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Petroleum biodegradation in the Tertiary reservoirs of the North Sea
Mah~~ und Pctiolmwl Geoloyy, Vol. 14. No. 1, pp. 55-64. 1997 (” 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0264~8172*97 $17.00+0.00 PII: 80264-6172(96)000463
Petroleum biodegradation of the North Sea
in the Tertiary reservoirs
Asif Ahsan and Dag A. Karlsen Petroleum Geochemistry Oslo 3, Norway
Program,
Department
of Geology,
University
of Oslo, N-0316,
Richard L. Patience* Statoil
a.s., N-4035, Stavanger,
Received
19 February
Norway
7996; revised 28 June
1996; accepted
23 July
1996
A suite of 17 oil samples from the Tertiary reservoirs (Palaeocene-Eocene) of the North Sea was subjected to routine geochemical analytical techniques in order to evaluate the degree of biodegradation. The methods included latroscan TLC-FID, gas chromatography of the saturated and aromatic hydrocarbon fractions, gas chromatography-mass spectrometry of the saturated hydrocarbon fraction and stable carbon and hydrogen isotope analyses. In addition, API gravities and weight percentages of asphaltene were also determined. Based on these data it was found that oils in the suite were derived from the Jurassic aged Kimmeridge Clay Formation originating from source rock horizons at similar maturity levels. The oils in the suite were taken from reservoirs with present day temperatures of 49-l 14 “C. Differences in the chemical composition of the oils (e.g. reduced relative amounts of saturated hydrocarbons, increased polar amounts and reduced relative concentrations of n-alkanes), were attributed to secondary alteration processes of which the most likely process to occur at temperatures less than 80°C and consistent with the observed chemical characteristics of the oils is biodegradation. The differences in chemical composition are thus attributed to different degrees of biodegradation. The oils were ranked and classified in terms of the degree of biodegradation using a range of restricting parameters based on the gas chromatographic concentrations of different alkane species. Based on these parameters, oils were divided into four classes. These classes are (a) non to slightly biodegraded; (b) mildly biodegraded; (cl moderately biodegraded; and (d) severely biodegraded. In cases where the degree of biodegradation was found to have reached levels where unambiguous identification of alkanes was not possible, unresolved complex mixture (UCM) parameters were utilised to assess the extent of biodegradation. The degree of biodegradation was subsequently compared to the type of reservoir sandstone facies from which oils in this study were taken. It was found that at similar temperatures, oils in the Balder Formation (Upper Late Palaeocene) were biodegraded to a higher degree compared to oils from either The Forties (Lower Late Palaeocene) or Sele (Middle Late Palaeocene) Formation. The higher degree of biodegradation in Balder reservoirs is attributed to relatively greater openness to meteoric water invasion in these reservoirs. 0 1997 Elsevier Science Ltd. All rights reserved. Keywords: North Sea; Tertiary reservoirs; petroleum
Petroleum biodegradation is just one of several processes that can alter the properties of a reservoired petroleum (Park, 1984). This process is brought about by bacteria which gain access to a reservoir through meteoric water flushing (Palmer, 1993). Aerobic bacteria have generally been accepted to be responsible for this process, whereas the role of anaerobic bacteria in biodegrading hydrocarbon species has been debated in great detail (Palmer, 1993). Connan et al., 1993 maintained that even anaerobic bacteria can be effective under certain conditions in initiating biodegradation. This view is also supported by Wilkes et al. (1995). Hollerbach (1987) suggested that
*Author
to whom correspondence
should be addressed
biodegradation;
unresolved complex mixture
anaerobes can biodegrade the products of petroleum already biodegraded by aerobes. Petroleums undergoing biodegradation become higher in viscosity and lower in API gravity and will hence be difficult to produce. It is because of this fact that biodegraded petroleums are generally undesirable from a commercial point of view. Hunt (1979) estimated that approximately 10% of all the world’s petroleum reserves have been lost to biodegradation and an additional 10% altered. Thus, statistically speaking, the chances of encountering a biodegraded oil is 1 in 10. Not only is the commercial value of oils affected as a result of this process. but several well established source and maturity parameters which are based on the ratios of different hydrocarbon species are also affected. For these reasons, an understanding of
Marine and Petroleum
Geology 1997 Volume
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1 55
Petroleum
biodegradation
in Tertiary
reservoirs:
the geologic controls on petroleum biodegradation is of utmost importance. There is a definite sequence of depletion of hydrocarbon species which takes place during successive stages 1984; of biodegradation (Volkman et al., 1983; Connan, Park, 1984). One may state as a first approximation that the simpler the structure of a compound the more easily it is biodegraded. Normal alkanes are depleted prior to isoalkanes which in turn are depleted prior to cycloalkanes. Within the same compound class, lower molecular weight compounds are depleted prior to higher molecular weight compounds (Phillippi, 1977; Schaefer, 1980; Magoon and Claypool, 1981). Multiringed cycloalkanes (steranes and triterpanes) are found to be most resistant to biodegradation, but even these compounds altered under extreme biodegradation (Seifert and Moldowan, 1979; Rulkotter and Wendisch. 1982; Volkman et rd.. 1983; Connan, 1984). The conditions for biodegradation to occur are summarised by Connan (1984) based on the work by Phillippi ( 1977) and Barker (1980) as follows: I. Oxygenated waters either by hydrodynamism or by compaction. 2. Biodegradation occuring at the oil-water contact as bacteria live in the water phase and do not thrive in the oil. 3. Sufficient supply of nutrients (nitrate and phosphate) and dissolved oxygen in moving waters. 4. Presence of microbes. 5. Subsurface temperatures allowing activity of bacteria. The maximum subsurface temperatures of biodegraded petroleums in several basins are given in Tuhk 1.
The differences in maximum temperature limits at which this process can take place in different basins is perhaps due to different strains of bacteria involved in degrading petroleums in these basins. These temperatures range from 88 ‘C in the Dampier Basin (Australia) to 54 ‘C in the South Sumatra-South Borneo Basin (Indonesia). Temperature or alternatively the depth of a reservoir is most frequently cited to be the key factor controlling biodegradation (Phillippi, 1977; Barker. 1980; Connan. 1984). However, in the North Sea some shallow Tertiary reservoirs are either not at all biodegraded or are biodegraded very little (e.g. some oils from Block 16 of the UKCS). This observation has led us to doubt temperature as the only controlling factor in biodegradation. We therefore propose that in basins like the North Sea the relative openness of a given reservoir and hence its depositional facies could play a first order control on biodegradation. In order to investigate this hypothesis, a suite of 17 oil
Table 1 Maximum subsurface temperatures in several basins (after Connan, 1984) 54°C 62°C 71°C 77°C 80°C 82°C 85°C 88°C
56
of biodegraded
oils
South Sumatra-South Borneo San Joaquin Basin (USA) Beaufort Basin (Canada) Los Angeles Basin (USA) Aquitaine Basin (France) Gulf Coast (USA) Mackenzie Delta (Canada) Dampier Basin (Australia)
Marine and Petroleum
Geology 1997 Volume
A. Ahsan et al. samples from the Tertiary reservoirs of the North Sea was selected as a data base as these oils were found to be biodegraded to different degrees (as will be illustrated later). The objectives of this study were first to rank the oils in terms of the degree of biodegradation and subsequently to correlate the degree of biodegradation to the type of reservoir facies from which these oils were obtained. For the ranking purposes. a restricted range of parameters based on the GC FID concentrations of different alkane species were used. The concentrations were normalised to the concentration of the internal standard squalane. whereas the parameters for evaluating the degree of biodegradation were taken partly from the literature and partly established by us.
Sample set and analytical techniques used 16 oil samples were procured from the Tertiary reservoirs in different quadrants from the United Kingdom Continental Shelf (UKCS). whereas I sample was obtained from a similar age reservoir on the Norwegian Cotinental Shelf (NOCS). F&ure 1 is a location map of oil samples for this study. The selection was made in order to compile a suite of oils ranging from non-biodegraded to severely biodegraded. 8 oils (F,G,J,K,M,N,P and Q) were taken from quadrant 9 of UKCS, 5 oils (C,D,E,H and 0) from quadrant 16, whereas 1 oil came from each of quadrants 30, 22, 21 and I (NOCS). The oils come mostly from different depths in the Sele (middle Late Palaeocene), Balder (upper Late Palaeocene) and Forties Formations (lower Late Palaeocene) and are mostly DST oils with the exception of G and K which are storage oils. The oils were analysed using a thin layer chromatography-based technique (Karlsen and Larter, 1989) to ascertain the bulk composition of the oils. Whole oils (i.e. undeasphaltened oils) were used for such analyses. The same oils were also separated into three fractions i.e. (1) Saturated hydrocarbons; (2) Aromatic hydrocarbons; and (3) Polars using medium performance liquid chromatography (MPLC) of the deasphaltened fraction. The MPLC was used as described by Radke et al. (1980). The saturated and aromatic hydrocarbon fractions were analysed using capillary-gas chromatography (GC-FID) to study the abundance and distribution of the individual alkanes. The internal standard squalane was used for quantifying different alkane species. The saturated hydrocarbon fractions were finally analysed using gas chromatography-mass spectrometry (GCMS).
