AUTHORS

Georgia Pe-Piper � Department of Geology,Saint Mary’s University, Halifax, Nova Scotia,B3H 3C3, Canada; gpiper@smu.ca

Georgia Pe-Piper graduated from the Univer-sity of Athens, Greece, and gained her Ph.D.in volcanic geochemistry from the Universityof Cambridge, United Kingdom. She is bestknown for her work on igneous rocks, but formany years, her teaching responsibilities in-cluded petroleum geology. Her current interestsinclude the application of mineralogical studiesto the provenance and diagenesis of sedimen-tary rocks.

Shawna Weir-Murphy � Department ofGeology, Saint Mary’s University, Halifax, NovaScotia, B3H 3C3, Canada; present address:EnCana, 421 7th Ave. SW, P.O. Box 2850,Calgary, Alberta T2P 2S5, Canada

Shawna Weir-Murphy graduated from SaintMary’s University in 2004 with an M.Sc. de-gree in applied science (geology) with the co-operative education option. Her thesis wason the Orpheus graben, offshore Nova Scotia.Since 2004 she has been employed as a ge-ologist with EnCana in the former Atlantic andMackenzie delta groups and currently in theIntegrated Oil Division working on a steam-assisted gravity drainage project.

ACKNOWLEDGEMENTS

This work was supported principally by anNaturalSciences and Engineering Research Council ofCanada Discovery Grant to G.P.P., but our workon Cretaceous sedimentary rocks has also beensupported by ExxonMobil and partners in theSable project, Petroleum Research-Atlantic Can-ada, and an NSERC Collaborative Research andDevelopment grant. We thank Lila Dolanskyand Patricia Stoffyn (electron microprobeanalysis), Randolph Corney (drafting), DavidJ. W. Piper (manuscript review and editing),and the Canada–Nova Scotia Offshore PetroleumBoard for the provision of samples and data.Reviews by W. Steven Donaldson, David N.Awwiller, David E. Eby, Rick Abegg, Charles T.Feazel, and Ken Wolgemuth greatly improvedthis manuscript.

Early diagenesis of inner-shelfphosphorite and iron-silicateminerals, Lower Cretaceousof the Orpheus graben,southeastern Canada:Implications for the originof chlorite rimsGeorgia Pe-Piper and Shawna Weir-Murphy

ABSTRACT

Wells in the Orpheus graben encountered the most proximalpart of the deltaic LowerCretaceous rocks of the Scotian Basin.More distal sandstones are important gas reservoir rocks, withgood reservoir quality where Fe-rich chlorite (chamosite) rimson framework grains have inhibited quartz cementation. Cut-ting samples from the Orpheus graben show the presence ofFe-rich sheet silicates (berthierine or chamosite) and early dia-genetic phosphorite. Thesemineralswere analyzed by electronmicroprobe, and their texturesweremappedwith backscatteredelectron images. Studies in the North Sea have shown a re-lationship between high phosphorus and the presence of goodchlorite rims in reservoir rocks. The mineralization of pore-water phosphorus, instead of its return to seawater, is favoredby Fe-rich sediments and sorption on iron oxides during shal-low sea-floor diagenesis. The Fe, Ti, and P contents are un-commonly high in Scotian Basin shales compared with globalaverage shale compositions. The uncommon occurrence ofinner-shelf phosphorite in this study is interpreted to be a con-sequence of the same high Fe content of the sediment that alsofavors the formation of Fe-rich sheet silicates. In rapidly de-posited deltaic sandstones of the offshore reservoirs, the domi-nance of type 3 kerogen led to sulfate depletion occurring atdepths of tens of meters and a corresponding great thickness

AAPG Bulletin, v. 92, no. 9 (September 2008), pp. 1153– 1168 1153

Copyright #2008. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received October 18, 2007; provisional acceptance February 21, 2008; revised manuscriptreceived April 28, 2008; final acceptance May 5, 2008.

DOI:10.1306/05050807118

for overlying Eh (oxidation potential)-controlled diageneticzones. The thick Fe-reduction zone allowed the formation ofearly diagenetic berthierine, which on burial formed the cha-mosite rims that resulted in the improved reservoir quality insandstones. The distribution of phosphorus minerals may bean indicator of conditions suitable for berthierine formation.

INTRODUCTION

Lower Cretaceous sandstones are the principal reservoirs foroil and gas of the Scotian Basin, offshore eastern Canada. Thesandstones were deposited in an overlapping series of smalldeltas on the eastern Scotian Shelf (Cummings et al., 2006) fedby braided rivers drainingmountainous terrain in Atlantic Can-ada (Pe-Piper andMacKay, 2006). Themost proximal preserveddeltaic sediments in the ScotianBasin are found in theOrpheusgraben, which acted as a small pull-apart basin in the EarlyCretaceous (Pe-Piper and Piper, 2004). Several wildcat wellswere drilled in the 1970s through theCretaceous sediments ofthe Orpheus graben (Figure 1). Although there were no oil orgas shows in theOrpheus graben, the wells provide importantinformation on the up-dip more proximal facies of the reser-voir sandstones in the gas fields around Sable Island (Figure 1).

