From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, May 6, 2011
Link to streaming video TBA: available 05.06.2011 at 8:55am
Dr. Ursula Hammes
Bureau of Economic Geology
The Upper Jurassic (Kimmeridgian to Lower Tithonian) Haynesville and Bossier Shales of East Texas and northwest Louisiana are currently one of the most important shale-gas plays in North America, exhibiting overpressure and high temperature, steep decline rates, EURs estimated at 3 to 7 Bcf per well of each formation, and play resources estimated together in the hundreds of trillions of cubic feet. These shale-gas plays have been studied extensively by companies and academic institutions within the last year, but to date the depositional setting, facies, diagenesis, pore evolution, petrophysics, best completion techniques, and geochemical characteristics of the Haynesville and Bossier shales are still poorly understood. Our work represents new insights into Haynesville and Bossier Shale facies, deposition, geochemistry, petrophysics, reservoir quality, and stratigraphy in light of paleographic setting and regional tectonics.
Haynesville and Bossier Shale deposition was influenced by basement structures, local carbonate platforms, and salt movement associated with the opening of the Gulf of Mexico basin. The deep basin was surrounded by carbonate shelves of the Smackover/Haynesville Lime Louark sequence in the north and east and local platforms within the basin. The basin periodically exhibited restricted environment and reducing anoxic conditions, as indicated by variably increased molybdenum content, presence of framboidal pyrite, and TOC-S-Fe relationships. These organic-rich intervals are concentrated along and between platforms and islands that provided restrictive and anoxic conditions during Haynesville and part of Bossier times.
The mudrock facies range from calcareous-dominated facies near the carbonate platforms and islands to siliceous-dominated lithologies in areas where deltas prograded into the basin and diluted organic matter (e.g., northern Louisiana and northeast Texas). These facies are a direct response to a second-order transgression that lasted from the early Kimmeridgian to the Berriasian. Haynesville and Bossier shales each compose three upward-coarsening cycles that probably represent third-order sequences within the larger second-order transgressive systems and early highstand systems tracts, respectively. Each Haynesville third-order cycle is characterized by unlaminated mudstone grading into laminated and bioturbated mudstone. Most of the three Bossier third-order cycles are dominated by varying amounts of siliciclastic mudstones and siltstones. However, the third Bossier cycle exhibits higher carbonate and an increase in organic productivity in a southern restricted area (beyond the basinward limits of Cotton Valley progradation), creating another productive gas-shale opportunity. This organic-rich Bossier cycle extends across the Sabine Island complex and the Mt. Enterprise Fault Zone in a narrow trough from Nacogdoches County, Texas, to Red River Parish, Louisiana. Similar to the organic-rich Haynesville cycles, each third-order cycle grades from unlaminated into laminated mudstone and is capped by bioturbated, carbonate-rich mudstone facies. Best reservoir properties are commonly found in facies with the highest TOC, lowest siliciclastics, highest level of maturity, and highest porosity. Most porosity in the Haynesville and Bossier is related to interparticle nano- and micropores and, to a minor degree, by porosity in organic matter.
Haynesville and Bossier gas shales are distinctive on wireline logs—high gamma ray, low density, low neutron porosity, high sonic traveltime, moderately high resistivity. A multimin log model seems to predict the TOC content from logs. Persistence of distinctive log signatures is similar for the organic-rich Bossier Shale and the Haynesville Shale across the study area, suggesting that favorable conditions for shale-gas production extend beyond established producing areas.