Results and discussion Source of’the oils For this study, source rock extract data for the North Sea oils given in the literature (Cornford ef ~1.. 1983: Ostfeldt, 1987) were compared with data obtained on oils through in-house analyses. Typical North Sea oils derived from the Kimmeridge Clay Formation (KCF) show the presence of 28, 30-bisnorhopane (Grantham rf ul., 1980; Cornford et d., 1983; Hughes ef ul., 1985; Dahl and Speers, 198.5: Schou et ul., 1985). Oils in this study also show the presence of this triterpane species. In Figuw 2, the relative distribution of C,, C2X and C,, zeta-2OR steranes is plotted in a ternary diagram along with samples taken from Barnard and Bastow’s study (Barnard
14 Number
1
Petroleum biodegradation
figure 1 Location map of the samples which is marked by arrows
in the data set. Individual
samples
are indicated
50
2s
in Tertiary reservoirs: A. Ahsan et al.
by bold capital
letters within
75
each quadrant
100
Figure 2 C2,, CZ8and CZs (cKYc+ZOR)sterane distribution of oils shown as ternary diagram. ‘m’ and ‘0’ represent Barnard and Bastow (1991) samples which are oils from the Tertiary and the Mesozoic reservoirs, whereas ‘+’ represent samples for the present study
and Bastow, 1991). It is apparent from this figure that normal Tertiary reservoir samples taken from Barnard and Bastow’s study and represented by ‘m’ plots close to the oils in this study are represented by ‘+ ‘. Some samples
from Barnard and Bastow’s study show a shift towards 100% C9 apex which is interpreted by the authors to be due to a increasing degree of biodegradation. None of the oils in the present study are biodegraded to such an
Marine and Petroleum
Geology 1997 Volume
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Petroleum biodegradation Table 2 Geochemical parameters explanation of parameters Samples A
~:y~~,” 1.14 1.45 1.14 1.02 0.97 0.74 0.81
in Tertiary reservoirs: A. Ahsan et al. calculated by GC-FID of the saturated hydrocarbon fractions, ‘Carbon preference index. See text for
c::E;:
mz:
c:,“/x;;
Prln-C,,
Phln-C,,
1.83 1.92 1.56 1.48 1.59 3.24 3.15
2.67 2.19 1.96 1.64 1.83 12.08 10.95
9.18 5.05 4.19 2.18 2.16 11.42 6.81
0.58 0.56 0.68 0.65 0.67 0.76 0.74 12.81 14.04 340.05 74.89
0.46 0.48 0.64 0.58 0.63 0.67 0.67 5.44 8.62 80.63 179.15
Pr/Ph
PrNCMc,,
1.49 7.*jO 13.30 7.50 13.00 7.33 6.86 2.49 4.42 1.51 1.34 1.07 0.86 0.85
N
1.07 1.05 1.04 0.98 1.24 1.44 1.41 1.28 0.99 1.23 1.24 0.92 1.13 1.18
1.35 1.24 1.29
0
CPI’
0.96 1.28 1.42
0.85 0.75 0.74
P Q
extent that a shift towards 100% Cz, could be observed. Cornford et al. (1983) showed the Pr/Ph (pristanel phytane) ratios for the KCF derived oils to range from 0.60 to 1.90 with an average of 1.24. Pr/Ph for oils in the present study, excluding three oils, ranging from 0.92 to 1.49 with an average also of 1.24 (Tuhk 2). This average of I .24 is based on 14 oils as the remaining three oils (oils 0, P and Q) in the suite are either depleted in pristane and phytane or their identification is doubtful. High Pr/Ph values (> 3) indicate an oxidising environment, whereas lower values indicate an intermediate or reducing environment (Powell and Mckirdy. 1973; Didyk et al., 1978). The first geochemical parameter in Tut&s 2 and 3 shows the ratio of the sum of n-alkanes from n-C,,, to r&Y,, to the sum of n-alkanes from n-C,, to n-C,, (Cti = sum of n-alkanes ranging from n-C,,, to n-CIS). Likewise the second, third and fourth parameters represent the same geochemical parameter as the first one, but with different n-alkane ranges. These parameters are calculated for oils A to G, because beyond oil G. identification of n-alkanes has either become impossible or at least doubtful. The fifth parameter is the ratio of Pristane to n-C,,. The sixth parameter is the ratio of Phytane to n-Clx. The seventh parameter represents the ratio of Pristane to the height of the hump of unresolved complex mixture (UCM). whereas the eighth parameter represents the ratio of the height of the hump at the retention time of n-C,, to the height of the hump at the retention time of n-C,,. The ninth and the tenth parameters represent the ratio of Pristane to Phytane and the carbon preference index (CPI) respectively. Mstfeldt (1987) has used the CzsC30 (norhopanel hopane) ratio to correlate the oils to their source rocks. This ratio varies for oils for the present study from
0.37 to 0.62 with an average of 0.48, whereas the oils believed to be derived from KCF are shown by Mstfeldt to have a C2,,,‘Clo ratio ranging from 0.39 to 0.72. The values for data obtained on oils from a(stfeldt’s study and those on oils for the present study cover an almost identical range. However, oils and extracts from organic rich carbonates (Zumberge, 1984; Connan and Cousteau, 1986; Price rt al., 1987) and some evaporites (Connan and Cousteau, 1986) are shown to have unusually high concentrations of Cz,-hopane relative to C,,,-hopane. Riva et al. (1989) have suggested that C2q;C3,, ratios might provide a scale of ‘carbonaticity’. Brooks (1986) noted that high CIY hopane content was found to be associated with bisnorlupanes and oleanane, both of which are considered to be terrestrial indicators. However. the oils analysed in the present study showed no sign of coming from such source rock types. Bailey rt ul. (1990) have used carbon isotope and biomarker data in order to source correlate the oils in different regions of the North Sea. According to these authors, the variation in the character of the Viking Graben versus Central Graben oils in the North Sea is attributed to variations in organofacies. North Sea petroleums derived from the KCF were shown to vary in stable carbon isotope values for whole oils from - 30.4 to - 28.6%0 6’C PDB, whereas stable carbon isotope values for whole oils for the present study varies from -28.0 to -29.7 760. Based on the comparison of GCFID (Figuvr 3) GC MS and isotope data on oils in this study and the data presented in the literature (of oils and source rocks), it is concluded that oils in the present study are sourced from the Upper Jurassic to the Lower Cretaceous (Oxfordonian-Ryazanian stage) source rock, regionally termed as either Kimmeridge Clay, Draupne, Mandal, Farsund
Table 3 Table shows parameters chosen for ranking samples A-D with ratio numbers and assigned values in parentheses to each ratio number, whereas the last column shows the aggregate of assigned values for each sample Samples
z:z~,”
z::/z;
A B C D
1.14 1.45 1.14 1.02
1.83 1.92 1.56 1.48
58
(2) (I) (2) (3)
Marine and Petroleum
Wz,” (2) (I) (3) (4)
2.67 2.19 1.96 1.94
(I) (2) (3) (4)
I-,;:o/c~~
Prln-C,,
Phln-C,,
9.18 5.05 4.19 2.81
0.58 0.56 0.68 0.65
0.46 0.48 0.64 0.58
Geology 1997 Volume
(I) (2) (3) (4)
14 Number
(2) (I) (4) (3)
1
(I) (2) (4) (3)
PWCM,,,
Total
-% 7.50 13.30 7.50
10 12 21 24
(1) (3) (2) (3)
Petroleum
biodegradation
’
Normal Oils
HCtiQ Oils
in Tertiary 0
P . L 0
I .
L ??
NM -
I .
K .
IT
H -*
J .
I3 .
30
30
30
API gravity A .
HCIV) Oils H
F .
C .
F .
.
.
P .
.
:l
E .
E Mf
D
I
I
30
JO
I 20
API gravity
D . c .
I LO . .
(‘ .
Normal Oils
GO
J
)
u .
F. .
JO
API gravity
Normal Oils
K - M’i N . ??L .
A .
1
I
p H .
A. Ahsan et al.
Heav; Oils
0 .
3P ”
reservoirs:
.
I
I
30
40
API gravity
Figure 3 Four sterane and triterpane maturity parameters plotted against API gravity values of oils. Horizontal lines on graphs are plotted at the average values for the corresponding parameter whereas vertical line divides oils into heavy (~25 API) and normal (>25 API) oils
Tau Formation (Cooper and Barnard, 1984; Field, 1985; Thomas et al., 1985; Cornford et al., 1986).
or
Maturity
of' the oils
In order to assess the maturity level of the oils, we have used the following triterpane and sterane parameters: Diahopane/(Diahopane + Normoretane) (Cornford et al., 1986) C,,-Hopane/(C,,-Hopane+C,,-Moretane) (Seifert and Moldowan, 1980) 29Ts/(29Ts + Norhopane) (Peters and Moldowan, 1993) P/?/cc%+ pfl CZg steranes (Seifert and Moldowan, 1986). Figure 3 illustrates that based on all four parameters the differences in maturity level are not large enough to cause subtle differences in composition of the oils. The average values of each of these maturity parameters is represented by a horizontal line whereas, a vertical line divides normal (API >25) and heavy (API ~25) oils in Figure 4. Based on a combination of maturity level and API values, we have divided the oils into four classes. These classes with their oils and maturity parameters are
from the table that shown in Table 4. It is apparent both normal and heavy oils are derived from source rock horizons at comparable maturity levels. Based on four maturity parameters, the distribution of oils in different categories is different. This is however not surprising as the compounds on which these four maturity parameters are based respond to thermal stresses in a non-consistent manner. However, the point stressed here is that regardless of the maturity parameter used, there are always heavy oils that have a similar maturity to normal oils. Had maturity been the sole factor affecting the API gravities for oils then the API gravities should have been consistent with the maturity level of the source rock from which a given oil is derived i.e. an oil expelled from a source rock at lower maturity level should have a lower API gravity and vice versa. A Uniform source further implies that the differences in the API gravities, bulk and molecular level compositions are attributable to some factor other than maturity. Uniform source and similar maturity levels further suggest that the reservoired oils should have similar chemical composition. This is in fact not the case with oils in the present study. Furthermore, as discussed below, the reservoir temperature from which
Marine and Petroleum
Geology 1997 Volume
14 Number
1 59
Petroleum
biodegradation
in Tertiary
reservoirs:
A. Ahsan et al. Pr
oderately
Biodegraded
Severely
Retention
Biodegraded
time -+
Figure 4 GC-FID traces of saturated hydrocarbon fraction of oils A, H, L and 0 which are representative of four classes of biodegradation. ‘S’ is internal standard squalane. Oil names are represented at the upper right corner of each chromatogram. The biodegradation classes to which each individual oil belongs are also shown on chromatograms
oils have been taken vary in present day estimated temperatures from 113.5 to 48.6 C. The difference in chemical compositions both at bulk and molecular level can thus most likely be attributed to different degrees of secondary alteration processes to which oils in the present study have been subjected.
Biodegradation
purameters
used to rank the oils
Having established that the oils are derived from KCF at similar maturity levels, the next step was to look for the reasons for the differences in the chemical compositions. Reservoir depths have been converted into reservoir temperatures using an approximate geothermal gradient of 35 C/km+ 5 C (temperature at the sediment-water interface) (Cornford et ul., 1983 and references therein). Based on this geothermal gradient, the temperatures of the reservoirs from which the oils have been procured vary from 113.5 C (oil B; Tuhle 5) to 48.6 C (oil L; Table 5). The majority of oils in the suite have been taken from temperature regimes which are at present less than 80 ‘C. At these temperatures, the most likely secondary alteration process to occur is considered to be biodegradation
(Tublr I). However, there are oils in the suite which are taken from reservoirs at temperatures less than 80 C, but which do not show any signs of biodegradation (oils C, D. E and F: these oils have all their II-alkane intact; Tubk 2).
