PRODUCTIVE LOWER WILCOX STRATIGRAPHIC TRAPS FROM AN ENTRENCHED VALLEY IN KINKLER FIELD, LAVACA COUNTY, TEXAS

Abstract

ABSTRACT Subsurface data around Kinkler field define a lower Wilcox A delta shale-filled valley which causes multiple stratigraphic traps in the incised strata, the fill, and the overlying beds. The channel is 11/2 mi (2.8 km) wide, 4 mi (7.4 km) long, and 360 ft (110 m) thick, trending N 30°E and curving to the north at its updip end. The erosional surface becomes more areally and vertically extensive in Halletsville field 2 mi (3.7 km) south, and correlates with the Lavaca channel erosional surface in Valentine field 9 mi (17 km) to the southwest. Kinkler field was drilled originally as a seismically-defined anticlinal structure, and the discovery well completed from a sand deposited within the shale-filled channel. Two additional producers, as well as 5 dry or marginal wells from this zone delineate the sand. It is 21/2 mi (4.6 km) long, about 1500 ft (457 m) wide, and reaches a net sand thickness of 39 ft (12 m). We call this a bay margin sand. Another productive sand occurs directly below the erosional surface. This upward-fining sand is part of a lower Wilcox A delta-distributary-channel complex which can be correlated across several square miles. Because individual reservoirs are discontinuous we think this is a point bar with clay drapes separating depositional lobes. A compaction closure exists over the east margin of the channel. The structure is caused by counter-regional dip into the shale-filled channel on the west in combination with regional dip to the southeast. Although the amount of closure, 25 ft. (7.6 m) is small, the structure may have influenced overlying meander points and channel migration. Two overbank sands produce oil 700 ft (213 m) above the channel, and the compaction feature may have influenced the deposition of a gas productive upper Wilcox sheet sand 3000 ft (914 m) above. The inferred geologic history suggests the lower Wilcox A section is an upward coarsening progradational deltaic sequence ending in a delta plain environment. Sea level lowering of several hundred feet caused the river to incise deeply into the flat-lying surface at about the end of lower Wilcox A deposition resulting in an entrenched valley. Subsequent rapid marine transgression created a drowned valley which received fine clastics both from its marine and fluvial-deltaic ends. Sand spits developed along the valley margin as filling took place. A present-day analog of the Kinkler field model is Lavaca Bay, a Pleistocene entrenched valley and its documented record of erosion and sedimentation (Wilkinson and Byrne, 1977)