Environmental framework, structural evolution and petroleum potential of the Cambrian Wilberns Formation West-Central Texas

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. Analyses of core samples indicate that the upper portion of the Wilberns Formation consists of a lower sandstone facies overlain by a dolomitic interval. The suite of sedimentary structures present in both facies suggests tidal flat deposition and the electric log signatures are similar to those produced by channel and tidal sand bodies. Thus, it is postulated that the Wilberns Formation was deposed on extensive tidal flats in tidal channels. The Fort Chadbourne fault system is a linear zone of deformation trending from Sutton County northward into northeastern Nolan County. This structural zone has uplifted and faulted the Wilberns Formation. The presence of en echelon faults and folds as the predominant structures along the Fort Chadbourne system suggests wrench faulting. The orientation of the faults (N-S) and folds (NE-SW) suggests that the wrench system had left lateral movement produced from compressive forces active during the Ouachita orogeny. Thin section analyses of core samples produced a general paragenetic sequence for the upper sandstone units of the Wilberns Formation consisting of at least four diagenetic stages as follows: (1) Burial and compaction leading to reduction of primary porosity and development of quartz overgrowths. (2) Precipitation of dolomite cement into remaining pore space. (3) Uplift and exposure leading to dissolution of calcareous materials, resulting in development of secondary porosity. (4) Re-burial and precipitation of hematite, pyrite and clays as pore lining and pore fill. 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; it is found only along the Ford Chadbourne fault system.