Alaskan North Slope Petroleum Geochemistry for the Shublik Formation, Kingak Shale, Pebble Shale Unit, and Torok Formation: INTRODUCTORY PAPERS
0
Citation
0
Reference
20
Related Paper
Abstract:
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, C15+ 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 w st 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.Keywords:
Pebble
Cite
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 norhopane. The Group II oils are devoid of the C30 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 indigeneous to the Lisburne.
Phytane
Asphaltene
API gravity
Petroleum geochemistry
Cite
Citations (0)
Shale Gas
Cite
Citations (24)
Carbon isotopic studies of kerogen assemblages and petroleums from the North Slope-Colville trough area of Alaska have permitted firmer source-oil correlation assignments. As a section, the Mesozoic contains a suite of potential source beds including the Shublik Formation, Kingak Shale formation, and Lower Cretaceous units and, most notably, a post-Neocomian, highly radioactive zone (HRZ). The maturation and generation history of these sediments has been broadly controlled by the Brookian orogeny. Using well data, trends in generalized source richness, hydrocarbon proneness, and organofacies have been recognized. In projecting these data into the deeper Colville trough, a considerable variation in hydrocarbon generating potential was noted over the Mesozoic section. Several particularly attractive oil-prone units were recognized. The generic relationship of a wide range of North Slope petroleums--including early, normal, and post-mature or biodegraded examples--was established. A majority of the principal accumulations could be assigned to the previously defined Barrow-Prudhoe oil family. This widespread generic series included petroleums from Upper Cretaceous, Kuparuk River, Ivishak, and Lisburne reservoirs. Lesser, but distinct, Simpson/Seabee-type oil groupings were also recognized. Effective source-to-oil correlation was achieved by a comparison of the carbon isotopic compositions of the kerogen pyrolyzates and the crude oils. The possible contributions of the various source units were assessed in terms of isotopic match, source potential, and volumetrics. Assuming continuity of source characteristics into the deeper Colville trough, a Triassic/Jurassic combination constituted the closest source match to the major oil accumulations. End_of_Article - Last_Page 678------------
Cite
Citations (0)
The potential for conventional and/or unconventional hydrocarbon exploration requires the presence of organic-rich, thermally mature rock units containing oil or gas-prone kerogen. Thick black, organic rich shale intervals are well exposed along roadside cuts and river banks at several localities in the eastern part of the Mamfe Basin. Earlier described as anoxic lake bottom deposits, these fine grained rocks constitute the probable pod of active source rock in this basin and belonging to the middle stratigraphic unit of the three that make up the basin’s sedimentary fill. Samples collected from representative outcrop sections (Etoko mile 21, Bachuo Ntai, and Satom Bridge) in the study area were subjected to geochemical analytic techniques; Total Organic Carbon (TOC), Rock-Eval Pyrolysis and Vitrinite reflectance (%Ro) values were calculated. TOC data obtained range from 1.06% to 16.10% indicating good to excellent hydrocarbon generative potentials, Rock-Eval Pyrolysis data plotted along Kerogen Types I, II and III with oil and gas generative potentials. 4 out of 9 samples fall within the oil window from the calculated %Ro while temperatures corresponding to the peak of kerogen pyrolysis (Tmax) and Production Index (PI) for the 9 samples range from 398oC to 463oC indicating that the organic matter (OM) are immature to post mature.The black shale unit of this part of the basin therefore contains very high amounts of good to excellent quality of thermally matured organic matter which can produce and expel oil and gas respectively.
Outcrop
Hydrocarbon exploration
Cite
Citations (9)
Siltstone
Cite
Citations (33)
Cite
Citations (6)
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 S1 + 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 (S1 + 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 ss steranes, etc.). A combined review of isotopic, molecular, and geological data has, however, allowed the finalizing of a diagnosis the origin of each crude oil including those that have been recognized as biodegraded oils. End_Page 243------------------------
Pebble
Petroleum geochemistry
Cite
Citations (0)
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 s 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.
Phytane
Petroleum reservoir
Cite
Citations (0)
Forty-one crude oil samples from the North Slope of Alaska have variable diamondoid and biomarker concentrations, indicating different extents of oil cracking. Some of the samples are mixtures of high- and low-maturity components containing high concentrations of both diamondoids and biomarkers. Compound-specific isotope analysis of diamondoids (CSIAD) shows that the Shublik Formation accounts for the higher maturity component in several mixed oil samples, whereas biomarkers, especially those providing information on the age of the source rock, show either a Cretaceous Hue-gamma ray zone (GRZ) or Triassic Shublik source for the lower maturity component. Oil samples in this study mainly correlate to six source rocks based on their biomarker characteristics and CSIAD. Chemometrics of selected source-related biomarker and isotope ratios helps to classify the oil samples into different genetic families. The source rocks include carbonate and shale organofacies of the Triassic Shublik Formation, Jurassic Kingak Shale, Lower Cretaceous Pebble shale, Lower Cretaceous Hue-GRZ, and Cenozoic Canning Formation. Oil presumed to originate from a seventh source rock interval, the Carboniferous–Permian Lisburne Group, was not clearly differentiated from well-established Shublik oil by any geochemical age-related parameter or CSIAD, which suggests that the Lisburne is not an effective source rock for any of the studied oil samples. Four oil samples collected from wells located north of the Barrow arch show unique biomarker characteristics, but age-related biomarker parameters indicate likely Triassic source rock organofacies that is not represented by any of the samples from south of the arch. The source rock for these four oil samples appears to be a clay-rich equivalent of the calcareous Shublik Formation that occurs to the north of the Barrow arch.
Cite
Citations (21)
Marl
Basin modelling
Petroleum geochemistry
Organic geochemistry
Cite
Citations (8)