--> Abstract: Geologic Assessment of Undiscovered Conventional and Continuous Resources of the Upper Cretaceous Austin Chalk, by Krystal Pearson; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Geologic Assessment of Undiscovered Conventional and Continuous Resources of the Upper Cretaceous Austin Chalk

Krystal Pearson1

(1) U.S. Geological Survey, Denver, CO.

An assessment of the technically recoverable undiscovered 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. This assessment also included the partially time-equivalent Tokio and Eutaw Formations, which represent a change in lithology from dominantly chalk reservoirs in Texas and Louisiana, to dominantly sandstone reservoirs in Arkansas, Mississippi, and Alabama. Various considerations determine whether a region will be assessed using either the continuous or the conventional methodologies. Assessment units were defined on the basis of geology, trap, seal, migration, production trends, estimated ultimate recovery, and reservoir characteristics, as well as on whether the unit produces oil and gas from conventional or continuous reservoirs.

The Upper Cretaceous Austin Chalk forms a low-permeability, onshore U.S. Gulf of Mexico reservoir that produces oil and gas from fractures that are oriented parallel to the Lower Cretaceous shelf edge. Horizontal drilling links these fracture systems to create an interconnected network that drains the reservoir. Field and well locations along the production trend are controlled by fracture networks. Heavily fractured chalk is present along both regional and local fault zones. Fractures are also genetically linked to movement of the underlying Jurassic Louann Salt; tensile fractures that created the most effective reservoirs formed in the vicinity of salt-related structures. Undiscovered accumulations should also be associated with structurally controlled fractures, as some of the Austin Chalk overlies the Lower Cretaceous shelf edge and remains under-explored.

The Upper Cretaceous Eagle Ford Shale is the primary source rock for Austin Chalk hydrocarbons. This transgressive marine shale varies in thickness and lithology across Texas and Louisiana, and contains both oil- and gas-prone kerogens. The Eagle Ford began generating oil and gas in the early Miocene, and vertical migration through fractures was sufficient to charge the reservoir. In addition, the Upper Jurassic Smackover Formation was deposited over portions of Louisiana, Alabama, Mississippi, and Arkansas, and it is likely the main source rock where present. Hydrocarbon generation and migration in the Smackover Formation began in the Early Cretaceous, and migration of Jurassic-sourced hydrocarbons into the Austin required more complex migration pathways.