The Surface Geochemical Expression of Some Utah Oil and Helium Fields
Soil and soil gas geochemical surveys were conducted over the Lisbon/Lightning Draw SE and Covenant oil fields and the Harley Dome helium field in Utah. Some of the methods represent unique approaches to surface geochemistry including our ability to predict reservoir fluid composition (e.g. CO2 vs. hydrocarbons) through hydrocarbon and fixed gas analysis of soil gas samples and Synchronous Scanned Fluorescence (SSF) analysis of soil samples. These methods could be used to reduce the exploration risk for other petroleum and helium plays in Utah. Soils over the the 7,500 to 9,000 foot deep Lisbon and Lighting Draw fields are anomalous in heavy hydrocarbons and CO2-rich soil gas microseeps over Lightning Draw SE reflect the high CO2 concentrations in the reservoir gas (27-36% CO2). Soils over both fields show a strong cadmium-uranium-molybdenum association that may reflect heavy metal deposition in a reduced hydrocarbon chimney. Headspace and fluorescence analysis of soil samples over the 6,400-foot deep Covenant oil field reveals a spatial association of wet gas (%nC4) anomalies and light oil microseeps directly over and to the southwest of the field. Tertiary normal faults may be the preferred migration pathway for 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. High contrast helium anomalies are evident in 3 foot deep soil gas samples directly over the Harley Dome, which has 93% N2 and 7% He in the reservoir gas contained within a Jurassic Entrada sandstone at 4,000 foot depth. The most intense helium concentrations are over the apex of the dome near a normal fault. A soil gas survey is scheduled for the 9,000 foot deep, CO2-rich, Navajo sandstone hosted, Providence oil field early in 2018. If the results become available by May, they will be presented in this paper. These and other geochemical orientation surveys have demonstrated that, in addition to light hydrocarbon analysis of soil gas samples, the incorporation of fixed gas analysis (CO2. O2, CO, H2, He, N2) and SSF analysis of soils may lead to better prediction of reservoir fluid compositions from surface samples. SSF analysis of live oil microseeps can predict the API gravity of oil in reservoirs and light hydrocarbon ratios and fixed gas concentrations can predict whether the source is oil or gas dominated.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018