Natural sealed fractures in mudrocks : a case study tied to burial history from the Barnett shale, Fort Worth basin, Texas, USA.

Abstract : The Mississippian Barnett Shale (Texas, USA), consisting of organic-rich shales and limestones, hosts the largest gas fields of North America. A study was conducted on sealed fractures from core and outcrop samples of the Barnett Shale of the Fort Worth Basin which aims to: 1) characterize the phases occurring in the fractures from samples having experienced different burial histories; 2) establish a paragenesis to relate the timing of fracture origin and healing with the burial history of the basin; and 3) contribute to the understanding of the mechanisms causing overpressures in shales. Four fracture generations were distinguished. Type 1 veins formed during early mechanical compaction (at few 10s to 100 meters of depth) of still not fully cemented sediments. Type 2 veins formed during moderate burial (≤ 2 km), from slightly modified seawater, and could correspond to the fast deposition of the Pennsylvanian lithologies during the onset of the Ouachita compression. Type 3 veins formed during deep burial (> 3 km) from silica-rich basinal brines possibly derived from clay diagenesis. Type 4 veins formed at very deep burial (>4 km), from hot and O-rich fluids, carrying light oil (20-30 API) and record the opening of the fluid system after organic matter maturation and hydrocarbon migration. The results issued from the vein sample characterization were inserted in the framework of the reconstructed burial history for the Fort Worth Basin, which allowed to set in time and space the vein development and to discuss their genetic mechanism and fluid origin. Differences were highlighted between the timing and thermal regimes under which fractures formed in Barnett lithologies from different areas of the basin, this suggesting that extrapolation of outcrop observations to subsurface must be used with due care.
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Marta Gasparrini, William Sassi, Julia F. W. Gale. Natural sealed fractures in mudrocks : a case study tied to burial history from the Barnett shale, Fort Worth basin, Texas, USA.. Marine and Petroleum Geology, Elsevier, 2014, 55 (August), pp.122-141. 〈10.1016/j.marpetgeo.2013.12.006〉. 〈hal-01069984〉

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