Alaska North Slope Oil-Rock Correlation Study:

Analysis of North Slope Crude

Edited by Leslie B. Magoon and George E. Claypool


The Alaska North Slope oil-rock correlation study was organized because several oil companies requested oil and rock samples for geochemical analyses that were recovered during the exploration drilling in the National Petroleum Reserve in Alaska (NPRA). Samples acquired with public funds could not be given to private organizations unless some guarantees could be provided that the information acquired from these samples could be made available to the public. For this reason, in August 1981, we sent out over 40 invitations to research laboratories in industry, government, and academia.

Requirements to participate in this study included: (1) participation in an AAPG-sponsored research conference, (2) presentation of the data interpretations at the 1983 Annual AAPG Meeting in Dallas Texas, and (3) contribution of a manuscript, to include all acquired data and interpretations, that would be included in a symposium volume. If a research group wished to participate, they were to write a letter of intent that included their proposed analytical program and a statement indicating that the requirements would be adhered to by their group. Even with these stringent requirements, 30 research groups wished to participate. A balanced cross section of research groups are participating and are as follows: 15 from oil companies, 7 from commercial laboratories, 7 from government laboratories, and 1 university laboratory. These groups are listed in Table 1.

In January 1982, each research group was sent 8 oils and 15 rocks recovered from NPRA drilling and 1 oil from the Prudhoe Bay field. Each group then proceeded to analyze these samples as they indicated in their letter of intent.

  1. Page xi

    The Alaska North Slope oil-rock correlation study was organized because several oil companies requested oil and rock samples for geochemical analyses that were recovered during the exploration drilling in the National Petroleum Reserve in Alaska (NPRA). Samples acquired with public funds could not be given to private organizations unless some guarantees could be provided that the information acquired from these samples could be made available to the public. For this reason, in August 1981, we sent out over 40 invitations to research laboratories in industry, government, and academia.

    Requirements to participate in this study included: (1) participation in an AAPG-sponsored research conference, (2) presentation of the data interpretations at the 1983 Annual AAPG Meeting in Dallas Texas, and (3) contribution of a manuscript, to include all acquired data and interpretations, that would be included in a symposium volume. If a research group wished to participate, they were to write a letter of intent that included their proposed analytical program and a statement indicating that the requirements would be adhered to by their group. Even with these stringent requirements, 30 research groups wished to participate. A balanced cross section of research groups are participating and are as follows: 15 from oil companies, 7 from commercial laboratories, 7 from government laboratories, and 1 university laboratory. These groups are listed in Table 1.

    In January 1982, each research group was sent 8 oils and 15 rocks recovered from NPRA drilling and 1 oil from the Prudhoe Bay field. Each group then proceeded to analyze these samples as they indicated in their letter of intent.

    1. Page 3

      The North Slope petroleum province consists of a Mississippian to earliest Cretaceous continental platform sequence and an overlying Cretaceous to Quaternary successor basin sequence. Both sequences are highly deformed along a foreland fold and thrust belt to the south but are relatively undeformed along a passive margin to the north. Most oil and gas accumulations in this province occur along the east-plunging Barrow arch—a structural axis separating the foreland basin from the passive margin.

      Preliminary evaluation of geologic and geochemical information indicated that four rock units, the Shublik Formation, Kingak Shale, pebble shale unit, and Torok Formation, are likely source rocks for the oils considered in this study. Together, these four rock units represent 140 Ma (Middle Triassic to late Early Cretaceous) of continuous marine deposition totaling more than 20,000 ft of rock in the southern part of the province while they are interrupted by one or more unconformities and range from several hundred to several thousand feet in thickness to the north. These rocks consist of an estimated 90% shale and siltstone and 10% sandstone deposited in a variety of sedimentary environments and at various rates of sedimentation. The Shublik Formation and pebble shale unit represent relatively slow rates of sedimentation in a shelf environment that was periodically anoxic. The Torok Formation and Kingak Shale represent relatively high rates of deposition along a shelf-slope-basin depositional profile in water depths calculated to be 1,000 to 3,000 ft or greater. Source rock characteristics may vary along the profile of deposition.

      Analysis of basin development suggests that North Slope oil and gas formed between the time of sufficient source rock burial (~100 Ma) and the time of tilting of oil-water contacts in oil fields in the Prudhoe Bay region (~40 Ma). Sediment loading and subsidence associated with northeastward basin fill made the Prudhoe Bay region the focus of migrating hydrocarbons. Later, during Tertiary time, regional eastward tilting changed migration directions from the Prudhoe area to the Barrow area.

    2. Page 31

      Petroleum source-rock richness, type, and thermal maturity for four rock units under the Alaskan North Slope are determined from four geochemical analyses (organic-carbon content, Cl5+ hydrocarbon content, elemental analyses, and vitrinite reflectance) of samples from 84 wells and 16 outcrops. Contour maps of organic-carbon content indicate that the average richness for the Shublik Formation, Kingak Shale, pebble shale unit, and Torok Formation is 1.7,1.5, 2.4, and 1.2 wt%, respectively. The organic-carbon content of the Shublik Formation, Kingak Shale, and pebble shale unit increases from west to east and downdip in the Prudhoe Bay area. Elemental analyses of kerogen plotted on a van Krevelen diagram indicate: (1) the Shublik Formation is Type II/III in the west but Type I in the Prudhoe Bay area; (2) the Kingak Shale is Type II/III across the Slope but Type II in the Prudhoe Bay area; (3) the pebble shale unit and Torok Formation both tend toward Type III even though the former is higher in organic-carbon content. Contour maps of vitrinite reflectance drawn on the pebble shale unit unconformity and at the top of the Torok indicate all four units are immature to marginally mature over the Barrow arch and mature to overmature in the Colville trough. Carbon isotope data for the saturated and aromatic fractions of the C15+ hydrocarbons from rock extracts suggest possible source-rock correlations with similar data for four North Slope oil types (Umiat, Simpson, pebble shale, and Kingak), but no obvious correlation of the Barrow-Prudhoe oil type with any of the four source rocks.

    3. Page 49

      A cooperative Alaskan North Slope oil-rock correlation study was undertaken by 30 private, government, and academic institutions from seven countries. The interpretations reported by all participants were based on independent analyses of the same 9 oil samples and 15 rock samples. Reports by 26 of the participants, plus two supporting papers and this summary paper, are included in this volume.

      A variety of analytical techniques were used, but certain analyses (organic carbon, pyrolysis assay, solvent extraction, liquid chromatography, carbon isotopes, gas chromatography, and gas chromatography-mass spectrometry) were common to the analytical programs of a majority of the participating laboratories. The results on the same samples reported by different laboratories were generally comparable, except for certain of the more subjective or method-dependent procedures. Statistical comparison of selected results showed that most labs are internally consistent but that the variability among laboratories is greater than expected.

