Relationship between reservoir diagenetic evolution and petroleum emplacement in the Ula Field, North Sea Tor Nedkvitne*, Dag A. Karlsen, Knut Bjorlykke and Steve R. Lartert Department of Geology, University of Oslo, PO Box 1047, Blindern, 0316 Oslo 3, Norway Received 31 March 1992; revised 8 July 1992; accepted 10 July 1992 The diagenetic and petroleum filling histories of the Ula Field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements. This study shows that diagenesis continues actively after the arrival of petroleum in the sandstones, although the reaction rates and petroleum saturation remain obscure. Microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110°C (i.e. =2.5 km) and that this cementation has continued to the present day. Synchronously with quartz cementation, authigenic albite was formed as inter- and intragranular cements. Large amounts of petroleum inclusions locally occur in the albite and quartz cements. These inclusions were trapped in the cements as the main charge of petroleum arrived at the structure. Diffusion, through thin water layers between the petroleum and the mineral surface, would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite. It is evident from analysis of the petroleum inclusions in early formed K-feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth, before the rapid subsidence in the last 25 my. Compared with petroleum in the free porosity of the Ula Formation, the petroleum in the K-feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature. Later trapped petroleum inclusions, in quartz and albite, have characteristics found both in K-feldspar and in the Ula Formation DST oil and, thus, is likely to reflect the progressive change in the Ula Field petroleum charge which occurred during the time period of quartz diagenesis. Keywords: diagenesis; petroleum filling; fluid inclusions; reservoir geochemistry Introduction Microthermometric studies of fluid inclusions in authigenic minerals have been applied previously to assess temperatures of important cementations (e.g. quartz and carbonates) in clastic reservoirs (Haszeldine et al., 1984; McLimans,1987; Jourdan et al., 1987; Glasmann et al., 1989; Burley et al., 1989; Walderhaug, 1990; Saigal and Bj0rlykke, 1992). Microthermometry used as a tool to provide absolute temperatures of inclusion formation is of great value to diagenesis, as traditional microscopy techniques can only give textural relationships between the authigenic minerals (i.e. only the relative chronology of authigenic events). Earlier hypotheses proposed that diagenetic reactions terminate as a result of petroleum accumulation (Hancock and Taylor, 1978; Hawkins, 1978; Sommer, 1978). However, petroleum inclusions in authigenic cements from clastic petroleum reservoirs are widely documented (Murray, 1957; Pagel et al., 1986; *Present address: Saga Petrolcum as, PO Box 490, 1301 Sandvika, Norway tPresent address: Newcastle Research Group in Fossil Fuels and Environmental Geochemistry (NRG), Drummond Building, University of Ncwcastle upon Tync NE1 7RU, UK 0264,-8172/93/030255-16 ¢c'1993 Butterworth-Heinemann Ltd Walgenwitz et al., 1990; Tilley et al., 1989; Walderhaug, 1990). This documents that diagenesis is active in the remaining water in the petroleum column during petroleum emplacement although our knowledge of the petroleum saturation required for inclusion formation remains obscure. Homogenization temperatures of petroleum inclusions have been used to give valuable information about petroleum arrival in reservoirs (Horsfield and McLimans, 1984; McLimans, 1987; Walderhaug, 1990). Techniques for analysing the composition of fluids trapped in the inclusions (cf. Roedder, 1984), when used together with homogenization temperatures, can give information about the compositional evolution of fluids in reservoirs. This evolution of petroleum fluids can, through the use of the burial history, be linked to an absolute chronology. The Ula oilfield (Home, 1987; Spencer et al., 1986; Larter et al., 1990) was chosen as a candidate for studying the relationship between diagenesis and petroleum filling due to the presence of both petroleum and aqueous inclusions in authigenic K-feldspar, albite and quartz cements. Microthermometric data and analysis of the petroleum composition in inclusions entrapped in the different types of cements have been Marine and Petroleum Geology, 1993, Vol 10, June 255
Transcript
Relationship between reservoir diagenetic evolution and petroleum emplacement in the Ula Field, North Sea
Tor Nedkvitne*, Dag A. Karlsen, Knut Bjorlykke and Steve R. Lartert Department of Geology, Universi ty of Oslo, PO Box 1047, Blindern, 0316 Oslo 3, Norway
Received 31 March 1992; revised 8 July 1992; accepted 10 July 1992
The diagenetic and petroleum filling histories of the Ula Field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements. This study shows that diagenesis continues actively after the arrival of petroleum in the sandstones, although the reaction rates and petroleum saturation remain obscure. Microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110°C (i.e. =2.5 km) and that this cementation has continued to the present day. Synchronously with quartz cementation, authigenic albite was formed as inter- and intragranular cements. Large amounts of petroleum inclusions locally occur in the albite and quartz cements. These inclusions were trapped in the cements as the main charge of petroleum arrived at the structure. Diffusion, through thin water layers between the petroleum and the mineral surface, would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite. It is evident from analysis of the petroleum inclusions in early formed K-feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth, before the rapid subsidence in the last 25 my. Compared with petroleum in the free porosity of the Ula Formation, the petroleum in the K-feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature. Later trapped petroleum inclusions, in quartz and albite, have characteristics found both in K-feldspar and in the Ula Formation DST oil and, thus, is likely to reflect the progressive change in the Ula Field petroleum charge which occurred during the time period of quartz diagenesis.
Microthermometric studies of fluid inclusions in authigenic minerals have been applied previously to assess temperatures of important cementations (e.g. quartz and carbonates) in clastic reservoirs (Haszeldine et al., 1984; McLimans,1987; Jourdan et al., 1987; Glasmann et al., 1989; Burley et al., 1989; Walderhaug, 1990; Saigal and Bj0rlykke, 1992). Microthermometry used as a tool to provide absolute temperatures of inclusion formation is of great value to diagenesis, as traditional microscopy techniques can only give textural relationships between the authigenic minerals (i.e. only the relative chronology of authigenic events). Earlier hypotheses proposed that diagenetic reactions terminate as a result of petroleum accumulation (Hancock and Taylor, 1978; Hawkins, 1978; Sommer, 1978). However, petroleum inclusions in authigenic cements from clastic petroleum reservoirs are widely documented (Murray, 1957; Pagel et al., 1986;
*Present address: Saga Petrolcum as, PO Box 490, 1301 Sandvika, Norway tPresent address: Newcastle Research Group in Fossil Fuels and Environmental Geochemistry (NRG), Drummond Building, University of Ncwcastle upon Tync NE1 7RU, UK
Walgenwitz et al., 1990; Tilley et al., 1989; Walderhaug, 1990). This documents that diagenesis is active in the remaining water in the petroleum column during petroleum emplacement although our knowledge of the petroleum saturation required for inclusion formation remains obscure. Homogenization temperatures of petroleum inclusions have been used to give valuable information about petroleum arrival in reservoirs (Horsfield and McLimans, 1984; McLimans, 1987; Walderhaug, 1990). Techniques for analysing the composition of fluids trapped in the inclusions (cf. Roedder, 1984), when used together with homogenization temperatures, can give information about the compositional evolution of fluids in reservoirs. This evolution of petroleum fluids can, through the use of the burial history, be linked to an absolute chronology.
