Sterane compositional traits of Bowser and Sustut basin crude oils: Indications for three effective petroleum systems, KG Osadetz, C Jiang, CA Evenchick, F Ferri

Tags: Bowser Lake Group, crude oil, compositional variations, Bowser Lake, Sustut Group, mass chromatogram, Bowser Basin, Assemblage, petroleum systems, petroleum resource, steranes, petroleum system, source rock, C28, Tango Creek Fm, outcrop, Ritchie Ritchie, depositional environment, Buckinghorse Creek, compositions, Tsatia Mountain, compositional variation, Sustut Basin Bowser Basin Stikinia Belt, source rocks, Canada, Sustut Basin, Geological Survey of Canada, Sustut basins, pristane, northern Bowser, petroleum assessment, intrusive rocks, Muskaboo Creek Assemblage, oil stains, A. Evenchick3, Hazelton Group, crude oils, Brothers Peak Fm
Content: STERANE COMPOSITIONAL TRAITS OF BOWSER AND SUSTUT BASIN crude oilS: INDICATIONS FOR THREE EFFECTIVE PETROLEUM SYSTEMS By K.G. Osadetz1, C. Jiang2 ,C. A. Evenchick3, F. Ferri4, L. D. Stasiuk1, N. S. F. Wilson1 and M. Hayes4
KEYWORDS: Crude oil, natural gas, petroleum stains, seepages, Bowser-Sustut basins, organic geochemistry, solvent extracts, biomarkers, steranes, terpanes, gas chromatography-mass spectrometry. ABSTRACT Crude oils extracted from Bowser Lake and Sustut groups have distinctive compositions that are inferred to be indicative of at least three effective petroleum systems that have generated, expelled and accumulated crude oil. Compositional differences among the three effective petroleum systems are illustrated by compositional variations of steranes, complicated molecules that have retained structural similarities to their inferred biological precursor, cholesterol. Oil stains occur widely, both geographically and stratigraphically. One compositional oil family is inferred to be derived from Stikine assemblage, the sub-Hazelton succession. This petroleum is derived from pre-Jurassic marine carbonate source rocks deposited in hypersaline to mesohaline environments. A second compositional oil family is derived from Mesozoic open marine source rocks, that are inferred to be within the upper Hazelton or lower Bowser Lake Group, as the lowest stratigraphic occurrence of these oils lies in marine slope deposits of Bowser clastic wedge. A third oil family is inferred to be derived from lacustrine Mesozoic source rocks occurs in northern Bowser and Sustut Basins, where it is probably derived from thick, often coaly, non-marine Bowser Lake successions. The occurrence and composition of these crude oils expand the petroleum prospectivity of the Bowser and Sustut basins by reducing petroleum system risks and indicating a possible petroleum system for Hazelton Group, which is now attributed petroleum potential. INTRODUCTION This report results from work performed as part of the project "Integrated Petroleum Resource Potential and Geoscience Studies of the Bowser and Sustut Basins", a collaborative research project of the BC Ministry of Energy and Mines (Oil and Gas Emerging Opportunities and Geoscience Branch), and the Geological Survey of
Canada (Evenchick et al., 2003). The current multi-year project is multidisciplinary in scope and broad in geographic coverage, including the length and breadth of both the Bowser and Sustut basins. Primary activities include geological framework, energy resource studies, and petroleum resource assessment. Previous petroleum assessment work of the region identified substantial petroleum potential while recognising that there are several poorly understood, but significant risks (Hannigan et al., 1995). That study showed that there were significant play level risks associated with the inferred petroleum potential of the Bowser Basin. More recent GSC/BCMEM research resulted in a profound shift in perceptions of organic and thermal maturity patterns in the Bowser and Sustut basins (Evenchick et al., 2002). The first regional organic maturity dataset illustrates that large areas, including the lowest stratigraphic levels of the Bowser Basin, have sufficiently low organic maturity levels to be favourable for the formation and preservation of crude oil. This fundamentally changed previous views that considered the high thermal maturity of some of the stratigraphically highest coals as a negative indication for hydrocarbon potential in all stratigraphic levels and all the geographic regions of the basins. The recent discovery of petroleum within the basin (Osadetz et al., 2003a), as seepages or stains, provides information that eliminates play level risks associated with petroleum system function and reservoir occurrence. The results presented herein indicate that there are at least three operational petroleum systems, each with sources and crude oils of different molecular composition and stratigraphic characteristics operating in Bowser and Sustut basins. The results are consistent with the revised observations and models of thermal maturity and history. Integration of these data, models, and interpretations will increase the robustness of petroleum resource assessments in this potential frontier petroleum province. 1Geological Survey of Canada: Calgary, 3303 33 St. NW, Calgary, AB, Canada, T2L 2A7 2Humble Geochemical Services Division, 218 Higgins Street, Humble, Texas 77338 U.S.A. 3Geological Survey of Canada: Pacific, 101-605 Robson St. Vancouver, BC, Canada, V6B 5J3; 4Oil and Gas Emerging Opportunities and Geosciences Branch, British Columbia Energy and Mines, PO Box 9323, 6th Floor, 1810 Blanshard St., Victoria, BC, Canada, V8W 9N3