These oils are from the Sele Formation. Still, there is one Sele oil (oil 0) taken from a depth corresponding to a temperature of 65 C. which is biodegraded. Thus, we suggest that for the oils studied, the depth of a reservoir (and hence temperature) is not the only first degree control on biodegradation. Having established that biodegradation is the most likely origin of the differences in the chemical compositions both at bulk and molecular level, the oils were ranked in terms of degree of biodegradation. Volkman cf 01. (1983), Connan (1984) and Park (1984) have attempted to assess the degree of biodegradation based on the depletion of specific biomarker compounds. This type of ranking strategy requires GC MS data, whereas in the present study. ranking is based on GCFID data obtained on the saturated hydrocarbon fraction. Phillippi (1977). Schaefer and Leythaeuser (1980) Magoon and
Table 4 Maturity assessment of oils in the present study is based on the four parameters as shown in the table above and Figure 4. Oils are grouped into four classes on the bases of maturity and API gravity. See text for more explanation
Maturity
Parameters
Diahopane/ (diahopane+norhopane) C30 hopane/ C30hopane+C30 moretane 29 Ts/(29Ts+norhopane) BP-- (flP+a&C29 Steranes
Normal oils more mature than the average
Heavy oils more mature than the average
Normal oils less mature than the average
A. B, C
I, L, 0, P, Q
DE, F, G
C. D, E, G
1, L, M, N, 0, P, Q
A, 9, F
A. 8. F, G
J, K, t. M, N H, 1,K, M. N Q
C, D. E A, B
C, Q, E. F, G
60 Marine and Petroleum Geology 1997 Volume 14 Number 1
Heavy oils less mature than the average H, J, K, M, N H,
J. K
H, 1,0. P, Q J. L, 0, P
Petroleum
biodegradation
in Tertiary
reservoirs:
Table 5 Quadrants, formations, calculated present day temperatures, depth of reservoirs degree of biodegradation to which each sample in the suite has been subjected
Samples
N J Q P 0 F K M E C D G H A B
Quadrant 21 9 9 9 9 16 9 9 9 16 16 16 22 9 16 30
Formation
Temperature
Tay Sand Balder Balder Balder Balder Sele Balder Balder Balder Sele Sele Sele Balder Balder Forties Forties Forties
48.6 57.5 63.2 64.2 64.5 65.0 65.1 67.2 67.2 77.6 78.6 79.7 81.8 -
1.25 1.50 1.66 1.69 1.70 1.71 1.72 1.78 1.78 2.08 2.10 2.14 2.19 -
113.2 113.5
3.09 3.10
Claypool (198 I), have quoted the relative rates of depletion of specific alkanes as a result of biodegradation to be normal alkane > isoalkane > cycloalkane. These authors and Snowdon and Powell (1979) argued that short chain n-alkanes are removed prior to long chain IIalkanes. On the other hand, Perry and Cerniglia (1973) presented evidence to suggest that the n-alkanes in the C,,, to C,, region are degraded at a faster rate than the C2 to C, or C,9+ compounds. For ranking the oils in the present study, the view held by Phillippi (1977) Snowdon and Powell (1979) Schaefer and Leythaeuser (1980) Magoon and Claypool (1981) has been adopted. Thus four n-alkane concentration ratio parameters have been established. These parameters are: (1) Z:l$C:y (where Z:Ii = sum of the ti-alkane concentration from n-C,o to MC,,); (2) Zf:/X$; (3) Xf,$::; and (4) Z;i:/C$ (Tub/r 3). Jobson et al. (1972) and Bailey et al. (1973) maintained that the gradual removal of n-alkanes by bacteria can be monitored by measuring the ratios of pristane to n-C,, and phytane to n-C,, which increase with increasing degree of biodegradation. For ranking the oils in the present study, further assessment of biodegradation was also based on Pr/n-C,, and Phjn-C,, in case of advanced degrees of biodegradation. The hump has been found to increase in intensity with increasing degree of biodegradation (Gough and Rowland, 1990 and references therein). This concept has been utilised to assess the degree of biodegradation in cases where n-alkane homologues are depleted. The intensity of the UCM in millivolts was measured by subtracting the base line intensity at the retention time of nC,, or n-C,7 of a non-biodegraded oil from the UCM intensity of a biodegraded oil at the retention time of nC,, or n-C,,. In cases where n-C,, was found depleted, assessment was based on the ratio of pristane to UCM at the retention time of n-C,, (since all the oils were run under identical GC-FID conditions, it was therefore assumed that n-C,,. had it been present, would have the same retention time in cases where it was not found present). There are oils in the suite for which identification of even pristane and phytane was not possible. Therefore assessment of these oils was based on the UCMc,,/UCMc2, ratios. In conclusion, 8 biodegradation
(“C)
Depth (km)
A. Ahsan et al.
and the
Degree of biodegradation
1 2 2
assessment parameters, based on the literature review were established. These parameters and their corresponding values are shown in Tuhk 2. Interpretation
of’geochemid
parameters
Oils A-G were ranked using the first 7 parameters in Table 2. In these oils. all the n-alkane homologues were found present. Out of a total of 7 oils, 4 (oils A, B, C and D) were ranked first. This is due to the fact that none of these oils showed any biodegradation effect as assessed by parameters discussed above. The four n-alkane parameters indicated that the sum of the range of lighter nalkane concentration was higher than the sum of the range of n-alkane concentration chosen as denominator e.g. oil A has ZZ$Zft ratio equal to 1.14. Some oils (E, F and G) were found to be slightly effected by biodegradation as these have X$Zfl: ratios less than 1. It was because of this reason that oils A, B, C. D and E, F, G were considered separately in the ranking procedure. In order to rank oils A to D, each parameter was given equal weight and all the parameters were considered simultaneously. Each ratio number for a given oil corresponding to a given parameter was assigned a value ranging from l-4 depending upon the ratio number of a given parameter. In cases of n-alkane homologues’ parameters, the highest ratio number was assigned a value of 1, and the lowest ratio number a value of 4. When 2 or more oils were found to have the same ratio number, these oils were then assigned the same number. The same interpretation is valid for Pr/UCM,,,, whereas for Pr/nC,, and Ph/n-C,,, oils having the highest ratio number were assigned a value of 4. Subsequently these values (l4) for each parameter corresponding to each oil were aggregated and an oil carrying the least total was ranked higher in the list than the oil carrying the next highest total. This ranking strategy is illustrated in Table 3 for oils A-D. It is worth mentioning here that if only the first four n-alkane parameters are taken into account, as these parameters are most affected during slight biodegradation, then oils A and B carry an equal sum of 6, whereas oils C and D carry a sum of 11 and 15, respectively. Based on this, ranking of A and B was not possible.
Marine and Petroleum
Geology 1997 Volume 14 Number
1 61
Petroleum
biodegradation
in Tertiary
reservoirs:
A. Ahsan et al.
However oils C and D were ranked in the same order irrespective of whether the first four n-alkane or all parameters in Table 2 were used. While ranking oils E-G, it was found that oils E and F carried the same total for the assigned values, whereas oil G carried the highest total. It was for this reason that oils E and F were compared to each other without taking oil G into account. It was found that oil E carried the lesser total and therefore stood higher in the ranking list than oil F. Oils H-K were ranked using Prin-C,,. Ph/nC,, and Pr/UCM,,, with the same ranking strategy as adopted for oils A-G. Since oils L-N and 0-Q were ranked using only one parameter PrUCMc,, and UCM,,,/UCM,,,, respectively, ranking these oils was quite straight forward. An oil bearing higher values for parameters was placed higher in the ranking list than oils of lower values. For oils L-N. two parameters and UCM,,,/UCM,,, were available; PriUCMc,, however, Pr/UCMc,, used as the combination of two parameters always gave the same total for all the oils. Classification of oils was thus based on the type or combination of parameters. Oils A-G were ranked using the first 7 parameters with oils A-D showing no effect on any of the parameters. These oils show none of their nalkane ranges to be affected. However. there are still minor differences in their n-alkane, pristane and phytane parameters. Oils E-G show slight effect on their first parameter. Oils A-G were therefore classified as non- to slightly biodegraded. Oils H-K were assigned to belong to a mildly biodegraded class. Oils L-N were classified as moderately biodegraded, whereas oils O-Q as severely biodegraded. Figure 4 shows the GCFID traces of saturated hydrocarbon fraction of oils A, H, L and 0 which are representative of four classes of biodegradation.
carbon fraction (6°C Sat) to see how these data conform to the classification obtained through CC-FID data of the saturated hydrocarbon fraction. Fipw 5 shows bulk data plotted versus four classes of biodegradation. The figure shows that there is generally a good correlation between classes of biodegradation and the bulk properties of oils. However, wt % asphaltene of oils do not show a consistent behaviour with increasing degree of biodegradation. The asphaltene content of oils is low. generally in fractions to 3% of the total oil weight. However, weight measurement on such low-in-asphaltene oils is subject to large uncertainties, especially when the weight of oils taken to measure asphaltene content is low. Stahl (1980) studied the effects of biodegradation under simulated laboratory conditions on the isotopic compositions of saturated, aromatic and asphaltene fraction of oils and found that the (5’C of the saturated fraction of oils shifted towards heavier values with increasing degree of biodegradation. His result is to some extent substantiated by our study of biodegraded oils under natural conditions. Figm 5 further shows that there is a substantial change in the bulk properties of oils between none-. slightly- and mildly-biodegraded oils. However, once oils have been mildly biodegraded these differences in the bulk properties are not as pronounced with further increase in biodegradation. Hence our parameters are more sensitive to the changes that accompany biodegradation than the bulk properties of oils.
Conclusions All the oils at depth levels greater than 2.0 km are non to very slightly biodegraded. However, I oil (I) which belongs to the Balder Formation has been found to be mildly biodegraded even at a depth of 2.19 km (Table 5). All the oils at depth levels less than 2.00 km have been found to be biodegraded to different degrees regardless of their formations. However, oils in the Balder Formation are biodegraded to a higher degree than oils in the Sele Formation at similar depth levels. The difference
Biodegradation, bulk and isotope composition data We have used wt% asphaltene, API gravity. % polars, % saturates (%Sat; these two percentages have been obtained through Iatroscan) and 6’C of saturated hydro-
48
31 -
73 c
c 0
0
0
43
.?’ 38
k
33
;;: d
28
t
0
v : 2 & 0
23 IX
: 00 I7
0 0
27 ~
08
I
k!