The Ula oilfield (Home, 1987; Spencer et al., 1986; Larter et al., 1990) was chosen as a candidate for studying the relationship between diagenesis and petroleum filling due to the presence of both petroleum and aqueous inclusions in authigenic K-feldspar, albite and quartz cements. Microthermometric data and analysis of the petroleum composition in inclusions entrapped in the different types of cements have been
Marine and Petroleum Geology, 1993, Vol 10, June 255
Diagenetic evolution and petroleum emplacement, Ula Field: T. Nedkvitne et al. 2°(X) '
I ~ ULA F ~ E L D ~
CENTRAL
GRABEN
O ' %
°0 I \\\ Figure 1 Location map of the Ula Field
3020 ̀ 57°30"
56°30 '
obtained, thus allowing this relationship to be studied.
Geological setting The Ula Field is located 280 km south-west of Stavanger at the eastern margin of the Central Graben along the Hidra Fault Zone (Figure 1). The field consists of Upper Triassic - Upper Jurassic sandstones in an anticlinal structure. The structure is a result of Upper Jurassic rifting followed by inversion in the Cretaceous and Tertiary (Brown et al., 1992). Local and regional geological overviews are found in Home (1987) and Spencer et al. (1986), and Latter et al. (1990) describe the petroleum accumulated in the field.
The Upper Triassic Skagcrrak Formation oil bearing zones are cored only in a few wells, and the diagenesis in these fluvial sandstones will not be described in detail. The Bryne Formation of Middle Jurassic age unconformably overlies the Upper Triassic sediments and represents a thin sheet (20-30 m) of deltaic and shallow marine sandstones which is not present everywhere. Disconformably overlying the Bryne Formation is the Upper Jurassic Ula Formation, which is the developed reservoir. The Ula Formation is a rather homogeneous sandstone sequence deposited on a storm dominated shallow marine shelf (Brown et al., 1992) and the sandstones are generally extensively bioturbated (cf. Home, 1987). The accumulation of these sands, and analogous sands in the area, was closely controlled by syndepositional tectonic movements on adjacent faults, with maximum thicknesses obtained close to active faults and in rim synclines (Fterseth and Pederstad, 1988). Home (1987) subdivided the Ula Formation into five reservoir units (Figure 2), which generally follow upwardly coarsening sedimentary facies, except for reservoir unit I which is a fining-up sequence. Brown et al. (1992) made a further refinement of the reservoir units into a and b subunits. The reserw)ir units I - I1 - I l i (Home, 1987) are laterally extensive throughout the reservoir, and units 11-III represent sandstones with the best reservoir properties. The Ula Formation is overlain by the Farsund Formation and the organic rich Mandal Formation mudstones of Upper Jurassic age. The latter
-~ ~ GRAIN HORIZ. ~ ~ ~ SIZE POROSITY I PERM.
~ ~ Si F M Vc 10 20 10 IE4 I i i i i i I l , i I I
] ::::i:::: , 3420 : : ' : i ) .
¢ e. . ~
3440 e~'~'~elc ~ ~: , } "
c¢.eccc i "
3460 :.i.:il i
"°ti
~ SANDSTONE ~ , , ~ SHELL DEBRIS
SILTSTONE ¢ } BIOTURBATION
CALCITE CEMENT ~ ~ BIOTURBATION OPHIOMORPHA TYPE
Figure2 Sedimentological log of the Ula Formation in well 7/12-6 showing the subdivided reservoir units of Home (1987). Four of the five reservoir units are present in this well. Porosity and permeability are measured from individual core plugs
is the main petroleum source rock in the area, and samples at 4 km have a maturity equivalent to around 0.8% R0 near peak oil generation (Spencer et al., 1986; Latter et al., 1990).
The depth to the reservoir crest is ~3400 m, and the reservoir has experienced nearly 2 km of burial during the last 25 my, with the highest burial rates in the Pile-Pleistocene (Figure 3). Today, the reservoir temperature and pressure are 143°C and 483 bar at 3450 m, respectively (Home, 1987). The oil saturation
T I M E (m.y.b.p)
15 30 45 60 75 90 11)5 1211 135 150 i i i
~. I k m 50 c I . . . .
. • K-feldspar
/ / I trap •
~[~ bubble point pressure
Figure3 Compaction corrected burial history for the Ula Formation. Temperatures of 50 and 100°C are marked by assuming a surface seafloor temperature of 5°C and the present geothermal gradient of 40°C/km. Important diagenetic and petroleum emplacement events are marked
256 Marine and Petroleum Geology, 1993, Vol 10, June
Diagenetic evolution and petroleum emplacement, Ula Field: T. Nedkvitne et al.
in the reservoir is in the range 75-97% (Home, 1987), whereas the bubble point of the petroleum in the Ula Formation is 159 bar at 143°C (Home, 1987).
Petrography The Jurassic/Triassic samples consist of very fine to very coarse grained sandstones of arkosic to subarkosic composition. The plagioclase/K-feldspar ratio is variable, but towards the deeper part of the Skagerrak Formation (well 7/12-6) the plagioclase content decreases to almost zero. Mica is abundant, especially in finer grained sediments, and it is typically concentrated in burrow walls and along stylolites in the Ula Formation.
Calcite cemented sand beds of 0.1-1.0 m thickness occur in reservoir units I I - I I I (Figure 2) and are usually associated with bioclastic debris. Sand grains in the calcite cemented beds are loosely packed, indicating cementation at shallow burial depth before extensive compaction. The calcite has a poikilotopic texture, but minor sparry texture occurs in the central parts of the thickest beds, where, exposed to the open pore space, the calcite cement consists of euhedral crystals without
dissolution features (Figure 4A). Minor pressure solution of calcite is seen at the edges of the calcite cemented beds in direct contact with clay rich lamina.
Authigenic quartz occurs as euhedral overgrowths (Figure 4B), and the total amount can reach up to 5% (point-counted) in clean and well sorted sandstones. No quartz cement is found in the carbonate cemented beds, suggesting that carbonate cementation occurred before quartz cementation. Intergranular pressure solution of quartz and stylolites is often observed (Figure 4C), and stylolites are commonly seen developed along clay laminae. Quartz is the major mineral dissolved along the stylolites, and only minor pressure solution of feldspar grains is observed.
Authigenic intergranular K-feldspar overgrowths (Figure 4D) are ubiquitous and may reach up to 3% in clean and well sorted sandstones. K-feldspar overgrowths also occur in carbonate cemented beds, although they are less abundant towards the central parts of the thickest cemented beds. Leaching of detrital and authigenic K-feldspar (Figure 4A,E,F) is observed in the high porosity well sorted sandstone facies in the upper part of the Ula Formation (reservoir units I and II), and the dissolution reaches nearly 2%
Table 1 Summary of fluid inclusion data
Core Homogenization Well depth (m) Cement Fluid temperatures (°C)
*No reproducible homogenization temperatures were obtained. Repeated measurements the first measurements (i.e. the lowermost) are listed 1.Samples from the o i l -water contact in well 7/12-A12
resulted in higher temperature readings, and
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Diagenetic evolution and petroleum emplacement, Ula Field: T. Nedkvitne et al.
Figure 4 See caption on facing page
258 Marine and Petroleum Geology, 1993, Vol 10, June
Diagenetic evolution and (point-counted) in the most well sorted sandstones. K-feldspar leaching is not found in the calcite cemented beds, suggesting that dissolution took place subsequent to calcite cementation. The degree of leaching decreases gradually downwards in unit III and rapidly upwards in the finer grained and less sorted unit I, and is not observed in the Bryne and Skagerrak Formations.
