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Petrographic Analysis of Tuscaloosa Marine Shale (Upper Cretaceous) Core Recovered from Eads Poitevent # 1

Abstract

Petrographic Analysis of Tuscaloosa Marine Shale (Upper Cretaceous) Core Recovered from Eads Poitevent # 1

Mary K. Fearn*, Dr. Raphaël Gottardi*, Dr. Davide Oppo*

*School of Geosciences, University of Louisiana at Lafayette; Lafayette, Louisiana, USA, [email protected]

Abstract

Keywords: Cretaceous, Tuscaloosa Formation, Tuscaloosa Marine Shale, Shale, Unconventional Resource

The Upper Cretaceous Tuscaloosa Marine Shale (TMS) is a potentially major oil play that extends in the subsurface of south-central Louisiana and south-central Mississippi. It is included in the Tuscaloosa Group, a complete second-order depositional sequence in which TMS is the high-stand phase. There is still considerable debate as to whether the TMS is an extension of the time-equivalent Eagle Ford Formation of Texas. The Cretaceous experienced several periods of anoxia (Oceanic Anoxic Events) and relative sea-level variations at global scale and that are preserved in the sedimentary record of large parts of the Gulf of Mexico. The importance of these widespread marine anoxic events and the associated deposition of black shales are apparent in petroleum exploration and during the deposition of TMS.

The stratigraphic analysis of the Eads Poitevent #1 core (St. Tammany Parish in Lacombe Bayou Field) allowed to reconstruct the facies characteristic of TMS in this area, the depositional environment and the reservoir potential. An in-depth knowledge of fossils and sedimentary structures acts as a support to our petrographic observations. These observations can be compared to wireline logs and petrophysical data in order to obtain a complete reservoir characterization of the TMS.

The presence of a black shale succession is observed predominantly in the basal portion of the core, where a silty shale is interbedded with horizons of well classed fine sand. The cyclic sedimentary beds are interpreted as turbidite deposits mainly formed by Bouma’s horizons C and D, thus possibly identifying the TMS depositional environment in this core as a distal deep water lobe. The frequency of sand layers increases moving up-section possibly testifying a sea level fall and the gradual transition to the upper sandstone member of the Tuscaloosa Group. Further analyses are needed to validate these preliminary interpretations and to fully reconstruct the depositional history.