Geologic Considerations for Assessing and Producing a Conventional-Continuous Hybrid-System Oil and Gas Reservoir: Austin Chalk, U.S. Gulf Coast
The U.S. Geological Survey (USGS) recently completed a geology-based assessment of the undiscovered, technically recoverable oil and gas resources in the Upper Cretaceous Austin Chalk of the U.S. Gulf Coast. The assessment was based on the geologic elements and petroleum processes used to define a total petroleum system, which includes petroleum source rocks, reservoir and seal rocks, and petroleum traps. Using this petroleum-system framework, the USGS defined three conventional and one continuous (unconventional) assessment unit.
USGS assessment methodology defines conventional accumulations as those with good permeabilities and porosities, well-defined boundaries, and hydrocarbon-water contacts. In contrast, low-permeability continuous reservoirs have diffuse boundaries and lack obvious traps and seals, although they may be affected by large-scale structures, such as anticlines and fracture networks. Reservoir characterization as either conventional or continuous has strong implications for assessment of undiscovered resources and production strategy. If characterizing multiple reservoirs within a single formation, one must consider: 1) conventional traps (such as faults and salt structures); 2) geology associated with large-scale fracture systems (e.g. anticlines and proximity to the Lower Cretaceous shelf edge); and 3) petrophysical, mineralogical, and diagenetic properties of the reservoir rock.
The Austin Chalk is a low primary-permeability reservoir that requires interconnected fracture networks for production of oil and gas, which is sourced by the underlying Eagle Ford Shale. It behaves as a hybrid-system, wherein varied geologic settings contribute to both conventional and continuous hydrocarbon accumulations. As unconventional resource plays are becoming increasingly important, so is an understanding of the geology that contributes to their production. Current models characterize the Austin as a primarily continuous reservoir; this is in contrast to former geologic models, as well as production methods such as vertical drilling, that construed it as a conventional reservoir. This shift in philosophy is likely due to current drilling focused at the margins of major structures. Recent work provides a more refined understanding of the Austin Chalk as a hybrid-system reservoir, wherein, lateral drilling programs and reservoir stimulation techniques are advantageous only in reservoirs that are dominantly continuous.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California