Authigenic albite is associated with detrital plagio- clase grains and occurs as intragranular replacement and intergranular overgrowth cements (Figure 4G). A clearly authigenic origin is inferred from the euhedral crystal phases, microprobe analysis showing a pure end-member (Ab>99) (cf. Gold, 1987; Milliken, 1989; Morad et al., 1990), and due to the occurrence of petroleum inclusions in the cement. The appearance of authigenic albite is well defined in backscattered and cathodoluminescence images (Figure 4G, H). The amount of authigenic albite reaches about 2% in well sorted sandstones, but none was observed in the carbonate cemented beds.
The overall content of kaolinite in the Ula Formation is rather small (<1%), but in some intervals it is more abundant ( = 2 - 3 % ) . In the Skagerrak Formation kaolinite is missing whereas authigenic illite and chlorite are abundant. In the Ula Formation, authigenic chlorite and illite occur only in trace concentrations.
Fluid inclusions
Microthermometric data, presented in Table 1 and as histograms (Figure 5), are measurements of primary inclusions from the Ula Formation. The homogenization temperatures obtained from all the inclusions, except for K-feldspar inclusions, are reproducible measurements (up to +0.5°C). Very precise homogenization temperature measurements of aqueous inclusions were, in many instances, difficult to obtain. This is because the vapour phase (bubble) in the inclusions commonly attaches to the mineral wall before homogenization, and spurious optical effects at the inclusion wall made accurate readings difficult on minute inclusions. This was not a problem for the
Figure 4 (A) SEM image illustrating euhedral crystals of calcite (C) showing no dissolution features. Note that the detrital (DF) and overgrowth (AF) of feldspar are leached. From 3708.60 m (core depth) in well 7/12-A18. (B) SEM image of quartz over- growth (O) and a cavity (arrow) developed at the interface between the detrital grain and the overgrowth. From 3708.60 m (core depth) in well 7/12-A18. (C) Optical micrograph of a stylolite (arrows). Quartz grains (Q) are markedly more dissolved along the stylolite than feldspar grains (F). Scale bar is 500/~m. From 3711.88 m (core depth) in well 7/12-A18. (D) A backscattered electron image superimposed on a cathodoluminescence image showing K-feldspar overgrowth (AF) on a detrital K-feldspar grain (DF). From 3734.60 m (core depth) in well 7/12-A18. (E) SEM image illustrating a K-feldspar grain where the overgrowth (arrow) is partly dissolved. From 3708.60 m (core depth) in well 7/12-A18. (F) SEM image of a corroded (i.e. leached) K-feldspar overgrowth (AF) outlining a totally dissolved detrital grain (SP). From 3708.60 m (core depth) in well 7/12-A18. (G) Backscattered electron image of a detrital plagioclase (DP) partly replaced by authigenic intragranular albite (A). Authigenic intergranular albite surrounds the grain with euhedral overgrowth (B). From 3430.00 m in well 7/12-6. (H) Cathodoluminescence image of the same area as in Figure 4G. Authigenic albite shows weak luminescence while the detrital phase has a bright luminescence
petroleum emplacement, Ula Field." T. Nedkvitne et al.
12 Petroleum inclusions
Aqueous inclusions
QUARTZ CEMENT
4; 6'0 8; 1;0 120 ,,;4
Present reservoir temperature
0 i i
20 160 180 200 i i
220 240 260
8-
0 20 40 180 260 2~0 2~0 2~0
ALBITE CEMENT
4'0 6'0 8'0 160 1~0 4c~
4 ] K-FELDSPAR CEMENT
01 20 40 60 80 100 120
i i i
40 160 180 2;0 2~0 240 2~0
HOMOGENIZATION TEMPERATURE (°C)
Figure 5 Histograms of homogenization temperatures. Present reservoir temperature 143°C
petroleum inclusions because here the vapour phase always remained in the central part of the inclusion, where the image was generally good. The petroleum inclusions with homogenization temperatures in Table 1 were carefully examined by both UV fluorescence and on the freezing stage and showed no evidence of a free water phase (i.e. they were trapped as a homogenous fluid). However, as few heterogeneous (oil and water) inclusions were observed, the presence of a very thin water film at the interface between the oil and the inclusion wall cannot be excluded for the inclusions presented in Table 1. The authigenic origin of the inclusions was tested by the use of cathodoluminescence on the cement (cf. Burley et al., 1989). Petroleum inclusions also occur in the calcite, but exclusively towards the edges of the cemented beds. These inclusions had a very strong tendency to stretch and leak in the laboratory, and no reliable homogenization data was obtained.
Melting studies of frozen aqueous inclusions designed to measure salinity (cf. Burley et al., 1989) were attempted. However, these measurements were influenced by only two coexisting phases (ice and liquid) at melting, i.e. the ice melted metastably (cf. Roedder, 1967), and is therefore not applicable for salinity measurements. The vapour phase appeared as a sluggish nucleation at much higher temperatures than melting (about +2 to + 6°C).
A few inclusions, with homogenization temperatures scattered in the range 177-250°C (Table 1; Figure 5), are interpreted to be trapped heterogeneous (oil and
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Diagenetic evolution and petroleum emplacement, Ula Field: T. Nedkvitne et al.
iiii~
0
Q.
t -
t - O t - O
[
260 Marine and Petroleum Geology, 1993, Vol 10, June
Diagenetic evolution and petroleum emplacement, Ula Field: T. Nedkvitne et al.
water) or stretched/re-equilibrated inclusions originally formed at temperatures <140°C (cf. Prezibindowski and Larese, 1987; McLimans, 1987; Burruss, 1989).
Inclusions in authigenic quartz Fluid inclusions in authigenic quartz are found in intragranular healed fractures and in intergranular overgrowths, but they are trapped most often at the interface (dust rim) between the clastic grain and the intergranular overgrowth cement (Plate 1B). The petroleum inclusions reach diameters of up to 40 tzm, and the largest inclusions usually have an irregular form. The aqueous inclusions (Plate 1C, 1D) are smaller, up to 15/zm, and their form is more regular (negative crystal form). Homogenization temperature measurements plotted as a histogram (Figure 5) show a unimodal distribution in the range 113-138°C. The highest temperature of the unimodal distribution for the aqueous inclusions approaches the present day reservoir temperature (143°C). Petroleum and aqueous inclusions occur next to each other in the same overgrowth, suggesting that they were trapped pene- contemporaneously. However, the homogenization temperatures of the petroleum inclusions are in the range 75-114°C (Figure 5). The lower range of homogenization temperatures of petroleum inclusions relative to the aqueous inclusions can be explained if the petroleum in the inclusions was trapped as an undersaturated oil (with respect to gas) (cf. Burruss, 1989). This is probably due to the fact that the bubble point pressure of the present undersaturated petroleum in the Ula Formation was reached at around 15 Ma, corresponding to a burial depth of about 2 km (Figure3).
