ABSTRACT

Gilbertown Field, located in Choctaw County, southwestern Alabama, is associated with the Gilbertown graben, one of many Mesozoic-Cenozoic extensional structures in the eastern part of the Gulf Coast basin that formed during the opening of the Gulf of Mexico. Hydrocarbons in this field are mainly trapped by fault closure in the Upper Cretaceous Eutaw Formation and Selma Group.

To evaluate the burial and maturation history of the Gilbertown area, twelve wells representing different structural settings related to the Gilbertown graben were chosen for analysis. Burial and subsidence curves were constructed from well log data using BasinMod. Vitrinite reflectance profiles were constructed from data based on well cuttings analysis.

Analysis of burial history indicates that the subsidence history of Jurassic and Tertiary strata in the Gilbertown area is typical of extensional basins. Factoring out the tectonic component of subsidence suggests that more than half of the total effective subsidence can be accounted for by sediment load and compaction. Thermal modeling demonstrates that source rocks in the Upper Cretaceous section are undermature. The most likely scenario is that oil was generated in the Smackover Formation, and much of that oil migrated along faults and fractures into what is now Gilbertown Field.

Production data from 33 fields in southwest Alabama indicate that a good correlation exists between maturity and the oil-to-gas production ratio. Gilbertown Field, however, produces very little gas compared to other fields whose source rock is of similar thermal maturity, and may represent early migration of hydrocarbons from the Smackover or selective trapping of oil relative to gas.