Terrence J. Donovan

Recognition of Petroleum-bearing Traps by Unusual Isotopic Compositions of Carbonate-cemented Surface Rocks

Unusual carbon and oxygen isotopic compositions characterize outcropping dolomitic sandstone of Permian age over the Davenport oil field, Oklahoma. The δC 13 PDB ranges from −5.1 to −11.3/mil and the δO 18 SMOW ranges from +28.8 to +48.8/mil. Areal distribution of both isotopic ranges is systematic and closely reflects the distribution of petroleum in the producing interval at a depth of 1,000 m (3,300 ft). The 20/mil positive oxygen range in these surface rocks suggests evaporative fractionation of ground water controlled by the near-surface expansion of vertically migrating dry natural gas from the underlying deposit. The rather light, though unexceptional, carbon values, unlike those reported for other hydrocarbon-derived carbonate rocks, indicate only partial chemical or biochemical oxidation of the gas, suggesting further that the leakage rate was relatively rapid. At the Cement oil field, Oklahoma, similar relations in surface rocks occur, but the carbon isotopic values are exceptionally light (−5.5 to −39.2/ml relative to PDB), indicating significant near-surface oxidation of slowly seeping hydrocarbons of low molecular weight. The occurrence of carbonate rocks derived from the chemical oxidation or bacterial conversion of hydrocarbons is well known (Thode and others, 1953; S. R. Silverman and others, unpub. data; Russell and others, 1967; Hathaway and Degens, 1968; Mamchur, 1969; Davis and Kirkland, 1970). Such carbonates typically have δC 13 PDB values more negative than about −25/mil. Similarly, unusual isotopic signatures have been documented in carbonate-cemented sandstone and calcitized gypsum cropping out over the Cement oil field in the southeastern part of the Anadarko basin of Oklahoma (Donovan, 1974). There, the rock cements display carbon isotopic ratios that are increasingly light and oxygen ratios that are increasingly heavy toward the crest of the anticlinal structure. These trends are limited areally and are systematic in their patterns, reflecting the subsurface distribution of petroleum. Both liquid and gaseous hydrocarbons have leaked from the Cement reservoirs; much of the hydrocarbon was oxidized and the carbon incorporated into the interstitial carbonate cements of the overlying rocks. Rapid vertical leakage of gas, expansive under depressurization, resulted in preferential evaporation of H 2 O 16 , compared with H 2 O 18 from near-surface ground waters. This evapo-escape to the atmosphere. Rayleigh distillation proceeds if escape rates cause evaporation of ground water at rates faster than the replacement or recharge rate of meteoric water. Rocks whose isotopic compositions reflect an intermediate mix of these contrasting processes have not yet been found but presumably exist. The existence of these kinds of isotopically anomalous carbonate cements would prove useful in petroleum exploration independently of other kinds of geochemical indicators.

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.

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.

Correlations among hydrocarbon microseepage, soil chemistry, and uptake of micronutrients by plants, Bell Creek oil field, Montana

Abstract Chelate-extractable iron and manganese concentrations in soils over and around the Bell Creek oil field suggest that compared to low average background values, there are moderate amounts of these elements directly over the production area and higher concentrations distributed in an aureole pattern around the periphery of the field. Adsorbed and organically bound iron and manganese appear to be readily taken up by plants resulting in anomalously high levels of these elements in leaves and needles over the oil field and suggesting correlation with corresponding low concentrations in soils. Iron and manganese appear to have bypassed the soil formation process where, under normal oxidizing conditions, they would have ultimately precipitated as insoluble oxides and hydroxides.

Integration of geologic, geochemical, and geophysical data of the Cement oil field, Oklahoma, using spatial array processing

Geologic, geochemical, and geophysical measurements were made at the Cement oil field, Oklahoma, test site using airborne and spaceborne sensors coupled with ground‐based data collection. The data collected include (1) airborne gamma‐ray spectrometry (214Bi, 208Tl, 40K, and total intensity channels), (2) low‐altitude aeromagnetic profiles, (3) precision gravity measurements, (4) images from the Landsat multispectral scanner (MSS) systems and U-2 photography, and (5) geologic and topographic maps. In order to reduce, analyze, display, and correlate the information, it was necessary to transform the data from vector space to raster space (a two‐dimensional image array) with fixed resolution and array dimension. With the data in array form, spatial array processing techniques were applied to (1) correct geometrically the data for proper registration, (2) perform areal interpolation and smoothing, (3) display the data as images, and (4) perform integration and correlation studies. Each data set was transforme...

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------------

The U.S. geological survey's project BIRDDOG (basic investigation of remotely detectable deposits of oil and gas)

An experimental program to develop an integrated remote sensing-geochemical petroleum exploration technique has been established by the U.S. Geological Survey. Briefly, the concept is based on previous and ongoing research within the U.S. Geological Survey which demonstrates that imperfect rock seals capping petroleum and natural gas deposits may permit large volumes of low-molecular-weight hydrocarbons to escape to the surface, with one or more of the following results: (1) the slowly seeping hydrocarbons chemically and biochemically oxidize; (2) liberated CO2 is incorporated into carbonates, forming pore-filling cements invested with unique isotopic signatures; (3) gas, expansive under depressurization, evaporates near-surface pore water, likewise precipitating isotopically distinctive pore cements; (4) the highly reducing hydrocarbons and associated compounds cause discoloration of surface strata by reduction and dissolution of iron. All these late diagenetic alteration phenomena reflect the subsurface distribution of petroleum. The BIRDDOG program in part attempts to explore for these kinds of surface anomalies using the following integrated approach.