Previous studies of the diagenetic history of theCretaceousreservoir rocks of the Scotian Basin have shown the diageneticgrowth of Fe-rich chlorite (chamosite) rims on sandstone grainpreserved the porosity from later quartz cementation (JansaandNogueraUrrea, 1990;Drummond, 1992). Several studiesin other petroleum basins such as the North Sea have sug-gested that authigenic Fe-rich clays (odinite) alter first duringsea-floor diagenesis to theberthierine and then on deeper burialto the chamosite rims that inhibit later cementation (Ehrenberg,1993; Aagaard et al., 2000). In theVenture field of the ScotianBasin (Figure 1), chamosite rims are common in thick-beddedsandstones that underlie transgressive surfaces (Drummond,1992; Reimer, 2002). During a wider study of the Cretaceousof the Orpheus graben (Weir-Murphy, 2004), we identifiedphosphatic rocks (phosphorites) intimately associatedwith Fe-rich chlorite, which is not previously reported in the LowerCretaceous rocks of the Scotian Basin. These phosphoritesoccur throughout the Lower Cretaceous Logan Canyon andMissisauga formations (Figure 2).

The objectives of this article are to document the occur-rence, petrology, and chemistry of phosphorites and associ-ated Fe-rich sheet silicate minerals, including berthierine andchamosite, in the Lower Cretaceous of the Orpheus graben

DATASHARE 30

Datashare 30 is accessible in an electronicversion on the AAPG Web site at www.aapg.org/datashare/index.html.

Editor’s Note

A color version of Figure 2 may be seen in theonline version of this article.

1154 Phosphorite and Origin of Chlorite Rims, Scotian Basin

and to evaluate their significance in authigenesisand early diagenesis. These findings are thenapplied to the more general issue of the de-positional and early diagenetic environment of theLower Cretaceous reservoir rocks of the Scotian

Basin and the origin of chlorite rims.

GEOLOGICAL BACKGROUND

The Scotian Basin

The Scotian Basin formed on a passive continentalmargin that rifted in theLateTriassic (McIver, 1972;Wade and MacLean, 1990). Early rift sedimentscomprise clastic red beds and salt. Through the Ju-rassic, there was mixed clastic and carbonate sedi-mentation, but in the Early Cretaceous, widespreaddeltaic sediments of the Missisauga, Logan Canyon,and Verrill Canyon formations prograded acrossthe continentalmargin. UpperCretaceous and Ter-tiary rocks consist principally of shales with minorchalks and marls. The sand-rich Missisauga For-mation is of Berriasian to Barremian age (Williamset al., 1990) and passes seaward into the shales oftheVerrillCanyonFormation.TheoverlyingAptianto Cenomanian Logan Canyon Formation is alsodeltaic, comprising two shale units (Naskapi and

Sable members) separated by two sandier units(Cree and Marmora members).

In the Orpheus graben, the Missisauga Forma-tion consists of coarse to pebbly sandstone inter-bedded with thin beds of shale, very fine grained

sandstone, marl, and limestone, deposited on anupper delta plain, which experienced episodic ma-rine transgressions (Weir-Murphy, 2004). To thesouth in the Sable subbasin, the depositional en-vironments range from lower delta plain to outercontinental shelf (Cummings and Arnott, 2005;Cummings et al., 2006). In the Orpheus graben,erosion at theunconformity at thebase of theLoganCanyon Formation has removedmuch of the uppermember of the Missisauga Formation (Wade andMacLean, 1990).

The Naskapi Member at the base of the LoganCanyon Formation is shale rich and present only lo-cally in the Orpheus graben. The lower Cree Mem-ber consists of thick beds of medium- to coarse-grained, quartz-rich sandstone interbeddedwith thinbeds of very fine- to fine-grained sandstone, siltymudstone, andmarl, locallywith thick basalt flowsand felsic pyroclastic rocks (Jansa and Pe-Piper,1985), notably in the Union et al. Jason C-20 andShell Argo F-38 wells (Figure 2). The upper CreeMember has fine- to medium-grained sandstoneinterbedded with very fine-grained sandstone and

Figure 1. Location map of the ScotianBasin showing the studied wells. The graystippled area is the Scotian Basin withgreater than 4-km (13,000-ft)-thickMesozoic–Cenozoic sediments (4-kmisopach from Wade and MacLean, 1990).

