--> --> The Surface Geochemical Expression of Some Helium & Petroleum Deposits

AAPG Rocky Mountain Section Meeting

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The Surface Geochemical Expression of Some Helium & Petroleum Deposits

Abstract

The surface geochemical expression of some helium and petroleum deposits will be emphasized using the results of microseep survey over deposits and prospects in North America, Tanzania and Mongolia. These geochemical surveys are unique in terms of their ability to predict reservoir fluid composition (e.g. helium vs. CO2 vs. hydrocarbons) from surface using extended range hydrocarbon (C1-C20) and fixed gas (He, H2, CO2, O2, N2) analyses of soil gas and soil samples. Helium is anomalous in soil gas directly over the Jurassic, Entrada sandstone hosted Harley Dome field (7% He) in eastern Utah. Both light hydrocarbons and helium are anomalous over the Pennsylvanian, Morrow sandstone hosted Shirley Rother field (3.2% He) in southeast Colorado. In southern Tanzania, a regional 400 km2 soil gas survey revealed helium leaking out of various parts of the Rukwa Basin which, in conjunction with seismic, will be used to select drill targets. In southern Michigan (Albion-Scipio), oil microseeps with similar aromatic hydrocarbon composition to the produced oil, were detected over a 1,200 meter deep oil-charged, hydrothermal dolomite reservoir. Wells drilled over and near the microseeps had initial production rates of 250 BBO/D whereas those drilled off the microseeps were dry or only had oil shows. Microseeps over the Mississippian, Leadville Limestone hosted Lighting Draw field in eastern Utah are anomalous in both light hydrocarbons and CO2 reflecting leakage of hydrocarbon and CO2-rich reservoir gases. Headspace and fluorescence analysis of soil samples over the Jurassic sandstone hosted Covenant oil field in southern-central Utah reveals wet gas (%C4) anomalies and light oil microseeps directly over and to the southwest of the field. Tertiary normal faults may be the preferred migration pathway for the ascending hydrocarbons. The oil microseeps are lighter than the Covenant oil and may have migrated from a different formation (Twin Creek Limestone?) above the Navajo sandstone-hosted Covenant field. In eastern Mongolia, thermogenic hydrocarbons leak up sub-vertical normal faults from a light oil source based on light hydrocarbon ratios (C1/C2+C3 versus C2/C3+C4) in the microseeps and fluorescence spectral patterns of organic extracts of shot-hole sediment samples.