ABSTRACT

In the central Texas Gulf Coast updip of the Lower Cretaceous shelf margin, the Late Paleocene/Eocene Lower Wilcox Formation consists predominantly of massive sandstones. An exception to this is in northern Lavaca County where the Lower Wilcox was incised by submarine canyons (e.g. the Yoakum shale channel) during Middle Wilcox time. The productive sandstones in Valentine field and other nearby fields are remnants of the Lower Wilcox sandstone between, and in places below, these shale filled canyons.

Regional speculative seismic data was examined northeast of the anamolous Lavaca County area to find analogous geological conditions where there was unleased acreage. In northeast Fayette County, terracing was noticed at the Lower Cretaceous Edwards and Sligo levels, with a corresponding "wipe-out" of reflectors within the Lower Wilcox above this terracing. Prospect-detailing seismic data indicated that the terracing had localized lateral continuity. A flat spot was mapped at Lower Wilcox, Edwards, and Sligo levels. This feature was found to influence even the surface topography. When regional dip was removed, the present-day flat spot became a closed structure.

A well drilled to test the Lower Wilcox reflection "wipe-out" zone resulted in a Lower Wilcox discovery with an initial potential of 2.5 MMcf gas and 50 barrels of condensate per day. Sidewall cores of the high resistivity spikes bounding the producing sand, encountered lignitic coals. A synthetic seismogram of the producing well showed that the top of the Lower Wilcox as picked on the regional seismic data correlated to the uppermost coal. The producing sand is genetically indistinguishable from the other Lower Wilcox sands. It has a blocky, massive character; decreasing upward grain size; and is bounded by thin coals. A fluvial, point-bar, delta plain depositional setting is indicated.

Examples of producing point-bar sands from Mississippi and Wyoming were studied and a development strategy was formulated. Because the discovery well had a water contact, two delineation wells were drilled updip. The producing sand pinched out, and both offset wells were dry holes.

The massive, Lower Wilcox sands did not correlate well between the three wells but individual coals and groups of coals made excellent correlations. Two short seismic lines were shot to tie the producing well and the two dry holes. During processing, a filter analysis revealed that coherent energy above 50 Hz was preserved and relatively noise free down to the depth of the reservoir. A wide band pass (i.e. 12-75 Hz) was therefore included allowing for much greater resolution. The previously acquired seismic data was also reprocessed to include the high frequencies. It was noted that the Lower Wilcox was composed of a series of cycles. The seismic cycles were transferred to the well logs, and cycle boundaries correlated with slightly thicker coals or groups of coals which were widespread in aerial extent. Within the producing sand seismic cycle a strong pinch-out was seen on the strike and dip seismic lines near the producing well. Strong pinch-outs were also noted within other seismic cycles.

Some sort of 3-D program is necessary to trace the pinch-outs within various seismic cycles across the Winedale structure. The VSP (Vertical Seismic Profile) would have been helpful; but in 1978 when these wells were drilled, it was still considered an experimental tool and not routinely available.