Regional Distribution of Rock Properties, Hydrocarbon-Water Contacts, and Hydrocarbon Migration Derived From Carbonate Petrophysical Analysis of the Austin Chalk Across the Onshore Gulf of Mexico Basin, U.S.A.

Abstract

The U.S. Geological Survey (USGS) is assessing undiscovered, technically recoverable hydrocarbon resources in the Austin Chalk across the onshore Gulf Coast region. In support of this work, regional investigation of the Austin using carbonate petrophysical analysis has been completed. Main contributions from this study provide petrophysical perspectives on (1) geographical distribution of porosity, (2) correlation between hydrocarbon production and the occurrence of overpressure, (3) presence or absence of oil-water and gas-water contacts, and (4) how the depositional contact between the Austin and the underlying Eagle Ford Shale corresponds to the presence of hydrocarbon accumulations in locations without major fault and fracture zones.

The geographical distribution of Austin Chalk porosity was investigated in relation to major structural elements of the onshore Gulf Coast. Findings indicate porosity values are elevated near fault and fracture zones as well as near the Lower Cretaceous shelf margin. In a regional context, results indicate no strong correlation between production and the occurrence of overpressure. This implies that, in general, production from reservoirs within the Austin are not primarily driven by pressure. To characterize the Austin as either a conventional or continuous play for USGS resource assessments, regional scale maps showing the presence or absence of oil-water and gas-water contacts were created.

Mechanisms of hydrocarbon charge of the Austin have not been well constrained and are a long-standing point of debate. The underlying organic-rich Eagle Ford Shale is a source for buoyant migration of hydrocarbons through the stratigraphic section as well as vertical migration along faults and fractures zones. However, in regions without the presence of fault and fracture zones, the permeability of the Eagle Ford Shale is not conducive to the volumes and distances that hydrocarbons would need to migrate in order to charge the prolific fields of the Austin. Results from this petrophysical investigation indicate that the nature of the contact between the Austin and the Eagle Ford Shale plays a role in determining whether hydrocarbon accumulations occur in locations without the presence of major fault and fracture zones. Specifically, the presence and characteristics of a gradational contact between the organic-rich Eagle Ford Shale and the Austin appears to be important for hydrocarbon migration and subsequent accumulation in the Austin.

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