      Most of the laboratories classified the 9 oils into two oil types, resembling oils from either Prudhoe Bay or Umiat fields, but a number of variations were proposed involving mixing, oil subtypes, and multiple or no oil types. Most of the laboratories conducted a separate geochemical evaluation of the hydrocarbon source potential of the 15 rock samples.

      Correlation among oils and rocks was difficult owing to the apparent non-source rock character of many of the rock samples. Nevertheless, oil-rock correlations were proposed by 21 of the 26 participating laboratories that contributed reports. Seventeen laboratories indicated the Triassic Shublik Formation as the main source of the Prudhoe-type oil, with eight of those calling upon contribution from the Jurassic Kingak Shale. The pebble shale unit was selected as the source of the Umiat-type oil by 14 labs, with seven labs indicating the Cretaceous Torok Formation as a co-source.

    1. Page 85

      Routine geochemical techniques are used to study 15 rock samples and 9 crude oils from the North Slope of Alaska. Results indicate that the Cretaceous pebble shale unit, the Jurassic Kingak, and the Triassic Shublik formations may contain locally effective or expended oil source beds but most often contain primarily gas-generating organic matter. Organic facies variations occur within formations, and many of the rock samples have matured beyond the oil preservation limit. This makes oil-rock correlation difficult, if not impossible. Some of the oils analyzed are biodegraded, making typing and correlation even more difficult. Most oil samples, however, appear to fall into two groups: a Prudhoe Bay type, possibly related to Kingak and Shublik source beds, and an Umiat type, which may have originated in pebble shale or even Torok source beds. The Simpson oils are possibly mixtures of both basic types, and the Dalton oil may be largely but not entirely indigenous to the Lisburne limestone.

      Despite the presence of thick, organic-rich, and thermally mature shales throughout the study area, oil convertibilities are very low and none of the samples analyzed represent a significant source sequence. This may explain the almost complete absence of oil production in the National Petroleum Reserve area. The source or sources of the Prudhoe Bay oil accumulation were not identified by the samples analyzed in this study.

    2. Page 95

      Fifteen rock and 9 oil samples from the North Slope of Alaska were analyzed by Shell Development Company as part of a joint oil-source rock correlation study sponsored by the U.S. Geological Survey (USGS). When sample size and/or organic richness permitted, the following rock and oil analyses were made: C10-C35 saturate hydrocarbons by gas chromatography, C7 composition by gas-liquid chromatography, 3,4, 5-ring naphthene distribution, and carbon isotope abundance measurements by mass spectrometry. Topped oils were analyzed for their gross composition (C15+). Vitrinite reflectance, visual kerogen, total organic carbon, and pyrolysis FID were determined for the rock samples. About one-half the rock samples are either at too high a maturity or are too lean in organic carbon to be viewed with total confidence for oil-source rock correlation interpretations, particularly in light of the limited number of samples in this study. At least two oils are thought to be transformed. Carbon isotopic ratios from oil and rock samples in this study indicate that the Triassic Shublik Formation is the source rock for one or more of the North Slope oils.

    3. Page 123

      A source-to-petroleum correlation between five possible source units and a group of nine oils representative of the Alaskan North Slope has been attempted. A program was pursued to examine the utility of a stable carbon isotopic comparison of source kerogen-kerogen pyrolyzates with the petroleums.

      Of the sediment spot horizons studied, only the Echooka was without discernible source potential. The remaining sediments showed varying degrees of source richness. Unfortunately, half the sediments proved to be unsuitable for kerogen pyrolyzate production, being of advanced thermal maturity and partially to fully spent. This included the majority of the pre-Cretaceous examined. Using the pebble shale-Torok Formation as a datum, maturity increased rapidly to the south, in the direction of the Colville Trough, and less rapidly east to west.

      Three limiting groups of oils were recognized using isotopic and compositional information. These groups were best typified by the Put River, Simpson, and Seabee petroleums, each showing a progressive I3C enrichment. Additionally, other oils showed characteristics suggestive of mixing phenomena. The Put River oil was believed to be representative of the major economic oil type of North Alaska.

      Despite large kerogen-kerogen pyrolyzate differentials, no convincing isotopic match was observed between the pebble shale or Torok formations and either the Put River or Simpson oils. Similarly, the limited Kingak Formation data showed equivocal correlation with the oils and again large kerogen-kerogen pyrolyzate differentials. It is therefore possible that direct correlation of source and petroleum on a whole kerogen 6l3C basis may be fallible. All three Shublik specimens were spent; however, two showed residual kerogen carbon isotopic values consistent with possible correlation with the Prudhoe oil type.

      Although the petroleums provided a good basis for correlation, it was regrettable that additional sediments of appropriate maturity and representative of regional plus stratigraphic facies variation were not available for this study.

    4. Page 139

      Nine oils and 16 core samples were analyzed as part of the Alaskan North Slope Oil-Rock Correlation Study organized by the U.S. Geological Survey (USGS). Carbon isotopic measurements and gas chromatographic analyses were performed on whole oils, oil fractions, and rock extracts. Preliminary screening and more detailed analyses were made on the kerogens to determine their quality as sources of petroleum, maturity, and isotopic composition.

      Oil analyses suggest that there are three different genetic oil types. The Barrow-Prudhoe oils were defined by Magoon and Claypool (1981). They are characterized by their relatively high sulfur content, low API gravity and pristane/phytane ratio, and light carbon isotopic composition. The Simpson-Umiat oil type is identified by its low sulfur content, higher API gravity and pristane/phytane ratio, heavier carbon isotopic composition, and anomalously high concentrations of cyclic and aromatic light hydrocarbons. The third type is not an oil but represents compounds that oils, primarily Simpson-Umiat type, have extracted from immature sediments during migration. This third component was called Torok-Nanushuk and is noted by an anomalous light hydrocarbon distribution and relatively heavy carbon isotopic composition.

      It is possible to describe each of the oil samples in terms of mixing these three oil types in varying proportions. The Put River D-3 oil is typical of the petroleum found at the Prudhoe Bay field and contains mainly Barrow-Prudhoe type mixed with a small Simpson-Umiat component. The South Barrow No. 19 is comprised of pure Barrow-Prudhoe oil. It has lost a significant proportion of its light hydrocarbons, and the influence of a Simpson-Umiat component may have been obscured. The Fish Creek No. 1 oil is severely biodegraded but can be classified as Barrow-Prudhoe from its isotopic composition. Maturity parameters indicate that the Dalton No. 1 oil is an immature equivalent of the Barrow-Prudhoe oils.