Inclusions in authigenic albite Petroleum and aqueous inclusions in authigenic albite occur in intragranular and intergranular (overgrowth) cement. The highest abundance of inclusions is found in intragranular albite (Plate 1E, 1H) and they occur in association with ribbon and skeletal textures of detrital plagioclase (Plate 1F, 1I). Inclusions in the inter-
Plate I (A) Optical micrograph of a quartz grain illustrating petroleum inclusions with emission of bright fluorescence (arrows) in the authigenic cement. From 3430.00 m in well 7/12-6 where the highest abundance of petroleum inclusions occur, i.e. equivalent to 5 on the semi-quantitative scale in Figure 8. Scale bar is 50/~m. (B) Optical micrograph showing petroleum inclusions (arrows) at the interface between the overgrowth (O) and the detrital grain. From 3430.00 m in well 7/12-6. Scale bar is 50/~m. (C),(D) Optical micrograph of aqueous inclusions (arrows) from 4079.80 m (core depth) in well 7/12-A12. Scale bar is 25/.~m. (E) Optical micrograph of a piagioctase grain from 3711.88 m (core depth) in well 7/12-A18. Petroleum inclusions with bright fluorescence are marked with arrows. Scale bar is 50/~m. (F) Backscattered electron image of the same area as in (E). Dark grey area on the grain shows authigenic albite (arrows) whereas the bright area (DP) represents detrital plagioclase. (G) Optical micrograph of a petroleum inclusion (arrow) in a K-feldspar overgrowth. From 3714.30 m (core depth) in well 7/12-A18. Scale bar is 50/~m. (H) Optical micrograph illustrating petroleum inclusions (arrows) in plagioclase grain from 3734.60 m (core depth) in well 7/12-A18. Scale bar is 50/~m. (I) Backscattered electron image of the same area as in (H) showing the intragranular albite (A) and the detrital plagioclase (DP). (J) Optical micrograph of skeletal remains of a partly dissolved feldspar grain (F). Intragranular porosity (IR) and intergranular porosity (IE) are marked. The sample is from the Brent Group sandstones in the Viking Graben at ~2 km burial and at 90°C. Scale bar is 50/~m
granular (overgrowth) albite occur most often at the interface between the detrital and authigenic phase as they do in quartz grains. The sizes of petroleum inclusions in albite reach a maximum diameter of 40/zm, whereas the aqueous inclusions found are not larger than ~30/zm. Homogenization temperatures of the fluid inclusions in the authigenic albite show a similar unimodal distribution to the inclusions trapped in quartz. The petroleum inclusions fall in the range 72-125°C whereas the aqueous inclusions are in the range 114-133°C (Plate 1). This implies that the inclusions in the albite were trapped almost synchronously with the inclusions in the quartz.
Inclusions in authigenic K-feldspar The fluid inclusions in the K-feldspar are not easily detected using optical microscopy, due to leaching affecting the clarity of the mineral forms. Aqueous inclusions were barely observed and no homogen- ization temperature measurements were attained. Nevertheless, the petroleum inclusions (up to 30/zm) are relatively easily detected by UV microscopy because of their UV excited fluorescence emission (Plate 1G). Petroleum inclusions in K-feldspar are less abundant than petroleum inclusions in quartz and albite, and they occur exclusively in overgrowths in well sorted high porosity sandstone facies in the upper part of the reservoir (reservoir units I and II in Figure 2). The homogenization temperatures of the petroleum inclusions are in the range 28-124°C (Figure 5) and they do not show the unimodal distribution which is observed for the inclusions in authigenic quartz and albite. The typical scattered distribution may indicate that the K-feldspar overgrowths have formed over a larger temperature range. However, the high end temperature measurements can also be related to stretched and re-equilibrated inclusions originally formed at low temperatures (cf. Prezibindowksi and Larese, 1987; McLimans, 1987; Burruss, 1989). This is supported by the fact that K-feldspar inclusions occur in fragile and partly dissolved overgrowths and many were prone to stretch and decrepitate during heating. Authigenic K-feldspar containing petroleum inclusions is found in calcite cemented beds whereas detrital quartz and plagioclase grains in the calcite cemented beds do not show signs of authigenic mineral growth. This is additional evidence supporting the relatively earlier K-feldspar diagenesis. A possible mechanism for lowering the homogenization temperature (i.e. increasing the density of the inclusion) would, however, be by selective leaching of gas from the inclusions (Hanor, 1980), but no relative shrinkage of the gas bubble was observed after decrepitation, suggesting this to be very unlikely.
Petroleum geochemistry of the fluids in the inclusions Table 2 and Figure 6 summarize the molecular and compositional characteristics of the bulk petroleum inclusions and the Ula Formation DST oil. The n-alkane distribution of the inclusions, although severely modified by evaporative depletion in the C1-C17 part, due to sample workup, confirmed that the fluorescing inclusion samples contain oil. The molecular ratios containing the isoprenoid hydro- carbons and lower molecular weight aromatic hydro-
M a r i n e and P e t r o l e u m G e o l o g y , 1993, Vo l 10, J u n e 261
Diagenetic evolution and petroleum emplacement, Ula Field: T. Nedkvitne et al. Table 2 Maturi ty and bulk properties of petroleum inclusions and Ula Formation oil
1" Methylnaphthalene ratio (Radke et al., 1986) 2 Dimethylnaphtha.lene ratio (Radke et a l , 1986) 3 Trimethylnaphthalene ratio 2 (Radke et al., 1986) 4 Methylphenanthrene ratio (Radke et al., 1986) 5 Methylphenanthrene index 1 (MPI 1) (Radke et al., 1986) 6 % Rc, calculated vitrinite reflectance from MPI (Radke et al., 1986) 7 Methylphenanthrene distribution factor (Kvaldheim et al., 1987) 8 20S1(20S + 2OR) of 5(x(H), 14~x(H), 17c~(H)-steranes (Mackenzie et al., 1985) 9 {3~/(1313 + c~(x) of C29 (20R and 20S) sterane isomers (Mackenzie et al., 1985)
10 C20/C2o + C28) triaromatic steroids (Mackenzie et al., 1985) 11 C28 triaromatic sterane/(C28 triaromatic sterane + C29 monoaromatic sterane) (Mackenzie et al., 1985) 12 Hopane/sterane (Mackenzie, 1984) 13 Hopane x/hopane x + normoretane) (Cornford et al., 1986) 14 Ts/'I'm (Seifert and Moldowan, 1981) 15 C2917cdH), 21~(H)-norhopane/(C3o17c((H), 2113(H)-hopane) (Mackenzie et al., 1985)
carbons should be used with caution due to the light end depletion of the extracts. Still, it appears that the distributions are representative of a mature oil with a pristane/phytane ratio typical of marine petroleum (cf. Connan and Cassou, 1980).
Several molecular parameters, usually interpreted to reflect maturity more than facies, e.g. isomerization at C20 in o¢0~ocC29 steranes, the ratio of O~B]~C29 steranes to o¢o~o{C29 steranes (Mackenzie et al., 1985), the ratio of hopane X to normoretane (Cornford et al., 1986), the steroid hydrocarbon ratio C28 20R-triaromatic/(C28 20R-triaromatic + C2950~(H ) ÷ 5f3(H) monoaromatic) (Mackenzie, 1984), the methylphenanthrene index 1 (Radke, 1987; 1988), the methylnaphthalene ratio (Radke et al., 1986), and the methylphenanthrene distribution factor (Kvaldheim et al., 1987) suggest that the petroleum in K-feldspar inclusions is less mature than the present day Ula Formation DST oil. Still, several parameters, especially among the medium range aromatic hydrocarbons (cf. Alexander et al., 1985; Radke et al., 1986; Radke, 1987) are more similar in these samples, as in the ratio of C20/(C20 ÷ C28 ) triaromatic steroids. Owing to evaporative loss during inclusion analysis the naphthalene parameters are more questionable.