Pe-Piper and Weir-Murphy 1155

Figure 2. Summary strati-graphic columns of theShell Crow F-52, Shell ArgoF-38, and Union et al.Jason C-20 wells, Orpheusgraben. Lithotypes basedon cutting samples and wire-line logs. The wire-line logsshown are the gamma ray(left) and sonic velocity(right). KB = kelly bashing.

1156 Phosphorite and Origin of Chlorite Rims, Scotian Basin

siltymudstone beds. Strata of the upperCreeMem-ber were probably deposited in a lower delta plainto a shoreface environment, based on lithology, seis-mic facies, and palynology (Weir-Murphy, 2004).The Sable Member is a thin shale-rich unit over-lain by the Marmora Member, which is composedof very fine- to fine-grained sandstone interbeddedwith medium-grained sandstone, silty mudstone,shale, marl, and coal beds. The lower part was de-posited in an upper shoreface or deltaic environ-ment that became progressively deeper as sea levelcontinued to rise, and the upper part of this mem-ber was deposited in a lower shoreface or open-shelf environment (Weir-Murphy, 2004).

Authigenic and Early Diagenetic Phosphorites andBerthierine and Chamosite

Phosphorite is a rock composed of carbonate-fluorapatite,with a varying admixture of terrigenousand biogenous impurities but containing at least15wt. % P2O5.Most ancient and present-day phos-phorites are known from shelf-edge environmentswhere upwelling results in high organic productiv-ity. Processes leading to the formation of phospho-rites include (1) inorganic precipitation from porewater (Froelich et al., 1988), (2) replacement of car-bonates (McArthur et al., 1980), and (3) bacterialprecipitation (O’Brien et al., 1981). However, phos-phorites less commonly form in coastal regions re-mote from upwelling (Schwennicke et al., 2000).The fluvial supply of phosphateunder conditions ofintense tropical chemicalweatheringmaybe impor-tant for the formation of shallow-water phosphates(Glenn et al., 1994). Phosphate-rich intervals in sed-imentary ironstones have been interpreted as form-ing onmaximum flooding surfaces (e.g., Taylor andMacQuaker, 2000), with the iron-redox cycling ofphosphorus being important for the precipitation ofphosphate cement (Ruttenberg andBerner, 1993).

Modern river-influenced shallow-marine sedi-ments in the tropics are characterized by the ver-dine facies (Odin, 1990), in which the authigenicFe-rich claymineral odinite precipitates. In deeperwaters of the outer shelf, the glaucony facies is de-veloped, in which glauconite is the dominant au-thigenic Fe-rich sheet silicate (Odin, 1990). On

burial and under reducing conditions with low ac-tivities of bicarbonate and sulfide (Taylor andCurtis,1995), odinite transforms to poorly crystalline ber-thierine, a mineral with a serpentine-like structure(Bailey, 1988). Above about 90jC, berthierine al-ters to chamosite (Ryan and Hillier, 2002).

METHODS

TheCretaceous section in theOrpheus grabenwellswas sampled from cuttings; no conventional coreis available (Figure 2). Samples weighing 30–40 gwere washed through 2 mm (0.07 in.) and 63 mmsieves. Cuttings coarser than 2mm (0.07 in.) wereclassified into lithologies using a binocular micro-scope, and representative cuttings were thin sec-tioned.Mineralswere identified, and their chemicalcomposition was determined using a JEOL-8200electronmicroprobewith fivewavelength spectrom-eters and a Noran 133 eV energy dispersion detec-tor. The beam was operated at 15 kV and 20 nA,with a beam diameter of 1–10 mm. Scanning elec-tron microscopy was conducted using an Electro-Scan E3, equipped with a Noran Voyager x-rayenergy dispersive spectrometer (EDS).Thenomen-clature of chlorite minerals is based on chemicalcomposition, by comparison with analyses in theliterature confirmedby x-ray diffraction (Brindley,1982; Iijima andMatsumoto, 1982; Toth and Fritz,1997; Mucke, 2006). Chlorite minerals with lessthan 2% total MgO are identified as berthierine,those with 2–10% MgO as chamosite, and thosewith greater than 10% MgO as Mg-chlorite.

