Donald E. Anders

Possible Late Middle Ordovician Organic Carbon Isotope Excursion: Evidence from Ordovician Oils and Hydrocarbon Source Rocks, Mid-Continent and East-Central United States

Oils generated by Middle Ordovician rocks are found throughout the Mid-Continent and east-central regions of the United States. Gas chromatographic characteristics of these oils include a relatively high abundance of n-alkanes with carbon numbers less than 20, a strong predominance of odd-numbered n-alkanes between C10 and C20, and relatively small amounts of branched and cyclic alkanes. Saturated and aromatic hydrocarbon fractions of 43 Ordovician oils from the Anadarko, Ardmore, Forest City, Illinois, Michigan, Salina-Sedgwick, and Williston basins and the Iowa shelf demonstrate a wide range in carbon isotope composition (^dgr13Csat = -24.9 ^pmil to -33.9 ^pmil, (^dgr13Carom = -24.3 ^pmil to -33.7 ^pmil). Saturated and aromatic hydrocarbons extracted from late Middle Ordovician shales (17 core samples) show ranges in ^dgr13C similar to that of the oils. The wide ranges in ^dgr13C for oils and rock extracts reflect a major, positive excursion(s) (6-9 per ^pmil) in organic matter ^dgr13C in late Middle Ordovician rocks. This excursion has at least a regional significance in that it can be documented in sections 480 mi (770 km) apart in south-central Kansas and eastern Iowa. The distance may be as much as 930 mi (1,500 km) if the carbon isotope variations observed in Michigan basin Ordovician oils and in organic matter from late Middle Ordovician rocks in southwestern Ontario are related to the same carbon isotope excursion. Organic-matter ^dgr13C in core samples from south-central Kansas and eastern Iowa is not directly related to variations in quantity or quality of organic matter, or maceral compositi n. The positive excursion in organic matter ^dgr13C is a possible result of increased organic matter productivity and/or preservation. The parallel shifts in organic and carbonate ^dgr13C in core samples from 1 E. M. Greene well, Washington County, Iowa, imply changes in the isotope composition of the ocean-atmosphere carbon reservoir. Differences in the magnitude of the carbon isotope shifts between organic matter (8.8 ^pmil) and carbonate (4.2 ^pmil) in this core suggest a decrease, either locally or regionally, in available dissolved CO2, possibly a result of high organic-matter productivity and/or limited circulation in the late Middle Ordovician seas.

Geochemical correlation of surface and subsurface oils, western Greece

Abstract Biodegraded seep oils, devoid of n-alkanes and most isoprenoid hydrocarbons, were successfully correlated with subsurface crude oils in western Greece primarily using carbon isotopes and the more microbially resistant tricyclic terpane and aromatic steroid hydrocarbon distributions. All the studied oils seem to fit into three major genetic groups which are consistent with the geologic, tectonic, and geographic settings and are presumably related to different source-rock facies. Group I oils are typical of the abundant oil shows in the Epirus province, northwest Greece. Minor but consistent biomarker differences necessitated subgrouping of the Epirus oils, implying derivation from slightly different sources. Group II oils are located in the Katakolon area, northwestern Peloponnese. Group III oils, which are clearly distinct from the other groups, are centered in Zakynthos Island. An oil seep at Trifos, west central Greece, and a subsurface oil from Paxi Island, because of uncertain correlations, are classed intermediate between Group I and II oils. An aromatic steroid hydrocarbon measurement, modified after Mackenzie et al. (1981), appears useful in determining the relative maturity of biodegraded and conventional oils and carbonate rock extracts.