REGIONAL GEOLOGICAL OVERVIEW The Bowser and Sustut basins are located in northcentral British Columbia (Figure 1), in the Intermontane Belt of the Canadian Cordillera, a region of sedimentary diagenesis or low metamorphic grade (mainly greenschist facies) relative to the adjacent Omineca and Coast metamorphic and plutonic belts. They overlie Devonian to early Middle Jurassic strata of the allochthonous terrane Stikinia. The basins comprise three stratigraphic successions with overlapping ages. The Bowser Lake Group is the lowest, ranging from upper Middle Jurassic to midCretaceous. It constitutes a major clastic depositional wedge that includes strata deposited in distal submarine fan, slope, shallow marine shelf, deltaic, fluvial, and lacustrine environments (e.g. Tipper and Richards, 1976; Evenchick et al., 2001). It was deposited directly on Stikinia, a volcanic arc that includes Jurassic upper Hazelton Group clastic successions. The Lower to midCretaceous Skeena Group occurs south of the Bowser Lake Group with uncertain stratigraphic relationships. Skeena Group strata were deposited in marine and nonmarine environments, and are intercalated with volcanic successions (Tipper and Richards, 1976). The mid-to Upper Cretaceous Sustut Group, a fluvial and lacustrine foreland basin succession, unconformably onlaps the Hazelton and Bowser Lake groups that are deformed in older Skeena Fold belt structures. The Skeena Fold Belt subsequently involved and deformed the Sustut Group (Eisbacher, 1974; Figure 2). All 3 successions and underlying Stikinia are deformed in the Skeena Fold Belt, a thin skinned contractional fold and thrust belt of Cretaceous and possibly early Tertiary age (Evenchick 1991). Northeast vergent open to close folds of about 100 to 1000 m wavelengths are the dominant structures at exposed levels, but larger wavelength folds often outlined by anticlinoria and synclinoria in Bowser Lake Group are associated with structural culminations and depressions inferred to be controlled by the involvement of Stikine Assemblage volcanic and clastic strata. The fold hinges trend northwest dominantly, but domains of northeast fold hinge trends occur in western Skeena Fold Belt (ibid.). Hinterland verging thrusts, in the vicinity of boundary between Bowser Basin and Sustut Basin (Evenchick and Thorkelson, in press), define a triangle zone (Gordy et al., 1977), similar to major productive and prospective structures in many thrust and fold belts (MacKay et al., 1996). CRUDE OIL OCCURRENCE New field work and the analysis of existing samples have identified, extracted, and characterized twenty crude oil occurrences from locations in Bowser and Sustut basins (Table 1, Figure 2). Numerous additional
indications of crude oil staining and petroleum fluid inclusions occur with the Bowser and Sustut basins, but only twenty samples (Table 1) are characterized here. Petroleum occurrences include: 1. Tsatia Mountain, (NAD 27, UTM Zone V E442468 N6380068), a breeched oil field in Muskaboo Creek Assemblage Bowser Lake Group (GSC Extract X9693 and X9694; Osadetz et al., 2003); 2. Sandstone in the roof of the triangle zone north of Cold Fish Lake (NAD 27, UTM Zone V E511100 N6396070), Tango Creek Formation Sustut Group (GSC Extract 9746); 3. Footwall of the Crescent Fault near the confluence of Buckinghorse Creek and Spatsizi River (NAD 27, UTM Zone V E525670 N6366320 ), Eaglenest (deltaic) assemblage of the Bowser Lake Group (GSC Extract X9731); 4. Amoco Ritchie a-3-J/104-A-6, one of the only two petroleum exploration wells in the basin, shows extensive oil stains which were extracted from samples at depths of 2115.7'; 4337.0'; 4723.4'; 6745.0' (GSC Extracts X9742-X9745); 5. Twelve diverse samples from the northern Bowser and Sustut Basins (between 57.4284o N to 57.7803oN and 127.7689o W to 130.0625o W; GSC Extracts X9790 to X9801); 6. New field examples of rocks potentially stained with crude oil have also been identified during sample drilling for paleomagnetic samples (Evenchick et al., 2003). Oil films were present in the circulating fluid during paleomagnetic sampling of all Stikinia rocks sampled at Oweegee Dome. Oil films were present in a large number of lithologies through this antiformal culmination, including limestone, volcanic flows, volcaniclastic turbidites and conglomerates in Hazelton Group and lower units, as well as from Bowser Lake Group sandstone at Mount Ritchie (Figure 2, ibid.). These samples remain to be extracted and characterized, but they might reasonably be expected to resemble oil samples from the Ritchie wells; 7. In addition, to the stains noted above there are confirmed flammable natural gas seeps of biogenic methane into Tatogga Lake (Osadetz et al., 2003) that will be reported elsewhere (Evenchick et al., in prep). All these indications demonstrate that petroleum (both crude oil and natural gas) occurs in Bowser and Sustut basin. The existing resource assessment suggests that significant petroleum resources and large pool sizes can be expected (Hannigan et al., 1995), but only
64 O 144 O
FORELAND
Features referred to in text Sustut Basin Bowser Basin Stikinia Belt boundary BELT
OMIN EC A
57 O 138 O 54 O 134 O Queen Char lotte Island
FORELA ND
f Co BELT
Limit
Eastern
o
YT (Yukon-Tanana) 60 O rdilleran
INTERMOCNOTAANSET
Deformation
INSULAR BELT
BELT BELT BELT
48 O 128 O 500 km
48
12 O
OO
49 O
Figure 1. Location of the Bowser and Sustut basins on a base map showing the morphogeological belts of the Canadian Cordillera.
58o N
104G Iskut Tatogga Lake
Tsatia Mountain
104H Tri angle zone Buckinghorse Creek
94E
57o N
104B
CUA.SN.AA.DA 56o N Stewart NTS map areas 93L Smithers 93M Hazelton 94D McConnell Creek 94E Toodoggone River 103I Terrace 103P Nass River 104A Bowser Lake 104B Iskut River 104G Telegraph Creek 104H Spatsizi River
104A Oweegee Dome Mt. Ritchie Ritchie wells 103P New Aiyansh 103I
94D 93M Smithers 93L
Terrace
Prince Rupert 54o N 130oW Tertiary and younger volcanic rocks Sustut Basin rocks (Cretaceous) Bowser Basin rocks (Jurassic, Cret.) Hazelton and Stuhini groups (Triassic and Jurassic volcanic arc rocks) and minor older strata
Kitimat 128 oW Teritary intrusive rocks Cretaceous intrusive rocks Jurassic intrusive rocks Triassic intrusive rocks
Houston 126 oW oil stain locations discussed in text hi ghwa ys towns, cities rivers, coastline
Figure 2. The locations of select crude oil samples discussed in text shown on a regional map of the Bowser and Sustut basins. The locations of all samples are listed in Table 1.