DESCRIPTION OF THE PHOSPHORITES

Stratigraphic Distribution

Phosphorite is present in 69% of the greater than2-mm(0.07-in.) cutting samples examined from theShell Crow F-52 well and in 35% of the samplesfrom the Shell Argo F-38 well. The cemented ornodular character of the phosphorites means thatthey are overrepresented in the greater than 2-mm(0.07-in.) cuttings (together with siderite-cemented

Pe-Piper and Weir-Murphy 1157

sediments) compared with the shales and looselycemented sandstones thatmake up the bulk of thesedimentary succession. The distribution of lime-stone and basalt cuttings, derived from discrete ho-rizons identified on wire-line logs, suggests that sin-gle cuttings or abundances of less than 1% up to200 m (656 ft) below the marker bed might becaused by down-hole contamination, but otherwisemultiple cuttings are interpreted as mostly in situ.The distribution of phosphorite cuttings suggeststhat phosphorite is most abundant in the MarmoraMember but is present at some horizons in theSable and Cree members of the Logan Canyon For-mation and the upper and middle members of theMissisauga Formation (Figure 2). Where only a sin-gle phosphorite cutting was found in a sample, itwas not studied further.

Mode of Occurrence

Macroscopically, the phosphorite cuttings have abrownish color and a variety of shapes. They occurcommonly as intraformational clasts or nodules, incoated grains, and as rare skeletal fragments (prob-ably reptile bones). The most common mode ofoccurrence is as a phosphatic cement in cuttings ofsiltymudstone (Figure 3a), siltstone, and very fine-grained quartz wacke (Figure 3b). This cement iscommonlynodular,with a crude concentric layering,defined by (1) subtle compositional variations inthe phosphate cement (Figure 3a), (2) small pyritecrystals, or (3) secondary porosity probably resultingfrom the dissolution of siderite (Figure 3c). Somenodules show residual areas of uncemented mud-stone with an irregular limonite rim (Figure 3d),which suggest that phosphate precipitation was dis-placive. Sparse authigenic glauconite is present insome phosphate-cemented samples (Figure 3b),occurring as irregular peloids. Themargins of someglauconite grains appear embayed and replaced byfrancolite (Figure 3b, inset). Acicular siderite crys-tals have replaced phosphatic cement in some nod-ules (Figure 4a). In other cases, the phosphate formscrystals of a few microns in size that appear to bediagenetic replacements of siderite (Figure 4b).

Coated grains (ooids) of iron-richminerals fromthe Marmora Member in the Shell Argo F-38 well

include alternating layers of siderite, berthierine, andphosphate (Figure 4c, d). Siderite forms discretelayers tens of microns wide in some coated grainsbut more generally appears coarsely crystalline, ap-parently replacing berthierine or some other pre-cursor that wasmixedwith berthierine (Figure 4c).Some coated grains show asymmetric growth(Figure 4d) and some concentric layers appearabraded (Figure 4c), demonstrating a seabed orshallow-burial origin for the grains (cf. Pufahl andGrimm,2003).Aphosphate- and siderite-cementedsandymudstone from theCreeMember of the ShellCrow F-52 well contains berthierine pellets andcoated grains (Figure 4e). Berthierine or chamositealso fills secondary porosity in some early sideritenodules (Figure 4f). In the more distal Shell PCIet al. Peskowesk A-99 well (Figure 1) (Pe-Piperet al., 2006), a bioturbated transgressive sandstonein the Cree Member also includes coated grainspredominantly of glauconite (Figure 4g, h). Somegrains contain layers of Fe-rich chlorite (chamosite)(Figure 4h), with other layers consisting predomi-nantly of kaolinite (Figure 4g). The outer surface ofthe coated grains is corroded and rimmed by Mg-rich siderite.

Several cuttings of phosphate-cemented mud-stone have also been identified in the undividedmiddle part of the LoganCanyon Formation of theUnion et al. Sambro I-29 well (Pe-Piper and Piper,2007), the most proximal well on the central Sco-tian Shelf (Figure 1).We have not found phospho-rite in our studies of 10 more distal wells on theScotian Shelf, although coated grains of iron-richminerals are present in most wells.

In the Shell Argo F-38 well, two phosphatic cut-ting samples showed a bonemicrostructure. Themin-eral assemblage in vascular canals is similar to thatin the host rocks of the Logan Canyon Formation,providing evidence that the bones are close to in situ.

Mineralogy

An x-ray diffraction analysis of six phosphorite sam-ples shows the presence of a carbonate hydroxyap-atite, with a CO2 content estimated to be 2% basedon the position of characteristic crystallographicspacings using themethod ofGulbrandsen (1970).

