Natural fractures of the Tuscaloosa marine shale

Abstract

The Tuscaloosa marine shale is an unconventional play whose core area is located in southwestern Mississippi and southeastern Louisiana. Its significance to the energy industry stems from its large oil and gas resources of approximately 1.5 billion bbl of oil (238.5 million m3 of oil) and 4.6 TCF of gas (1.29 billion m3 of gas) and proximity to existing infrastructure. Despite more than 80 wells being hydraulically fractured in the formation, resulting in a total of 13.82 million bbl of oil and 9.04 BCF of gas, challenges remain due to the shale’s high clay content and diverse mineral makeup. Besides, a well-developed network of natural fractures exists across the play, and its effect on hydrocarbon production is yet to be fully understood. This study uses an integrated approach to the characterization of natural fractures in the Tuscaloosa marine shale, incorporating electrical borehole image logs, shear-wave splitting data, and core descriptions from seven wells across the formation. The results show that the identified natural fractures are vertical and subvertical extension fractures, which can be fully mineralized and have heights between 1 and 3 ft (0.31 and 0.91 m). These fractures occur along the east-west direction, are associated with calcite-rich strata, and are capable of transecting the whole borehole. Smaller fractures terminate due to changes in lithology but commonly reactivate in parallel planes. The proposed methodology can help maximize hydraulic fracturing performance across the shale play by identifying stress direction and optimum lateral placement with respect to fracture location. A total of 500 closed fractures are identified in the lateral section of one well. It is also shown that the maximum horizontal stress orientation is consistent throughout the formation and adheres to the general stress trend in the Gulf Coast Basin.