Table 1: Crude oil stain and fluid inclusion sample locations
Extract # Latitude (oN) Longitude (oW) Depth
Map Unit
X9693
57.5613
129.9616
outcrop Bowser Lake Gp; Muskaboo Creek Assemblage
X9694
57.5613
129.9616
outcrop Bowser Lake Gp; Muskaboo Creek Assemblage
X9731
57.4408
128.5724
outcrop
Bowser Lake Gp; Eaglenest Assemblage
X9742
56.4188
129.1531
644.8 m
Bowser Lake Group
X9743
56.4188
129.1531 1321.9 m
Bowser Lake Group
X9744
56.4188
129.1531 1439.6 m
Bowser Lake Group
X9745
56.4188
129.1531 2055.8 m
Bowser Lake Group
X9746
57.5613
129.9616
outcrop
Sustut Group; Tango Creek Fm.
X9790
57.9247
129.1408
outcrop
Sustut Group; Tango Creek Fm.
X9791
57.5559
130.0481
outcrop
Bowser Lake Gp; Todagin Assemblage
X9792
57.1124
129.7686
outcrop
Bowser Lake Gp; Todagin Assemblage
X9793
57.4284
129.9309
outcrop
Bowser Lake Gp; Skelhorne Assemblage
X9794
57.4322
129.8974
outcrop
Bowser Lake Gp; Skelhorne Assemblage
X9795
57.3179
127.7667
outcrop
Sustut Group; Tango Creek Fm.
X9796
57.3137
127.7329
outcrop
Sustut Group; Tango Creek Fm.
X9797
57.1170
127.5309
outcrop
Bowser Lake Gp; Eaglenest Assemblage
X9798
57.7310
128.7963
outcrop
Sustut Group; Brothers Peak Fm.
X9799
57.7803
128.8793
outcrop
Sustut Group; Brothers Peak Fm.
X9800
57.5460
130.0625
outcrop
Bowser Lake Gp; Skelhorne Assemblage
X9801
57.3060
127.7689
outcrop
Sustut Group; Tango Creek Fm.
exploratory drilling will confirm the existence of a significant undiscovered petroleum resource. PETROLEUM CHARACTERIZATION USING MOLECULAR COMPOSITIONAL TRAITS Crude oil compositional characteristics reflect kerogen paleoecology, depositional environments, diagenesis, and maturity. Differential expulsion, migration, and post accumulation affects processes like catagenesis, biodegradation and water washing can also affect crude oil composition (Peters and Moldowan, 1993; Waples and Machihara, 1990; Seifert and Moldowan, 1986, 1981, 1978). Therefore it is important to distinguish source characteristics from migration and postaccumulation effects before inferring depositional and diagenetic characteristics directly from crude oils.
Extracts studied herein exhibit three distinguishable and distinctive compositional associations, or family groups, defined by persistent compositional characteristics that are exhibited by a variety of fractions and compounds, but which are especially well illustrated by steranes, the focus of this discussion. Phytolic acid side chains from chlorophyll molecules indicate a particular phototrophism. The acyclic isoprenoid compounds pristane (Pr) and phytane (Ph), which are ubiquitous compounds in crude oils, are commonly derived from these side chains of chlorophyll molecules, although other sources for these compounds exist (Figure 3; Volkman and Maxwell, 1986). Low Pr/Ph is commonly inferred to indicate water column anoxia, especially when accompanied by even-odd n-alkane predominance (Welte and Waples, 1973). High Pr/Ph ratios are commonly interpreted as indicative of oxic water columns (Volkman and Maxwell, 1986). Steranes, both regular and rearranged, are common biological markers in oils. Acid clays in the depositional
6.4 0 6.3 5 6.3 0 6.2 5 6.2 0 6.1 5 6.1 0 6.0 5 6.0 0 5.9 5 5.9 0 5.8 5 5.8 0 5.7 5 5.7 0 5.6 5 5.6 0 5.5 5 5.5 0 5
Pr Ph C20 C15
GSC Lab no. 9745 extract sample Ritchie A-3-J #1 C-428630 6745.0 F
C25
10
15
20
25
30
35
40
45
50
55
60
65
Time (min)
5.5 90 5.5 85
C25
GSC Lab no. 9742
extract sample
Ritchie A-3-J #68
5.5 80
C-428718
5.5 75
2115.7 F
5.5 70
5.5 65
5.5 60
5.5 55 C20 5.5 50
5.5 45
5.5 40 5.5 35
Pr Ph
5.5 30
5.5 25 C15 5.5 20
5.5 15
5.5 10
5
10
15
20
25
30
35
40
45
50
55
60
Time (min)
Figure 3. Illustrative saturate fraction gas chromatograms (SFGC) of two solvent extract samples from the Ritchie A-3-J well at 6745.0 feet (2055.8 m, top) and 2115.7 feet (644.8 m, bottom)(Figure 2). All figures show DETECTOR RESPONSE (y-axis) as a function of time since injection on the chromatographic column (x-axis). The obvious SFGC compositional differences between these two samples are interpreted as due primarily to biodegradation of the shallower crude oil sample by aerobic bacteria.
environment can control early diagenetic reactions that result in sterane rearrangement via unsaturated intermediates to produce diasteranes (Figure 4; Sieskind et al, 1979; Rubinstein et al., 1975). Low relative diasterane abundances, like those in some carbonate sourced petroleum systems are generally interpreted to indicate a clay starved depositional environment (i.e. a "carbonate" source rock), although many "carbonate" source rocks have diasterane/regular sterane ratios like those attributed to "clastic" source rocks (Osadetz et al., 1992). A biological source of diasteranes is unlikely, but the association of anomalously high diasteranes in Carbonate rocks with evaporitic depositional environments suggests a relationship (Clark and Philp, 1989). Strongly reducing depositional environments are required to preserve organic matter and form kerogen in sedimentary petroleum source rocks. Since strongly reducing conditions can persist below the sediment-water interface, even in the absence of water column anoxia, euxinic sediments do not directly indicate water column environmental conditions. However, some biological compounds to source rock kerogen, notably phytoloic acid side chains of chlorophyll and bactirohopane-tetrol
are commonly inferred to be affected by reduction and oxidation reactions in the water column, prior to, or at the earliest stages of, their incorporation into the sediment (Peters and Moldowan, 1993). Peters and Moldowan (1991) suggest that C32 to C34 hopane prominences could result from either redox reactions controlled by the deposition environment or from precursors molecules other than bacteriohopane-tetrol. The first alternative is preferred because C34 hopane and C35 hopane prominence commonly follows source rock depositional environment and paleoecology (Osadetz et al., 1992). This results in a water column chemistry indicator that is preserved in compounds that are a common trace component of crude oils (Figure 5; Peters and Moldowan, 1991). The higher the carbon number of predominant extended hopanes the stronger and more persistant the water column anoxia. RESULTS Select gross and molecular compositional results of the twenty examined samples appear in Table 2. All subsequent illustrations of gas chromatograms and mass chromatograms show detector response (y-axis) as a
m/z 217
C28 Regular
Sterane
C29
C27 Regular Sterane
Diasterane
C29 Regular Sterane
Response At Detector
5
10
15
20
25
30
35
40
45
Time
Figure 4. An example m/z 217 mass chromatogram showing the relative abundance of both regular and rearranged, or dia-, steranes in solvent extract sample X9693, from a sample of Muskaboo Creek Assemblage bioclastic sandstone at Tsatia Mountain.