1158 Phosphorite and Origin of Chlorite Rims, Scotian Basin

BecauseCO2 cannot be analyzed by electronmicro-probe, the phosphate composition from electronmicroprobe analyses (Figure 5) was calculated onthe basis of 2% CO2 (Table 1). The fluorine (F)content in all analyses is more than 2.5–3.5%(Figure 5), indicating that the phosphorite is fran-colite (McConnell, 1963). Electron microprobeanalyses also show high calculated OH abundance(Table 1), as noted for other marine francolite byAbed and Fakhouri (1996). Significant positivecorrelations between CaO and P2O5 and SrO andnegative correlations between CaO and Na2O,Al2O3 and FeOt (with representative correlationsillustrated in Figure 5) are observed. Phosphatecement appears to have higher Al, Na, Fe, F, and

S than bone and correspondingly lower P2O5

and CaO.Iron-rich silicateminerals withwhich the phos-

phorite is intimately associated have also been ana-lyzed by electronmicroprobe. Berthierinewith lessthan 2%MgO is present only in the shallowest sam-ples in Argo and Crow; at greater depths, Fe-richchlorite (chamosite) with 2–10% MgO is present(Figure 6). The Mg-rich chlorite is an important al-teration mineral in basalt and tuffs, for example inthe Union et al. Jason C-20 well, and also occurs as adetrital mineral. Present geothermal gradients in theOrpheus grabenwells fall between 20 and 45jC/km.Based principally on the thermal alteration indexof spores, Lyngberg (1984) estimated that the Ro

Figure 3. Backscattered electron images of cuttings. Open dots represent electron microprobe analyses. (a) A part of a concentrically zonedfrancolite nodule showing areas of uncemented mudstone (d) with silt-size quartz and feldspar and a clayey matrix. Marmora Member, ShellCrow F-52 well, 436 m (1430 ft). (b) A fragment of a phosphorite nodule with glauconite grains. The inset shows the corrosion of glauconite.Marmora Member, Shell Crow F-52 well, 436 m (1430 ft). (c) A fragment of a nodule of phosphate-cemented silty mudstone showingsecondary porosity (dark areas). Middle member, Shell Argo F-38 well, 1430m (4690 ft). (d) The detail of the area of uncementedmudstoneshown by a box in (a) with silt-size quartz and feldspar and a clayey matrix. Marmora Member, Shell Crow F-52 well, 436 m (1430 ft).Mineral abbreviations: fran = francolite; glt = glauconite; kln = kaolinite; kfs = K-feldspar; lm = limonite; py = pyrite; and qtz = quartz.

Pe-Piper and Weir-Murphy 1159

for the Lower Cretaceous is less than 0.5 in ShellCrow F-52 and generally between 0.5 and 1.1 inShell Argo F-38 and Union et al. Jason C-20. Using

theRo synthesis of Suggate (1998), this suggests thatthroughout the Lower Cretaceous in Shell CrowF-52 and at least in the Logan Canyon Formation

1160 Phosphorite and Origin of Chlorite Rims, Scotian Basin

of Shell Argo F-38 andUnion et al. Jason C-20, thetemperature never reached the 90–120jC (Aagaardet al., 2000) required for complete conversion ofberthierine to chamosite. Therefore, the iron sili-cate mineral phases are likely to consist of a com-plex mix of interlayered mineral phases, similarto the verdine facies of the Jurassic of Wyoming(Ryan and Hillier, 2002). The scatter in chemicalcompositions (Figure 6) compared with literaturedeterminations on well-crystallized phases sup-ports this interpretation. Our usage of the termsberthierine and chamosite is, therefore, likely loose,

indicating only the predominant component of amineralogically complex iron silicate.

DISCUSSION

Significance of Inner-Shelf Phosphorite in theOrpheus Graben

The Orpheus graben represents the sedimento-logically most proximal part of the Scotian Basin.

Figure 4. Backscattered electron images of cuttings. (a) Phosphate-cemented silty mudstone. Siderite forms clusters of euhedralcrystals. Marmora Member, Shell Crow F-52 well, 493 m (1617 ft). (b) Siderite-francolite mudstone. Marmora Member, Shell Argo F-38well, 579 m (1900 ft). (c) Berthierine-siderite-francolite-coated grains cemented by phosphorite (francolite). Marmora Member, ShellArgo F-38 well, 579 m (1900 ft). (d) Berthierine-siderite-francolite-coated grains cemented by phosphorite (francolite). MarmoraMember, Shell Argo F-38 well, 579 m (1900 ft). (e) Berthierine pellets and/or coated grains in a phosphate-siderite–cemented sandymudstone from different areas of the same sample. Cree Member, Shell Crow F-52 well, 692 m (2270 ft). (f) Siderite nodule with thesecondary porosity partially filled with chamosite. Cree Member, Shell Crow F-52 well, 756 m (2480 ft). (g) Coated grain with a quartzcore rimmed by kaolinite and glauconite, surrounded by Mg-siderite cement. Cree Member, Shell PCI et al. Peskowesk A-99 well,2276 m (7467 ft). (h) Coated grain with a quartz core rimmed by chamosite and then glauconite, surrounded by Mg-siderite cement.Cree Member, Shell PCI et al. Peskowesk A-99 well, 2276 m (7467 ft). Mineral abbreviations: brh = berthierine; chm = chamosite; fran =francolite; glt = glauconite; kaol = kaolinite; kfs = K-feldspar; qtz = quartz; and sd = siderite.

