Sweet Spots in Shale Oil and Gas Plays: Prediction of Petroleum Composition and Reservoir Pressure

Prediction of fluid composition and reservoir pressure are essential elements in the assessment of shale oil and gas plays. The most profitable part of a fairway can often be defined by the intersection of high reservoir pressure with the right gas-oil ratio. In this study, an in-house source rock kinetic model (Osborne and Barwise, 2011) was coupled with regional basin modeling in the Eagle Ford and Woodford fairways to predict fluid compositions and to evaluate the effect of petroleum generation on pore pressure. The in-house kinetic model accounts for petroleum retained in both organic and inorganic porosity. With this kinetic model, maps of thermal stress were converted to maps of gas-oil ratio, viscosity, and BTU content to predict flow of both petroleum and revenue from wells across the fairway. In both the Eagle Ford and Woodford, petroleum compositions are closer to an instantaneous product over a narrow thermal stress range rather than a cumulative product from expulsion and migration over a broad range of thermal stress. The petroleum is in near equilibrium with the thermal stress state of the rock and most petroleum was generated in situ and retained as the last generated product with limited lateral migration.

Several authors have proposed that petroleum generation creates most of the over-pressure in source rocks. Basin modeling performed in this study suggests that petroleum generation can account for much of the over-pressure within the Eagle Ford Shale gas fairway (as measured in psi above hydrostatic). However, for both the Anadarko and Maverick basins, the majority of regional over-pressure was generated from disequilibrium compaction during rapid burial associated with foreland subsidence. Late exhumation altered shale reservoir pore pressure states in both basins. Therefore, whereas retained petroleum properties can be linked closely to thermal stress, creation and retention of over-pressure is not strictly due to petroleum generation and a broader, basin-scale interpretation is required in order to define regions where revenue generation will be highest. Because it is often the foreland phase of rapid subsidence and burial that catalyzes both disequilibrium compaction and source rock maturation, the generation of petroleum and over-pressure are often coeval and their effects on reservoir pressure, effective stress, permeability, and reservoir deliverability can be difficult to differentiate.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California