Response At Detector
C30 Hopane
m/z 191
C23 Tricyclic C24 Tetracyclic
C31 C32 C34
C33
C35
5
10
15
20
25
30
35
40
45
Time
m/z 218
C29 C28 C27
Response At Detector
5
10
15
20
25
30
35
40
45
Time
Figure 5. An example m/z 191 (terpanes, Top) and 218 (regular steranes, Bottom) mass chromatogram in solvent 5extract sample X9693, from Muskaboo Creek Assemblage bioclastic sandstone at Tsatia Mountain.
function of time since injection on the chromatographic column (x-axis). Compounds are identified by both standard elution order and by full scan triple sector GCMS operating in MS-MS mode. All samples have been characterized optically or petrographically as being stains or petroleum fluid inlusions. The low hydrocarbon yields and low HC% values are due to the small recoverable volumes (Table 2). This is expected considering the exposure of both outcrops and old well cuttings to processes of dissipation and alteration resulting from their exposure to the elements, especially in light of the inferred low densities and expected volatility of the hydrocarbons (Osadetz et al., 2002). SELECT CRUDE OIL MOLECULAR COMPOSITIONAL TRAITS Molecular compositional differences among the samples are diverse, but they can be characterized by consideration of select compound variations. The observed variations are interpreted using standard techniques and previous studies to distinguish variations due to the alteration, specifically biodegradation, so that other variations due primarily to source can be isolated and interpreted. The variation of biological marker s compounds, like steranes and terpanes, within the sample set are observed and interpreted to be unaffected by alteration, allowing interpretation of compositional variations attributed to source rock age (regular steranes, Grantham and Wakefield, 1988), lithology (diasteranes/regular steranes, Seiskind et al., 1979; hopane prominence, Osadetz et al., 1992) and depositional environment (Peters and Moldowan, 1991) that form the basis of compositional family and petroleum system definition.
Figure 3 illustrates the range of Saturate Fraction Gas Chromatogram (SFGC) compositions observed in the samples. Both samples are from the Ritchie A-3-J well at 6745.0 feet (2055.8 m, Figure 3, top) and 2115.7 feet (644.8 m, Figure 3, bottom; Figure 2) and both have similar biological marker compositions. Solvent extract X9745 (top) exhibits a normal crude oil response dominated by normal alkanes, derived from cell wall phospholipids and the irregular isoprenoid, most noticeably pristane (Pr) and phytane (Ph). The low amplitude baseline "hump" defines the envelope of a complicated mixture of coeluting compounds. Solvent extract X9742 (bottom) exhibits a biodegraded crude oil response dominated by normal alkanes that are paired, for samples with longer elution times than nC20, with a homologous series of alkylcyclohexanes and methylalkylcyclohexanes that were identified by GC-MSMS experiments not discussed herein. Sample X9742 also exhibits a high amplitude baseline "hump" of more complicated coeluting compounds. Note especially the relative change in response of the "hump" relative to compounds eluting prior to and after nC20 between the two samples. The sterane compositions of these two samples are essentially similar (Table 2, Figures 4, 5, 6) illustrating that the lower molecular weight saturate fraction composition has been altered by the preferential removal of normal alkanes. Such compositional variations are commonly inferred to be indicative of biodegradation (Peters and Moldowan, 1993; Osadetz et al., 1992) The sample set compositional variation can be illustrated using a few key compounds. The most illustrative variations are shown by the regular and rearranged steranes (Figure 4). An example m/z 217 mass chromatogram showing the relative abundance of regular and rearranged, or dia-, steranes in solvent extract sample
TABLE 2: SELECT GROSS AND MOLECULAR COMPOSITIONAL RESULTS FROM BOWSER BASIN CRUDE OILS
Sample No. X9693 X9694 X9731 X9742 X9743 X9744 X9745 X9746 X9790 X9791 X9792 X9793 X9794 X9795 X9796 X9797 X9798 X9799 X9800 X9801
TOC (%) BD BD BD BD 0.10 0.01 5.05 0.05 4.69 0.88 0.74 1.07 19.15 8.07 3.26 43.28 0.78 0.53 0.52 3.19
(ppm of rock) 2385 22500 N/A 5500 396 771 579 1179 4442 1897 2857 4080 9238 8600 6657 29875 4903 3448 1832 3701
Extract (%
%%%
of TOC) HC (%) R+A (%) Sat/Aro Pr/Ph C29 St Dia/Reg C27 C28 C29 C27/C29 C28/C29
N/A
10.8 85.0
1.0 N/A
0.22
26 21 53 0.49 0.40
N/A
8.9
80.0
1.0 N/A
0.26
32 19 48 0.67 0.39
N/A
N/A N/A N/A N/A
0.76
42 26 31 1.35 0.83
N/A
4.6
87.7
0.3 N/A
0.54
30 29 40 0.76 0.73
40
7.9
86.8
2.0 N/A
0.56
25 33 42 0.59 0.80
771
10.7 57.1
0.8 0.44
0.64
22 33 44 0.50 0.75
1
27.3 58.2
0.6 1.11
0.55
26 34 40 0.66 0.86
236
5.4
75.0
4.0 0.69
0.52
30 29 41 0.73 0.71
9
4.7
90.6
0.1 1.29
0.57
16 18 66 0.24 0.28
22
3.6
86.4
1.0 0.86
0.65
41 27 31 1.32 0.87
39
3.9
85.0
0.4 0.67
0.76
41 27 32 1.28 0.86
38
4.9
73.5
0.3 0.97
0.61
40 27 33 1.22 0.83
5
6.2
90.2
0.7 1.67
0.38
21 24 55 0.38 0.43
11
7.4
84.5
0.2 4.49
0.27
7 19 73 0.10 0.26
20
8.2
82.0
0.4 2.59
0.32
16 22 62 0.26 0.35
7
15.7 83.7
0.2 4.56
0.23
5 25 70 0.07 0.35
63
2.0
89.5
0.5 0.7
0.54
36 26 37 0.97 0.71
65
12.0 81.5
0.3 0.56
0.37
25 30 46 0.53 0.64
35
8.1
84.3
0.9 1.04
0.77
38 27 35 1.10 0.77
12
10.2 86.3
0.3 1.87
0.19
9 19 72 0.13 0.27