      The oil from the Simpson Core Test is considered to best represent the Simpson-Umiat genetic type although it too lost all light hydrocarbons. The severely degraded Cape Simpson oil is believed to be a mixture of Simpson-Umiat and Barrow-Prudhoe. Umiat No. 4 oil is representative of the Simpson-Umiat type with a large component extracted from immature, Cretaceous reservoir rocks. Various parameters suggest that the Seabee condensate is of similar maturity to the Umiat oils and probably is the result of phase separation. It too has a substantial quantity of the extracted Torok-Nanushuk component. Finally, the South Barrow No. 20 oil is a slightly biodegraded mixture of all three oil types, but it is not possible to define the proportions clearly.

      From the samples used in this study, the pebble shale samples are considered to have the highest potential for oil generation because they are comprised predominantly of amorphous or herbaceous organic matter. The Torok samples contain mainly coaly fragments and are considered gas prone. One sample of Kingak contains an abundance of herbaceous organic material. Other examples of Kingak, Shublik, and Fortress Mountain units are overmature and contaminated with nonindigenous hydrocarbons.

      Because most of the extracts were either contaminated or generated from immature samples, rock/oil correlations were based on the carbon isotopic composition of kerogen isolates, oils, and oil fractions. From the suite of samples used in this study, only the Shublik could serve as a source for any of the oils. Evidence suggests, but does not conclusively prove, that the Shublik is indeed a major source of the Barrow-Prudhoe oils. There are sufficient data to show extensive lateral and vertical variation in organic facies in die pebble shale and Kingak unit and that these could also be major source contributors to producing oil fields. The pebble shale unit is believed to be the source of the Simpson-Umiat oils.

    1. Page 165

      Analysis of 9 Alaskan North Slope oils, using 12 correlation parameters, generally confirms the type classifications and geographic distributions reported by Magoon and Claypool (1981). Five of the 9 oil samples were characterized as Barrow-Prudhoe type oils and 4 as Simpson-Umiat type. Two Barrow-Prudhoe oils and one Simpson-Umiat oil were severely biodegraded; their characterizations were less firm than those for the undegraded oils. Three Barrow-Prudhoe oils and one Simpson-Umiat oil showed significant modifications that could be explained by lateral variations in the source beds. The differences were deemed sufficient to permit subtype classifications for these 4 oils. In addition to being a subtype, one of the Umiat oils appeared to be the separated light fraction of a full-range Umiat type oil.

      Organic carbon analysis of 15 core samples from various formations showed the Triassic Shublik, Jurassic Kingak, Neocomian pebble shale, and Cretaceous Torok formations to be organic rich. Six samples were too mature for use in oil-rock correlation. Based primarily on gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) data, the primary sources for the Barrow-Prudhoe type oil are believed to be shales within the Shublik and Kingak formations. Hydrous pyrolysis of immature samples from Cretaceous horizons confirmed that the immaturity of some of these samples was not a factor in their poor correlation with Barrow-Prudhoe type oil. Oil-rock correlation data for the Simpson-Umiat oils suggest that the Neocomian pebble shale is the primary source for this oil type.

    2. Page 185

      Organic geochemical analyses of 9 crude oils and 15 core samples from the North Slope of Alaska were performed in conjunction with an interlaboratory oil-source rock correlation study initiated by the U.S. Geological Survey (USGS). Correlation parameters employed included data derived from capillary gas chromatography of Cl5+ aliphatic hydrocarbons; GC-MS-DS analysis for sterane, diterpane, and triterpane biomarkers; and stable carbon isotopic analysis of aliphatic, aromatic, NSO, asphaltene, and kerogen fractions. Of the 9 oils analyzed, the 5 oils from Point Barrow, Dalton, Fish Creek, and Prudhoe Bay areas (the Prudhoe group of oils) represent one distinct family. Two of these oils are biodegraded. The remaining 4 oils from Umiat and Cape Simpson areas form a less well-defined family. This latter group of oils has a range in maturity (e.g., biomarker parameters show Cape Simpson oils to be less mature than Umiat) and includes two biodegraded oils and one condensate.

      Six of the 15 possible source rocks are thermally overmature with respect to oil generation (R0 > 1.6%) and have low quantities of extractable C15+ hydrocarbons. Thus, these samples did not contain sufficient geochemical information for a reliable source rock-oil correlation study. Samples that can be used for correlation purposes represent the following formations: pebble shale unit (4 samples), Torok (3 samples), Kingak and Shublik (1 sample each). Stable carbon isotope compositions, isoprenoid ratios, and terpane distributions suggest that the extracts from the pebble shale show the highest degree of correlation with the Umiat-Cape Simpson oils. Comparison of the Umiat-Cape Simpson oils with the Torok and Kingak samples shows good correlation when some geochemical parameters are considered and poor correlation when others are considered. None of the above rock samples correlate well with the Prudhoe group of oils. The Shublik Formation sample is only marginally reliable for correlation because of its high thermal maturity: R0 =1.6%. However, comparison of several geochemical parameters (in particular carbon isotope ratios, isoprenoid ratios, and diterpane distributions) suggest that this formation may have sourced the Prudhoe group of oils. A better correlation study would be possible if less mature Shublik samples were available because sterane and triterpane biomarkers would have been better preserved. Because of the small number of samples analyzed, positive or negative correlations must be tentatively regarded since organic inhomogeneities within stratigraphic units can result in variations in geochemical parameters. This is especially evident in the isotopic and biomarker variations observed in the pebble shale samples.

    3. Page 203

      Chemical analyses of 9 oils and organic matter in 15 rocks from the North Slope suggest that the oils comprise at least two chemically distinct types. Type A oils, including oils from Prudhoe Bay to Point Barrow, can be distinguished from Type B oils, including oils from Umiat Field to Point Barrow, by distinctive 5 a, 20R sterane carbon number distributions, sulfur, vanadium, and nickel concentration, V/(V + Ni) ratios, hopane and aromatic hydrocarbon distributions, and carbon isotope ratios.

      Of the 9 oils of this study, at least 4 are degraded. Of the 15 rocks provided by the U.S. Geological Survey for this study, 7 are post-mature. These sample problems limited the results of this work. Those samples having R0 values below 1.4% are all Lower Cretaceous except one, which is Jurassic. Although 12 samples have TOC values greater than 1.0%, only three samples (two pebble shale and one Kingak) contain indigenous extract yields above 500 ppm. Of these, only one pebble shale sample and the Kingak sample have over 200 ppm indigenous extractable hydrocarbons.