Although dependent on facies, the ratio of 180~(H)- trisnorneohopane to 17cKH)-trisnorhopane (Ts/Tm) (Seifert and Moldowan, 1981) is significantly lower in the K-feldspar inclusions, probably reflecting the lower maturity of this fluid. The hopane to sterane ratio is higher in the K-feldspar inclusions than in the Ula Formation DST oil, suggesting a slightly different source facies influence. However, this ratio is well within the range associated with 'North Sea oils' (cf. Mackenzie, 1984). Also the distribution of C27 , C28 , C2950¢(H), 1413(H), 1713(H) steranes (20S) is compatible in general with North Sea oils sourced from the Mandal/Kimmeridge Formation and equivalents (cf. Northam, 1985), and especially to petroleum in the Ula Field. The C29170¢(H), 2113(H)-norhopane/C3o170~(H) 2113(H)-hopane ratio, which is 0.81 in the K-feldspar inclusions and 0.43 in the Ula Formation oil, again suggests a source facies difference between the K-feldspar and quartz petroleum phases. The relative
proportions of tricyclic terpanes to pentacyclic triterpanes (Figure 6) is different in the K-feldspar petroleum inclusion fluids and the Ula Formation DST oil, showing that the differences are real. Tricyclic terpanes are found in oils of diverse source facies, except possibly those of purely terrestrial origin (cf. Aquino Neto et al., 1983; Mello et al., 1988).
To summarize, the facies which sourced the entrapped K-feldspar petroleum appear to be different to that sourcing the main oil in the free porosity of the Ula Formation. The petroleum in the K-feldspar inclusions is of lower maturity than the oil found today in the Ula Field. The quartz inclusions which formed subsequent to the K-feldspar inclusions contain petroleum with molecular parameters which, in general terms, can be interpreted as containing characteristics from both the K-feldspar inclusion petroleum and the present day oil charge. Thus the composition of this petroleum may reflect the progressive maturation of the Mandal Formation in areas more proximal to the drainage area of the Ula Field.
Analysis from the Iatroscan thin-layer chromatography/flame ionization detection (TLC/FID) technique shows that the percentages of saturated, aromatic and polar fractions are 39, 10 and 51%, respectively, for the petroleum in the K-feldspar inclusions, whereas for the petroleum in the quartz inclusions these are 51, 13 and 36%. These values are markedly different from the Ula Formation oil where the percentages of saturated, aromatic and polar fractions are 68, 25 and 7%, respectively. The difference in polar content between the inclusion and Ula Formation oil can probably be related to the trapping mechanism of the inclusions, which is discussed later. However, the enriched polar compounds in petroleum inclusions may also reflect analytical problems associated with the incomplete removal of non-inclusion absorbed phases on the grains.
The gas composition of the inclusions (Figure 7) shows that the gas trapped in the petroleum during K-feldspar diagenesis contains all the characteristics of a gas associated with a mature oil (cf. Astaf'ev et al., 1973; Connan and Cassou, 1980; Reznikov, 1969) with
262 Marine and Petroleum Geology, 1993, Vol 10, June
Diagenetic evolution and petroleum emplacement, Ula Field: T. Nedkvitne et al.
K - F E L D S P A R I N C L U S I O N OIL
n-Cl7
i
pentacyclic triterpanes / "%
] tricyclic ter~nes
i . , • , • , • , • , • i • , , i ,
Q U A R T Z I N C L U S I O N OIL
1
n-C17
n-Cl3
ULA F O R M A T I O N OIL
n-C13 i
f
A
a = 18cc(H)-22,29,30-trisnorneohopane (Ts) e = Hopane x
b = 17oc(H)-22,29,30-trisnorhopane (Tin) f = 17B(H),21(~(H)-30 normoretane
Figure 6 Gas chromatograms of total alkanes and m/z 191 ion chromatograms from inclusion and Ula Formation oils
Marine and Petroleum Geology, 1993, Vol 10, June 263
C,
C1
K-feldspar
[I C~ C3 n-C 4 . ~ . J ~ ' . fl-C s
c, Quartz
C3
C2
I n-C4
• ° °°
3500n
L-
Ula Formation
~2 C3
i .c, n-C. JL__J~ - ~..___A
i-Cs n-Cs
Figure 7 Gas chromatograms showing the relative distribution of C1-C~ in quartz, K-feldspar and Ula Formation oils
Fie~d: T. Nedkvitne et al. wells 7/12-3A, -6, -A03, -A18 and -4 were analysed by UV microscopy for the evaluation of the abundance and distribution of petroleum inclusions in the quartz cement. The absolute abundance of petroleum inclusions tends to increase towards the top of every well (Figure 8), and in the top of well 7/12-6 (field crest) the highest number density of petroleum inclusions occurs (Plate 1A). The ratio between the density of petroleum and aqueous inclusions increases from the oil-water contact (OWC) towards the crest of the structure, mainly because the absolute number of petroleum inclusions increases. The increase in abundance of petroleum inclusions towards the reservoir crest may reflect the early evolution of the petroleum column in the Ula Formation. No petroleum inclusions were observed below the observed present day OWC (wells -3A and -A12). In addition to this trend, a high relative concentration of petroleum inclusions is positively correlated with a well sorted high permeability sandstone facies extending laterally throughout the reservoir (i.e. the boundary between reservoir units I and II, and in the upper part of reservoir unit III) (Figure 8). This suggests the possibility of an early petroleum saturation in these sandstone facies (the proto-Ula accumulation). The relationship between petroleum saturation and inclusion abundance is discussed in the following.
Pore water analysis
The activity data for dissolved species from the six water samples (DST and RFT tests, 7/12-3A, -7, -A12) is plotted on a logarithmic diagram towards the stability of albite, kaolinite, illite and K-feldspar (Figure 9). The samples plot in a cluster in the illite field close to the
Diagenetic evolution and petroleum emplacement, Ula
subordinate iso-alkanes. In contrast with this, the gas in the quartz inclusions has a very wet signature. This may reflect the lower gas to oil ratio signature of the early oil expelled from the nearby Mandal Formation. The methane rich gas composition observed today in the field may reflect the addition of a recent dryer gas charge to the reservoir, possibly in response to the rapid burial. Isotopic data and solution gases from the Ula Formation did show evidence of a lateral gas influence into the field from the east (Larter et al., 1990), which had not equilibrated due to poor lateral mixing.
Distribution o f oil inclusions in the reservoir Sand grain fractions sampled from various depths at
Figure 8 Abundance of petroleum inclusions in quartz cement, indicated by the bar length (0-5), at different depths and wells in the reservoir. See Plate 1A for petroleum inclusion abundance equal to 5. The degree of cementation is shown by the number (1-3) right to the bar, where 1 displays the most cementation (=5%)
264 Marine and Petroleum Geology, 1993, Vol 10, June
7-
6.
.'r- 5-
3-
2-
Diagenetic evolution and petroleum emplacement, Ula Field. T. Nedkvitne et al. inclusion formation, a tentative depth of K-feldspar formation is estimated at ~1 km. /
K-FELDSPAR
ALBITE
KAOLINITE ILLITE
Iog(aRqaw)
Figure9 Activity data of dissolved species in the present formation water plotted towards the stability field of authigenic mineral phases assuming quartz saturation and using the present pressure-temperature conditions
triple-point between albite, K-feldspar and illite. One pore water sample (DST, 7/12-3A at about
3600 m) was analysed with respect to methane and shows a concentration of 1250 ppm (weight) dissolved methane at ~ 2 0 0 0 0 0 p p m salinity, suggesting the water to be close to saturation or fully saturated with respect to methane (Haas. 1978).