X9693 is from Muskaboo Creek Assemblage at Tsatia Mountain. Steranes are derived from cholestane-like molecules that act as common cell wall rigidifiers in eucaryotic organisms. The presence of diasteranes is an indicator of depositional environment and source rock lithology (Sieskind et al, 1979; Rubinstein et al., 1975). Two important groups of biological marker are the terpanes (m/z 191, Figure 5: Top) and regular steranes (m/z 218, Figure 5: Bottom). Their occurrence is also illustrated by mass chromatograms from solvent extract sample X9693, from Muskaboo Creek Assemblage at Tsatia Mountain. The m/z 191 mass chromatogram illustrates terpanes, which are primarily derived from bacteriohopanetetrol, a cell wall rigidifier in prokaryotic organisms (Peters and Moldown, 1993). The ratio of similar carbon number hopanes in the homologous group of compounds that occurs to the right of the C30 hopane peak as annotated double peaks is controlled by depositional environment physical conditions (Peters and Moldowan, 1991). This sample shows that C34 hopanes are prominent, due to the accumulation of the source rock in an environment where anhydrite of gypsum was accumulating (Osadetz et al., 1992). The m/z 218 mass chromatogram illustrates regular steranes that were probably derived primarily from cholesterol. The ratio of C28 to C29 steranes is known to increase with geological age in marine depositional environments, due to biochemical evolution in the marine biosphere (Grantham and Wakefield, 1998). The observed ratio of C28/C29 steranes, combined with the standard interpretation of the m/z 191 mass chromatogram indicates that the source rock of this oil stain is a sub-Hazelton carbonate source rock deposited in submarine hypersaline to mesohaline depositional environments, and probably occuring in the underlying Stikine succession. C28
60 40 20
40 60 80
X9743
X9745
Oil Family
X9744
From Lacust rine
X9799
X9742 X9791-9793
So urce
X9797
X9794
X9746
X9796 X9795 X9801
X9798 X9693
X9731
Oil Family From Mesozoic Marine Source
100 80
0 20
C290
X9790 20
X9694
X9800
Oil Family Tsatia Mountain From Sub-Hazelton So urce
40
60
80C27
Figure 6. A ternary diagram illustrating compositional variations and affinities of all twenty solvent extract samples using the relative abundance of C27-C28-C29 regular steranes (Table 2). The three oil families are identified.
A ternary diagram shows the variations and affinities of all twenty samples using the relative abundance of C27-C28-C29 regular steranes (Figure 6, Table 2). The biodegradation of some oils (Figure 3) does not affect regular sterane compositions, such that observed variations can be attributed primarily to source rock compositional differences. This figure illustrates the presence of the three oil families identified. One, composed of two extracts from Tsatia Mountain (X9693, X9694) is inferred using terpane and sterane compositional characteristics to be derived from a carbonate source in the underlying Stikine succession, as discussed above. Using similar standard interpretations (Peters and Moldowan, 1993) we interpret the steranes and terpanes of the other samples. The second compositional family includes samples from the Amoco Ritchie a-3-J/104-A-6 at depths of 2115.7'; 4337.0'; 4723.4'; 6745.0' (GSC Extracts X9742-X9745), the Tango Creek Formation sample from the Triangle zone (GSC Extract X9746), the Eaglenest assemblage sample from Buckinghorse Creek (GSC Extract X9731), as well as six samples from other locations in the northern Bowser basin region (GSC Extracts X9791-93 and X9798-80, Table 1) which have compositional characteristics that suggest derivation from an open Mesozoic marine source rock. This potential source facies probably lies in the upper Hazelton or lower Bowser Lake groups, as the lowest stratigraphic occurrence of these oils lies in slope and shelf facies of the Bowser Lake Group. A third oil family composed of six samples from the group of twelve diverse samples from northern Bowser Basin (GSC Extracts X9790, X9794-97 and X9801) is distinguished by having lower C27 regular steranes and generally higher C29 regular steranes compared to all other samples. This oil compositional family is inferred to have a non-marine, possibly lacustrine source in Bowser Lake or Sustut groups (Peters and Moldowan, 1993). The transitional position of samples X9794 and X9796, do not preclude the possibility that they could be mixtures of the two oil families inferred to have Mesozoic source rocks. This is possible since the geographic range of the two end-member compositions of the Mesozoic marine and non-marine source oils overlaps. However, other evidence presented below suggests that mixing is not important. A cross plot of the ratios C29 diasteranes to regular steranes and ratio of regular C28 steranes to C29 steranes (Figure 7) shows additional compositional variations within families using variation of diasteranes/regular steranes (Sieskind et al, 1979; Rubinstein et al., 1975). Those oils inferred to have a non-marine Mesozoic source have an overlapping range of diaterane/regular sterane ratios to those inferred to have Mesozoic marine sources, however the range of non-marine source oil compositions is illustrated by X9790, which is one of the samples that is most different from the oils inferred to have marine Mesozoic sources. Therefore, it is unlikely that the samples X9794 and X9796 are mixtures of any significant proportion. A cross plot of the ratios C29 diasteranes to
1.0
Steranes C28/C29
0.8
X9745
X9791
X9743
X9793
X9742
X9746
X9798
X9744
X9792 X9731 X9800
X9799
0.6 Oil Family
Tsatia
Mountain
X9794
0.4
X9693
X9694
Oil Family From Mesozoic Marine Source
X9797
X9796
X9801
X9795
0.2
X9790 Oil Family From Mesozoic Lacustrine Source
0.0
0.0
0.2
0.4
0.6
0.8
1.0
C29 Diasteranes/C27 Regular Steranes
Figure 7. A cross plot of the ratios C29 diasteranes to C27 regular steranes and ratio of regular C28 steranes to C29 steranes showing that the compositional variations within families, as typified by the variation of diasteranes/regular steranes.
