Geologic Evaluation of Regional Production Trends in the Upper Cretaceous Austin Chalk

Krystal Pearson
U.S. Geological Survey, Central Energy Resources Science Center, Box 25046, MS 939, DFC, Denver, Colorado 80225

The Upper Cretaceous Austin Chalk, which extends across Texas and Louisiana, is characterized by reservoirs that produce oil, gas, and in some cases, anomalously large amounts of water. Reservoirs typically have low matrix permeability and contain natural fractures. Horizontal drilling has been used to enhance and connect these fracture systems to drain the reservoir more effectively. Although the formation contains continuous (unconventional) reservoirs, it behaves as a hybrid system, wherein varied geologic settings yield both continuous and conventional accumulations.

Well data indicate that the primary Austin Chalk production trend is parallel to and just updip of the underlying Lower Cretaceous shelf edge. This trend includes Giddings and Pearsall fields, and smaller geologically similar fields. Recent drilling has expanded this belt eastward into Louisiana, as well as downdip of the paleoshelf edge; this new region of production has proved successful and includes significant fields such as Brookeland and Masters Creek.

The Austin Chalk is classified as a biomicrite according to Folk (1959); it is comprised primarily of coccoliths (Dravis, 1979). This low-permeability, low-porosity rock requires large connected fracture systems to store and produce hydrocarbons. Most of the large-scale fractures are parallel to regional strike with few dip-oriented fractures (Haymond, 1991). This one-dimensional fracture network requires many smaller, localized fractures to maintain fluid flow in the reservoir. The clay component of the formation also affects fracture intensity and connectivity (Corbet et al., 1987; Haymond, 1991).

The Austin Chalk is a low-porosity, low-permeability carbonate with a dual pore system comprised of a microporous matrix and a moderately interconnected fracture system (Dawson et al., 1995). Micropores range in size from 5 to 7 μm and matrix porosity commonly ranges from 3 to 10% (Dawson et al., 1995), generally decreasing with depth (Dravis, 1979). Permeability also decreases with depth and typically ranges from 0.1 to 0.5 mD (Dawson et al., 1995).

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012