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Gas geochemistry of the Mobile Bay Jurassic Norphlet Formation: Thermal controls and implications for reservoir connectivity

¹ ExxonMobil Exploration Company, 233 Benmar, Houston, Texas 77060; paul.j.mankiewicz{at}exxonmobil.com
² ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77252-2189
³ ExxonMobil Exploration Company, 16825 Northchase Dr, Houston, Texas 77060
⁴ ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77252-2189

Paul Mankiewicz received his B.S. and M.S. degrees in geology and doctorate in environmental science and engineering from the University of California, Los Angeles. He works as a geologic advisor for ExxonMobil Exploration Company, evaluating hydrocarbon systems for new opportunities worldwide. Prior to his current assignment, he worked at ExxonMobil's Upstream Research Company, conducting research in molecular and isotopic geochemistry for over 20 years.

Robert Pottorf received his Ph.D. in geochemistry from Penn State University in 1979 and has worked at ExxonMobil Upstream Research since that time. He has conducted research on the origins and distribution of nonhydrocarbon gases, fluid rock interactions related to porosity prediction, developed tools to assess hydrocarbon migration timing and pathways, and applied fluid-inclusion technologies to solve exploration and production problems throughout the world.

Mike Kozar received his B.A. degree in geology from the College of Wooster in 1983, and his M.S. degree from the University of Tennessee in 1986. He has worked for ExxonMobil in research and operation settings and has participated in projects ranging from regional exploration to detailed reservoir characterization studies. He has also instructed sequence stratigraphy, reservoir characterization, and seismic interpretation courses.

Peter Vrolijk earned his B.S. and M.S. degrees from the Massachusetts Institute of Technology and his Ph.D. in geology from the University of California, Santa Cruz, in 1987. In 1989, he joined Exxon Production Research (now ExxonMobil Upstream Research), doing research on a wide range of topics, including most recently fault-seal analysis and reservoir connectivity analysis.

ABSTRACT

The Mobile Bay gas field is located offshore Alabama in the northern Gulf of Mexico. Production is from eolian dunes of the Jurassic Norphlet sandstone at depths exceeding 6100 m (>20,000 ft) and temperatures greater than 200°C. Reservoir connectivity and compositional variation, including the distribution of nonhydrocarbon gases (H₂S and CO₂), are critical factors in production strategy. To evaluate the controls on compositional variation and connectivity, detailed molecular and isotopic analyses were conducted for 29 wells. Analysis of volatiles in fluid inclusions suggests that the field was originally filled with oil that subsequently cracked to gas. In addition to the thermal destruction (cracking) of oil, the process of thermochemical sulfate reduction (TSR) continues to destroy the remaining hydrocarbons through oxidation of gas and reduction of sulfate to form H₂S and CO₂. The variable extent of the TSR process at Mobile Bay results in a wide range of hydrocarbon and H₂S compositions. Condensates are almost exclusively composed of diamondoids whose composition appears controlled by H₂S concentrations.

In contrast to hydrocarbon and H₂S contents, CO₂ concentrations are relatively constant throughout the field. Carbon isotopic ratios for CO₂ correlate positively with those for wet-gas hydrocarbons but are heavier than expected for CO₂ originating from hydrocarbon oxidation via TSR. The narrow range of CO₂ contents and heavy isotope ratios suggests that CO₂ is regulated by water-rock equilibration and carbonate precipitation. The destruction of the hydrocarbon gas and mineralization of the carbon dioxide product create a volume reduction and an associated drop in reservoir pressure. This process creates several internal sinks (or exits) that may control the spill direction for gas in the field.