regular steranes and the ratio of pristane and phytane from the SFGC illustrates that the compositional distinction between the two interpreted Mesozoic oil families is also reflected by other compositional traits (Figure 8). Insufficient pristane and phytane were observed in the oils inferred to be sourced from the Stikine assemblage strata (X9693; X9694) to allow there characterization using pristane and phytane, however, they are distinguished from most Mesozoic sourced oils by their generally higher saturate to aromatic hydrocarbon ratio (Table 2). DISCUSSION The interpretation that biodegradation has altered the composition of some crude oils at or near the surface is important, but not surprising. The Amoco Ritchie a-3J/104-A-6 well has a porous interval containing a resistive fluid that is either "by-passed petroleum pay" or fresh water. The nature of the wireline-log resistivity anomaly is not diagnostic. Some crude oil extracts from this well are clearly biodegraded (Figure 3), as a result of aerobic bacterial degradation that implies a connection with oxygenated, probably fresh and meteoric water, which like hydrocarbons, is electrically resistive. In the same well Koch (1973) reported other petroleum shows including both dry and wet gas in cuttings samples at depths <2600' where the gas detector indicated >40 units, compared to background readings of 10-20 units. Therefore, the nature of the resistivity anomaly in the well
remains unresolved. Regardless, the combined observations are important since the wireline logs indicate the presence of porous zones in some of the deepest strata in Bowser Lake Group, while the oil stains indicate an effective petroleum system in the same region. The results are consistent with the revised thermal maturity model (Evenchick et al., 2002). The analysis of the molecular composition of these oil stains and seepages has identified at least three distinct compositional crude oil families. One oil family is inferred to be derived from carbonate source rocks in underlying Stikinia. The second and third oil families are inferred to be derived from Jurassic or younger sources in the Hazelton-Bowser-Sustut successions, one of which has the characteristics of a marine source, and the other which has compositional characteristics of a non-marine, or lacustrine, source. The source rocks of these petroleum systems have not been identified explicitly, nor have the oils been correlated to solvent extracts from potential source rocks, although potential source rock intervals have been identified in a variety of stratigraphic positions (Evenchick et al., 2002; Osadetz et al., 2003b). However, the significant number of petroleum stains and their association with structures that could be traps for petroleum, as at the Ritchie well, Tsatia Mountain and in the roof of the triangle zone all point toward a complete removal of play level risks for both petroleum system and reservoir. This suggests that a revised petroleum assessment would be even more encouraging than the existing one (Hannigan et al., 1995).
1.0
29 C Diasteranes/C27 Regular Steranes
0.8
X9792
X9800
Oil Family
X9744 0.6
X9791 X9793
From Mesozoic Marine Source
X9798
X9790
X9745
X9746
0.4
X9799
X9794
X9796 Oil Family
0.2
X9801
From Mesozoic Lacustrine
Source
X9795 X9797
0.0
0.0
1.0
2.0
3.0
4.0
5.0
Pristane/Phytane
Figure 8: A cross plot of the ratios C29 diasteranes to C27 regular steranes and ratio of pristane (Pr) and phytane (Ph) from the saturate fraction gas chromatogram (SFGC) that illustrates how compositional distinctions between the two Mesozoic oil families is reflected by additional compositional traits.
CONCLUSIONS Oil stains occur widely, both geographically and stratigraphically, with only the northern half of these basins being investigated. Twenty crude oil stains and petroleum fluid inclusions have been extracted from Bowser Lake and Sustut group rocks and their compositions have been characterized. The molecular compositions of these samples are interpreted to show that there are at least three compositionally distinct oil families representative of three effective petroleum systems in Bowser and Sustut basins. Molecular compositional differences can be characterized by sterane compostional variations using standard techniques and previous studies, once the effects of alteration -- specifically biodegradation, are discounted. One compositional oil family is inferred derived from the sub-Hazelton succession. A second compositional oil family derived from normal Meozoic marine source rocks is inferred derived from the upper Hazelton or lower Bowser Lake groups. A third oil family derived from lacustrine Mesozoic source rocks is inferred to occur in the Bowser Lake Group. The occurrence and composition of these crude oils expand the petroleum prospectivity of Bowser Basin by reducing petroleum system risks and indicating a possible
petroleum system for Hazelton Group. The preservation of crude oils is also a strong confirmation of the revised thermal maturity model for the basin (Evenchick et al., 2002). Existing petroleum resource assessments (Hannigan et al., 1995) do not attribute petroleum potential to sub-Bowser successions indicating a need for revision. ACKNOWLEDGEMENTS We thank Sneh Achal, Rachel Robinson, and Marina Milovic for their technical assistance in the production of the data employed here. GSC colleagues Mark Obermajer, Maowen Li and David Ritcey provided helpful discussions and technical assistance in the preparation of this paper. This is Geological Survey of Canada contribution #2004XXX.
