Timing of hydrocarbon generation and accumulation in fault-bounded compartments in the Norphlet Formation, offshore Alabama

Abstract

Analysis of the variation in depth of the gas-water contacts in the Bon Secour Bay and Lower Mobile Bay-Mary Ann Fields of offshore Mobile Bay, Alabama, together with fault seal analysis, implies that some of the pre-Cretaceous faults in these fields must be sealing. It is also likely that the large, post-Cretaceous Lower Mobile Bay Fault is dip-leaking. The gas currently reservoired in the Norphlet Formation has most likely been produced by thermal degradation of oils generated in the overlying Jurassic lower Smackover Formation. For a range of heat flow models appropriate for this area of the Gulf Coast, the onset of gas generation (defined as the time at which 10% of the maximum possible amount of gas has been generated) occurs at 104 −5 +10 Ma. The error bounds in this estimate are the 90% confidence limits, which were calculated by assuming that the times of onset of gas generation are log-normally distributed. The time of onset of gas generation is younger than the age of the sealing faults in the area, hence it is likely that gas generation occurred in situ in the Norphlet Formation, with re-migration occurring during the development of the post-Cretaceous Lower Mobile Bay Fault.