8
I
L 3
4
0
23 0 1Y-O
0 0
15 -g
0
II --B I I
I
I
I
7
3
1
0 0
0 0
0
I I
2
3
4
I
2
3
1
I
I
I
2
I
I
3
3
Figure 5 Bulk properties and carbon isotope values plotted versus biodegradation classes on x-axis. Numbers on x-axis indicate class of biodegradation with 1 being the non-slightly biodegraded and 4 the severely biodegraded. Some oils do not plot on some graphs as these have overlapping values
62
Marine and Petroleum
Geology 1997 Volume
14 Number
1
Petroleum biodegradation in the degree of biodegradation is suggested to be related to the formation and hence depositional environment of the reservoir rock formation from which a given oil has been taken. The relatively higher degree of biodegradation for Balder oils compared with the Sele and Forties Formations at similar depth levels is attributable to comparatively higher lateral openness of the Balder Formation to meteoric water invasion. It is therefore suggested that the potential for biodegradation in prospect evaluation should take into account the degree of lateral openness of the structure. Laterally continous sandstones are more likely to be biodegraded than isolated sandstones enclosed in clays.
References Bailey, N. J. L. Jobson, A. M. and Rogers, M. (1973) Bacterial degradation of crude oil, Comparision of field and experimental data Chem. Geol. 11,203-221 Bailey, N. J. L. Burwood, R. and Harriman, G. E. (1990) Application of pyrolysate carbon isotope and biomarker technology to organofacies and oil correlation problems in the North Sea Basin Org. Geochem. 16,1157-l 172 Barker, C. (1980) Application of Geochemistry in Exploration, Maidenhead, 28 July 1980 Barnard, P. C. and Bastow, M. A. (1991) Hydrocarbon Generation, Migration, Alteration, Entrapment and Mixing in the Central and the Northern North Sea (Eds W. A. England and A. J. Fleet) Petroleum migration Spec. Pub/. geol. sot. No. 59, 167190 Brooks, P. W. (1986) Unusual biological marker geochemistry of oils and possible source rocks, offshore Beaufort-Mackenzie Delta, Canada. Advances in Organic Geochemistry 1985 (Eds D. Leythaeuser and J. Rulkotter) Pergamon Press, Oxford. 401-406 Connan, J. (1984) Biodegradation of Crude Oils in Reservoirs Adv. Petrol. Geochem. 1.299-333 Connan, J. and Cousteau, H. (1986) Influence ofthe Geochemical characteristics of Heavy oils on their Recovery. AAPG Exploration for Heavy oil and Natural Bitumen Res. Conf (Santa Maria, Calif.) Connan, J. and Lacrampe-Couloume, G. (1993) The origin of the Lacq superior heavy oil accumulation and of the giant Lacq inferior gas field (Aquitaine basin, SW France). In: Applied Petroleum Geochemistry (Ed. M. L. Bordenave) Technip, Paris, pp.465-487 Cooper, B. S. and Barnard, P. C. (1984) Source rocks and oils of the Central and Northern North Sea, (Eds G. Demaison and R. J. Murris). q. v, 303-314 Cornford, C., Morrow, J. A., Turrington, A., Miles, J. A. and Brooks, J. A. (1983) Some geological controls on oil composition in the U. K. North Sea. Petroleum Geochemistry and Exploration of Europe (Ed. J. Brooks) Oxford, Blackwell, 3550 Cornford, C. Needham, E. J. and de Walque, L. (1986) Geochemical habitat of North Sea oils and gases. Habitat of hydrocarbons on the Norwegian Continental Shelf (Ed. A. M. Spencer et a/.) Petroleum Geochemistry in Exploration of the Norwegian Shelf, Graham and Trotman, London, 39-54 Dahl. B. and Soeers. G. C. (1985) Oraanic Geochemistrv of the dseberg Field (I) In: Petroleum Geology in Exploration of the Norwegian She/f (Eds B. M. Thomas et a/.). Graham and Trotman, London, pp. 185-196 Didyk, B. M., Simoneit, B. R., Brassel, S. and Eglinton, G. (1978) Organic geochemical indicators of paleoenvironmental conditions of sedimentation Nature 272.216222 Field, J. D. (1985) Organic geochemistry in exploration of the northern North Sea (Eds B. M. Thomas et a/.) a. v, 39-58 Gough, M. A. and Rowland, S. J. (1990) Characterization of unresolved complex mixtures of hydrocarbons in petroleum Nature 344,648650 Grantham, P. J., Posthuma, J. and De Groot, K. (1980) Variation and significance of the CZ7 and CZ8 triterpane content of a North Sea core and various North Sea crude oils In: Advances
in Tertiary reservoirs: A. Ahsan et al.
in Organic Geochemistry 7979 (Eds A. G. Douglas and J. R. Maxwell). Pergamon Press, Oxford, pp. 29-38 Hollerbach, F. R. (1987) Influence of Biodegradation on the Chemical Composition of Heavy Oil and Bitumen In: Exploration for Heavy Crude Oil and Natural Bitumen (Ed. R. F. Meyer). Am. Assoc. Pet. Geol. Studies in Geology No. 25, pp. 243-247 Hughes, W. B., Holba, H. G., Miller, D. E. and Richardson, J. S. (1985) Geochemistry of greater Ekofisk Crude oils. In: Petroleum Geochemistry in Exploration of the Norwegian Shelf (Eds B. M. Thomas et al.) q. v. 75-92 Hunt, J. (1979) Petroleum Geochemistry and Geology. W. H. Freeman and Company, San Francisco Jobson, A. M., Cook, F. D., Westlake, D. W. S. (1972) Microbial utilization of crude oil Appl. Microbial. 23, 1082-1089 Karlsen. D. and Larter. S. R. (1989) A raoid correlation method for petroleum population mapping within individual petroleum reservoirs. In: Application to petroleum reservoir description (Ed. Collinson, J. D.) 77-85 Magoon L. B., Claypool G. E. (1981) Two oil types on North Slope of Alaska; implications for exploration. Am. Assoc. Pet. Gee/. Bull. 65,644-652 Palmer, S. E. (1993) Effects of biodegradation and water washing on crude oil composition. In: Organic Geochemistry (Eds M. H. Engel and S. A. Macko) Plenum Press, New York, pp.511533 Park, P. J. D. (1984) Crude oil biodegradation under simulated and natural conditions, II: aromatic steroid hydrocarbons Org. Geochem. 6,605-617 Perry, J. J. and Cerniglia, C. E. (1973) Degradation of petroleum by filamentous fungi In: Microbial Degradation of oil PO//utants(Eds D. G. Ahearn and S. P. Meyers). Workshop, Center for Wetland resources, Lousiana State Univ., Baton Rouse, La. 89-94 Phillippi, G. T. (1977) On the depth, time and mechanism of origin of the heavy to medium gravity naphthenic crude oils Geochim. Cosmochim. Acta 41,33-35 Powell, T. G. and Mckirdy, D. M. (1973) Relationship between pristane to phytane, crude oil composition and geological environment in Australia Nature, Phys. Sci. 243, 37-39 Price, P. L., Sullivan, T. 0. and Alexander, R. (1987) The nature and occurrence of oil in Seram, Indonesia. In: Proceedings of the Indonesian Petroleum Association, Sixteenth Annual 1, Jakarta, Indonesia. Petroleum Association, Convention. 141-173 Radke, M., Schaefer R. G., Leythaeuser, D. and Teichmuller, M. (1980) Preparative Hydrocarbon Group Type Determinartion by Automated Medium Pressure Liquid Chromatography Anal. Chem. 52,406411 Riva, A., Riolo, J., Mycke, B., Ocampo, R., Callot, H. J., Albracht, P. and Nali, M. Molecular parameters in Italian Carbonate oils, reconstruction of past depositional environments. 74th International meeting on Organic Geochemistry, Paris, September 18-22. Abstract no. 335 Rulkotter, J. and Wendisch, D. (1982) Microbial alteration of 17~ (H)-hopanes in Madagascar asphalts: removal of C-IO methyl group and ring opening Geochim. Cosmochim. Acta. 46, 1534-1553 Shannon, P. M. and Naylor, D. (Eds) (1989) Petroleum Basin Studies. Graham and Trotman, London, 78-79 Schaefer, R. G. and Leythaeuser, D. (1980) Analysis of trace amounts of Hydrocarbons (C,-C,) from rocks and crude oil oils and its application in petroleum geochemistry. Adv. Org. Geochem. 1979 (Eds A. G. Douglas and J. R. Maxwell). Pergamon Press, Oxford Schou, L., Eggen, S. and Schoell, M. (1985) Oil-oil and oil-source rock correlation, Northern North Sea In: Petroleum Geochemistry in Exploration of the Norwegian shelf (Eds B. M. Thomas, A. G. Dare, S. S. Eggen, P. C. Home and R. M. Laisse). Graham and Trotman, London, pp. 101-117 Seifert, W. K. and Moldowan, J. M. (1979) The effect of biodegradation on steranes and triterpanes in crude oils Geochim. Cosmochim. Acta. 43,111-126 Snowdon, L. R. and Powell, T. G. (1979) Families of crude oils and condensates in the Beaufort-Mackenzie Basin Bull. Can. Petrol. Gee/. 27, 113-162 Stahl, W. J. (1980) Compositional changes and 13C/‘ZC fractionations during the degradation of hydrocarbons by bacteria Geochim. Cosmochim. Acta. 44,1903-1907
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Geology 1997 Volume
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Petroleum
biodegradation
in Tertiary
reservoirs:
Thomas, 6. M., Moeller, P. P., Whitaker, M. F. and Shaw, N. D. (1985a) Organic facies and hydrocarbon distribution in the Norwegian North Sea (Eds B. M. Thomas et a/.). q. v, 3-26 Volkman, J. K., Alexander, R., Kagi, R. I. and Woodhouse, G. W. (1983) Demethylated hopanes in petroleum geochemistry Geochim. Cosmochim. Acta. 46,785-794 Waples, D. W. and Machiara, T. (1991) Biomarkersfor Geologists, Am. Assoc. Petr. Geol. Methods in Exploration series, no. 9 Wilkes, H., Willsch, H. Rabus, R., Aeckersberg, F., Reuter, P. and Widdel, F. (1995) Compositional changes of crude oils upon
64 Marine and Petroleum
Geology 1997 Volume
A. Ahsan et al. anaerobic degradation by sulphate-reducing bacteria. Org. Geochem, 17th. International meeting on Organic Geochemistry. 321-323 Zumberge, J. E. (1984) Source rocks of the La Luna Formation (Upper Cretaceous) in the Magdalena Valley, Colombia (J. G. Palacas). Geochemistry and Source Rock Potential of Carbonate Rocks, Am. Assoc. Petrol. Geol. Studies in Geology No. 78. Tulsa, 127-133 Bstfeldt, P. (1987) Oil-Source rockcorrelation in the Danish North Sea (Eds J. Brooks and K. W. Glennie) q. v. 419-43
14 Number
1
Petroleum biodegradation of the North Sea
in the Tertiary reservoirs
Asif Ahsan and Dag A. Karlsen Petroleum Geochemistry Oslo 3, Norway
Program,
Department
of Geology,
University
of Oslo, N-0316,
Richard L. Patience* Statoil