APPENDIX 1: ANALYTICAL PROCEDURES Anhydrous Pyrolysis Rock-Eval/TOC is a useful screen for recognizing sources and stained lithologies. Rock samples suspected or identified as having crude oil stains or petroleum fluid inclusions were pyrolyzed using Rock-Eval/TOC (Table 2), to determine total organic carbon content (Table 2; Espitalie et al., 1985; Peters, 1986; Tissot and Welte, 1978, p. 443-447). The Rock-Eval/TOC analysis gives five parameters: S1, S2, S3, TOC and Tmax. The S1 parameter measures free or adsorbed hydrocarbons volatilized at moderate temperatures (300oC). S2 measures the hydrocarbons liberated during a ramped heating (300-550oC at 25oC/min.). The S3 parameter measures organic CO2 generated from the kerogen during rapid heating (300-390oC at 25oC/min.). Milligrams product per gram rock sample, the equivalent to kilograms per tonne, is the measure of all these parameters. Total Organic Carbon (TOC) is measured and reported in weight per cent. Tmax, the temperature corresponding to the S2 peak maximum temperature is measured in oC. Rock-Eval/TOC parameters have significance only above threshold TOC, S1 and S2 values. If TOC is less than about 0.3% then all parameters have questionable significance and the experiment suggests no potential. Oxygen Index (OI), S3/TOC, has questionable significance if TOC is less than about 0.5%. OI values greater than 150 mg/g TOC can result from either low TOC determination or from a mineral matrix CO2 contribution during pyrolysis. Both Tmax and Production Index (PI = S1/(S1+S2)), have questionable significance if S1 and S2 values are less than about 0.2. Results can be affected by mineral matrix effects. These either retain generated compounds, generally lowering the S1 or S2 peaks, while increasing Tmax, or by liberating inorganic CO2 and increasing S3 and OI. Mineral matrix effects are important if TOC, S1 and S2 are low, an effect not significant in this study. Solvent Extract Gross Composition The amount and composition of solvent extractable bituminous material, including crude oil stains and petroleum fluid inclusions was obtained by extracting the bitumen from the rock sample using the Soxhlet technique (Table 2). Solvent extracts were fractionated using packed column chromatography following a method effectively similar to that published by Snowdon (1978). The resulting gross composition can be used to identify crude oil stains or to characterize source rock richness and maturity. Normalized solvent extract hydrocarbon (HC)
yield, quoted in milligrams extract per gram organic carbon (mg/g TOC), is a richness indicator. HC yields less than 30 mg/g TOC suggests no source rock potential. Those between 30 and 50 mg/g TOC suggest marginal potential. HC yields between 50 and 80 mg/g TOC show good potential. Greater values indicate excellent potential. Hydrocarbon percentage criteria for maturity are commonly OM Type and lithology independent. Stained samples are those with more than 55% HC's and lower values are characteristic of petroleum source rocks, if sufficient material is available, which is not the case for this study. Less than 20% HCs' characterizes thermally immature sources, 25%, but less than 45% HCs' is the interval of marginal maturity with higher values occurring during the the main HC generation stage. Solvent Extract Molecular Composition The extractable bitumen was deasphalted by adding an excess of pentane (40 volumes) and then fractionated using open column liquid chromatography. Saturate hydrocarbons were analysed using gas chromatography (GC) and gas chromatography - mass spectrometry (GCMS). A Varian 3700 FID gas chromatograph was used with a 30 m DB-1 column coated with OV-1 and helium as the mobile phase. The temperature was programmed from 50oC to 280oC at a rate of 4oC/min and then held for 30 min at the final temperature. The eluting compounds were detected and quantitatively determined using a hydrogen flame ionization detector. The resulting saturate fraction chromatograms (SFGC) were integrated using Turbochrom software. GCMS was performed in both single ion monitoring and full scan modes on both saturate and aromatic hydrocarbon fractions of solvent extracts, although only select saturate fraction compositional characteristics obtained from single ion monitoring experiments are reported here. Single ion monitoring GCMS experiments were performed on a VG 70SQ mass spectrometer with a HP gas chromatograph attached directly to the ion source (30 m DB-5 fused silica column used for GC separation), or under similar analytical conditions. The temperature, initially held at 100oC for 2 min, was programmed at 40oC/min to 180oC and at 4oC/min to 320oC, then held for 15 min at 320 oC. The mass spectrometer was operated with a 70 eV ionization voltage, 300 mA filament emission current and interface temperature of 280oC. Terpane and sterane ratios reported herein were calculated using m/z 191 and m/z 217 and m/z 218 mass chromatograms. REFERENCES Clark, J. P., and Philp, R. P. (1989): Geochemical characterization of evaporite and carbonate depositional environments and correlation of associated crude oils in the Black Creek Basin, Alberta. Bulletin of Canadian Petroleum Geology, v. 37, no. 4., pages 401-416.