      None of the 15 rocks examined appear to be the single source for any of the 9 oils, although several similarities between the oils and the organic matter in some of the rocks do exist, particularly for Type B oils. Analysis of the molecular composition of the oils suggests (particularly for the Type A oils) that they may have been sourced by more organic fades. If this is the case, then no single source rock may correspond chemically to the oils studied. Based upon the correlations that are possible on this limited number of samples, Lower Cretaceous (particularly pebble shale), Jurassic, and Triassic rocks are all potential sources. In addition, the probability of lateral changes in organic fades requires that other geographically distinct potential sources also be considered as possible contributors.

      In summary, the very limited number of samples available for study from within the oil window does not allow a definitive conclusion as to the source for these oils. However, it is probable that the pebble shale, Kingak, and Shublik formations have all made contributions in various geographic areas.

    4. Page 233

      Nine oil and 15 rock samples from the Alaskan North Slope have been analyzed for the purpose of determining which, if any, of the sampled formations sourced the sampled oils and determining the extent of agreement of results among the participating laboratories.

      Results from chromatography of the gasoline-range hydrocarbon fractions of the oil samples did not lead to meaningful correlations because most of the oils contained insufficient gasoline-range hydrocarbons. Hydrocarbon distributions derived from C15+ chromatography of the oils were not useful for correlation owing to the significant degree of biodegradation suffered by some samples. Stable carbon isotope compositions and sterane and triterpane distributions suggest that the oils represent at least two families and appear to provide the best means for correlating the oils to the rocks.

    5. Page 243

      Fifteen rock samples and 9 crude oils from 16 wells of the North Slope of Alaska have been investigated by organic geochemical techniques in order to assess the source rock potential for oil in Cretaceous formations and to find the tentative origin of oils accumulated in reservoirs ranging from Mississippian-Pennsylvanian to Cenomanian-Albian.

      Vitrinite reflectance as well as organic geochemical data (chloroform extract, gas analyses, Tmax) show that the older formations analyzed (Kingak, Shublik, and Sadlerochit) are generally too mature to have generated the oils. Among the rock samples examined, only sediments from the pebble shale unit, the Torok, and the Kingak formations have maturities in agreement with oil genesis. In the two main formations, namely pebble shale and Torok, a maturity suite ranging from immature to mature sediments has been characterized.

      Quality of the kerogen, assessed by Rock-Eval pyrolysis, has been found to be at least fair in the Torok Formation and pebble shale unit. Surprisingly, hydrogen indexes (HI) often do not exceed 80. The genetic potential, measured by Sx + S2 from Rock-Eval pyrolysis data, peaks at 4 mg/g of rock but is generally within the 0.5 to 2.5 range. Cretaceous formations of North Slope Alaska are at least moderate source rocks (Sx + S2 = 2-6 mg/g rock). Basic organic geochemistry data (Rock-Eval, chloroform extract, and gas yield) demonstrated that the pebble shale unit is a better (fair to good) source rock than the Torok Formation (poor source rock).

      Detailed organic geochemistry by gas chromatography and computerized gas chromatography-mass spectrometer have allowed a maturation assessment of kerogen by molecular measurements on steranes and terpanes. Maturities of dispersed organic matter, assessed by molecular parameters, have been compared to maturities based on vitrinite reflectance measurement data. The vitrinite reflectance scale generally matched the organic geochemical classification; however, some discrepancies have been observed in Torok samples within the 0.5 to 0.6% R0 range.

      Maturity of oils, assessed by molecular measurements on steranes and terpanes, is variable. Comparison of maturities of oils to maturities of indigenous chloroform extracts provides a tool to approach migration of hydrocarbons. The moderate maturity of Umiat oil and Seabee condensate suggests a migration of limited extent which is, in addition, in good agreement with a Torok origin. The striking discrepancy between the maturity of pebble shale sediments in the Walakpa No. 1 well and the maturity of South Barrow oils indicates that the oils accumulated in pebble shale and Sag River sandstones originate from much deeper source rocks.

      Geochemical characteristics of alkanes and aromatics from the Torok and the pebble shale source rocks are closely related. Oil-to-source rock correlations are, in some cases, difficult to establish because crude oil properties obviously reflect molecular changes through migration (amount of alkanes, of tricyclic and tetracyclic terpanes, of ββ steranes, etc.). A combined review of isotopic, molecular, and geological data has, however, allowed the finalizing of a diagnosis regarding the origin of each crude oil including those that have been recognized as biodegraded oils.

    6. Page 281

      Nine oils and 15 rock samples from across the National Petroleum Reserve in Alaska (NPRA) were analyzed by standard geochemical techniques in order to characterize the Alaskan North Slope oils and to attempt to determine the origin of these oils through direct crude oil-source rock correlation.

      Results of this study indicate four genetic oil types. One major oil type (Type I) includes the oils from Prudhoe Bay, South Barrow, and Fish Creek. These oils are reservoired in rocks of the Sadlerochit Group, pebble shale units and Sag River Sandstone, and Nanushuk Group, respectively (ranging in age from Permian to Cretaceous). Type I oils have the following geochemical characteristics: high sulfur content (0.9-1.8%), C19/C23 tricyclic terpane ratios 0.7 to 0.13, pristane/phytane ratios 1.3 to 1.5, farnesane/C^ isoprenoid ratios 0.9 to 1.0, and stable carbon isotope ratios for the saturated hydrocarbons from δ 13C —28.4 to —29.4 and for the aromatic hydrocarbons between δl3C —28.7 and —29.3. The oils of Type I contain biomarkers that are similar in distribution to the extractable organic matter from the Kingak Shale and Shublik and Echooka formations rocks. However, a large (4-5 per mil) difference in stable carbon isotopes exists between the aromatic hydrocarbon fractions from the oils and the Kingak and Echooka rocks. This difference is too large to result from migration alone and suggests that the Kingak and Echooka rocks are not the major source of Type I oils regardless of some genetic similarities in organic matter. The best overall geochemical correlation exists between the oils of Type I and rocks of the Shublik.

      A second major North Slope oil type (Type II) includes oils from the Simpson and Umiat fields. The Simpson oils were encountered in shallow core tests and as a seep in a seismic-test hole. The Umiat oil is from Cretaceous reservoirs of the Nanushuk Group. The Simpson-Umiat Type II oils have the following geochemical characteristics: low sulfur <0.2%, C19/C23 tricyclic terpane ratios >1.2, pristane/phytane ratios 2.1 to 2.2, farnesane/Cl6 isoprenoid ratios 0.6 to 0.7, and 6t5C ratios for the saturated hydrocarbons between —28.1 and —28.7 and for the aromatic fraction between —26.7 and —27.7. These Type II oils have many geochemical characteristics similar to the extractable organic matter from the Torok Formation and the pebble shale unit. However, the poor source rock quality of the organic matter in the Torok suggests that these rocks are poor oil sources but may have generated some gas.

