Charles N. Threlkeld

Ubiquitous tar balls with a California-source signature on the shorelines of Prince William Sound, Alaska

Although the shorelines of Prince William Sound still bear traces of the 1989 Exxon Valdez oil spill, most of the flattened tar balls that can be found today on these shorelines are not residues of Exxon Valdez oil. Instead, the carbon-isotopic and hydrocarbon-biomarker signatures of 61 tar ball samples, collected from shorelines throughout the northern and western parts of the sound, are all remarkably similar and have characteristics consistent with those of oil products that originated from the Monterey Formation source rocks of California. The carbon-isotopic compositions of the tar balls are all closely grouped (δ 13 C PDB = -23.7 ± 0.2‰), within the range found in crude oils from those rocks, but are distinct from isotopic compositions of 28 samples of residues from the Exxon Valdezoil spill (δ 13 C PDB = -29.4 ± 0.1‰). Likewise, values for selected biomarker ratios in the tar balls are all similar but distinct from values of residues from the 1989 oil spill. Carbon-isotopic and biomarker signatures generally relate the tar balls to oil products used in Alaska before ∼1970 for construction and pavements. How these tar balls with such similar geochemical characteristics became so widely dispersed throughout the northern and western parts of the sound is not known with certainty, butthe great 1964 Alaska earthquake was undoubtedly an important trigger, causing spills from ruptured storage facilities of California-sourced asphalt and fuel oil into Prince William Sound.

Geochemistry of a naturally occurring massive marine gas hydrate

During Deep Sea Drilling Project (DSDP) Leg 84 a core 1 m long and 6 cm in diameter of massive gas hydrate was unexpectedly recovered at Site 570 in upper slope sediment of the Middle America Trench offshore of Guatemala. This core contained only 5–7% sediment, the remainder being the solid hydrate composed of gas and water. Samples of the gas hydrate were decomposed under controlled conditions in a closed container maintained at 4°C. Gas pressure increased and asymptotically approached the equilibrium decomposition pressure for an ideal methane hydrate, CH4.5-3/4H2O, of 3930 kPa and approached to this pressure after each time gas was released, until the gas hydrate was completely decomposed. The gas evolved during hydrate decomposition was 99.4% methane, ∼0.2% ethane, and ∼0.4% CO2. Hydrocarbons from propane to heptane were also present, but in concentrations of less than 100 p.p.m. The carbon-isotopic composition of methane was −41 to −44 permil((000), relative to PDB standard. The observed volumetric methane/water ratio was 64 or 67, which indicates that before it was stored and analyzed, the gas hydrate probably had lost methane. The sample material used in the experiments was likely a mixture of methane hydrate and water ice. Formation of this massive gas hydrate probably involved the following processes: (i) upward migration of gas and its accumulation in a zone where conditions favored the growth of gas hydrates, (ii) continued, unusually rapid biological generation of methane, and (iii) release of gas from water solution as pressure decreased due to sea level lowering and tectonic uplift.

Carbon-13/carbon-12 isotope fractionation of organic matter associated with uranium ores induced by alpha irradiation.

Analyses of stable carbon isotopes from two sample suites from sandstone uranium (tabular) ores show interesting variations. Organic carbon associated with high-grade uranium ore is heavy (δ13C = –16.9 to –19.6 per mil, where δ13C = 13C/12C relative to the Pee Dee belemnite standard) relative to the adjacent lower-grade samples (–22.7 to –26.4 per mil). It is suggested that the heavy isotopic values for the are samples are related to a radiation and chemical isotope effect that has occurred mainly because of an alpha-radiation dose of 1011 rads.

Biogenic and Thermogenic Origins of Natural Gas in Cook Inlet Basin, Alaska

Two types of natural gas occurrences are present in the Cook Inlet basin. The major reserves (1.8 x 10/sup 11/m/sup 3/) occur in shallow (less than 2300 m), nonassociated dry gas fields that contain methane with ..delta.. /sup 13/C in the range of -63 t -56 per mil. These gas fields are in sandstones interbedded with coals of the Sterling and Beluga Formations; the gas fields are interpreted as biogenic in origin. Lesser reserves (1.1 x 10/sup 10/ m/sup 3/) of natural gas are associated with oil in the deeper Hemlock Conglomerate at the base of the Tertiary section; associated gas contains methane with ..delta.. /sup 13/C of about -46 per mil. The gases associated with oil in the Hemlock Conglomerate are thermogenic in origin.

Character, origin and occurrence of natural gases in the Anadarko basin, southwestern Kansas, western Oklahoma and Texas Panhandle, U.S.A.