a.s., N-4035, Stavanger,
Received
19 February
Norway
7996; revised 28 June
1996; accepted
23 July
1996
A suite of 17 oil samples from the Tertiary reservoirs (Palaeocene-Eocene) of the North Sea was subjected to routine geochemical analytical techniques in order to evaluate the degree of biodegradation. The methods included latroscan TLC-FID, gas chromatography of the saturated and aromatic hydrocarbon fractions, gas chromatography-mass spectrometry of the saturated hydrocarbon fraction and stable carbon and hydrogen isotope analyses. In addition, API gravities and weight percentages of asphaltene were also determined. Based on these data it was found that oils in the suite were derived from the Jurassic aged Kimmeridge Clay Formation originating from source rock horizons at similar maturity levels. The oils in the suite were taken from reservoirs with present day temperatures of 49-l 14 “C. Differences in the chemical composition of the oils (e.g. reduced relative amounts of saturated hydrocarbons, increased polar amounts and reduced relative concentrations of n-alkanes), were attributed to secondary alteration processes of which the most likely process to occur at temperatures less than 80°C and consistent with the observed chemical characteristics of the oils is biodegradation. The differences in chemical composition are thus attributed to different degrees of biodegradation. The oils were ranked and classified in terms of the degree of biodegradation using a range of restricting parameters based on the gas chromatographic concentrations of different alkane species. Based on these parameters, oils were divided into four classes. These classes are (a) non to slightly biodegraded; (b) mildly biodegraded; (cl moderately biodegraded; and (d) severely biodegraded. In cases where the degree of biodegradation was found to have reached levels where unambiguous identification of alkanes was not possible, unresolved complex mixture (UCM) parameters were utilised to assess the extent of biodegradation. The degree of biodegradation was subsequently compared to the type of reservoir sandstone facies from which oils in this study were taken. It was found that at similar temperatures, oils in the Balder Formation (Upper Late Palaeocene) were biodegraded to a higher degree compared to oils from either The Forties (Lower Late Palaeocene) or Sele (Middle Late Palaeocene) Formation. The higher degree of biodegradation in Balder reservoirs is attributed to relatively greater openness to meteoric water invasion in these reservoirs. 0 1997 Elsevier Science Ltd. All rights reserved. Keywords: North Sea; Tertiary reservoirs; petroleum
Petroleum biodegradation is just one of several processes that can alter the properties of a reservoired petroleum (Park, 1984). This process is brought about by bacteria which gain access to a reservoir through meteoric water flushing (Palmer, 1993). Aerobic bacteria have generally been accepted to be responsible for this process, whereas the role of anaerobic bacteria in biodegrading hydrocarbon species has been debated in great detail (Palmer, 1993). Connan et al., 1993 maintained that even anaerobic bacteria can be effective under certain conditions in initiating biodegradation. This view is also supported by Wilkes et al. (1995). Hollerbach (1987) suggested that
*Author
to whom correspondence
should be addressed
biodegradation;
unresolved complex mixture
anaerobes can biodegrade the products of petroleum already biodegraded by aerobes. Petroleums undergoing biodegradation become higher in viscosity and lower in API gravity and will hence be difficult to produce. It is because of this fact that biodegraded petroleums are generally undesirable from a commercial point of view. Hunt (1979) estimated that approximately 10% of all the world’s petroleum reserves have been lost to biodegradation and an additional 10% altered. Thus, statistically speaking, the chances of encountering a biodegraded oil is 1 in 10. Not only is the commercial value of oils affected as a result of this process. but several well established source and maturity parameters which are based on the ratios of different hydrocarbon species are also affected. For these reasons, an understanding of
Marine and Petroleum
Geology 1997 Volume
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1 55
Petroleum
biodegradation
in Tertiary
reservoirs:
the geologic controls on petroleum biodegradation is of utmost importance. There is a definite sequence of depletion of hydrocarbon species which takes place during successive stages 1984; of biodegradation (Volkman et al., 1983; Connan, Park, 1984). One may state as a first approximation that the simpler the structure of a compound the more easily it is biodegraded. Normal alkanes are depleted prior to isoalkanes which in turn are depleted prior to cycloalkanes. Within the same compound class, lower molecular weight compounds are depleted prior to higher molecular weight compounds (Phillippi, 1977; Schaefer, 1980; Magoon and Claypool, 1981). Multiringed cycloalkanes (steranes and triterpanes) are found to be most resistant to biodegradation, but even these compounds altered under extreme biodegradation (Seifert and Moldowan, 1979; Rulkotter and Wendisch. 1982; Volkman et rd.. 1983; Connan, 1984). The conditions for biodegradation to occur are summarised by Connan (1984) based on the work by Phillippi ( 1977) and Barker (1980) as follows: I. Oxygenated waters either by hydrodynamism or by compaction. 2. Biodegradation occuring at the oil-water contact as bacteria live in the water phase and do not thrive in the oil. 3. Sufficient supply of nutrients (nitrate and phosphate) and dissolved oxygen in moving waters. 4. Presence of microbes. 5. Subsurface temperatures allowing activity of bacteria. The maximum subsurface temperatures of biodegraded petroleums in several basins are given in Tuhk 1.
The differences in maximum temperature limits at which this process can take place in different basins is perhaps due to different strains of bacteria involved in degrading petroleums in these basins. These temperatures range from 88 ‘C in the Dampier Basin (Australia) to 54 ‘C in the South Sumatra-South Borneo Basin (Indonesia). Temperature or alternatively the depth of a reservoir is most frequently cited to be the key factor controlling biodegradation (Phillippi, 1977; Barker. 1980; Connan. 1984). However, in the North Sea some shallow Tertiary reservoirs are either not at all biodegraded or are biodegraded very little (e.g. some oils from Block 16 of the UKCS). This observation has led us to doubt temperature as the only controlling factor in biodegradation. We therefore propose that in basins like the North Sea the relative openness of a given reservoir and hence its depositional facies could play a first order control on biodegradation. In order to investigate this hypothesis, a suite of 17 oil
Table 1 Maximum subsurface temperatures in several basins (after Connan, 1984) 54°C 62°C 71°C 77°C 80°C 82°C 85°C 88°C
56
of biodegraded
oils
South Sumatra-South Borneo San Joaquin Basin (USA) Beaufort Basin (Canada) Los Angeles Basin (USA) Aquitaine Basin (France) Gulf Coast (USA) Mackenzie Delta (Canada) Dampier Basin (Australia)
Marine and Petroleum
Geology 1997 Volume
A. Ahsan et al. samples from the Tertiary reservoirs of the North Sea was selected as a data base as these oils were found to be biodegraded to different degrees (as will be illustrated later). The objectives of this study were first to rank the oils in terms of the degree of biodegradation and subsequently to correlate the degree of biodegradation to the type of reservoir facies from which these oils were obtained. For the ranking purposes. a restricted range of parameters based on the GC FID concentrations of different alkane species were used. The concentrations were normalised to the concentration of the internal standard squalane. whereas the parameters for evaluating the degree of biodegradation were taken partly from the literature and partly established by us.
Sample set and analytical techniques used 16 oil samples were procured from the Tertiary reservoirs in different quadrants from the United Kingdom Continental Shelf (UKCS). whereas I sample was obtained from a similar age reservoir on the Norwegian Cotinental Shelf (NOCS). F&ure 1 is a location map of oil samples for this study. The selection was made in order to compile a suite of oils ranging from non-biodegraded to severely biodegraded. 8 oils (F,G,J,K,M,N,P and Q) were taken from quadrant 9 of UKCS, 5 oils (C,D,E,H and 0) from quadrant 16, whereas 1 oil came from each of quadrants 30, 22, 21 and I (NOCS). The oils come mostly from different depths in the Sele (middle Late Palaeocene), Balder (upper Late Palaeocene) and Forties Formations (lower Late Palaeocene) and are mostly DST oils with the exception of G and K which are storage oils. The oils were analysed using a thin layer chromatography-based technique (Karlsen and Larter, 1989) to ascertain the bulk composition of the oils. Whole oils (i.e. undeasphaltened oils) were used for such analyses. The same oils were also separated into three fractions i.e. (1) Saturated hydrocarbons; (2) Aromatic hydrocarbons; and (3) Polars using medium performance liquid chromatography (MPLC) of the deasphaltened fraction. The MPLC was used as described by Radke et al. (1980). The saturated and aromatic hydrocarbon fractions were analysed using capillary-gas chromatography (GC-FID) to study the abundance and distribution of the individual alkanes. The internal standard squalane was used for quantifying different alkane species. The saturated hydrocarbon fractions were finally analysed using gas chromatography-mass spectrometry (GCMS).
