Overpressure in mudrocks: a case study on natural fractures from the Barnett Shale (Fort Worth Basin, Texas, USA).
Résumé
The development and sealing of natural fractures within organic-rich shale successions is becoming a fertile field of investigation due to the rising interest of shale exploration in unconventional gas and oil plays. Several processes are kno"W11 to generate overpressures and consequent fracturing within these lithologies. The conversion of organic matter in hydrocarbons can create pressure buildup in the trapped fluids. Fracturing may also be related to the build up oftectonic stress at different times of the basin history. During burial, diagenetic phenomena responsible for water expulsion and overpressure may occur, such as compaction of clays, smectite to illite transformation and gypsum conversion to anhydrite. Also, anomalous temperature regimes may le ad to the thermal expansion of pore waters and cause overpressures in the host sediments. Regardless of the fracture origin, subsequent pressure release may induce cementation of various mineral phases and formation of veins, wh ose petrographie and geochemical features help in discriminating among the different genetic mechanisms. Understanding the timing, burial depth and genetic mechanism for fracture creation or reactivation in shales, as well as the nature of the fluids precipitating the healing minerais may be relevant in both basins and reservoirs research for predicting fracture pattem distribution, evolution of porosity/permeability through time and joint relationships with organic matter maturation. Previous authors characterized the fracture pattern of the Barnett Shale from the Fort Worth Basin (Texas, USA) in terms of orientation, persistence, sealing and subcritical fracture index. This study was conducted on sealed fractures from the Barnett Shale and aims to: 1) Characterize the phases occurring in mineralized veins from samples having experienced different burial histories; 2) Establish a vein paragenesis and relate the timing of fracture origin to the burial history of the basin; 3) Contribute to the understanding ofthe mechanisms responsible for overpressure generation in shales. Different types of mineralized fractures were distinguished in the Barnett samples from the investigated locations: San Saba quarry outcrops (maximum burialless than 1 kn1), Mesquite#l core (maximum burial around 3 km, oil window) and Blakely core (maximum burial of about 5 km, gas window). Fractures from the San Saba quarry and Mezquite#l core record mainly early compaction and moderate burial overpressures, in line with the minor burial undergone and the major distance from the Ouachita thrust front. Conversely, four fracture generations were distinguished within the Blackly core, that recorded a deeper burial during basin evolution and was closer to the compressive Ouachita thrust front. The results from the vein characterization were inserted in the reconstructed burial history of the basin, placing constraints on vein development in time and space, and providing a basis on which to discuss their genetic mechanism and fluid origin. Comparison of fracture formation in the Barnett Shale from the Fort Worth Basin with shales in other basins suggests a control of local sedimentation, tectonics and burial on fracture development and healing pro cesses. Differences in maximum burial depths, proximity with thrust fronts and type of organic matter within the sediments are among the parameters, which could explain the differences recorded between these basins.