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Abstract: Anomalous Pressure: Key to Unlocking Unconventional Hydrocarbon Resources

Ronald C. Surdam

Anomalously pressured, compartmentalized gas accumulations represent a huge and largely untapped gas resource. At present, these unconventional gas accumulations tend to defy exploitation by conventional exploration and production technologies. Based on work in Rocky Mountain Laramide basins (e.g., Western Canada [Alberta], Powder River, Bighorn, Wind River, Greater Green River, Denver, and San Juan basins), a new paradigm has been created for gas exploration in basins characterized by anomalously pressured, compartmentalized gas accumulations. This paradigm is defined as the "determination and, if possible, three-dimensional evaluation of the pressure boundary between normal and anomalous pressure regimes, and the detection and delineation of porosity/permeability sweet pots' (i.e., areas of enhanced storage capacity and deliverability) below this boundary are the two most important elements in exploring for basin-center, or deep-basin gas accumulations" (Surdam, 1995). The evaluation of the regional pressure boundary and detection of underlying storage sweet spots are paramount when designing any new exploration strategies to exploit anomalously pressured gas accumulations and constitute a new way to search for and exploit such deposits. Certainly, there are other critical aspects, but these two elements are absolutely essential; they are as important to finding the gas-saturated, anomalously pressured section of a basin as structural closure and stratigraphic pinch-outs are to finding conventional hydrocarbons in the water-saturated, normally pressure section.

The new paradigm is a product of a dynamic, process-oriented conceptual model for the formation and destruction of anomalously pressured, compartmentalized gas accumulations. Another product of the process-oriented conceptual model is an innovative exploitation technology and the diagnostic tools necessary to (1) expedite the discovery of additional gas reserves, (2) substantially increase the rate of such discovery, and (3) eliminate much of the exploration risk presently associated with developing unconventional accumulations. This technology and resultant strategy eliminate many of the problems presently plaguing operators in anomalously pressured basins. Specifically, this strategy gives the explorationists the ability to (1) determine the position of the pressure boundary; (2) ev luate the 3-D aspects of the pressure boundary surface, with special emphasis on areas characterized by positive relief; (3) establish which depositional facies have the greatest potential for enhanced storage and deliverability below the pressure boundary (sweet spots); (4) document the potential determinative elements that control sweet-spot development in the targeted lithofacies (e.g., fractures, early migration of liquid hydrocarbons, overpressuring, chlorite rims, and/or dissolution of early carbonate cement); and (5) detect porosity/permeability sweet spots using 2-D and 3-D models of well log and/or seismic data.

The new technology, exploration paradigm, and diagnostic tools provide the keys to unlocking anomalously pressured, compartmentalized gas accumulations. Armed with new knowledge and diagnostic tools, explorationists, with substantially less risk, will be able to more efficiently and effectively convert unconventional gas resources to conventional energy reserves.

AAPG Search and Discovery Article #90949©1995-1996 AAPG International Distinguished Lecturers