Results and discussion Source of’the oils For this study, source rock extract data for the North Sea oils given in the literature (Cornford ef ~1.. 1983: Ostfeldt, 1987) were compared with data obtained on oils through in-house analyses. Typical North Sea oils derived from the Kimmeridge Clay Formation (KCF) show the presence of 28, 30-bisnorhopane (Grantham rf ul., 1980; Cornford et d., 1983; Hughes ef ul., 1985; Dahl and Speers, 198.5: Schou et ul., 1985). Oils in this study also show the presence of this triterpane species. In Figuw 2, the relative distribution of C,, C2X and C,, zeta-2OR steranes is plotted in a ternary diagram along with samples taken from Barnard and Bastow’s study (Barnard
14 Number
1
Petroleum biodegradation
figure 1 Location map of the samples which is marked by arrows
in the data set. Individual
samples
are indicated
50
2s
in Tertiary reservoirs: A. Ahsan et al.
by bold capital
letters within
75
each quadrant
100
Figure 2 C2,, CZ8and CZs (cKYc+ZOR)sterane distribution of oils shown as ternary diagram. ‘m’ and ‘0’ represent Barnard and Bastow (1991) samples which are oils from the Tertiary and the Mesozoic reservoirs, whereas ‘+’ represent samples for the present study
and Bastow, 1991). It is apparent from this figure that normal Tertiary reservoir samples taken from Barnard and Bastow’s study and represented by ‘m’ plots close to the oils in this study are represented by ‘+ ‘. Some samples
from Barnard and Bastow’s study show a shift towards 100% C9 apex which is interpreted by the authors to be due to a increasing degree of biodegradation. None of the oils in the present study are biodegraded to such an
Marine and Petroleum
Geology 1997 Volume
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1 57
Petroleum biodegradation Table 2 Geochemical parameters explanation of parameters Samples A
~:y~~,” 1.14 1.45 1.14 1.02 0.97 0.74 0.81
in Tertiary reservoirs: A. Ahsan et al. calculated by GC-FID of the saturated hydrocarbon fractions, ‘Carbon preference index. See text for
c::E;:
mz:
c:,“/x;;
Prln-C,,
Phln-C,,
1.83 1.92 1.56 1.48 1.59 3.24 3.15
2.67 2.19 1.96 1.64 1.83 12.08 10.95
9.18 5.05 4.19 2.18 2.16 11.42 6.81
0.58 0.56 0.68 0.65 0.67 0.76 0.74 12.81 14.04 340.05 74.89
0.46 0.48 0.64 0.58 0.63 0.67 0.67 5.44 8.62 80.63 179.15
Pr/Ph
PrNCMc,,
1.49 7.*jO 13.30 7.50 13.00 7.33 6.86 2.49 4.42 1.51 1.34 1.07 0.86 0.85
N
1.07 1.05 1.04 0.98 1.24 1.44 1.41 1.28 0.99 1.23 1.24 0.92 1.13 1.18
1.35 1.24 1.29
0
CPI’
0.96 1.28 1.42
0.85 0.75 0.74
P Q
extent that a shift towards 100% Cz, could be observed. Cornford et al. (1983) showed the Pr/Ph (pristanel phytane) ratios for the KCF derived oils to range from 0.60 to 1.90 with an average of 1.24. Pr/Ph for oils in the present study, excluding three oils, ranging from 0.92 to 1.49 with an average also of 1.24 (Tuhk 2). This average of I .24 is based on 14 oils as the remaining three oils (oils 0, P and Q) in the suite are either depleted in pristane and phytane or their identification is doubtful. High Pr/Ph values (> 3) indicate an oxidising environment, whereas lower values indicate an intermediate or reducing environment (Powell and Mckirdy. 1973; Didyk et al., 1978). The first geochemical parameter in Tut&s 2 and 3 shows the ratio of the sum of n-alkanes from n-C,,, to r&Y,, to the sum of n-alkanes from n-C,, to n-C,, (Cti = sum of n-alkanes ranging from n-C,,, to n-CIS). Likewise the second, third and fourth parameters represent the same geochemical parameter as the first one, but with different n-alkane ranges. These parameters are calculated for oils A to G, because beyond oil G. identification of n-alkanes has either become impossible or at least doubtful. The fifth parameter is the ratio of Pristane to n-C,,. The sixth parameter is the ratio of Phytane to n-Clx. The seventh parameter represents the ratio of Pristane to the height of the hump of unresolved complex mixture (UCM). whereas the eighth parameter represents the ratio of the height of the hump at the retention time of n-C,, to the height of the hump at the retention time of n-C,,. The ninth and the tenth parameters represent the ratio of Pristane to Phytane and the carbon preference index (CPI) respectively. Mstfeldt (1987) has used the CzsC30 (norhopanel hopane) ratio to correlate the oils to their source rocks. This ratio varies for oils for the present study from
0.37 to 0.62 with an average of 0.48, whereas the oils believed to be derived from KCF are shown by Mstfeldt to have a C2,,,‘Clo ratio ranging from 0.39 to 0.72. The values for data obtained on oils from a(stfeldt’s study and those on oils for the present study cover an almost identical range. However, oils and extracts from organic rich carbonates (Zumberge, 1984; Connan and Cousteau, 1986; Price rt al., 1987) and some evaporites (Connan and Cousteau, 1986) are shown to have unusually high concentrations of Cz,-hopane relative to C,,,-hopane. Riva et al. (1989) have suggested that C2q;C3,, ratios might provide a scale of ‘carbonaticity’. Brooks (1986) noted that high CIY hopane content was found to be associated with bisnorlupanes and oleanane, both of which are considered to be terrestrial indicators. However. the oils analysed in the present study showed no sign of coming from such source rock types. Bailey rt ul. (1990) have used carbon isotope and biomarker data in order to source correlate the oils in different regions of the North Sea. According to these authors, the variation in the character of the Viking Graben versus Central Graben oils in the North Sea is attributed to variations in organofacies. North Sea petroleums derived from the KCF were shown to vary in stable carbon isotope values for whole oils from - 30.4 to - 28.6%0 6’C PDB, whereas stable carbon isotope values for whole oils for the present study varies from -28.0 to -29.7 760. Based on the comparison of GCFID (Figuvr 3) GC MS and isotope data on oils in this study and the data presented in the literature (of oils and source rocks), it is concluded that oils in the present study are sourced from the Upper Jurassic to the Lower Cretaceous (Oxfordonian-Ryazanian stage) source rock, regionally termed as either Kimmeridge Clay, Draupne, Mandal, Farsund
Table 3 Table shows parameters chosen for ranking samples A-D with ratio numbers and assigned values in parentheses to each ratio number, whereas the last column shows the aggregate of assigned values for each sample Samples
z:z~,”
z::/z;
A B C D
1.14 1.45 1.14 1.02
1.83 1.92 1.56 1.48
58
(2) (I) (2) (3)
Marine and Petroleum
Wz,” (2) (I) (3) (4)
2.67 2.19 1.96 1.94
(I) (2) (3) (4)
I-,;:o/c~~
Prln-C,,
Phln-C,,
9.18 5.05 4.19 2.81
0.58 0.56 0.68 0.65
0.46 0.48 0.64 0.58
Geology 1997 Volume
(I) (2) (3) (4)
14 Number
(2) (I) (4) (3)
1
(I) (2) (4) (3)
PWCM,,,
Total
-% 7.50 13.30 7.50
10 12 21 24
(1) (3) (2) (3)
Petroleum
biodegradation
’
Normal Oils
HCtiQ Oils
in Tertiary 0
P . L 0
I .
L ??
NM -
I .
K .
IT
H -*
J .
I3 .
30
30
30
API gravity A .
HCIV) Oils H
F .
C .
F .
.
.
P .
.
:l
E .
E Mf
D
I
I
30
JO
I 20
API gravity
D . c .
I LO . .
(‘ .
Normal Oils
GO
J
)
u .
F. .
JO
API gravity
Normal Oils
K - M’i N . ??L .
A .
1
I
p H .
A. Ahsan et al.
Heav; Oils
0 .
3P ”
reservoirs:
.
I
I
30
40
API gravity
Figure 3 Four sterane and triterpane maturity parameters plotted against API gravity values of oils. Horizontal lines on graphs are plotted at the average values for the corresponding parameter whereas vertical line divides oils into heavy (~25 API) and normal (>25 API) oils
Tau Formation (Cooper and Barnard, 1984; Field, 1985; Thomas et al., 1985; Cornford et al., 1986).
or
Maturity
of' the oils
In order to assess the maturity level of the oils, we have used the following triterpane and sterane parameters: Diahopane/(Diahopane + Normoretane) (Cornford et al., 1986) C,,-Hopane/(C,,-Hopane+C,,-Moretane) (Seifert and Moldowan, 1980) 29Ts/(29Ts + Norhopane) (Peters and Moldowan, 1993) P/?/cc%+ pfl CZg steranes (Seifert and Moldowan, 1986). Figure 3 illustrates that based on all four parameters the differences in maturity level are not large enough to cause subtle differences in composition of the oils. The average values of each of these maturity parameters is represented by a horizontal line whereas, a vertical line divides normal (API >25) and heavy (API ~25) oils in Figure 4. Based on a combination of maturity level and API values, we have divided the oils into four classes. These classes with their oils and maturity parameters are
from the table that shown in Table 4. It is apparent both normal and heavy oils are derived from source rock horizons at comparable maturity levels. Based on four maturity parameters, the distribution of oils in different categories is different. This is however not surprising as the compounds on which these four maturity parameters are based respond to thermal stresses in a non-consistent manner. However, the point stressed here is that regardless of the maturity parameter used, there are always heavy oils that have a similar maturity to normal oils. Had maturity been the sole factor affecting the API gravities for oils then the API gravities should have been consistent with the maturity level of the source rock from which a given oil is derived i.e. an oil expelled from a source rock at lower maturity level should have a lower API gravity and vice versa. A Uniform source further implies that the differences in the API gravities, bulk and molecular level compositions are attributable to some factor other than maturity. Uniform source and similar maturity levels further suggest that the reservoired oils should have similar chemical composition. This is in fact not the case with oils in the present study. Furthermore, as discussed below, the reservoir temperature from which
Marine and Petroleum
Geology 1997 Volume
14 Number
1 59
Petroleum
biodegradation
in Tertiary
reservoirs:
A. Ahsan et al. Pr
oderately
Biodegraded
Severely
Retention
Biodegraded
time -+
Figure 4 GC-FID traces of saturated hydrocarbon fraction of oils A, H, L and 0 which are representative of four classes of biodegradation. ‘S’ is internal standard squalane. Oil names are represented at the upper right corner of each chromatogram. The biodegradation classes to which each individual oil belongs are also shown on chromatograms
oils have been taken vary in present day estimated temperatures from 113.5 to 48.6 C. The difference in chemical compositions both at bulk and molecular level can thus most likely be attributed to different degrees of secondary alteration processes to which oils in the present study have been subjected.
Biodegradation
purameters
used to rank the oils
Having established that the oils are derived from KCF at similar maturity levels, the next step was to look for the reasons for the differences in the chemical compositions. Reservoir depths have been converted into reservoir temperatures using an approximate geothermal gradient of 35 C/km+ 5 C (temperature at the sediment-water interface) (Cornford et ul., 1983 and references therein). Based on this geothermal gradient, the temperatures of the reservoirs from which the oils have been procured vary from 113.5 C (oil B; Tuhle 5) to 48.6 C (oil L; Table 5). The majority of oils in the suite have been taken from temperature regimes which are at present less than 80 ‘C. At these temperatures, the most likely secondary alteration process to occur is considered to be biodegradation
(Tublr I). However, there are oils in the suite which are taken from reservoirs at temperatures less than 80 C, but which do not show any signs of biodegradation (oils C, D. E and F: these oils have all their II-alkane intact; Tubk 2).
