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USGS2015

Unconventional energy resources: 2015 review. Shale gas and liquids

Introduction As the source rocks from which petroleum is generated, organic-rich shales have always been considered an important component of petroleum systems. Over the last few years, it has been realized that in some mudrocks, sufficient hydrocarbons remain in place to allow for commercial development, although advanced drilling and completion technology is typically required to access hydrocarbons from these reservoirs. Tight oil reservoirs (also referred to as continuous oil accumulations) contain hydrocarbons migrated from source rocks that are geologically/stratigraphically interbedded with or occur immediately overlying/underlying them. Migration is minimal in charging these tight oil accumulations (Gaswirth and Marra 2014 ). Companies around the world are now successfully exploiting organic-rich shales and tight rocks for contained hydrocarbons, and the search for these types of unconventional petroleum reservoirs is growing. Unconventional reservoirs range in geologic age from Ordovician to Tertiary (Silverman et al. 2005 ; EIA 2013a ).

Natural Resources Research↗

USGS2005

4D seismic to image a thin carbonate reservoir during a miscible CO2 flood: Hall-Gurney Field, Kansas, USA

Time-lapse (TL) seismic monitoring has proven a valuable tool for reservoir management. If available in a timely and cost-effective manner, it can be incorporated into a dynamic reservoir management approach, spec-ifically in terms of assessing sweep efficiency through constraining reservoir simulations, quantifying compartmentalization, and enhancing the placement of infill wells. Thus, cost-effectiveness, rapid turnaround time, and sensitivity of the seismic response to subtle changes in reservoir properties evident in production data are critical assets of a robust and economic TL application. Successful applications of TL monitoring have been predominantly offshore—Gulf of Mexico and North Sea—in thick clastic reservoirs characterized as low technical risk. Changes in TL seismic images as a direct result of enhanced oil recovery operations in low compressibility, “stiff” carbonate reservoirs are so small that distinguishing reservoir fluid changes from background noise on seismic attribute maps is challenging at best and many times impossible. Consequently, reservoirs predicted to produce weak TL anomalies associated with enhanced oil recovery (EOR) operations are classified as high-risk cases and languish in what has been designated the “stretch portfolio” of time-lapse application in reservoir management.

Kansas↗