Figure 5. Examples ofchemical variation in phos-phorite composition withmode of occurrence.

Pe-Piper and Weir-Murphy 1161

Inner-shelf phosphorite deposition is quite uncom-mon; normally phosphorites are associated with

shelf-edge upwelling, providing abundant P in ma-rine organic matter. The abundance of P in terres-

trial organicmatter ismuch lower. Several reportedexamples of inner-shelf phosphorites have been

interpreted to involve uncommon bacterial or algalaction (Schwennicke et al., 2000), providing high

Table 1. Representative Electron Microprobe Analyses of Phosphate Minerals

Sample

2-Argo

1690

3-Argo

1690

4-Argo

1690

6-Argo

1900B

7-Argo

3030B

13-Argo

3030A

14-Argo

3030A

11-Sambro

3170A

12-Sambro

3170A

Form Bone Bone Bone Nodule Bone Cement Cement Cement Cement

Electron Microprobe DataAl 0.12 0.12 0.12 0.12 0.12 1.05 0.95 1.48 1.16

Fe 0.85 0.89 0.92 1.19 0.84 1.44 1.25 1.89 1.61

Ca 34.25 34.20 34.01 34.79 34.81 32.04 32.44 31.10 32.76

Na 0.49 0.53 0.51 0.49 0.53 0.73 0.75 0.79 0.75

Sr 0.45 0.46 0.48 0.47 0.42 0.20 0.24 0.27 0.25

P 15.65 15.68 15.52 15.64 15.24 12.38 12.62 12.42 13.01

Ce 0.45 0.45 0.49 0.26 0.26 0.25 0.23 0.38 0.35

La 0.60 0.65 0.63 0.53 0.58 0.57 0.57 0.61 0.62

S 0.32 0.30 0.45 0.22 0.38 0.92 1.01 0.83 0.79

F 2.75 2.74 2.88 2.70 2.75 3.43 3.56 3.38 3.38

Cl 0.11 0.13 0.14 0.36 0.20 0.09 0.08 0.10 0.09

O 42.97 43.00 42.68 43.78 43.64 42.06 42.95 44.12 42.25

Total 99.01 99.15 98.83 100.55 99.77 95.16 96.65 97.37 97.02

OxidesAl2O3 0.23 0.23 0.23 0.23 0.23 1.98 1.80 2.80 2.19

FeOT 1.09 1.14 1.18 1.53 1.08 1.85 1.61 2.43 2.07

CaO 47.92 47.85 47.59 48.68 48.71 44.83 45.39 43.52 45.84

Na2O 0.66 0.71 0.69 0.66 0.71 0.98 1.01 1.06 1.01

SrO 0.53 0.54 0.57 0.56 0.50 0.24 0.28 0.32 0.30

P2O5 35.86 35.93 35.56 35.84 34.92 28.37 28.92 28.46 29.81

Ce2O3 0.53 0.53 0.57 0.30 0.30 0.29 0.27 0.45 0.41

La2O3 0.70 0.76 0.74 0.62 0.68 0.67 0.67 0.72 0.73

SO3 0.80 0.75 1.12 0.55 0.95 2.30 2.52 2.07 1.97

F 2.75 2.74 2.88 2.70 2.75 3.43 3.56 3.38 3.38

Cl 0.11 0.13 0.14 0.36 0.20 0.09 0.08 0.10 0.09

Distribution of Volatiles: Water OnlyH2O 8.81 8.82 8.51 9.60 9.84 11.40 11.87 13.59 10.38

Total (H2O) 100.00 100.14 99.78 101.62 100.87 96.44 97.98 98.89 98.18

Distribution of Volatiles: 2% CO2, Remainder WaterCO2 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00

H2O 7.17 7.18 6.87 7.96 8.20 9.77 10.24 11.95 8.75

Total 100.36 100.50 100.15 101.99 101.23 96.80 98.34 99.25 98.54

1162 Phosphorite and Origin of Chlorite Rims, Scotian Basin

levels of P input frommarine organic matter. How-ever, no independent geological evidence for such aprocess in the Orpheus graben, where the organicmatter is overwhelmingly terrigenous (Lyngberg,1984), exists.

The Role of Sea-Floor Diagenesis

Early iron-silicateminerals in the Scotian Basin areglauconite and berthierine (Figure 7). Berthierineprobably forms during sea-floor diagenesis fromodinite, an iron-rich clay precursor formed in theinner-shelf verdine facies (Bailey, 1988).Glauconiteis characteristic of fully marine deeper waters thatexperience little sedimentation (Odin, 1990) andis associated with calcite fossil detritus; it is morecommon in the more distal Shell PCI PeskoweskA-99 well and in other wells in distal parts of thebasin (Drummond, 1992; Pe-Piper et al., 2004). Theonly glauconite found in the verdine facies of theOrpheus graben are a few scattered grains, some ofwhich appear to be corroded under early diageneticconditions and are replaced by francolite (Figure 3b).

