ABSTRACT

The Grandison area, centering in southeastern Lafourche Parish, on the Mississippi River delta in South Louisiana, is a local basin surrounded by salt domes and anticlines. Hingeline flexures and normal faults help to make correlative zones 2,000 feet lower in the center of the basin than on the flanking anticlines. The main producing sands of the basin either grade into shale or are truncated onto the flexures and against the faults. One gas sand produces over a distance of more than twelve miles along its pinchout on the flanks of four major anticlines. Isopach maps were valuable in predicting where the pinchout would occur.

The foraminiferal assemblages in the marine zones reflect ecological changes that occur across the faults and flexures and on the flanks of the anticlines. The most extensive producing sand contains gas wherever the ecological conditions under which it was deposited are constant; but where these conditions change across the closely controlled Coffee Bay fault and onto the anticlines, no accumulation occurs.

Two separate reflection seismic surveys failed to find the Coffee Bay fault because they mapped from dips plotted on cross sections. After the fault had been proved by drilling it was found on the record sections by careful examination. These show that much of the displacement across this zone is by steep dip rather than by faulting alone, and that the amount of actual fault movement is much less than the amount of stratigraphic thickening onto the downthrown block.

Although the central part of the Grandison complex is a structural basin at depth, above 7,000 feet it is a broad, very gentle dome that makes a prominent photogeologic anomaly that has aided exploration.