Discussion
Temperatures and depths of cernentation Inclusions which were trapped as a single phase and saturated with respect to gas (e.g. methane), and at a later stage not re-equilibrated, will have a homogenization temperature equal to the trapping temperature (Hanor, 198(I). Pore waters in a petroleum filled reserw)ir will have soluble hydrocarbon concentrations dose to or at saturation (McAuliffe, 1979), consistent with the present methane saturation in the formation water. The occurrence of petroleum inclusions in authigenic K-feldspar, albite and quartz in the Ula Formation is evidence that petroleum was Present in the Ula Ficld reservoir when these authigenic minerals were formed. It is therefore a reasonable assumption that the homogenization temperatures for the aqueous inclusions within the unimodal distribution adequately describe the trapping temperature. This suggests that extensive quartz and albite formation started at ~ l l 0 ° C and continued as ongoing processes to the present day. The present geothermal gradient is ~40°C/km and applying this gradient suggests that extensive quartz and albite cementation started at =2 .5 km burial depth.
In the case of K-feldspar diagenesis, it is difficult to evaluate an exact temperature of formation. The homogenization temperature data are ambiguous probably due to stretching of the inclusion during burial, but petrographic data provide some broad constraints. K-feldspar overgrowths are, in contrast with authigenic quartz and albite, present in the poikilotopic calcite cement in which no evidence for extensive compaction was found, again implying an early origin. If we assume the lowest homogenization temperatures to be near the temperature of petroleum
Trapping mechanism of fluid inclusions Both aqueous and petroleum inclusions are trapped at similar sites in the different cement phases (e.g. healed fractures, dust rims, etc.), The only difference is that the largest petroleum inclusions tend to be bigger than the aqueous inclusions. Entrapment of aqueous inclusions can easily be explained by intergrowth of individual crystals (Roedder , 1984, p. 12). The mechanism by which petroleum inclusions become trapped is, however, more obscure (cf. Kvenvolden and Roedder, 1971) as is the relationship between petroleum saturation and inclusion formation and abundance.
There is no positive correlation between restricted cementation with increasing numbers of petroleum inclusions in the reservoir, which would have indicated that the quartz cementation was considerably inhibited by petroleum. In fact, a high abundance of petroleum inclusions occurs in the most cemented sandstones (Figure 8). This suggests that most of the surface area of most quartz grains had a strong preferential affinity for water during cementation. This is in agreement with wettability studies of siliciclastic reservoirs which show a dominance of water-wet situations (Cuiec, 1987).
Secondary migration of petroleum to the reservoir as a bulk phase requires a petroleum head and high petroleum saturation in the "petroleum rivers' leading to the field (Berg, 1975; England el al., 1987). This would imply that there was a high degree of petroleum saturation ( > 5 0 % ) in the sandstones when petroleum inclusions were trapped if the inclusions were formed from a conventional high saturated oil column. A buoyant petroleum slug opposed by displacement capillary pressures controls oil intrusion into fractures and cavities in the grains. Thus, the height of the oil column above the OWC would control the extent to which oil intrudes into small (pre-inclusion) fractures and cavities. The displacement capillary pressure of oil in a 100% water-wetted system can be obtained from mercury injection capillary pressure data (cf. Schowalter, 1979; Jennings, 1987). The height of the oil column as a function of capillary pressure can be derived using an equation from Jennings (1987)
H= Pc
(W,,,- Wo)CF
where H = height of oil column above OWC (m), Pc = capillary pressure (kPa) at a given mercury saturation, W,,, = density gradient of the formation water (kPa/m), We = density gradient of the oil (kPa/m) and CF = conversion factor needed to convert from a mercury-a i r system to an o i l -wate r system.
The IV,,. and We in the reservoir are, respectively, 11.5 and 6.5 kPa/m and the following equation and values are used to calculate the conversion factor:
CF= (THg)(COSOHg) To w(COSOo,'w)
0.48 x 0.766
().()28 × 1
where Trig = interfacial tension of mercury to air (N/m), 0Hg = contact angle of mercury to air, T, ........
Marine and Petroleum Geology, 1993, Vol 10, June 265
Diagenetic m e , , , , , , , , , , , ,, , , , , .
• 500 f ........ i ................. i ~= ~o ~oop
Z 300 I 200
~ 60 lOOI " .
• , . . . . . . . . . ,~.:.-,, ,D ~- 40 i i, i 0.1 1 .~]
,.,.,
Z0
i i i i i i l ~ l i i i i i i ' i l f 0.1 1 10
DIAMETER OF CAPILLARY, i.e. FRA(2TURE (p.m)
Figure 10 Height of the oil co lumn versus the d iameter of capillary (i.e. fractures, cavities) intruded by oil
evolut ion and pe t ro leum emplacement, Ula Field: T. Nedkvi tne e t al .
interfacial tension of oil to water (N/m) and 0o/,,. = contact angle of oil to water. The height of the oil column plotted against the diameter of the capillary intruded bv oil (i.e. fractures, cavities), is shown in Figure 10.
Detailed scanning electron microscopy studies show cavities in quartz developed at the interface between the detrital mineral and the authigenic overgrowth (i.e. dust rim) (Figure 4B), and this interface is a common site for oil inclusions (Plate 1B). These oil-filled cavities may, after a later stage of cementation, contain petroleum as inclusions. The width of these cavities reaches up to about 5 p,m and accordingly these will be intruded by oil when the oil column is about 5 m high (Figure 10). With larger oil columns the oil will intrude into smaller cavities, and thus the potential for formation of petroleum inclusions increases. This may tentatively explain the high abundance of petroleum inclusions in quartz cement towards the crest of the reservoir, as here the height of the oil column has, at any time, presumably been larger than elsewhere in the reservoir.
During the early stage el filling England et al. (1987) hypothesized that rcserw)irs contain fairly variable oil saturation. This may be similar at the earliest stage of filling to the situation in carrier beds where mass balance arguments have suggested that as little as 2 - 3 % of the overall carrier bed porosity may control petroleum movement through it (England and Mackenzie, 1987: Larter and Horstad, 1992). Although it is clear to us that quartz diagenesis may proceed to some extent through residual water films even in high petroleum saturations (~-80%), two additional plausible mechanisms need to also be considered to explain the alternating oil-wet/water-wet environment required at a potential inclusion site. Perhaps the most feasible mechanisms for achieving this are as follows. (1) With a partially filled reservoir it is easy to envisage situations where the reservoir consists of discontinuous o i l -water oil slugs, perhaps produced by mechanical disruption of the petroleum rivers during earthquake activity which must have been common in the Ula trap during rapid burial. Thus a given diagenetic environment may see high water saturation, then high oil saturation, then high water saturation again as these separated oil slugs are subsequently driven through the
reservoir. (2) Alternatively, and by a very different mechanism, earthquakes may slur the pe t ro leum- water interface in a turbulent manner to produce fine colloidal dispersions of oil in water. These may be sufficiently stable to allow small amounts of the colloidal oil to diffusively aggregate at sites where the surface can become oil-wet. This may be indicated in the polar rich nature of the inclusion extracts. This mechanism circumvents problems associated with intruding oil into the blind fractures of pores. Although the three mechanisms proposed have different bulk oil saturation requirements to work, it does seem clear that all three require at least local high oil saturations (i.e. at least local oil slugs) to operatc.