      A third genetic oil type (Type HI) is represented by the oil-show in the Dalton test well obtained from rocks of the Lisburne Group of Mississippian to Permian age. The Dalton oil is geochemically similar in many respects to Type I oils, but dissimilarities in sulfur, hydrocarbon, and asphaltic contents indicate probable genetic source differences. Geochemically, the Dalton oil resembles oils derived from carbonate rocks. Organic-rich carbonate units within the Lisburne are suspected as the source of the Dalton oil.

      A fourth oil type (Type IV) is represented by the condensate from the Seabee well reservoired in the Torok Formation of Cretaceous age. Geochemical comparison data on this sample are minimal (carbon isotopes only) because of its narrow boiling range. The carbon isotopes for the hydrocarbons from the Seabee condensate are most like carbon isotopes for hydrocarbons from the Torok and the pebble shale unit.

    7. Page 305

      As a tool in petroleum exploration, oil-to-oil correlation offers a means of detecting the migration of oil within or between formations. Oil-to-source rock correlations identify specific source beds that generated the petroleum. In order to accomplish these tasks, a variety of techniques have been used, including gas chromatographic analysis of various hydrocarbon boiling point ranges, mass spectral group type analysis, carbon isotope ratios, and gas chromatographic-mass spectral analysis of biological marker hydrocarbons.

      This report deals with a comparison of the results of these techniques in an attempt to correlate 9 oils and 15 source rock samples from five different formations from the National Petroleum Reserve in Alaska. Oils were compared to each other using the above-mentioned chemical parameters in an attempt to subdivide them into groups. Source rock samples were first evaluated to determine if they were capable of generating oil, prior to their comparison to the crude oils. Although individual data types allowed some differentiation of the oils, the data yielded no consistent grouping of oils or pairing of oils with source rocks. Additional work will be necessary in order to determine the source of the North Slope oils.

    8. Page 319

      Fifteen sediments, 8 oils, and a condensate from the North Slope of Alaska have been investigated with a range of geochemical techniques. In addition to organic carbon determination and Rock-Eval pyrolysis, the sedimentary rock extracts, the oils, and the condensate have been extensively examined with computerized gas chromatography-mass spectrometry (GC-MS), including the application of high-resolution (- 2,500) mass spectrometry. In this way, information on the distributions of aliphatic and aromatic biological marker hydrocarbons and of phenanthrene, methylphenanthrene, and dimethylphenanthrene isomers was aquired. Additional information on bulk composition was derived from the GC-MS analysis of the aromatic hydrocarbon fractions of the oils and the condensate with low electron energy (12 eV) mass spectrometry. These results were integrated with carbon, hydrogen, and sulfur isotope data.

      The results could be best explained by a model, which envisages that most of the oil presently accumulated in the North Slope of Alaska derived from the Jurassic Kingak Shale. In the Coastal Plain, this shale presently lies within the zone of oil generation, but in the Northern Foothills of the Brooks Range it is overmature and hence below it. In the model, most of the oil from this shale has migrated laterally and northward into the Sadlerochit Sandstone (Permian) and Lisburne Limestone (Mississippian) reservoirs around the Barrow arch. Minor contributions from the units overlying and underlying the Kingak Shale are also predicted (Neocomian pebble shale unit and Permo-Triassic Shublik Formation). Only when the deeper reservoirs were full did oil, following the same migration pathway, migrate further upward into younger and shallower reservoirs. Thus, the increase in oil maturity with decreasing reservoir depth is explained. This maturity trend was apparent from the oils’ sulfur contents, carbon and sulfur isotope compositions, bulk compositions (percent hydrocarbons, percent asphaltenes), and the distributions of their methylphenanthrene isomers (methylphenanthrene index). The most mature oils are also the shallowest oils and may therefore have migrated the longest distance. Any increase in the relative amounts of lighter and less polar components could either be the result of increased maturation, increased migration, or both.

      Not only have the distributions of biological marker compounds been considered, but also their absolute concentrations (Mg/g Corg, ppm in hydrocarbon fractions) by the addition of a known amount of an internal standard. This has shown that die concentrations of these components in the Ci5+ hydrocarbon fractions of sedimentary rock extracts decrease sharply with the onset of petroleum generation and that in immature rocks these are an order of magnitude greater than the concentrations of the same components in the hydrocarbon fractions of the oils. That the most unstable components (fastest decrease in concentration) considered (monoaromatic steroid hydrocarbons) suggest a correlation of the oils with immature sediments, while more stable components of the oils (e.g., the hopanes) match better with those of shales well within the zone of petroleum generation, implies that expulsion of components from shales occurs over a wide range of maturities. This in turn suggests oil accumulations could be averaged mixtures of organic fluids representing a range of maturities. The decreases in concentrations of the biological markers with the onset of petroleum generation mean their distributions could exaggerate the contributions of immature sources to a given oil pool.

      Such a case is thought to exist in the North Slope of Alaska. Although the shallower oils are more mature on a number of measurements, the biological markers suggest the reverse. If the shallower oils were more mature, they should have lower concentrations of biological markers. The addition of small amounts of immature oil, from either the pebble shale unit or Torok Formation in the region where the oils have accumulated, may have had a major influence on the biological marker patterns without contributing significantly to the vast bulk of the oils.

      Magoon and Claypool (1981) proposed a division of the North Slope oils into two families: Simpson-Umiat and Barrow-Prudhoe. This was mainly based on carbon and sulfur isotopic compositions and sulfur content. It now appears that both families came from similar sources but that the Simpson-Umiat oils are products of a later generation than the Barrow-Prudhoe oils. The Simpson-Umiat oils are therefore more mature and have migrated further into shallower reservoirs, where the addition of small amounts of immature oils from different sources is also more likely. The condensate derived from highly mature sources. It was found in the Northern Foothills, and the most favored source is the deep Kingak Shale (>4,000 m) in this region.

    9. Page 379

      The major objective of the U.S. Geological Survey-sponsored cooperative North Slope Alaska oil-rock correlation study was to establish, using a diversity of geochemical techniques, which formation(s) served as source(s) of the crude oils from the North Slope of Alaska. A second objective was to allow intercomparison and calibration of geochemical technologies among the university, petroleum industry, government, and private geochemical laboratories participating in the study.

