TIMING OF HYDROCARBON GENERATION AND ACCUMULATION IN FAULT-BOUNDED COMPARTMENTS IN THE NORPHLET FORMATION, OFFSHORE ALABAMA

Analysis of the variation in depth of the gas-water contacts in the Bo n Secour Bay and Lower Mobile Bay-Mary Ann Fields of offshore Mobile B ay, Alabama, together with fault seal analysis, implies that some of t he pre-Cretaceous faults in these fields must be sealing. It is also l ikely that the large, post-Cretaceous Lower Mobile Bay Fault is dip-le aking. The gas currently reservoired in the Norphlet Formation has mos t likely been produced by thermal degradation of oils generated in the overlying Jurassic lower Smackover Formation. For a range of heat flo w 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 cal culated by assuming that the times of onset of gas generation are log- normally distributed. The time of onset of gas generation is younger t han 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-mi gration occurring during the development of the post-Cretaceous Lower Mobile Bay Fault.