ABSTRACT: Occurrence of Oil in the Austin Chalk at Van Field, Van Zandt County, Texas: A Unique Geologic Setting

James T. Lowe, Deborah B. Carrington

Important new production has been established in the shallow Austin Chalk formation in the mature Van field. The geologic setting and reservoir characteristics of the Austin Chalk in Van field are unique. The Austin Chalk is buried to a depth of only 2100-2500 ft and has thus retained primary microporosity unlike the typical deep fractured chalk reservoirs.

The Van structure is a complexly faulted domal anticline created by salt intrusion and is approximately 2000 ft higher than surrounding structures in the area. A major northwest-dipping fault acts as the primary trapping mechanism. The field has produced 0.5 billion BO from thick Woodbine sands since its discovery in 1929. Occurrence of oil in the Austin Chalk has been known since the field discovery, but prior completions were low rate oil producers (<10 BOPD). Recent development of a large fracture stimulation technique has resulted in increased production rates of up to 300 BOPD.

The Austin Chalk reservoir limits were determined by isopaching feet of minimum productive resistivity having porosity above a cutoff value. The resistivity/porosity isopach showed a direct correlation between Austin Chalk productivity and the Austin Chalk structure and faulting pattern. Structural evidence along with oil typing indicate that the oil in the Austin Chalk has migrated upward along fault planes and through fault juxtaposition from the Woodbine sands 200 ft below the Austin Chalk.

Thin-section and scanning electron microscopy work performed on conventional cores showed that the Van Austin Chalk formation is a very fine grained limestone composed primarily of coccoliths. Various amounts of detrital illite clay are present in the coccolith matrix. All effective porosity is micro-intergranular and ranges from 15 to 35%. Based on the core analyses, the main porosity reducing agent and therefore control on reservoir quality is the amount of detrital clay present filling the micropores. Permeability is very low with values ranging from 0.01 to 1.5 md. There is no evidence of significant natural fractures or microfractures in the core. Artificial fractures are therefore required to create the permeability needed to sustain commercial production rates.

Development of the Austin Chalk at Van field continues through additional drilling and well recompletion. This program will add significant oil reserves to the Van field.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990