The Fe-rich coated phosphatic grains resemble

those described by Soudry (2000). Asymmetricgrowth and abrasion of concentric layers suggest

that they developed during very shallow burial andwere periodically reworked to the sea floor wherethey experienced erosion (e.g., Figure 4d). Suchcoated phosphatic grains generally formunder lowrates of sedimentation,with the phosphate precipi-tating during intermittent burial in the upper 5–20 cm (2–8 in.) of sediment (Pufahl and Grimm,2003). Some of the coated grains have a berthierinenucleus and all of them have concentric zones thatconsist either of siderite or a mixture that may in-clude any combination of siderite, berthierine, andfrancolite (Figure 4c, d). Thus, the overall petro-graphic evidence suggests that both francolite andsiderite have precipitated on berthierine in thecoated grains.

The sequential growth of early diageneticmin-erals can be inferred from textural relationships(Figure 7). The coated grains show an alternatinggrowth of berthierine, siderite, and francolite, butin some samples, euhedral siderite (Figure 4a) andlimonite rims at the edge of francolite cement(Figure 3d) postdate francolite. Pyrite framboidscoexist with francolite in many samples, generallydisplacing francolite.

The growth and progressive diagenetic evolu-

tion of coated grains and nodules can be evaluated

Figure 6. Chemical variation in chlorite, chamo-site, berthierine, and related minerals. See theMethods section for the mineral nomenclature.

Pe-Piper and Weir-Murphy 1163

in terms of geochemical zones that develop duringthe progressive burial of marine sediments con-taining organicmatter (Berner, 1981; Froelich et al.,1988) (Figure 7). Phosphate precipitation occurswithin the suboxic zone within whichmanganesereduction, iron reduction, and sulfate reductionpassing progressively downward are seen. Odinitehas been interpreted to be reduced to berthierinein the iron-reduction zone prior to the develop-ment of high activities of HS� and HCO�

3 deeperin the sediment. Thus, phosphate and berthierineformunder very similar early diagenetic conditions.At greater depths, once pore-water sulfate is de-pleted,microbialmethanogenesis of organicmatterwill increase pore-water alkalinity, and siderite, an-kerite, or ferroan dolomite will precipitate.

During sea-floor diagenesis, the release of dis-solved P to seawater is impeded by sorption ontoiron oxides and hydroxides, with P being releasedagain as iron oxides and hydroxides become re-duced on burial (Cha et al., 2005). Thus, the trap-ping of P during early diagenesis is favored by thehigh Fe content of sediments (Ruttenberg andBerner, 1993). Lower Cretaceous shales of theScotian Basin (Pe-Piper et al., in press) have Fe con-tents 1.5 times that of the global average shale(Figure 8). Furthermore, the deltaic shales andfluvial mudstones are uncommonly rich in Ti as aresult of abundant detrital ilmenite and its break-down products. Alteration of ilmenite releases reac-tive Fe, whichmay have been significant in the earlydiagenetic environment (Pe-Piper et al., 2005).

Figure 7. Schematicrepresentation of the se-quence of diageneticminerals.

1164 Phosphorite and Origin of Chlorite Rims, Scotian Basin

Co-occurrence of Berthierine and Phosphorite:Significance for Hydrocarbon Exploration

Studies in the North Sea have shown that verdine-facies Fe-rich sheet silicates are an authigenic orearly diagenetic precursor of chlorite (chamosite)rims that preserve porosity in deeply buried sand-stones by inhibiting quartz overgrowths (Ehrenberg,1993; Aagaard et al., 2000). Such chlorite rims areimportant for improved reservoir quality in theMis-sisauga sandstones of the Sable subbasin (Drum-mond, 1992), and sandstones with chlorite rimsaremost common in thick-bedded sandstones thatunderlie transgressive surfaces (Gould, 2007).

In this study, we have shown that, on the innershelf, berthierine formed during sea-floor diagen-esis under similar conditions to phosphorite as evi-denced by their co-occurrence in coated grains, andthat this is likely in the Fe-reduction zone. The re-lease of pore-water P back into the seawater is in-hibited by sorption on iron oxides and hydroxidesin the uppermost part of the sediment columnabove the Fe-reduction zone. We therefore hy-pothesize that elevatedmineralization of phospho-rus, instead of the return of dissolved P to seawater,is favored by the same sea-floor diagenetic condi-

tions that result in berthierine, which is the pre-cursor of chlorite rims.