These oils are from the Sele Formation. Still, there is one Sele oil (oil 0) taken from a depth corresponding to a temperature of 65 C. which is biodegraded. Thus, we suggest that for the oils studied, the depth of a reservoir (and hence temperature) is not the only first degree control on biodegradation. Having established that biodegradation is the most likely origin of the differences in the chemical compositions both at bulk and molecular level, the oils were ranked in terms of degree of biodegradation. Volkman cf 01. (1983), Connan (1984) and Park (1984) have attempted to assess the degree of biodegradation based on the depletion of specific biomarker compounds. This type of ranking strategy requires GC MS data, whereas in the present study. ranking is based on GCFID data obtained on the saturated hydrocarbon fraction. Phillippi (1977). Schaefer and Leythaeuser (1980) Magoon and
Table 4 Maturity assessment of oils in the present study is based on the four parameters as shown in the table above and Figure 4. Oils are grouped into four classes on the bases of maturity and API gravity. See text for more explanation
Maturity
Parameters
Diahopane/ (diahopane+norhopane) C30 hopane/ C30hopane+C30 moretane 29 Ts/(29Ts+norhopane) BP-- (flP+a&C29 Steranes
Normal oils more mature than the average
Heavy oils more mature than the average
Normal oils less mature than the average
A. B, C
I, L, 0, P, Q
DE, F, G
C. D, E, G
1, L, M, N, 0, P, Q
A, 9, F
A. 8. F, G
J, K, t. M, N H, 1,K, M. N Q
C, D. E A, B
C, Q, E. F, G
60 Marine and Petroleum Geology 1997 Volume 14 Number 1
Heavy oils less mature than the average H, J, K, M, N H,
J. K
H, 1,0. P, Q J. L, 0, P
Petroleum
biodegradation
in Tertiary
reservoirs:
Table 5 Quadrants, formations, calculated present day temperatures, depth of reservoirs degree of biodegradation to which each sample in the suite has been subjected
Samples
N J Q P 0 F K M E C D G H A B
Quadrant 21 9 9 9 9 16 9 9 9 16 16 16 22 9 16 30
Formation
Temperature
Tay Sand Balder Balder Balder Balder Sele Balder Balder Balder Sele Sele Sele Balder Balder Forties Forties Forties
48.6 57.5 63.2 64.2 64.5 65.0 65.1 67.2 67.2 77.6 78.6 79.7 81.8 -
1.25 1.50 1.66 1.69 1.70 1.71 1.72 1.78 1.78 2.08 2.10 2.14 2.19 -
113.2 113.5
3.09 3.10
Claypool (198 I), have quoted the relative rates of depletion of specific alkanes as a result of biodegradation to be normal alkane > isoalkane > cycloalkane. These authors and Snowdon and Powell (1979) argued that short chain n-alkanes are removed prior to long chain IIalkanes. On the other hand, Perry and Cerniglia (1973) presented evidence to suggest that the n-alkanes in the C,,, to C,, region are degraded at a faster rate than the C2 to C, or C,9+ compounds. For ranking the oils in the present study, the view held by Phillippi (1977) Snowdon and Powell (1979) Schaefer and Leythaeuser (1980) Magoon and Claypool (1981) has been adopted. Thus four n-alkane concentration ratio parameters have been established. These parameters are: (1) Z:l$C:y (where Z:Ii = sum of the ti-alkane concentration from n-C,o to MC,,); (2) Zf:/X$; (3) Xf,$::; and (4) Z;i:/C$ (Tub/r 3). Jobson et al. (1972) and Bailey et al. (1973) maintained that the gradual removal of n-alkanes by bacteria can be monitored by measuring the ratios of pristane to n-C,, and phytane to n-C,, which increase with increasing degree of biodegradation. For ranking the oils in the present study, further assessment of biodegradation was also based on Pr/n-C,, and Phjn-C,, in case of advanced degrees of biodegradation. The hump has been found to increase in intensity with increasing degree of biodegradation (Gough and Rowland, 1990 and references therein). This concept has been utilised to assess the degree of biodegradation in cases where n-alkane homologues are depleted. The intensity of the UCM in millivolts was measured by subtracting the base line intensity at the retention time of nC,, or n-C,7 of a non-biodegraded oil from the UCM intensity of a biodegraded oil at the retention time of nC,, or n-C,,. In cases where n-C,, was found depleted, assessment was based on the ratio of pristane to UCM at the retention time of n-C,, (since all the oils were run under identical GC-FID conditions, it was therefore assumed that n-C,,. had it been present, would have the same retention time in cases where it was not found present). There are oils in the suite for which identification of even pristane and phytane was not possible. Therefore assessment of these oils was based on the UCMc,,/UCMc2, ratios. In conclusion, 8 biodegradation
(“C)
Depth (km)
A. Ahsan et al.
and the
Degree of biodegradation
1 2 2
assessment parameters, based on the literature review were established. These parameters and their corresponding values are shown in Tuhk 2. Interpretation
of’geochemid
parameters
Oils A-G were ranked using the first 7 parameters in Table 2. In these oils. all the n-alkane homologues were found present. Out of a total of 7 oils, 4 (oils A, B, C and D) were ranked first. This is due to the fact that none of these oils showed any biodegradation effect as assessed by parameters discussed above. The four n-alkane parameters indicated that the sum of the range of lighter nalkane concentration was higher than the sum of the range of n-alkane concentration chosen as denominator e.g. oil A has ZZ$Zft ratio equal to 1.14. Some oils (E, F and G) were found to be slightly effected by biodegradation as these have X$Zfl: ratios less than 1. It was because of this reason that oils A, B, C. D and E, F, G were considered separately in the ranking procedure. In order to rank oils A to D, each parameter was given equal weight and all the parameters were considered simultaneously. Each ratio number for a given oil corresponding to a given parameter was assigned a value ranging from l-4 depending upon the ratio number of a given parameter. In cases of n-alkane homologues’ parameters, the highest ratio number was assigned a value of 1, and the lowest ratio number a value of 4. When 2 or more oils were found to have the same ratio number, these oils were then assigned the same number. The same interpretation is valid for Pr/UCM,,,, whereas for Pr/nC,, and Ph/n-C,,, oils having the highest ratio number were assigned a value of 4. Subsequently these values (l4) for each parameter corresponding to each oil were aggregated and an oil carrying the least total was ranked higher in the list than the oil carrying the next highest total. This ranking strategy is illustrated in Table 3 for oils A-D. It is worth mentioning here that if only the first four n-alkane parameters are taken into account, as these parameters are most affected during slight biodegradation, then oils A and B carry an equal sum of 6, whereas oils C and D carry a sum of 11 and 15, respectively. Based on this, ranking of A and B was not possible.
Marine and Petroleum
Geology 1997 Volume 14 Number
1 61
Petroleum
biodegradation
in Tertiary
reservoirs:
A. Ahsan et al.
However oils C and D were ranked in the same order irrespective of whether the first four n-alkane or all parameters in Table 2 were used. While ranking oils E-G, it was found that oils E and F carried the same total for the assigned values, whereas oil G carried the highest total. It was for this reason that oils E and F were compared to each other without taking oil G into account. It was found that oil E carried the lesser total and therefore stood higher in the ranking list than oil F. Oils H-K were ranked using Prin-C,,. Ph/nC,, and Pr/UCM,,, with the same ranking strategy as adopted for oils A-G. Since oils L-N and 0-Q were ranked using only one parameter PrUCMc,, and UCM,,,/UCM,,,, respectively, ranking these oils was quite straight forward. An oil bearing higher values for parameters was placed higher in the ranking list than oils of lower values. For oils L-N. two parameters and UCM,,,/UCM,,, were available; PriUCMc,, however, Pr/UCMc,, used as the combination of two parameters always gave the same total for all the oils. Classification of oils was thus based on the type or combination of parameters. Oils A-G were ranked using the first 7 parameters with oils A-D showing no effect on any of the parameters. These oils show none of their nalkane ranges to be affected. However. there are still minor differences in their n-alkane, pristane and phytane parameters. Oils E-G show slight effect on their first parameter. Oils A-G were therefore classified as non- to slightly biodegraded. Oils H-K were assigned to belong to a mildly biodegraded class. Oils L-N were classified as moderately biodegraded, whereas oils O-Q as severely biodegraded. Figure 4 shows the GCFID traces of saturated hydrocarbon fraction of oils A, H, L and 0 which are representative of four classes of biodegradation.
carbon fraction (6°C Sat) to see how these data conform to the classification obtained through CC-FID data of the saturated hydrocarbon fraction. Fipw 5 shows bulk data plotted versus four classes of biodegradation. The figure shows that there is generally a good correlation between classes of biodegradation and the bulk properties of oils. However, wt % asphaltene of oils do not show a consistent behaviour with increasing degree of biodegradation. The asphaltene content of oils is low. generally in fractions to 3% of the total oil weight. However, weight measurement on such low-in-asphaltene oils is subject to large uncertainties, especially when the weight of oils taken to measure asphaltene content is low. Stahl (1980) studied the effects of biodegradation under simulated laboratory conditions on the isotopic compositions of saturated, aromatic and asphaltene fraction of oils and found that the (5’C of the saturated fraction of oils shifted towards heavier values with increasing degree of biodegradation. His result is to some extent substantiated by our study of biodegraded oils under natural conditions. Figm 5 further shows that there is a substantial change in the bulk properties of oils between none-. slightly- and mildly-biodegraded oils. However, once oils have been mildly biodegraded these differences in the bulk properties are not as pronounced with further increase in biodegradation. Hence our parameters are more sensitive to the changes that accompany biodegradation than the bulk properties of oils.
Conclusions All the oils at depth levels greater than 2.0 km are non to very slightly biodegraded. However, I oil (I) which belongs to the Balder Formation has been found to be mildly biodegraded even at a depth of 2.19 km (Table 5). All the oils at depth levels less than 2.00 km have been found to be biodegraded to different degrees regardless of their formations. However, oils in the Balder Formation are biodegraded to a higher degree than oils in the Sele Formation at similar depth levels. The difference
Biodegradation, bulk and isotope composition data We have used wt% asphaltene, API gravity. % polars, % saturates (%Sat; these two percentages have been obtained through Iatroscan) and 6’C of saturated hydro-
48
31 -
73 c
c 0
0
0
43
.?’ 38
k
33
;;: d
28
t
0
v : 2 & 0
23 IX
: 00 I7
0 0
27 ~
08
I
k!
