the oo ids and pe lo ids(Figure 19b), has a comple te ly inver ted fabr ic wi th a poros i ty o f 37% (main ly

mold ic) and a Vp of 4105 m/s . D uring the fabr ic invers ion , the rock mu st have

)

F igure 19E x a m p l e f o r r e c o n s t r u c t i o n o f a p o r o s i ty - v e l o c i t y p a t h . F i g u r e s ( a ) a n d ( b ) a r e p o r o s i t y s c a n s( p o r o s i t y = b l a c k ; p a r ti c l e s = w h i t e ) o f p h o t o m i c r o g r a p h s , s h o r t s i d e eq u a l s 1 r a m . a ) O o i d - g r a i n s t o n e a tt i m e o f d e p o s i t i o n ( C a t C a y, B a h a m a s ) w i t h i n t e r p a r t i c le p o r o s i t y, r e p r e s e n t i n g s t a r t i n g c o n d i t i o n s o ft h e p a th ; p o r . = 4 5 % , Vp= 1779 m/s . b ) S am ple U nd a 65 m (a l s o s ho w n in F ig . 17d) , po r . = 37% ,Vp= 4 1 0 5 m / s , V = 1 64 0 m / s . F o r m e r o o i d ( ? ) - g r a i n s t o n e w i t h c o a r s e m o l d i c p o r o s i t y. A f t e r a f e wm a r i n e a l t e r a t i o n s a n d a n i n t e n s e b l o c k y c e m e n t a t i o n , al l g r a in s w e r e d i s s o lv e d a n d l e f t m o l d s b e h i n d( b l a c k ) . c) R e c o n s t r u c t e d v e l o c i t y - p o r o s i t y p a t h f o r a n o n s k e l e t a l g r a i n s t o n e f r o m d e p o s i t i o n ( a ) t o t h ed i a g e n e t i c s t a g e o b s e r v e d in ( b ) . T h e t r a n s f o r m a t i o n o f p o r e t y p e s t o g e t h e r w i t h t h e d i s s o l u t i o n o f g r a i n st h a t t o o k p l a c e a f t e r c e m e n t a t i o n l e d t o a f a b r i c i n v e r s i o n , w h i c h r e s u l t e d i n a c h a r a c t e r i s ti c l o o p o f t h ep a t h . T h e m o l d i c f r a m e w o r k ( b ) p r o v i d e s h i g h e l a s t i c r i g i d i t y a n d t h u s h i g h v e l o c i t y a t h i g h p o r o s i t y.


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Vol. 14 l, 1993 Con trols on Sonic Velocity in Carbo nates 317

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Controls on Sonic Velocity in Carbonates

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