Abstract Natural gas production in the Anadarko basin comes from three geographically separated areas that can be differentiated by age of reservoir and by inferred nature of organic, thermal origin of the gases. In the central basin, non-associated gases are produced mainly from Upper Mississippian and Pennsylvanian sandstones. Gas samples are from reservoirs as much as 6588 m deep. Gases become isotopically heavier ( δ 13 C 1 -values range from −49.8 to −33.2‰) and chemically drier (C 2+ -values range from 1–33%) with increasing level of thermal maturity. Gases were generated mainly from interbedded shales with type-III kerogen during the mature and post-mature stages of hydrocarbon generation. Deviations from the trend are due to vertical migration and mixing of gases generated at different levels of thermal maturity over the past 250 Myr. In the giant Panhandle-Hugoton field, non-associated gases are generally produced from Permian carbonates at depths of δ 13 C 1 -value is −43.2‰, mean C 2+ -value is 14%). Because organic-rich, mature source rocks are not present in the area, gases probably were generated in the central basin from Pennsylvanian or older source rocks during the mature stage of hydrocarbon generation. This interpretation implies migration over distances as much as several hundred kilometers. In the Sooner Trend, associated gases are produced from Silurian, Devonian and Mississippian carbonates at depths as great as 2950 m and were generated from type-II kerogen during the mature stage of hydrocarbon generation. Associated oil usually correlates with extracts of the Upper Devonian and Lower Mississippian Woodford Shale. Gases are isotopically lighter (mean δ 1 3 C 1 -value is −43.9‰) and chemically wetter (mean C 2+ value is 14%) than those derived from type-III kerogen at an equivalent level of thermal maturity.

Carbon isotopic comparisons of oil products used in the developmental history of Alaska

Abstract Studies of the fate of oil released into Prince William Sound, AK, as a result of the 1989 Exxon Valdez oil spill, have led to an unexpected discovery. In addition to oil-like residues attributed to the spill, the ubiquitous presence of flattened tar balls, the carbon isotopic compositions of which fall within a surprisingly narrow range [ δ 13 C PDB =−23.7±0.3‰ ( n =65)], were observed on the shorelines of the northern and western parts of the sound. These compositions are similar to those of some oil products [−23.7±0.7‰ ( n =35)] that were shipped from California and used in Alaska for fuel, lubrication, construction, and paving before ∼1970. These products include fuel oil, asphalt, and lubricants [−23.8±0.5‰ ( n =11)], caulking, sealants, and roofing tar [−23.7±0.7‰ ( n =16)], and road pavements and airport runways [−23.5±0.9‰ ( n =8)]. Fuel oil and asphalt [−23.5±0.1‰ ( n =3)], stored at the old Valdez town site and spilled during the 1964 Alaskan earthquake, appear to be the source of most of the beached tar balls. Oil products with lighter carbon isotopic compositions, between −25 and −30‰ ( n =18), appear to have been used more recently in Alaska, that is, after ∼1970. The source of some of the products used for modern pavement and runways [−29.3±0.2‰ ( n =6)] is likely Alaskan North Slope crude oil, an example of which was spilled in the 1989 oil spill [−29.2‰ ( n =1)].

Methane Hydrate in Slope Sediments on West Coast of Central America: ABSTRACT

Offshore Mexico and Guatemala slope sediments are classic sites of deep-sea gas hydrate occurrence. Gassy, frozen sediment was recovered in cores from this region on Legs 66, 67, and 84 of the Deep Sea Drilling Project. In addition, a massive 3 to 4-m thick layer of nearly pure methane hydrate at a depth of 250 m was cored on Leg 84 and preserved for study. The gas from the hydrate is 99+% methane with a few tenths percent carbon dioxide and traces (10-4v/v) of ethane. Most of the sites with gas hydrate in the sediments have methane with ^dgr13C of -70 to -60 ^pmil, indicating origin from methane-generating bacteria. The massive gas hydrate contained methane with ^dgr13C of -40 ^pmil, and the surrounding sediment had bicarbonate in the por water with ^dgr13C of +35 ^pmil. The 75 ^pmil separation in ^dgr13C between coexisting methane and bicarbonate is consistent with kinetic fractionation during bacterial reduction of carbon dioxide to methane, with continuous replenishment of carbon dioxide by fermentation processes. The areal extent of the massive gas hydrate is not known, but the single point yields a gas-in-place estimate of 5.2 × 108m3/km2 or 48 bcf/mi2. End_of_Article - Last_Page 245------------