Reservoirs in the Eugene Island Block 330 (EI-330) field are believed to have been filled through a complex pattern of cross-fault spilling through juxtaposed sands, and possible vertical migration up associated growth faults. Fault-plane-section analysis of a major basin-bounding fault shows that given the present configuration of juxtaposed sands, cross-fault flow cannot account for the filling of most EI-330 reservoirs; however, a syntectonic process of across-fault spiral migration of hydrocarbons during active slip on the fault could account for the filling of the reservoirs. Hydrocarbons could have entered the shallow reservoirs about 0.68 Ma, before most of the present downthrown sand-on-shale traps formed. A Pennzoil three-dimensional seismic survey, shot over the highly faulted eastern anticlinal structure, enabled the detailed mapping of a buried, down-to-the-north fault and the adjacent sands. This buried fault, fault F, divides the main reservoirs of the EI-330 field into two separate fault blocks. Fault-plane-section analysis, combined with pressure and geochemical data from the two fault blocks, reveals that fault F is generally nonsealing to lateral fluid migration between juxtaposed sands of the same age. The fault is sealing to lateral fluid flow in the majority of cases where sands of different ages are juxtaposed. In one case, the capillary pressure differential between two juxtaposed sands was higher than can be attributed to permeability differences of common Gulf Coast sands, suggesting that material in the fault zone is the most likely cause of the lack of fluid flow between the juxtaposed sands.