ABSTRACT

The South Hallettsville Field, Lavaca County, Texas, produces gas and condensate from lower Wilcox sandstones and shales which have been interpreted as either channel turbidite deposits in outer-shelf to slope locations or as delta to pro-delta sands and muds. Thirteen core samples from the General Crude Oil Company No. 1 A.G. Henkes Gas Unit were analyzed by x-ray diffraction methods to determine whether a semiquantitative estimate of clay mineral content would aid in determining the depositional environment. Discrete illite, kaolinite, and chlorite are of particular interest because the presence of these minerals is interpreted as being due to original deposition.

Three shale samples, from 10,194 to 10,206 ft (3107 to 3111 m), and 10 sandstone samples, 10,180 to 10,192 ft (3103 to 3107 m), were selected from one cored section. This sequence of samples is of particular importance because it contains a shale and the immediately overlying sandstone.

If a turbidity-type event occurred, the weight percents of non-diagenetic clays in this sequence should: 1) decrease significantly as the boundary is crossed between the shale and the overlying sandstone; and 2) gradually increase in progressively shallower samples within the sandstone. However, the weight percents for kaolinite and chlorite do not vary significantly regardless of whether there is a change in lithology, shale to sandstone, or a change in depth; the illite content gradually decreases with shallower depths. This sequence is more compatible with a deltaic environment of deposition.