      Two major groups of oils were identified. Group I includes oils from the Umiat and Simpson areas. These oils are generally low in sulfur, nitrogen, and asphaltenes, have medium gravities, have nickel/vanadium ratios ≥ and pristane/phytane ratios >1.5, and are high in diasteranes. Group II includes oils from the Barrow, Fish Creek, and Prudhoe Bay areas. These oils are generally high in sulfur, nitrogen, and asphaltenes, have low gravities, have nickel/vanadium ratios <1 and pristane/phytane ratios <1.5, and are moderate in diasteranes.

      Both groups of oils are of mixed but predominantly marine source input and are quite mature thermally. The triterpane patterns of the Group I oils contain certain C 30 pentacyclics, are low in tricyclics, and have hopane > norhopane. The Group II oils are devoid of the C 30 pentacyclics found in Group I oils, high in tricyclics, and have norhopane > hopane.

      The Kingak Shale appears to be the principal source of the Group I oils. However, the Group I, Seabee No. 1 condensate is believed to originate in the Torok Formation. The Group II oils have their principal source in the Shublik Formation but have to varying degrees the Kingak as a co-source. The Barrow pebble shale sandstone oil has major Kingak input; the Barrow Sag River Sandstone, Fish Creek, and Prudhoe Bay oils have minor Kingak input. The Dalton area oil produced from the Lisburne Group, included in Group II because of its overall similarity to these oils, is believed to be indigenous to the Lisburne.

    10. Page 403

      Using inspection properties alone it was possible to divide 7 of the 9 oil samples into one of the two groups defined by Magoon and Claypool. The Umiat No. 4, Simpson Core Test, and the Cape Simpson Area oils fall in the Simpson-Umiat group while the Put River, Dalton No. 1, Fish Creek No. 1, and the South Barrow No. 19 oils fall in the Barrow-Prudhoe group. The Barrow No. 20 oil is considered to be a mixture of both types, whereas the Seabee No. 1 condensate is anomalous and may represent an entirely different group.

      Gas chromatographic-mass spectrometry (GC-MS) and stable carbon isotope ratios of the whole oil and the oil fractions show concurrence with the above grouping but suggest there may be more of a mixing or continuum of properties than is implied by the concept of grouping.

      Three of the oils, Dalton, Fish Creek, and the Cape Simpson area, have undergone varying degrees of biodegradation and have lost their normal alkanes. The Simpson Core Test sample shows evidence of heavy biodegradation (presence of normethyl hopanes), but n-alkanes are still present. This suggests an earlier period of biodegradation followed by subsequent topping up with normal oil. A similar process is suggested to explain the composition of the Dalton oil.

      The majority of the sediment samples had good organic carbon contents (TOC > 1.0 wt%) but poor hydrocarbon source potentials. This and the general high maturity suggest that oil or gas generation has already occurred in most of the sediments examined, the shallower Torok and pebble shale samples excepted. The presence of inert carbon and the humic nature of most of the samples may be partially responsible for the low potentials observed.

      The very small amounts of extracted hydrocarbons recovered from the Kingak and Torok in Seabee No. 1 and the Shublik and Echooka formations in Inigok No. 1 were similar in nature and considered to be nonindigenous, probably contaminants.

    11. Page 443

      The oils differ considerably in their content of C15_ compounds as well as compound class composition. These variations can be attributed to secondary processes such as biodegradation and thermal alteration. Umiat oil can be separated from all other oils by its lack of tricyclic triterpanes. All other oils revealed sterane and triterpane fingerprint patterns that are similar in principal characteristics. Stable isotope ratios clearly separate the Umiat oil from all others, suggesting that the Barrow-Prudhoe oils form one genetic group.

      The rocks in the NPRA are isotopically and by their biomarker patterns significantly different from the Barrow-Prudhoe type oils but could have sourced the Umiat type. The general similarity of most NPRA oils with the Prudhoe oils suggests that the NPRA oils migrated from the Prudhoe Bay area along the Barrow arch.

    12. Page 459

      Samples of 6 oils, a condensate, 15 sediments from NPRA, and one Prudhoe Bay oil (Put River D-3) were evaluated to distinguish oil types, determine sources of the oils, and characterize the samples geochemically. Carbon isotope ratios were determined for all samples and fractions. Two distinctive groups of oils were recognized, with evident mixing between them. The Prudhoe Bay oil was distinguishable from these and was assigned to a third group. The D-3 oil showed the same unique features as other Prudhoe oils. The Dalton, Fish Creek, and South Barrow No. 19 oils comprise the most homogeneous group among the NPRA oils. This group is characterized by clustering of all carbon isotopic values (range 0.4 per mil, PDB), an average of -29.7 for aromatics, and of -29.5 for saturates. Aromatics that are isotopically lighter than saturates represent a rare occurrence in petroleums. The second group is represented by two oils from the Cape Simpson area, with average delta values of -28.1 for aromatics and -29.0 for saturates. The overall range of values increases to 1.2 per mil, with aromatics isotopically heavier than saturates. Published data suggest that the Cape Simpson oils are not an end-member group but are a mixture of the Fish Creek and an “Umiat” oil, which occurs in southeastern NPRA. The South Barrow No. 20 oil is also interpreted as a mixture of the two types.

      Source rock evaluation consisted of Rock-Eval, vitrinite reflectance, pyrolysis/light hydrocarbon gas chromatography (PGC), Soxhlet extraction, liquid chromatography, and carbon isotope ratio determinations on kerogen and extract fractions. The pebble shale had the best hydrocarbon generation potential based on Rock-Eval, but the hydrogen index values were low (less than 200), considering maturation level. This suggests that the pebble shale is not a prolific oil source in the area sampled. However, PGC data indicate that the Kingak is a major source rock.

      The high maturity of the Shublik, Echooka, and two of three Kingak samples, shown by vitrinite reflectance (Rc) values greater than 1.44%, prevents adequate assessment by Rock-Eval. Pyrolysis-GC indicated that the Shublik and Kingak originally contained oil-prone kerogen, whereas the Torok and Echooka did not. The pebble shale probably contains a mixture of kerogen types. Vitrinite reflectance indicates that the Torok Formation and pebble shale are approaching optimum maturity for oil generation in most locations, as is the Kingak in one well.

      Carbon isotope ratios of kerogens and rock extracts suggest a Shublik source for Fish Creek group oils. Kerogen from two Shublik samples has a mean delta value of -29.1, similar to the oil. Kerogens from all other formations range from -23.8 to -25.5, insufficiently similar to any of the oils to represent feasible sources. Thus, the actual source of the Cape Simpson oils is not represented among the sediment samples.