lntragranular authigenic albite cement occurs associated with skeletal-like remains of detrital plagioclase (Figure 4(;; Plate IF, ll). This replacement texture is described as albitization and has been said to occur as a continuous dissolution-precipitation mechanism (neomorphism), where authigenic albite has substituted the detrital plagioclasc phase (Boles, 1982; Morad, 1986, Morad el al., 1990). Morad el al. (1990) suggested that thc replacement proceeded in weakness planes such its grain fractures and traces of cleavage and twin planes, and not as solid state diffusion. This study shows that there are numerous petroleum inclusions inside the intragranular albite cement (Plale IE and IH), which strongly supports the idea that albite replacement did not take place as a solid state diffusion. The result from the calculation (Figure 10) suggests that oil can intrude relatively narrow fractures. It is, however, unlike b, that oil will intrude cleavage and twin plancs bccausc that requires an unrealistically high oil cohnnn. Gold (1987) and Milliken (1989) suggested that a hmger period of time may separate the dissolution of detrital plagioclase and precipitation (replacement) of authigenic albite. This implies that the intragranular albitc precipitates as overgrowths nucleated on residual remains of earlier, partly dissolved, detrital plagioclasc grains, and is therefore in principle the same process as the formation of intergranular albite overgrowths. Even a small oil column will allow oil to intrude into mtragranular porosity in skeletal remains of partly dissolved feldspar grains (Plate l J). A multiple intcrgrowth of albitc crystals after oil emplacement in the intragranular porosity will accordingly probably enclose oil as inclusions (i.e. small drusy cavities) and is therefore a very reasonable mechanism explaining the entrapment of the large numbers of petroleum inclusions which typically occur in intragranular albitc cement (Plate 11, IH). The implication is that the infill cement of authigenic albite took place some lime after plagioclase dissolution.
Diagenesis during petroleum emplacement
As suggested by textural relationships, the first formed authigenic aluminium silicate in the U[a Formation is K-feldspar overgrowth. Petroleum inclusions in K-feldspar cement are less abundant than albite and quartz petroleum inclusions and occur only in the upper part of the reservoir, reflecting an elevated OWC of the smaller petroleum accumulation in the reservoir during that stage. The formation mechanism of authigenic K-feldspar is not obvious and not understood, but may possibly be related to gravity-driven hydrogeological
266 Marine and Petroleum Geology, 1993, Vol 10, June
Diagenetic evolution and petroleum emplacement, Ula Field: 7. Nedkvitne et al.
flow (i.e. meteoric) mixed with salt deposits (cf. Duffin el al., 1989).
The quartz and albite cementation took place with at least some local high oil saturation in the sandstones. The relative permeability of water in sandstones exceeding 50% oil saturation is very low (Honapour et al., 1986), suggesting restricted pore water circulation (advective water flow) influenced from outside through the reservoir during albite and quartz cementation. Thus mass transfer of soluble compounds (ions) will dominantly be controlled by diffusion in the remaining pore water and most likelv in thin water layers at the interface between the oil and minerals. Less soluble components such as aluminium and silica could not build up high concentration gradients (i.e. restricted mobility), suggesting these elements to be derived relatively locally and not transported from outside the reservoir.
lntergranular pressure solution and stylolitization are often observed in the reservoir (Figure 4C) and represent an important internal silica source for the quartz cementation. It is, however, difficult to verify this quantitatively, because there are no good methods to measure the amount of silica released due to stylolitization and intergranular pressure solution. Silica derived from opaline marine grains (diatoms, radiolaria, sponge spicules) can be an internal silica source, but only a few relicts of such grains were observed, suggesting this source to be minor. In poorly sorted and clay rich sandstones small amounts of authigenic quartz are found, but high degrees of pressure solution of quartz are still observed in these sandstones. This can be explained by lower precipitation rates of silica in poorly sorted sandstones due to the presence of clay coatings on the grains (Tada and Siever, 1989), and implies a short distance diffusion of silica from poorly sorted to better sorted sandstones.
The fluid inclusion data show that albite formed almost synchronously with quartz cementation (Figure 5). Intergranular, and probably also intragranular, albite formed as overgrowth crystals. This requires a source of aluminium in addition to silica. The aluminium must be derived from a local source due to its very low mobility. Minor pressure solution of feldspar is observed, bul this cannot explain the aluminium required to form as much as 2vo1.% authigenic albite occurring in the clean and well sorted sandstones. A local aluminium source could be the selectively leached (i.e. hydrolysed) K-feldspars which have occurred subsequent to K-feldspar overgrowth formation. There is no evidence of time equivalent dissolution (i.e. hydrolysis) of other minerals, and this suggests the observed K-feldspar leaching to have been caused by lowering of the potassium activity in the pore water. The cause of a potassium activity depletion in the pore water is not obvious and not understood, and the fate of potassium from the K-feldspar leaching is not observed as precipaled authigenic potassium minerals in the Llla Formation. A selective leaching of K-feldspar in reservoir sandstones at deep burial depth has been proposed to be enforced by illitization in adjacent finer grained sediments (cf. Boles and Franks, 1979; Nedkvitne and Bj~arlykke, 1992). Fibrous i[lite occurs in thc Triassic sandstones underlying the Ula Formation, and possibly illitization in these sandstones represents a plausible potassium sink. The degree of K-feldspar leaching decreases, however, towards the
Triassic sandstones, suggesting that vertical diffusion downwards is very unlikely. The tortuosity of the water layer at the interface between the mineral and oil is greater in the poorly sorted than in the well sorted sandstones. A potassium depletion enforced from outside the leached area will therefore build up a higher flux in the well sorted sandstone facies than in the poorly sorted sandstone facies. Diffusion (i.e. flux) is thus expected to be much more efficient in the laterally orientated well sorted sandstone facies, in accordance with the higher leaching of K-feldspar observed in these sandstone facies.
The diagenetic reactions described here would be expected to buffer the pore water in the reservoir with respect to the mineral phases, and analysis of the present formation water shows a composition which is close to equilibrium with albite and K-feldspar (Figure 9). The pore water composition plots in the illite field, about 0.5 pH units away from the triple point between albite, K-feldspar and illite. This deviation from the triple point is probably within the uncertainty of the pH estimate.
Petroleum filling o f the reservoir
The occurrence of K-feldspar petroleum inclusions shows that some petroleum was present in the reservoir before or during the formation of authigenic K-feldspar. These inclusions are exclusively found in the high porosity (about 20%) well sorted facies in the upper part of the reservoir (units l - I I ) , tentatively identifying these sandstones to bc the parts of the reservoir where the earliest petroleum accumulated during K-feldspar formation. Although it is clear that some free petroleum phase was present here, as discussed earlier, we cannot assess the gross petroleum saturation in these reservoir units ( I - I I ) at this time. It is probable, based on the extract yield of biomarker alkanes in the K-feldspar inclusion versus those in the more mature DST petroleum in the pores today (Figure 6), that this oil charge was a very minor factor. This conclusion is based on the fact that the concentration of biomarker alkanes decreases with maturity, thus it is probable that the K-feldspar biomarker alkanes were in greater relative abundance than in the later charge. As the later charge shows little evidence of tricyclic alkanes we conclude that the K-feldspar inclusion equivalent petroleum charge was small. It is possible that these reservoir units represent a proto- accumulation, probably without a well developed OWC [perhaps discontinuous, similar to that proposed by England el al. (1987)] during the early stage of field filling. As the K-feldspar petroleum phase would have been emplaced at a shallow burial depth when the Mandal Formation source rock was immature at the eastern margin of the Central Graben, the earliest petroleum to arrive must either have migrated from a more axial and deeper part of the Central Graben or alternatively from stratigraphically deeper source rocks.
