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PASHIN, JACK C., and ANDREW K. RINDSBERG, Geological Survey of Alabama, Tuscaloosa, AL

Abstract: Joint Systems Along the Northeast Margin of the Mississippi Interior Salt Basin, Alabama

Joints are commonly overlooked structures in Mesozoic-Cenozoic strata of the eastern Gulf Coast basin, yet significant hydrocarbon accumulations are in fractured reservoirs in this area. As part of an investigation of naturally fractured chalk reservoirs, we conducted a detailed surface investigation of fracture systems along the northeastern margin of the Mississippi interior salt basin.

Joints are common in outcrops of Cenozoic strata, and include classic systematic joints and cross joints. Joints are concentrated in swarms. Brittle strata, such as siliceous mudstone, are more intensely jointed and contain more hackle plumes than poorly consolidated sand and clay. Orientation data for 575 joints reveal two orthogonal joint systems in the study area. System 1 joints strike with a vector-mean azimuth of 295°, which is subparallel to the peripheral faults and associated fold axes near the margin of the salt basin. System 2 joints strike at 343°, which is parallel to many modern hill slopes. Cross-cutting relationships indicate that system 2 joints postdate regional faulting.

System 2 joints apparently are forming in response to neotectonic stresses associated with regional unroofing and valley incision and probably do not extend downward to reservoir depths (~1,000 - 4,000 m). The peripheral faults were active at least from the Early Cretaceous through the Miocene, and orientation data suggest that system 1 joints formed in the same regional stress field as the faults. Therefore, system 1 joints and cross joints are predicted to exist at reservoir depths. These fractures should affect hydrocarbon recovery by connecting fault-parallel synthetic shear fractures, which constitute the main chalk reservoirs, and by facilitating flow in other types of reservoirs. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana