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The Wilberns Formation (Upper Cambrian) in the subsurface of west-central Texas is composed predominantly of sandstone units. Subsurface mapping of the Wilberns shows the sandstones to be persistent throughout
most of the study area. The Fort Chadbourne fault system is a linear zone of deformation trending from Sutton County northward into northeastern Nolan County. This structural zone contains uplifted and faulted Wilberns Formation. En echelon faults and folds as the predominant structures along the Fort Chadbourne fault system suggest wrench faulting. The orientation of these faults (north-south) and folds (northeast-southwest) suggests that the wrench system had left-lateral movement that was produced from compressive forces active during the Ouachita orogeny.
Based on thin-section analyses of core samples, the general paragenetic sequence for the upper sandstone units of the Wilberns Formation consists of at least 4 diagenetic stages: (1) reduction of primary porosity and development of quartz overgrowths resulting from burial and compaction; (2) precipitation of dolomite cement into remaining pore space; (3) dissolution of calcareous material and development of secondary porosity in response to uplift and exposure; and (4) precipitation of hematite, pyrite, and clays as pore lining and pore fill after reburial (hydrocarbon migration probably occurred during this stage).
This sequence of diagenetic events is responsible for the excellent reservoir quality of the sandstone units within the Wilberns Formation found along the Fort Chadbourne fault system.
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