Several studies have serendipitously found rela-tionships between P and either chlorite rims or Feabundance.A study of Jurassic sandstone reservoirsoff Norway by Ehrenberg et al. (1998) observedthat anomalously high P values occur in close strati-graphic proximity to zones with well-developedchlorite rims in rapidly depositedmedium-grainedsandstones. Although these authors speculated thatthe high P was caused by the high input of marineorganic matter from bacterial blooms, no evidencefor high input of marine organic matter was ob-served. Neither is there evidence for significant ma-rine organic matter in the Orpheus graben rocks(Lyngberg, 1984). Gould (2007) reported a goodcorrelation between the quality of chlorite rims inthe Venture field of the Sable subbasin and theabundance of phosphorus in the host rock. Litho-geochemical data from theCretaceous of theScotianBasin (Pe-Piper et al., in press) show a good cor-relation between Fe and P in sandstones and par-ticularly inmudstones (Figure 8), withmanymud-stones showing Fe and P abundance substantiallyabove the global average for shales. Such a relation-ship is consistent with our hypothesis that Pminer-alization during diagenesis is favored by sorptionon iron oxides and hydroxides.

We therefore propose that the presence of theelevated abundance of phosphorus in reservoir sand-stones of both the Scotian Basin and theNorth Seais an indication of sea-floor diagenetic conditionsthat favor the precipitation of the berthierine pre-cursor of chamosite rims. The widespread distri-bution of such rims in sandstone units up to 10 m(30 ft) thick (Drummond, 1992) can be related toabrupt changes in sedimentation rate at overlyingtransgression surfaces. The dominance of terres-trial organic matter (type 3 kerogen) in the rapidlydeposited thick-bedded reservoir sandstones of theSable Basin deltas (Powell, 1982) resulted in veryslow rates of sulfate reduction in pore water, withsulfate depletion at depths of several tens ofmeters,as noted for themodernMississippi (Berner, 1978)and Amazon (Burns, 1997) deltas. The abrupt de-crease in sedimentation rate at transgression sur-faces would result in the progressive shallowing of

Figure 8. Covariation in phosphorus and iron in the LowerCretaceous rocks of the Scotian Basin (plotted from the data inPe-Piper et al., in press). The gray box shows the range of esti-mates of average shales from the literature (Mason, 1982; Grometet al., 1984; Taylor and McLennan, 1985; Brownlow, 1996, theirtable 7-3).

Pe-Piper and Weir-Murphy 1165

the sulfate-depletion level and the overlying Eh-controlled diagenetic zones (Figure 7), as demon-strated in recent sediments by Burns (1997). Thisprocess would result in the conditions suitable forthe diagenetic formation of berthierine to migrateupward through the upper several meters or moreof the packet of reservoir sandstones. Much of thephosphorus would migrate upward in solution tothe thinning oxic zone that was eventually repre-sented by only the transgressive surface, althoughsome remineralized phosphorus would be trappedby the precipitation of berthierine and then car-bonateminerals. The behavior of P in porewater ismuch better understood in themodern ocean thanthe behavior of Fe because dissolved ferrous ironand reduced iron phases like iron monosulfide arevery susceptible to changes in Eh during sampling.More work on the distribution of both Fe and P inpore water and early diagenetic minerals is neededto fully understand the origin of berthierine in thesea-floor diagenetic system.

CONCLUSIONS

1. Uncommon inner-shelf phosphorite is found atseveral stratigraphic horizons in the Lower Cre-taceous proximal deltaic sediments of the Or-pheus graben, including skeletal material, fran-colite cement, and coated grains. Phosphoritedeveloped on transgressive surfaces.

2. The phosphorite coexists with verdine-facies

berthierine, which occurs in coated grains, as re-placements of pellets and intraclasts, and as neo-morphic minerals in secondary porosity in sid-erite concretions.

3. The uncommon occurrence of inner-shelf phos-phorite is a consequence of the high Fe contentof sediment, with the release of pore-water Pback to seawater inhibited by sorption on ironoxides and hydroxides. Scotian Basin shales con-tain uncommonly high Fe and Ti, favored by theilmenite-rich fluvial detritus brought to the delta.

4. Rapidly deposited deltaic sandstones in themoredistal parts of the Scotian Basin contain domi-nant type 3 kerogen that led to sulfate depletionoccurring at depths of tens of meters and a cor-

responding great thickness for overlying Eh-controlleddiageneticzones.The thickFe-reductionzone allowed the formation of early diageneticberthierine, which on burial formed the chamo-site rims of reservoir sandstones in the Ventureand other gas fields. The chamosite rims inhib-ited later quartz cementation.

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1168 Phosphorite and Origin of Chlorite Rims, Scotian Basin