Mapping Lower Austin Chalk Primary and Secondary Porosity Using Modern 3-D Seismic and Well Log Methods in Zavala County, Texas

Abstract

Establishing fracture distribution and porosity trends is key to successful well design. The Austin Chalk has historically been referred to as an unpredictable producer due to high fracture concentration and lateral variation in stratigraphy, however recent drilling activity targeting the lower Austin Chalk has been very successful. The Upper Cretaceous Austin Chalk (AC) and Eagle Ford (EF) units are considered by many to act as a single hydrocarbon system so both units are investigated. Communication between these two units is largely through expulsion or dewatering fractures, extensional faults or along the AC/EF unconformity. Total porosity for the Eagle Ford is composed of a primary matrix component and secondary fracture porosity. For the Austin Chalk, the secondary porosity includes both dissolution and fracture components which complicate wireline and seismic interpretation.

The current study interprets 40 square miles of modern 3D seismic data for horizons and faults using amplitude, coherence and ant tracking seismic attributes. Post stack acoustic impedance (AI) inversion is applied to the time migrated seismic volume with control from two wells; this input data is similar to that available to independent operators active in the area. Wireline acoustic impedance plotted against density-porosity reveal strong correlations that allow calibration of seismic AI into primary, secondary and total porosity from which time slices and surface maps are created. Relationships are identified between porosity and geological features of interest, such as faulted and brittle zones, that may prove useful in guiding future well development in the lower Austin Chalk.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018