Alan A. Roberts

Determination of Organic Carbon in Modern Carbonate Sediments

ABSTRACT In the routine analysis for organic carbon in modern unconsolidated sediments the initial step commonly is to remove the carbonate carbon by acid treatment and to analyze directly the organic carbon in the residue. We find, however, that as much as 44% of the organic carbon in modern carbonate sediments from Florida Bay, Florida, is solubilized and lost during the acid treatment. Therefore, the amount of carbon in the acid solution in these, and in similar modern sediments, must also be included in the analysis for an accurate determination of the percentage of total organic carbon in the sample.

Low-altitude aeromagnetic reconnaissance for petroleum in the Arctic National Wildlife Refuge, Alaska

Variations in the Earth’s magnetic field arising from local concentrations of shallow subsurface magnetite were mapped in the Arctic National Wildlife Refuge and elsewhere in northern Alaska. The anomalies were delineated with a magnetic horizontal gradiometer mounted on a low‐flying (300 ft or ∼90 m above ground) fixed‐wing airplane. Limited data from stable carbon‐isotope and remanent magnetism measurements of rock cores from the Cape Simpson region strongly suggest that the magnetic anomalies result from the chemical reduction of iron oxides in the presence of seeping hydrocarbons. The magnetic contrast between sedimentary rocks of normally low magnetic susceptibility and those locally enriched with this epigenetic magnetite results in distinctive high‐wavenumber and low‐amplitude total‐field anomalies. Magnetometers extended from each wing tip and in a tail stinger permit calculation of the resultant horizontal gradient vector relative to the flight path. This calculation allows more meaningful interp...

Aeromagnetic Detection of Diagenetic Magnetite over Oil Fields: GEOLOGIC NOTES

High-wave-number magnetic anomalies measured as part of an airborne magnetic survey over the Cement oil field, Oklahoma, are interpreted as reflecting abundant near-surface magnetite formed by the reduction of hydrated iron oxides and/or hematite as a direct result of petroleum microseepage. 3 figures.

Baseline concentrations of hydrocarbons in barrier-island quartz sand, northeastern Gulf of Mexico

The amount and approximate molecular distribution of indigenous hydrocarbons in coastal sand provide a baseline above which petroleum pollution is measurable. Organic carbon, alkanes, total hydrocarbons, alkane/organic carbon and hydrocarbon/organic carbon ratios, and gas chromatographic analyses were used to characterize marine, lagoon, and pond facies of the barrier-island sand. Results from six localities provide no evidence for petroleum pollution. Such pollution would be indicated by complex molecular distributions resembling petroleum and by alkane/organic carbon ratios greater than 0.003 and hydrocarbon/organic carbon ratios greater than 0.01.

Epigenetic Zoning in Surface and Near-Surface Rocks Resulting from Seepage-Induced Redox Gradients, Velma Oil Field, Oklahoma: ABSTRACT

ABSTRACT Surface and near-surface Permian sandstone has been drastically altered over the productive part of the structurally complex Velma oil field as a consequence of petroleum microseepage. Buried Permian sandstone along the northwest-southeast-trending anticline is cemented with abundant pyrite, ferroan calcite and ferroan dolomite. At the surface along the anticlinal crest, iron disulfide is scarce; isotopically anomalous carbonate-cemented sandstone is overlain by sandstone that is massively impregnated by hematite cement. Permian sandstone is normally reddish brown throughout southern Oklahoma, but along the anticlinal flank it has been bleached yellow and white owing to iron loss; some units contain abundant solid bitumen. The mineralogy in the vertical section over the anticline follows the calculated stability relations for iron sulfide, iron carbonate, and iron oxide along a gradient from strongly reducing conditions at depth to oxidizing conditions at the surface. Reducing conditions were provided by seeping hydrocarbons from subsurface reservoirs of this multizone giant field. Production depths range from 120 to 2,975 m. The principal evidence that these alterations are seepage-induced is provided by reports of oil seeps in the early literature, by zones of solid bitumen cements, and by C13 PDB values for carbonate cements that range from -7.8 to -36.7 ppt.

Mapping of Fracture Zones by Helium Emanometry and Possible Relationship of Helium Anomalies to Hydrocarbon Reservoirs in Western Pennsylvania: ABSTRACT

Helium is ubiquitous within the earths crust. Local helium concentrations in the subsurface are observed in association with ores containing uranium and thorium, oil and gas reservoirs, and thermal fluids. In addition, permeable fracture zones tend to intensify local concentrations of helium because of its relative ease of migration along these conduits. A survey of helium in soil-gas samples was conducted across a previously mapped fault zone and a drilling-defined hydrocarbon reservoir in Greene and Fayette Counties, southwestern Pennsylvania, to demonstrate the utility of such surveys for mapping fracture zones and for locating hydrocarbon reservoirs. In the study area, a northwest-trending fracture zone cuts across both an area of hydrocarbon accumulation and an area thought to be relatively barren of hydrocarbons. The survey indicated that the helium content of the soil was anomalously high in a 30-m wide zone above the mapped fault and fracture system. Similar results were obtained from traverses made across both the accurately mapped fault zone and the hypothesized extensions of that zone. These data support the concept that helium can be used to locate fault and fracture zones where location is precluded by more conventional mapping procedures. Although helium surveys have delineated known hydrocarbon reservoirs in other areas, this preliminary survey, which consisted of a few traverses rather than an extensive grid pattern, failed to produce a significant anomaly above a known reservoir in southwestern Pennsylvania. This may be due to the low de sity of sample distribution and the high average background helium value for these traverses. End_of_Article - Last_Page 1446------------

Experimental Low-Altitude Aeromagnetic Reconnaissance for Petroleum in Arctic National Wildlife Refuge, Alaska, Using Horizontal Gradients--A Progress Report: ABSTRACT

Variations in the earths magnetic field arising from areally restricted increased amounts of shallow-buried magnetite over hydrocarbon deposits have been mapped in the Arctic National Wildlife Refuge and elsewhere is northern Alaska. The anomalies have been delineated with a low-flying (90 m; 295 ft) magnetic horizontal gradiometer mounted on a fixed-wing airplane. Limited data from stable carbon isotope and remanent magnetism measurements of rock cores form the Cape Simpson region strongly suggest that the magnetic anomalies result from the chemical reduction of iron oxides in the presence of seeping hydrocarbons. Relatively large magnetic contrast between typical sedimentary rocks and those locally enriched with this epigenetic magnetite results in distinctive high wav -number and low-amplitude total field anomalies. Magnetometers extended from each wingtip and in a tail stinger permit calculation of the resultant horizontal gradient vector relative to the flight path. This calculation provides data for the unmeasured area between adjacent flight lines spaced at 1.5 km (.9 mi), thereby allowing End_Page 451------------------------------ generation of accurate computer-enhanced images or maps. Problems related to diurnal variations and solar storms at high magnetic latitude are largely overcome because changes in the total magnetic field do not significantly affect the magnetic gradient. Analysis of an experimental survey, covering 4,418 line km (2,745 line mi), suggests that the Marsh Creek anticline in the Arctic National Wildlife Refuge is prospective for oil and/or gas. Additional magnetic anomalies were also identified. Although the effect of permafrost on epigenetic processes has not been investigated, the data suggest that special purpose aeromagnetic surveying may be a useful and relatively inexpensive way to explore for oil and gas in this hostile environment. End_of_Article - Last_Page 452------------