ABSTRACT: A Unique Austin Chalk Reservoir, Van Field, Van Zandt County, Texas
James T. Lowe
Significant shallow oil production from the Austin Chalk was established in the Van field, Van Zandt County, in East Texas in the late 1980s. Development of the reservoir is still in progress.
The Van field structure is a complexly faulted domal anticline created by salt intrusion. The Woodbine sands, which underlie the Austin Chalk, have been and continue to be the predominant reservoir rocks in the field. Evidence indicates that faults provided vertical conduits for migration of Woodbine oil into the Austin Chalk where it was trapped along the structural crest. The most prolific Austin Chalk production is on the upthrown side of the main field fault, as is the Woodbine.
The composition of the Austin Chalk in the Van field is fairly typical of chalks elsewhere. It is a soft, white to light gray (except where oil stained) limestone composed mostly of coccoliths with some pelecypods. Unlike the Austin Chalk in the Giddings and Pearsall fields, the chalk at Van was not as deeply buried and therefore did not become brittle and susceptible to tensional or cryptic fracturing. The shallow burial in the Van field was also important in that it allowed the chalk to retain primary microporosity. The production comes entirely from this primary porosity.
In addition to the structural position and underlying oil source from the Woodbine, the depositional environment and associated lithofacies are also keys to the reservoir quality in the Van field as demonstrated by cores from the upthrown (productive) and downthrown (less productive) sides of the main field fault. The core from the upthrown well, J. A. Bracken 35 #12, contains only minor interstitial clay that has been removed by intensive burrowing. The core from the downthrown well, J. J. Murphy 31 #9, inversely contains significantly more clay and has minor to no burrowing.
It appears that at the time of Austin Chalk deposition, the main field fault was active and caused the upthrown side to be a structural high and a more agreeable environment for benthonic organisms such as pelecypods and worms. The resulting bioturbation enhanced the reservoir's permeability enough to allow migration and entrapment of the oil.
Future success in exploration for analogous Austin Chalk reservoirs will require the combination of a favorable environment of deposition (i.e., neritopelagic zone), a nearby Woodbine oil source, and a faulted trap that will provide the conduit for migration.
AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990