ZIMMERMAN, RONALD K., Louisiana State University, Basin Research Institute, Baton Rouge, LA

ABSTRACT: Fracture Genesis in the Austin Chalk of Central Louisiana: Implications for Hydrocarbon Exploration

Fractured Upper Cretaceous carbonate (chalk) reservoirs are present in a 50+ mile wide band extending across central Louisiana. The Austin Chalk in particular appears to host sufficient hydrocarbon-filled vertical extension fractures, at depths of 5,000-20,000 ft, to become an attractive target for horizontal exploration drilling.

Deep-seated divergent wrench faulting and detached normal tensional faulting were probably responsible for generating most of the area's shear and extension fractures. Wrench fault zones, specifically, were apparent loci for continued adjustments during post-Austin time; their associated faults and fractures affected competent carbonate beds and, within the Austin Chalk especially, produced a distinct architectural overprint. Other important fracture contributors to the overall pattern were those that stemmed from normal faulting or were caused by processes related to basin genesis such as sedimentary loading, down-warping, and basinward gravity sliding toward the Lower Cretaceous shelf edge.

Fracture genesis emanating from the two types of fault deformation has produced a dominant fracture network extending vertically and laterally through the more competent strata of the Upper Cretaceous, from relatively shallow depths updip, to the deep areas of the Mesozoic shelf edge on the south. Thus, that portion of the network associated with the divergent wrench faulting, appears to have not only provided potential reservoir porosity improvement but also hydrocarbon migration pathways

from distant and nearby source generating areas in coeval and older rocks. Although reservoir enhancement has occurred mainly in Upper Cretaceous carbonate beds the migration pathways lead into even some Tertiary sandstone reservoirs.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.