8
I
L 3
4
0
23 0 1Y-O
0 0
15 -g
0
II --B I I
I
I
I
7
3
1
0 0
0 0
0
I I
2
3
4
I
2
3
1
I
I
I
2
I
I
3
3
Figure 5 Bulk properties and carbon isotope values plotted versus biodegradation classes on x-axis. Numbers on x-axis indicate class of biodegradation with 1 being the non-slightly biodegraded and 4 the severely biodegraded. Some oils do not plot on some graphs as these have overlapping values
62
Marine and Petroleum
Geology 1997 Volume
14 Number
1
Petroleum biodegradation in the degree of biodegradation is suggested to be related to the formation and hence depositional environment of the reservoir rock formation from which a given oil has been taken. The relatively higher degree of biodegradation for Balder oils compared with the Sele and Forties Formations at similar depth levels is attributable to comparatively higher lateral openness of the Balder Formation to meteoric water invasion. It is therefore suggested that the potential for biodegradation in prospect evaluation should take into account the degree of lateral openness of the structure. Laterally continous sandstones are more likely to be biodegraded than isolated sandstones enclosed in clays.
References Bailey, N. J. L. Jobson, A. M. and Rogers, M. (1973) Bacterial degradation of crude oil, Comparision of field and experimental data Chem. Geol. 11,203-221 Bailey, N. J. L. Burwood, R. and Harriman, G. E. (1990) Application of pyrolysate carbon isotope and biomarker technology to organofacies and oil correlation problems in the North Sea Basin Org. Geochem. 16,1157-l 172 Barker, C. (1980) Application of Geochemistry in Exploration, Maidenhead, 28 July 1980 Barnard, P. C. and Bastow, M. A. (1991) Hydrocarbon Generation, Migration, Alteration, Entrapment and Mixing in the Central and the Northern North Sea (Eds W. A. England and A. J. Fleet) Petroleum migration Spec. Pub/. geol. sot. No. 59, 167190 Brooks, P. W. (1986) Unusual biological marker geochemistry of oils and possible source rocks, offshore Beaufort-Mackenzie Delta, Canada. Advances in Organic Geochemistry 1985 (Eds D. Leythaeuser and J. Rulkotter) Pergamon Press, Oxford. 401-406 Connan, J. (1984) Biodegradation of Crude Oils in Reservoirs Adv. Petrol. Geochem. 1.299-333 Connan, J. and Cousteau, H. (1986) Influence ofthe Geochemical characteristics of Heavy oils on their Recovery. AAPG Exploration for Heavy oil and Natural Bitumen Res. Conf (Santa Maria, Calif.) Connan, J. and Lacrampe-Couloume, G. (1993) The origin of the Lacq superior heavy oil accumulation and of the giant Lacq inferior gas field (Aquitaine basin, SW France). In: Applied Petroleum Geochemistry (Ed. M. L. Bordenave) Technip, Paris, pp.465-487 Cooper, B. S. and Barnard, P. C. (1984) Source rocks and oils of the Central and Northern North Sea, (Eds G. Demaison and R. J. Murris). q. v, 303-314 Cornford, C., Morrow, J. A., Turrington, A., Miles, J. A. and Brooks, J. A. (1983) Some geological controls on oil composition in the U. K. North Sea. Petroleum Geochemistry and Exploration of Europe (Ed. J. Brooks) Oxford, Blackwell, 3550 Cornford, C. Needham, E. J. and de Walque, L. (1986) Geochemical habitat of North Sea oils and gases. Habitat of hydrocarbons on the Norwegian Continental Shelf (Ed. A. M. Spencer et a/.) Petroleum Geochemistry in Exploration of the Norwegian Shelf, Graham and Trotman, London, 39-54 Dahl. B. and Soeers. G. C. (1985) Oraanic Geochemistrv of the dseberg Field (I) In: Petroleum Geology in Exploration of the Norwegian She/f (Eds B. M. Thomas et a/.). Graham and Trotman, London, pp. 185-196 Didyk, B. M., Simoneit, B. R., Brassel, S. and Eglinton, G. (1978) Organic geochemical indicators of paleoenvironmental conditions of sedimentation Nature 272.216222 Field, J. D. (1985) Organic geochemistry in exploration of the northern North Sea (Eds B. M. Thomas et a/.) a. v, 39-58 Gough, M. A. and Rowland, S. J. (1990) Characterization of unresolved complex mixtures of hydrocarbons in petroleum Nature 344,648650 Grantham, P. J., Posthuma, J. and De Groot, K. (1980) Variation and significance of the CZ7 and CZ8 triterpane content of a North Sea core and various North Sea crude oils In: Advances
in Tertiary reservoirs: A. Ahsan et al.
in Organic Geochemistry 7979 (Eds A. G. Douglas and J. R. Maxwell). Pergamon Press, Oxford, pp. 29-38 Hollerbach, F. R. (1987) Influence of Biodegradation on the Chemical Composition of Heavy Oil and Bitumen In: Exploration for Heavy Crude Oil and Natural Bitumen (Ed. R. F. Meyer). Am. Assoc. Pet. Geol. Studies in Geology No. 25, pp. 243-247 Hughes, W. B., Holba, H. G., Miller, D. E. and Richardson, J. S. (1985) Geochemistry of greater Ekofisk Crude oils. In: Petroleum Geochemistry in Exploration of the Norwegian Shelf (Eds B. M. Thomas et al.) q. v. 75-92 Hunt, J. (1979) Petroleum Geochemistry and Geology. W. H. Freeman and Company, San Francisco Jobson, A. M., Cook, F. D., Westlake, D. W. S. (1972) Microbial utilization of crude oil Appl. Microbial. 23, 1082-1089 Karlsen. D. and Larter. S. R. (1989) A raoid correlation method for petroleum population mapping within individual petroleum reservoirs. In: Application to petroleum reservoir description (Ed. Collinson, J. D.) 77-85 Magoon L. B., Claypool G. E. (1981) Two oil types on North Slope of Alaska; implications for exploration. Am. Assoc. Pet. Gee/. Bull. 65,644-652 Palmer, S. E. (1993) Effects of biodegradation and water washing on crude oil composition. In: Organic Geochemistry (Eds M. H. Engel and S. A. Macko) Plenum Press, New York, pp.511533 Park, P. J. D. (1984) Crude oil biodegradation under simulated and natural conditions, II: aromatic steroid hydrocarbons Org. Geochem. 6,605-617 Perry, J. J. and Cerniglia, C. E. (1973) Degradation of petroleum by filamentous fungi In: Microbial Degradation of oil PO//utants(Eds D. G. Ahearn and S. P. Meyers). Workshop, Center for Wetland resources, Lousiana State Univ., Baton Rouse, La. 89-94 Phillippi, G. T. (1977) On the depth, time and mechanism of origin of the heavy to medium gravity naphthenic crude oils Geochim. Cosmochim. Acta 41,33-35 Powell, T. G. and Mckirdy, D. M. (1973) Relationship between pristane to phytane, crude oil composition and geological environment in Australia Nature, Phys. Sci. 243, 37-39 Price, P. L., Sullivan, T. 0. and Alexander, R. (1987) The nature and occurrence of oil in Seram, Indonesia. In: Proceedings of the Indonesian Petroleum Association, Sixteenth Annual 1, Jakarta, Indonesia. Petroleum Association, Convention. 141-173 Radke, M., Schaefer R. G., Leythaeuser, D. and Teichmuller, M. (1980) Preparative Hydrocarbon Group Type Determinartion by Automated Medium Pressure Liquid Chromatography Anal. Chem. 52,406411 Riva, A., Riolo, J., Mycke, B., Ocampo, R., Callot, H. J., Albracht, P. and Nali, M. Molecular parameters in Italian Carbonate oils, reconstruction of past depositional environments. 74th International meeting on Organic Geochemistry, Paris, September 18-22. Abstract no. 335 Rulkotter, J. and Wendisch, D. (1982) Microbial alteration of 17~ (H)-hopanes in Madagascar asphalts: removal of C-IO methyl group and ring opening Geochim. Cosmochim. Acta. 46, 1534-1553 Shannon, P. M. and Naylor, D. (Eds) (1989) Petroleum Basin Studies. Graham and Trotman, London, 78-79 Schaefer, R. G. and Leythaeuser, D. (1980) Analysis of trace amounts of Hydrocarbons (C,-C,) from rocks and crude oil oils and its application in petroleum geochemistry. Adv. Org. Geochem. 1979 (Eds A. G. Douglas and J. R. Maxwell). Pergamon Press, Oxford Schou, L., Eggen, S. and Schoell, M. (1985) Oil-oil and oil-source rock correlation, Northern North Sea In: Petroleum Geochemistry in Exploration of the Norwegian shelf (Eds B. M. Thomas, A. G. Dare, S. S. Eggen, P. C. Home and R. M. Laisse). Graham and Trotman, London, pp. 101-117 Seifert, W. K. and Moldowan, J. M. (1979) The effect of biodegradation on steranes and triterpanes in crude oils Geochim. Cosmochim. Acta. 43,111-126 Snowdon, L. R. and Powell, T. G. (1979) Families of crude oils and condensates in the Beaufort-Mackenzie Basin Bull. Can. Petrol. Gee/. 27, 113-162 Stahl, W. J. (1980) Compositional changes and 13C/‘ZC fractionations during the degradation of hydrocarbons by bacteria Geochim. Cosmochim. Acta. 44,1903-1907
Marine and Petroleum
Geology 1997 Volume
14 Number
1 63
Petroleum
biodegradation
in Tertiary
reservoirs:
Thomas, 6. M., Moeller, P. P., Whitaker, M. F. and Shaw, N. D. (1985a) Organic facies and hydrocarbon distribution in the Norwegian North Sea (Eds B. M. Thomas et a/.). q. v, 3-26 Volkman, J. K., Alexander, R., Kagi, R. I. and Woodhouse, G. W. (1983) Demethylated hopanes in petroleum geochemistry Geochim. Cosmochim. Acta. 46,785-794 Waples, D. W. and Machiara, T. (1991) Biomarkersfor Geologists, Am. Assoc. Petr. Geol. Methods in Exploration series, no. 9 Wilkes, H., Willsch, H. Rabus, R., Aeckersberg, F., Reuter, P. and Widdel, F. (1995) Compositional changes of crude oils upon
64 Marine and Petroleum
Geology 1997 Volume
A. Ahsan et al. anaerobic degradation by sulphate-reducing bacteria. Org. Geochem, 17th. International meeting on Organic Geochemistry. 321-323 Zumberge, J. E. (1984) Source rocks of the La Luna Formation (Upper Cretaceous) in the Magdalena Valley, Colombia (J. G. Palacas). Geochemistry and Source Rock Potential of Carbonate Rocks, Am. Assoc. Petrol. Geol. Studies in Geology No. 78. Tulsa, 127-133 Bstfeldt, P. (1987) Oil-Source rockcorrelation in the Danish North Sea (Eds J. Brooks and K. W. Glennie) q. v. 419-43
14 Number
1