    1. Page 481

      Nine crude oil and 15 core samples from the North Slope were analyzed by geochemical methods to identify relationships between the oils and their source rocks. Based on bulk geochemical parameters, the crude oils can be separated into the two groups reported by Magoon and Claypool (1981): a Barrow-Prudhoe group and a Simpson-Umiat group. One exception to this classification is an oil produced from the Cretaceous Kongakut Formation at South Barrow that has some features of each type and may be a mixture of the two. Standard geochemical techniques were used to evaluate the oil-source potential of rocks from Cretaceous to Permian age. These data identify the pebble shale unit of the Cretaceous Kongakut Formation, the Jurassic Kingak Shale, and the Triassic Shublik Formation as having the most favorable oil-source potential. Gas chromatography-mass spectrometry (GC-MS) of steranes and terpanes was used to correlate oils with potential source rocks. Based on these results, both the pebble shale unit and the Cretaceous Torok Formation are suggested as possible sources of the oil from the Umiat Basin and Simpson shelf. A positive correlation could not be made for source rocks of the Barrow-Prudhoe oils, although based on pristane/phytane ratio the Triassic Shublik Formation is suggested as a possible source.

    2. Page 509

      Nine crude oil/condensate samples and 16 rock samples have been analyzed from a number of formations from the North Slope of Alaska. Medium pressure liquid chromatography (MPLC) was applied to achieve separation of the oils and extracts into saturated and aromatic hydrocarbons and NSO compounds. The aromatic fractions were then separated further into four subtractions by high pressure liquid chromatography (HPLC).

      Capillary gas chromatography was performed on the alkane and aromatic fractions to obtain general fingerprints and values such as CPI, pristane/phytane and n-C17/pristane.

      Major tri- and tetra-aromatic components, including compounds containing organic sulfur, were identified from retention data and by using dual detector gas chromatograph with flame ionization detector (FID) and sulfur sensitive detector (FPD) after gas chromatographic separation of one of the subtractions on a 100-m capillary column.

      Capillary gas chromatography-mass spectrometry (GC-MS) was performed on the alkane and aromatic fractions. Triterpane (m/z 191,205) and regular and rearranged steranes (m/z 217,218,259) were monitored in the alkane fractions. Stereochemical ratios such as %20S- and %14β(H), 17β(H)-steranes and %22S-hopanes and Tm/Ts (17α(H)-trisnorhopane/18α(H))-trisnorneohopane) together with the more source specific C29/C30 17α(H)-hopane ratio, were calculated from integration of the mass chromatograms. The aromatic fractions were monitored for the mono- and triaromatic steranes (m/z 239,253,231, respectively) and the methylated phenanthrenes (m/z 178 nl4) and dibenzothiophenes (m/z 184 nl4). The whole oils have also, prior to any separation or dilution, been analyzed by capillary gas chromatography, with temperature programming from —50°C to 270° C. In addition, aromatic hydrocarbon fingerprints were also employed as an aid in correlation of oils and source rocks.

      All except one of the sediments seem to be quite mature with a vitrinite reflectance within or beyond the oil producing zone (Ro > 0.5) and a CPI for most of the samples from 0.9 to 1.1. The two shallowest samples appeared to be the least mature, both from vitrinite reflectance, alkane/isoprenoid, sterane, and hopane biological marker ratios.

      The crude oils appear, from the gas chromatograms of the alkane fraction, to be quite dissimilar. Three of the oils contained hardly any n-alkanes, probably owing to biodegradation, and could not be correlated to the other oils or to the possible source rocks on the basis of alkane/isoprenoid ratios. Of the “non-biodegraded” crude oils, one was a condensate with only a trace of n-alkanes above C17-C18. The sterane/triterpane maturation parameters showed only minor differences between the oil samples. The source-specific parameters could, however, be applied to subdivide the oil samples into two or three different groups. Aromatic hydrocarbon distributions also allowed a subdivision into two or three different groups.

      Further correlation between the crude oils and the source rocks was based on the combined use of molecular and bulk parameters together with the geological information and suggests possible mixed source for certain oils. No positive correlation on several different parameters could be made between the oils and any of the source rocks.

    3. Page 557

      Seven of 9 oils for this study have experienced varying degrees of alteration. As a result, whole oil gas chromatography and other conventional analyses show little resemblance in composition. Gas chromatography-mass spectrometric analysis of sterane and terpane components of the oil and rock samples permitted the determination of genetic relationship among the rocks and oils.

      The North Slope oils can be divided into three groups by the sterane compositions and the sterane and terpane distribution patterns. Group 1 oils, which are least mature and generally referred to as Simpson-Umiat type (Magoon and Claypool, 1981) probably originated from the Lower Cretaceous pebble shale unit. Group 2 oils, reservoired in a wide range of geological formations ranging from Permo-Triassic to Cretaceous, were derived from either the Kingak Shale, the Shublik Formation, or possibly the Sadlerochit Group. Group 3, the most mature oil type, was probably sourced from the deeper Lisburne Group, from which no rock sample was included in this study.

    4. Page 571

      A total of 9 oils and 15 rock samples from the National Petroleum Reserve, Alaska (NPRA), have been characterized and analyzed using a variety of organic geochemical techniques by a number of laboratories involved in a multidisciplinary study of these samples. Results presented in this paper will concentrate on two aspects of the study. The first is the determination of biological marker distributions (i.e., steranes and triterpanes) in both the oils and the rock samples using the technique of gas chromatography-mass spectrometry; the second is characterization of the organic matter in rock samples using microscale pyrolysis techniques combined with gas chromatography and gas chromatography-mass spectrometry.

      Sterane and triterpane fingerprints of the 9 oils permit them to be divided into two main groups. The distributions of steranes and triterpanes in the core samples show that some of these samples can be eliminated as source rocks on a maturity basis. Other cores have distributions of steranes and triterpanes that are sufficiently different from the oils to eliminate them as possible sources for the oils examined in this study. Detailed characterization of organic-rich rocks, or source rocks, by pyrolysis-gas chromatography and pyrolysis-gas chromatography-mass spectrometry allowed distinctions to be made between the rocks on the basis of their source material and, in certain cases, their relative maturities.

    5. Page 593

      For purposes of oil-rock correlations, we compared distributions and ratios of the following aliphatic hydrocarbons in 15 rock and 9 oil samples from the North Slope of Alaska: n-alkanes (C14 to C28), pristane and phytane, tricyclic and pentacyclic terpanes, and steranes and diasteranes. From the results we classified most of the oil samples into two groups, called Prudhoe type and Umiat type. Rock samples were also classified into two groups. Because of similarities in molecular marker patterns, we believe that the hydrocarbons in the two groups of rocks are genetically related to the two oil types: The Shublik Formation and Kingak Shale are among the likely sources for Prudhoe-type oils, and the pebble shale unit and the Torok Formation are the most likely sources for Umiat-type oils.

