Regional stress regime study of East Texas using orientation of fractures in the Weches Formation

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Abstract

Fractures in the Weches Formation in a roughly 50 mile radius around Nacogdoches, Texas were documented and analyzed using data obtained at various outcrops. This data was used to infer the direction of horizontal stresses that created the fractures. These joints are typically steeply dipping (700 – 900) and iron filled. Crack-seal formation, steep dip and a lack of shear movement suggests these fractures were predominantly opening mode joints. Slickenlines are visible on some fracture planes; however, their bearings show random orientation indicating unloading movement during erosion. Limonite veins and iron ledges are a late stage diagenetic event which indicates a late origin for fractures during exhumation. Orientation data of 540 fractures from 14 different outcrops show three main joint sets: N75-85W, N75-85E, and N40E. These sets stretch over distances of up to 90 miles. These orientations are believed to be the result of late tertiary to recent stress states caused by local faulting, Gulfward extension, and extension into the East Texas basin. These trends indicate a regional roughly east-west direction of maximum horizontal stress that changes gradually from the east, where it points N75-85W, to a more northeasterly direction closer to the East Texas basin. Until now previous fracture have used borehole samples rather than outcrop. These results show similar trends to other fracture analyses in East Texas supporting previous studies that show there is quaternary movement of faulting near the East Texas basin and that present day Gulfward extension has an effect on the stress regime in East Texas.

Authors

• Cory Dean Ellison, Stephen F. Austin

• Chris Barker, Stephen F. Austin