Eisbacher, G.H.(1974): Sedimentary history and tectonic evolution of the Sustut and Sifton basins, north-central British Columbia; Geological Survey of Canada, Paper 73-31, 57 pages. Espitalie, J., Deroo, G. and Marquis, F. (1985): Rock Eval Pyrolysis and Its Applications. Preprint; Institut Francais du Petrole, Geologie No. 27299, 72 p. English translation of, La pyrolyse Rock-Eval et ses applications, Premiere, Deuxieme et Troisieme Parties, in Revue de l'Institut Francais du Petrole, v. 40, p. 563-579 and 755-784; v. 41, pages 73-89. Evenchick, C.A. (1991): Geometry, evolution, and tectonic framework of the Skeena Fold Belt, north-central British Columbia; Tectonics, v. 10, page 527-546. Evenchick, C.A., Poulton, T.P., Tipper, H.W., and Braidek, I. (2001): Fossils and facies of the northern two-thirds of the Bowser Basin, northern British Columbia; Geological Survey of Canada, Open File 3956. Evenchick, C.A., Ferri, F., Mustard, P.S., McMechan, M., Osadetz, K. G., Enkin, R., Hadlari, T., and McNicoll, V. J. (2003): Recent results and activities of the Integrated Petroleum Resource Potential and Geoscience Studies of the Bowser and Sustut Basins project; in current research, Geological Survey of Canada, A-13, 11 pages. Evenchick, C.A., Hayes, M.C., Buddell, K.A., and Osadetz, K.G. (2002): Vitrinite reflectance data and preliminary organic maturity model for the northern two thirds of the Bowser and Sustut basins, north-central British Columbia. Geological Survey of Canada, Open File 4343 and B.C. Ministry of Energy and Mines, Petroleum Geology Open File 2002-1. Evenchick, C.A., Osadetz, K.G., Ferri, F., Mayr, B., and Snowdon, L. R., (in prep.): A Natural Seepage of Biogenic Methane in the Intermontane Belt (Bowser Basin) of the Canadian Cordillera. Bulletin of Canadian Petroleum Geology, v. XX, no. X., p. XXX-XXX. Evenchick, C.A. and Thorkelson, D.J. (in press): Geology of the Spatsizi River map area, north-central British Columbia, Geological Survey of Canada Bulletin 577. Gordy., P.L., Frey, F.R., and Norris, D.K. (1977): Geological guide for the C.S.P.G. 1977 Waterton - Glacier Park Field Conference. - Candian Society of Petroleum Geologists, Calgary.Grantham, P. J., and Wakefield, L. L.(1988): Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time, Organic Geochemistry, v. 12, no. 1, pages 61-73. Hannigan P. K., Lee, P. J. and Osadetz, K. G. (1995): Oil and gas resource potential of the Bowser-Whitehorse area of British Columbia, Report to BCEMR, March 1995, 72 pages. Koch, N. G. (1973): Central Cordilleran Region; in R. G. McCrossan ed., The Future Petroleum Provinces of Canada - Their Geology and Potential, Canadian Society of Petroleum Geologists, Memoir 1, pages 37-71. MacKay, P. A., Varsek, J. L., Kubli, T. E., Dechesne, R. G., Newson, A. C., Reid, J. P. (1996): Triangle Zones and Tectonic Wedges. Bulletin of Canadian Petroleum Geology, v.44, No.2, pages. I-1-I-5. Osadetz, K.G., Evenchick, C. A. , Ferri, F. , Stasiuk, L. D., and Wilson, N. S. F. (2003a): Indications for effective petroleum systems in Bowser and Sustut basins, northcentral British Columbia. in Geological fieldwork, 2002;
B.C. Ministry of Energy and Mines, Paper 2003-1, pages 257-264. Osadetz, K.G., Snowdon, L.R., and Obermajer, M. (2003b): ROCK-EVAL/TOC results from 11 Northern British Columbia boreholes. Geological Survey of Canada, Open File 1550 and B.C. Ministry of Energy and Mines, Petroleum Geology Open File 2003-1 (CD-ROM). Osadetz, K. G., Brooks, P. W., and Snowdon, L. R. (1992): Oil families and their sources in Canadian Williston Basin (southeastern Saskatchewan and southwestern Manitoba). Bulletin of Canadian Petroleum Geology, 40, pages 254273. Peters, K. E. (1986): Guidelines for evaluating petroleum source rock using programmed pyrolysis. American Association of Petroleum Geologists, Bulletin, v. 70/3, p. 318-329. Peters, K. E., and Moldowan, J. M. (1993): The Biomarker Guide, Prentic-Hall, Englewood Cliffs, N. J., 363 pages. Peters, K. E., and Moldowan, J. M. (1991): Effects of source, thermal maturity, and biodegradation on the distribution and isomerization of homohpanes in petroleum. Organic Geochemistry, v. 17, no. 1, pages 47-61. Rubinstein, I., Sieskind, O., and Albercht, P. (1975): Rearranged sterenes in a shale: occurrence of simulated formation. Journal of the Chemical Society, Perkin Transactions I, pages 1833-1835. Seifert, W. K., and Moldowan, M. J. (1986): Use of biological markers in petroleum exploration; in R. B. Johns (ed.), Biological Markers in the sedimentary record. Amsterdam: Elsevier, pages 261-290. Seifert, W. K., and Moldowan, J. M. (1981): Paleoreconstruction of biological markers. Geochimica et Cosmochimica Acta, v. 45, pages 783-794. Seifert, W. K., and Moldowan, J. M. (1978): Applications of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochimica et Cosmochimica Acta, v. 42, pages 77-95. Sieskind, O., Joly, G., and Alberecht, P. (1979): Simulation of the geochemical transformation of sterols: superacid effect of clay minera. Geochimica et Cosmochimica Acta, V. 43, pages 1675-1679. Snowdon, L. R. (1978): Organic geochemistry of the Upper Cretaceous/Tertiary delta complexes of the Beaufort Mackenzie sedimentary basin. Geological Survey of Canada, Bulletin 291. Tipper, H.W. and Richards, T.A. (1976): Jurassic stratigraphy and history of north-central British Columbia; Geological Survey of Canada, Bulletin 270, 73 pages. Tissot, B. P., and Welte, D. H. (1978): Petroleum formation and occurrence; Springer-Verlag, Berlin, 538 pages. Volkman, J. K., and Maxwell, J. R. (1986): Acyclic isoprenoids as biological markers; in R. B. Johns (ed.), Biological Mrakers in the Sedimentary Record. Methods in Geochemistry and Geophysics, 24, Amsterdam: Elsevier, pages 1-42. Waples, D. W., and Machihara, T. (1990): Application of sterane and triterpane biomarkers in petroleum exploration. Bulletin of Canadian Petroleum Geology, v. 38, pages 357-380. Welte, D. H., and Waples, D. W. (1973): Uber die Bevorzugung geradzahliger n-Alkane in Sedimentgesteinen. Naturwissenschaften, v. 60, pages 516-517.

KG Osadetz, C Jiang, CA Evenchick, F Ferri

File: sterane-compositional-traits-of-bowser-and-sustut-basin-crude.pdf
Title: STERANE COMPOSITIONAL TRAITS OF BOWSER AND SUSTUT BASIN CRUDE OILS: INDICATIONS FOR THREE EFFECTIVE PETROLEUM SYSTEMS
Author: KG Osadetz, C Jiang, CA Evenchick, F Ferri
Author: K.G. Osadetz1, C. Jiang2 ,C. A. Evenchick3, F. Ferri4, L. D. Stasiuk1, N. S. F. Wilson1 and M. Hayes4
Published: Fri May 28 14:22:07 2004
Pages: 14
File size: 0.47 Mb


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