Subsurface fracture analysis using FMI logs: implications for regional state of stress prediction in the Black Warrior Basin, Alabama

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University of Alabama Libraries

The Black Warrior Basin is a Paleozoic foreland basin located at the southern tip of the exposed northeast-trending Alleghanian fold-thrust belt. It is bounded by the northwest-trending Ouachita fold-thrust belt to the southwest. The eastern Black Warrior Basin is broken by a multitude of thin-skinned normal faults that generally strike northwest, parallel to the Ouachita trend and perpendicular to the Alleghanian trend. However, little is known about the regional state of stress in the Black Warrior Basin. This study uses FMI (Fullbore Formation MicroImager) and other conventional open hole logs to define the orientation of subsurface natural fractures. Using well log data from the Gorgas #1, the present-day stress orientation was interpreted from induced fractures (borehole breakouts and drilling-induced tensile fractures), indicating a maximum horizontal stress orientation (SHmax) of N 65°E. The image log interpretation revealed three sets of conductive and resistive fractures with strikes of (i) 45°-75°, (ii) 300°-320°, and (iii) 0°-10°. Fracture sets (i) and (ii) were interpreted as a part of the previously identified and documented regional east-northeast (ENE) joint system and the cross-fold joint system, respectively. Fracture set (iii) consists of only resistive (healed) fractures detected in the Lower Pottsville and older strata. It is postulated that the origin of these NNE oriented fractures, on the basis of the subsidence pattern of the Pennsylvanian strata in the basin and the similarity with the regional joint system observed in the Arkoma basin, is the Ouachita orogeny. The interpreted NNE Ouachita convergence direction indicates that thin-skinned normal faults in the Pottsville Formation are not related to the Ouachita thrust belt system, but are instead related to the NE-SW oriented extensional stress field created at a right angle to the northwest-converging Alleghanian orogeny.

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Geology, Geophysics, Petroleum geology