Marine and Nonmarine Gas-Bearing Rocks in Upper Cretaceous Blackhawk and Neslen Formations, Eastern Uinta Basin, Utah: Sedimentology, Diagenesis, and Source Rock Potential

The Upper Cretaceous Blackhawk and Neslen Formations in the eastern Uinta basin contain large amounts of thermogenic gas that was generated from interbedded humic-rich source rocks. The geometry and distribution of hydrocarbon source and reservoir rocks are controlled by depositional environment. The Blackhawk, composed of laterally extensive sandstone and locally interbedded carbonaceous siltstone and minor coal, reflects deposition in nearshore marine and backshore environments. The Neslen contains organic-rich siltstone and mudstone with lesser amounts of carbonaceous shale, coal, and lenticular sandstone that formed in coastal and lower alluvial-plain depositional settings. Potential reservoir sandstones are composed dominantly of monocrystalline quartz grains and sedimentary lithic fragments. Mechanical compaction during early burial was followed by the precipitation of quartz, carbonate, and barite later in the burial history. Variations in porosity and permeability (2-10%; < 1 md) reflects the presence of authigenic clay, mineral cements, and dissolved lithic grains. Natural fractures, cemented with carbonate, barite, and kaolinite, occur locally. Active hydrocarbon generation occurred in the Neslen and Blackhawk during the Oligocene and Miocene when these units were near their maximum burial depth and temperature. The rate of hydrocarbon generation decreased from the late Miocene to the present, owing to widespread cooling that occurred in response to regional uplift and erosion associated with the development of the Colorado Plateau. Temporally equivalent rocks in other areas of the basin may have experienced similar diagenetic and hydrocarbon generation histories.

Sedimentology and petroleum occurrence, Schoolhouse Member, Maroon Formation (Lower Permian), northwestern Colorado

The Lower Permian Schoolhouse Member of the Maroon Formation (formerly considered the Schoolhouse Tongue of the Weber Sandstone) forms a partly exhumed petroleum reservoir in the Eagle basin of northwestern Colorado. The Schoolhouse consists mainly of yellowish gray to gray, low-angle to parallel bedded, very fine to fine-grained sandstone of eolian sand-sheet origin; interbedded fluvial deposits are present in most sections. The sand-sheet deposits of the Schoolhouse Member are sedimentologically and petrologically similar to those in the underlying red beds of the main body of the Maroon Formation, and the Schoolhouse is considered the uppermost sand sheet in the Maroon depositional sequence. The bleached and oil-stained Schoolhouse Member is distinguished from the unde lying Maroon red beds on the basis of its diagenetic history, which is related to regional hydrocarbon migration and development of secondary porosity. Geological and geochemical data suggest that Schoolhouse Member oils have upper Paleozoic sources, including the intrabasinal Belden Formation. Late Paleozoic faults have served as local conduits for vertical petroleum migration. Large-scale (>200 km) lateral migration from sources in the Permian Phosphoria Formation is also possible but less likely. Belden oil was generated and migrated before about 75 Ma. Subsequently, the Schoolhouse Member reservoir was uplifted, then partly exhumed on the monoclinal flank of the Laramide (latest Cretaceous-Paleogene) White River uplift. Based on this study, exploration models for Maroon Formation and Weber Sandstone reservoirs in northwestern Colorado should be expanded to more fully consider Belden source rocks and the controls of late Paleoz ic structures on hydrocarbon migration and trapping. Stratigraphic units of diagenetic origin comparable to the Schoolhouse Member are likely in other basin provinces, and their proper interpretation is critical for reconstructing the histories of associated petroleum systems.

Oil-source correlation study in northeastern Alaska

Abstract The occurrence of numerous oil-stained outcrops across the coastal plain of the Arctic National Wildlife Refuge (ANWR) in northeastern Alaska indicates that commercial hydrocarbons could be present in the subsurface of this region. In addition, this region is flanked by two important oil provinces—the Prudhoe Bay area to the west and the Mackenzie delta to the east. To begin to understand the petroleum resource potential of ANWR, we evaluated the source rock quality and thermal maturity of five rock units ranging in age from Triassic to early Tertiary: Shublik Formation, Kingak Shale, pebble shale unit, Hue Shale and Canning Shale. We also compared ANWR oils using stable carbon isotope ratios, tricyclic terpane ratios, and saturate/aromatic hydrocarbon ratios. The organic carbon content of the five rock units range from an average of 1.6 to 4.0 wt%. Cretaceous rocks from the coastal plain are thermally immature (vitrinite reflectance ANWR oils are divided into three groups: (1) Jago oil type, includes oils from Angun Point, Katakturuk River and Jago River; (2) Manning oil type, from Manning Point near the coast of the Beaufort Sea; and (3) Kavik oil type, from Kavik west of the Canning River. None of the three oil types of ANWR compares favorably with the economically important oils from Prudhoe Bay and the National Petroleum Reserve of Alaska (NPRA). The most promising source rock for the otherd ANWR oil types could not be type II units of the Hue Shale. Possible source rocks for the other ANWR oil types could not be established.