Petroleum inclusions in authigenic quartz and albite are much more common in the reservoir than petroleum inclusions in K-feldspar. Nevertheless, petroleum inclusions in albite and quartz are more abundant in high porosity and well sorted sandstone facies (units I - I I ) , where the oil inclusions in authigenic K-feldspar occur, indicating that these
Marine and Petroleum Geology, 1993, Vol 10, June 267
D i a g e n e t i c e v o l u t i o n a n d p e t r o l e u m e m p l a c e m e n t ,
sandstones were also more prone to trap petroleum inclusions during albite and quartz cementation than elsewhere in the reservoir. The general trend is, however, that the abundance of petroleum inclusions in the quartz cement decreases downwards, possibly reflecting vertical changes in the OWC during cementation. The petroleum in the albite and quartz cement was trapped in the temperature range 110-143°C, thus its composition will reflect petroleum present in the reservoir from about 10 Ma to the present day. During that period of time the Ula Field and surrounding source basin have experienced 1.0 km of subsidence, a factor which is expected to influence the petroleum generation in the region, e.g. a progressive maturation of the Mandal Formation in the drainage area of the Ula Field. The change in the relative ratio of tricyclic terpanes to pentacyclic triterpanes (Figure 6) and several other geochemical parameters (Table 2) in the quartz inclusion oil may possibly reflect the fact that the petroleum in the trap became more similar to the present Ula Formation oil during the 1.0 km of burial, which is likely as this represents the main period of filling of the field (Larter et al., 1990). Other parameters remain invariate or show the opposite trend but we feel this tentative conclusion is supportable. Maturity modelling (Larter et al., 1990) suggests the main charge of reservoir filling to be recent (i.e. in the last 5 Ma), and that the maturity of the petroleum approximately equates to that in the present day Mandal Formation in the source kitchen to the east.
Ula F ie l d : T, N e d k v i t n e et al.
(4) The geochemistry of petroleum inclusions in K-feldspar overgrowths is evidence of a relatively early arrival of some oil to the reservoir, i.e, a proto-Ula accumulation in reservoir units 1-11. This is supported by K-feldspar overgrowths containing petroleum inclusions in calcite cemented beds formed before quartz and albite cementation.
(5) The petroleum in K-feldspar inclusions is not compatible with the Mandal Formation source rock sediments as known to us. It seems relatively clear that this petroleum was reservoired before the Neogene burial when the potential Mesozoic source rocks were immature in the more local drainage areas. Thus, a long distance migration from the graben axis, or alternatively from Palaeozoic sources more local to the field, took place.
Acknowledgements
The authors thank BP Petroleum Development Norway A/S for extensive technical and financial support and access to samples and formation water data. We are grateful to the Ula Field licence partners (Statoil, Norsk Conoco A/S, K/S A/S Pelican & Co) and Svenska Petroleum A/S for permission to publish. Tor Nedkvitne and Dag Karlsen acknowledge NAVF and Amoco Norway Oil Company A/S for support. We also thank Dr R. S. Haszeldine and an anonymous reviewer for their helpful comments.
Conclusions
(1) Microthermometric measurements of fluid inclusions suggest that authigenic quartz and albite in the Ula Formation are formed as a continuous process in the temperature range of about 110°-143°C, corresponding to a depth of about 2 .5-3 .4 kin. The high abundance of petroleum inclusions indicates that cementation took place after oil arrived in the sandstones. Although the relationship between oil saturation and petroleum inclusion formation is not clear, we do feel that the occurrence of these inclusions indicates that at least local high oil saturation, even if discontinuous, was present in the reservoir at that time.
(2) There is no positive correlation between reducing cementation and large numbers of petroleum inclusions. Even if there was some pre-petroleum authigenic cement (i.e before the main oil charge), this would indicate that cementation was not stopped by the petroleum even if diagenesis was slowed down. Thus, most of the quartz mineral surfaces were water-wet during cementation, and it is probable that the mass transfer during albite and quartz diagenesis was by short distance diffusion in a thin water layer at the interface between the oil and mineral phases.
(3) Selective K-feldspar dissolution is observed in well sorted sandstone facies in the upper part of the Ula Formation. The dissolution occurred after oil emplacement and was probably enforced by potassium depletion in adjacent sediments. Less well understood is the aluminium sink from leaching, but it may be consumed locally by the formation of authigenic albite.
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Appendix: Analytical procedures Sandstone samples from wells 7/12-6, -3, -4, -A03, -A18 and -AI2 were examined by optical microscopy and high resolution backscattered electron microscopy (BSEM) with an energy dispersive electron microprobe (EDAX) and cathodoluminescence facilities. Bulk mineralogy was obtained by X-ray diffraction analysis.
Microthermometric measurements of aqueous and petroleum inclusions in authigenic quartz, albite, K-feldspar were made with a Chaixmeca and a Linkam THM 600 heating/freezing stage. The temperature measurements were calibrated with a series of pure synthetic substances with known melting points (cf. Roedder 1984, p. 206). The composition of the petroleum inclusions was analysed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC-MS) (cf. Karlsen et at., 1992). Two analytical procedures were followed. (1) Higher boiling
Ula F ie l d : T. N e d k v i t n e et al.
hydrocarbons (C9+) were analysed by crushing cements of chromic acid cleaned sand fractions in an agate mortar under dichloromethane (DCM, b.pt. 40°C), concentrating the extract by evaporation of the solvent before injection. (2) Gas in the inclusions (C~-C~) was analysed using a crusher (steel crushing chamber with brass cylinder) equipped with a septum, through which the sample was withdrawn with a gas-tight syringe. The chromatograms obtained represent bulk analysis of the inclusions (e.g. typically 2-3 g sand fractions) in quartz and K-feldspar cements. Fractionation of the different minerals was achieved using heavy liquid separation techniques based on density differences. The purity of the fractions was tested by element image analysis of thin sections of the different fractions. This showed very little contamination of the quartz and K-feldspar fractions (< 1%). The albite fraction obtained was contaminated by both quartz and K-feldspar (about 15 and 10%, respectively) and is not presented here. The latroscan TLC/FID technique (cf. Karlsen and Larter, 1991) was used to analyse the saturated, aromatic and polar fractions of the petroleum in the inclusions. A more detailed description of all analytical procedures used in the hydrocarbon analysis is given in Karlsen et al. (1992).
The abundance and distribution of petroleum inclusions in the reservoir have been evaluated using a screening procedure, where sandstones sampled from different wells and at variable depths were disintegrated to yield sand grain fractions. These fractions were cleaned in DCM and chromic acid during ultrasonic treatment, removing organic coatings, other surface material and carbonates. The petroleum inclusions were clearly visible by UV microscopy due to their fluorescence emission (Plate 1A). The abundance of petroleum inclusions in the quartz grain fraction was differentiated semiquantitatively by visual observation on a scale of 0-5, where 5 is highest abundance of petroleum inclusions (Plate 1A). During the same procedure we evaluated the degree of quartz cementation (overgrowths on grains) on a scale of 1-34 where 3 means almost no cementation (<1%). An evaluation of the degree of cementation was necessary because the number of inclusions per grain increases with increasing overgrowth on the grain.
Activities of dissolved species from six water samples (DST and RFT samples, 7/12-3A,-7,-A12) have been calculated using Pitzer's method using the computer program SOLMINEQ.88 (cf. Kharaka et al., 1988). The calculation was carried out assuming quartz
. saturation and using the present pressure-temperature conditions of 483 bar and 143°C. The in situ pH of the formation water was calculated using calcite saturation.
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