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Sweet Spots in Shale Gas and Liquids Fairways: Understanding Petroleum Composition and Reservoir Pressure.

Harris Cander
BP America, Houston, Texas

Prediction of fluid composition and reservoir pressure is critical for assessing value in shale gas and liquids plays. The most profitable parts of a fairway can sometimes be defined by the intersection of high reservoir pressure with the right gas-oil ratio. This study used regional basin modeling and an in-house source rock kinetic model in the Eagle Ford Shale to: (1) predict fluid compositions and viscosity; (2) evaluate the relative effects on pore pressure caused by disequilibrium compaction and petroleum generation. The in-house kinetic model accounts for petroleum retained and stored in both organic and inorganic porosity. This model was used to derive maps of petroleum composition and phase in order to predict flow of both petroleum and revenue from wells across the fairway. In the Eagle Ford Shale, petroleum compositions are closer to an instantaneous expelled composition over a narrow thermal stress range rather than a cumulative composition from expulsion and migration over a broad range of thermal stress. Lateral migration within the Eagle Ford is minimal because petroleum is flushed vertically during maturation. The final retained petroleum is in near equilibrium with the thermal stress state of the rock.
High reservoir pressure is greatly beneficial to well rates in shale fairways and can be critical for commerciality in liquid-rich parts of the fairway. Over-pressure provides energy to the reservoir, retards the process of petroleum going two-phase, and holds open pore throats by reducing effective stress. The origin of overpressure in source rocks has been a topic of debate for decades. Several authors have proposed that petroleum generation creates enough over-pressure in source rocks to cause micro-fractures that aid the process of primary migration. 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, this study also shows that 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. 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. The results of this study are similar to results from a previous study of the Woodford Shale in the Anadarko Basin and point out that reservoir and fluid properties of source rocks can only be understood within the context of the overall basin history.

 

AAPG Search and Discovery Article #90152©2012 AAPG Southwest Section Meeting, Fort Worth, Texas, 19-22 May 2012