Petroleum Geochemistry of oils and rocks in the Arctic National Wildlife Refuge, Alaska - Abstract

Abstract Thirteen oil seeps or oil-stained outcrops in or adjacent to the coastal plain of the Arctic National Wildlife Refuge (ANWR) in northeastern Alaska indicate that commercial quantities of hydrocarbons may be present in the subsurface. The area ia flanked by two important petroleum provinces, the Prudhoe Bay area on the west and the Mackenzie delta on the east. Organic carbon content, organic matter type and pyrolysis hydrocarbon yield show that all rock units, except the Hue Shale which contains type II organic matter, are predominantly type III organic matter. Pre-Cretaceous rocks that crop out in the Brooks Range could not be adequately evaluated because of the high thermal maturity of their organic matter. Thermal maturity threshold of oil, condensate, and gas from calculated vitrinite reflectance gradients in the Point Thomson area are 4,000, 7,300, and 9,330 m, respectively (12,000, 22,500, and 28,000 ft). TTI calculations for the Beli-1 and Point Thomson-1 wells indicate that maturity first occurred in the south and progressed north. The Hue shale matured in the Beli-1 well during the Eocene and in the Point Thomson-1 well in late Miocene to early Pliocene time. In the Point Thomson area, the condensate and gas recovered from the Thomson sandstone and basement complex is probably from the pebble shale unit, and the oil recovered from the Canning Formation probably came from the Hue Shale. The gas recovered from the three wells in the Kavik area is probably thermal gas from overmature source rocks in the immediate area. None of the oils in or adjacent to ANWR correlate with oils from the Prudhoe Bay area or farther west. However, the Jago oil type, which includes oil from Marsh Creek anticline, Jago River, and Angun Point, do correlate well with the Hue Shale.

Depositional Environments, Diagenesis, and Hydrocarbon Potential of Nonmarine Upper Cretaceous and Lower Tertiary rocks, Eastern Uinta Basin, Utah: ABSTRACT

Core studies of nonmarine rocks from the Natural Buttes field, Utah, indicate that depositional environment and diagenetic alteration control the geometry and quality of low-permeability gas reservoirs in the eastern part of the Uinta basin. The Tuscher Formation (Upper Cretaceous) is composed of fine to medium-grained, moderately to well-sorted sandstones and less abundant carbonaceous and coaly shale that formed on the lower part of an alluvial braidplain. The Wasatch Formation (Paleocene and Eocene) unconformably overlies Cretaceous rocks and consists of fine-grained lenticular cross-bedded sandstones, argillaceous siltstones, and variegated mudstones, which were deposited in lower delta-plain settings along the margin of Lake Uinta. Cretaceous and Tertiary sandstones ave been modified by minor quartz overgrowths, by the precipitation and subsequent dissolution of ferroan and nonferroan calcite, by poikilotopic anhydrite, and by the formation of authigenic illite, mixed-layer illite-smectite, kaolinite, chlorite, and corrensite. Most authigenic carbonate and anhydrite formed during early burial, before significant compaction. During later stages of diagenesis, precipitation of authigenic clay in secondary pores created by carbonate dissolution reduced porosity and permeability. Large amounts of natural gas generated in situ are stratigraphically trapped in these lenticular, diagenetically modified sandstones. Source rocks in the Tuscher Formation have reached the advanced stages of thermogenic gas generation (0.7% Ro) but are only moderatel mature with respect to liquid hydrocarbon generation. Interbeds of lacustrine Green River shale are in the early stages of gas generation (0.5% Ro) and are source rocks for gas produced from the Wasatch Formation. End_of_Article - Last_Page 860------------