Southeastern Section - 60th Annual Meeting (23–25 March 2011)

Paper No. 7
Presentation Time: 1:30 PM-5:30 PM


PEARSON, Krystal M., U.S. Geological Survey, Box 25046 MS 939 Denver Federal Center, Lakewood, CO 80225,

The Upper Cretaceous Austin Chalk of the onshore U.S. Gulf of Mexico forms a low-permeability reservoir that produces unconventional (continuous) oil from major fractures that are oriented parallel to the underlying Lower Cretaceous shelf edge. Heavily fractured chalk is present along both regional and local fault zones, and field and well locations along the production trend are controlled by fracture networks. Horizontal drilling links these fracture systems to create an interconnected network that drains the reservoir.

An assessment of the technically recoverable undiscovered oil and gas resources of the Austin Chalk in the onshore portion and State waters of the Gulf of Mexico was recently conducted by the U.S. Geological Survey (USGS). A continuous assessment unit (AU) was defined that encompasses the giant Giddings and Pearsall fields and smaller related fields. In this AU, the chalk produces primarily oil with associated gas in a “continuous” reservoir, where traditional trap styles such as anticlines and faults are not necessary for the accumulation and production of oil and gas. A number of local structures in the AU likely enhance some petroleum accumulations and aid production due to associated fractures. A “sweet spot” is created where there are significant numbers of interconnected fracture systems. This sweet spot defines the region where production is, or is expected to be, elevated compared to the surrounding area. If small structures and faults exist outside of the current sweet spot, they will lead to an expanded area of exploration; otherwise, exploration will be limited to infill drilling.

The USGS, in assessing the continuous Austin Chalk play, estimated it to contain one of the nation’s largest accumulations of undiscovered unconventional oil. The undiscovered Austin Chalk resources greatly contribute to the oil and gas potential of the Gulf Coast region, which is a critical component of domestic energy sources.