Abstract: Characteristics of Lower Wilcox Reservoirs, Valentine and South Hallettsville Fields, Lavaca County, Texas

By

Robert R. Berg

In Valentine field, sandstones of the lowermost Wilcox Group produce oil at depths of about 9100 ft (2776 m) in a stratigraphic trap from two sandstones locally called the "Technik" and "Kubena" zones. Full-diameter cores from the "Technik" zone show that it consists of thin-bedded turbidites. The "Technik" reservoir is 25 ft (8 m) thick, and the upper part is composed of thicker beds on the order of 2 to 4 ft (0.6 to 1.2 m). Each bed displays sequences of massive or massive to laminated bedding. These sandstones represent turbidite sequences of the "AP and the "ABE" types and were probably channel deposits. The lower part is composed of thinner beds about 1 ft (0.3 m) thick of more complete sequences of the "ABCE" type. Some adjacent sandy shales are moderately bioturbated. The "Technik" non-reservoir facies is 31 ft (9.5. m) thick and consists of thinly- shales and turbidite sandstones which are typically incomplete sequences of the "AE", "BE" and "CE" types. Shales are not bioturbated. The non-reservoir facies may be characterized as overbank deposits. The "Kubena" non-reservoir facies is similar to the "Technik."

The "Technik" reservoir sandstones appear to represent the fill of outer-shelf channels along which sands were transported across the shelf margin and into the deeper basin. The non-reservoir facies represents overbank deposits, probably contemporaneous levee sediments adjacent to channels.

In South Hallettsville field, Lower Wilcox sandstones produce gas at depths of from 10,000 to 11,400 ft (3050 to 3477 m). The beds are 2 to 10 ft (0.6 to 3 m) thick and consist of massive "A" sandstones that show textural grading. These deposits are stacked, channel turbidites that occur as isolated bodies in an expanded section of black shale. Some sandstones and their underlying shales are highly contorted, probably as a result of soft-sediment deformation.

Regional seismic control shows that Lower Wilcox sandstones are contained within a thick lens of shale that was deposited on the Early Eocene shelf margin and slope. This expanded section, called the Hallettsville slope sequence, is unconformable on Upper Cretaceous rocks. The underlying erosion surface appears to have been formed by slumping and mass movement of sediment deposited at the Upper Cretaceous shelf margin. Sediments of the Hallettsville sequence also were unstable and suffered deformation shortly after deposition.

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