Application of Borehole Image Interpretation Techniques for Mechanical Stratigraphy of Gas Shale Reservoirs
There are many emerging unconventional gas shale plays across North America following the success of the Barnett Shale in Fort Worth Basin. The key to unlocking these plays is not only in drilling horizontal wells or optimizing hydraulic fracturing techniques, but - most importantly - in understanding the many geological factors that can potentially affect the successful exploitation of a gas shale reservoir. Formation thickness, mechanical rock properties, lateral continuity of favorable facies, faults and other geological hazards are among such factors, to name only a few.
This work presents case histories of the application of borehole image interpretation techniques, developed specifically to evaluate gas shale reservoirs. Our study is focused on the use of borehole image data to better understand the control of mechanical stratigraphy on the architecture of natural fracture network. The approach also takes advantage of a variety of formation evaluation techniques including geochemical logs to drive lithological facies which are then integrated with the borehole image data. Many of the natural fractures in the studied gas shale reservoirs often terminate abruptly at sharp lithologic boundaries. Fractures in laminated siliceous mudstone are restricted to the more competent layers, and typically die out at rheologically weaker zones. Nevertheless, there are many throughgoing fractures where the role of the regional structural setting on determining their attitude, morphology and spacing is still rather ambiguous.
The result of this study is presented in form of a mechanical stratigraphy column with intermittent fractured and un-fractured intervals. This suggests that detailed classification of different lithofacies based on wireline geochemical logs can have critical implications for a more precise prediction of style, geometry and distribution of natural fracture population at the reservoir scale.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009