    1. Page 621

      Nine oils from 8 wells and one seismic shot hole and 15 solvent extracts from 8 wells in the Prudhoe Bay and North Slope region were analyzed using the following techniques: (1) column chromatographic fractionation; (2) saturate fraction gas chromatography; (3) gasoline range stripping of 25 compounds (oils only); (4) aromatic family analysis (oils and selected extracts); (5) kerogen isolation and CHN analysis (cores only); (6) total organic carbon (cores only).

      Normal oil-source correlation procedures for this laboratory consist of (1) normal-alkane and acyclic isoprenoid pattern interpretation and matching; (2) cluster and factor analysis of 26 gasoline range compounds; and (3) cluster and factor analysis of 16 classes of compounds in the aromatic fraction. Ancillary data such as kerogen type and quantity, level of thermal alteration, and yield and distribution of saturate, aromatic, NSO, and asphaltene classes are used to make volumetric estimates and to confirm the nature and possible distribution of the source rocks. For the North Slope core samples, low extract yields and moderate to high levels of contamination of the cores precluded useful interpretation of much of the saturate fraction GC data, and the dry storage of the cores precluded obtaining the gasoline range fraction by helium stripping. Biodegradation of two of the oil samples resulted in removal of most of the n-alkanes and acyclic isoprenoids.

      The Shublik Formation sample from Ikpikpuk No. 1 at a reflectance level of 1.5% and the Kingak Shale sample from North Kalikpik No. 1 at a reflectance of 0.8% (hydrocarbon yields of 46.4 and 68.4 mg/g, respectively) were the only two samples that exceeded the 30 mg/g cutoff for a potential petroleum source rock. The Kingak sample had only about 1% total organic carbon and is thus probably quantitatively insignificant as a source. Three of four other samples that had reflectance levels above 1.5% still have organic carbon contents of 1.8 to 4.5% and could have acted as sources as they passed through the optimum oil generation stage. Other samples were very low (<9 mg/g), and the extract properties have been essentially masked by contamination.

    2. Page 639

      A number of standard and several unique analytical techniques were applied to the North Slope intercalibration correlation study. Although carbon isotopic compositions, compound type classifications, and n-alkane and aromatic distributions by fused silica, capillary gas chromatography were determined, a unique total scanning fluorescence technique is emphasized. Fluorescence fingerprints for both oils and hexane Soxhlet extracts of shale were determined using a software package developed in house and a Perkin Elmer 650-40 computer-aided fluorometer. Fluorescence data are presented in three dimensions and/or as a contour plot of intensity. Fluorescence patterns are compared on a point-to-point basis to obtain a similarity index (i.e., S.I. = 1.00 for identical spectra and S.I. ≤0.000 for completely dissimilar spectra). Oil-oil correlation parameters were unable to clearly delineate separate groups within the 9 oils. On the contrary, most parameters indicated a near continuum in oil compositions. Oil 001 was typical of Prudhoe Bay oil and oil 024 was typical of Simpson-Umiat oils and were chosen as potential end members in this suite of oils based on isotopic and fluorescence analyses. Oils 002,003,004,005,007, and 008 appeared to be of intermediate or altered compositions, and oil 006 had no apparent generic relationship to the other oils. Source rock-oil correlations suggested that the Kingak and Shublik shales were sources of Prudhoe Bay type oils and that the pebble shale was the main source of Simpson-Umiat oils. Intermediate oils represent various admixtures of sources from these three units. The Torok was interpreted as being infiltrated with low-level nonindigenous bitumen sourced in deeper formations and was not a source bed. Both oils and source rocks were typed reasonably independently of maturation and degradation effects by the fluorescence technique. This technique shows promise as a correlation tool.

    3. Page 651

      Gas chromatography-mass spectrometry (GC-MS) was used to correlate sediment extracts with oils from the NPRA area of Alaska. Based on the distribution of triterpanes, there are two oil types. The first group of oils is similar to the Shublik and Kingak Shale extracts. The other is similar to the pebble shale extract.

    4. Page 663

      New computer techniques recently developed in our laboratory and applied to the correlation of oil families have for the first time been applied to the correlation of oils and source rocks from the Alaskan North Slope. The computer process utilizes the entire information content of gas chromatograph-mass spectrometric (GC-MS) analyses of whole oils and rock extracts in the correlation process. Previous correlation efforts utilizing GC-MS data have been limited to the use of preselected biomarkers or have required laborious and time-consuming procedures to find new biomarkers.

      The reproducibility of common sample preparation procedures including extraction procedures (Soxhlet, shakeout, and sonication) and fractionation by liquid chromatography (alumina and silica gel) were evaluated at the beginning of this study. This evaluation involved the use of stable-labeled deuterated 2- to 5-ring aromatic hydrocarbon spikes. The spikes were added to the oils or sediments prior to extraction and/or liquid chromatography. The results of these studies indicated that liquid chromatographic fractionation is not sufficiently reproducible for our correlation technique and an exhaustive extraction procedure is required to recover indigenous organic matter from sediments. Whole crudes and sediment extracts prepared using a Soxhlet extraction procedure were, therefore, used for the correlations. Sediment extracts were screened by gas chromatograph-flame ionization detector prior to GC-MS analysis to insure adequate extractable organics for correlation purposes. GC-MS data were collected under very carefully controlled conditions to minimize instrumental variability. Oil-oil and oil-rock correlations were then made using the computerized correlation process. The oils were grouped into two families whose members were shown to have undergone different degrees of alteration.

      Seven of the 15 sediment samples provided contained sufficient extractable organic matter to warrant correlation of the GC-MS data, but only 3 of these sediments have vitrinite reflectance maturities within the oil generation window. Correlation of the oils provided (excluding the Seabee condensate) with these 7 sediments showed no unique correlations. Several marginal correlations exist where a sediment correlates equally well with several oils, but these oils do not belong to the same group (or family). This may mean that each group has mixed sources, that we have not yet sampled the source rocks, and/or that the algorithm used needs further development to perform oil-rock correlations.

    5. Page 671

      Fluorescence analysis in a wide range of excitation and emission was applied to 9 oils and 14 sediment extracts from the Alaskan North Slope. Correlation coefficients calculated from the spectra led to grouping of oils and extracts. Similarities between some spectra of die oils and spectra of the extracts are seen. The classification found with this method is essentially in agreement with other geochemical correlation data.

Purchase Chapters

Recommended Reading