Fracture Diagenesis and Its Effects from Reservoir Permeability in Tensleep Sandstones, Wyoming

Peigui Yin
EORI, University of Wyoming, Laramie, WY

Fractures in reservoir sandstones have been characterized intensively on intensity, aperture, orientation, and connectivity, but there is a gap between these characterizations and fracture permeability. This gap is fracture diagenesis, which determines whether a fracture serves as permeability conduit or barrier. Fracture diagenesis, counting all chemical, physical, and biogenetic changes after fracture generation, includes cementation, granulation, dissolution, and replacement within fractures and their associated belts in the matrix sandstones.

Five types of natural fractures were observed in Wyoming Tensleep sandstones: gouge-filled, gouge and mineral-filled, mineral-filled, partially-open, and open fractures. The filled fractures are pervasive in most of the highly-permeable Tensleep sandstones, whereas open, partially open and mineral-filled fractures are usually observed in tight Tensleep sandstones. Most the original fracture cements are anhydrite, which probably resulted from re-distribution of anhydrite in the deep marine carbonate-evaporate sections. Granulation of the fracture-filling minerals indicates multi-episodes of fracture reactivation. Dissolution of the anhydrite cement has contributed some partially-open or open fractures in the Tensleep sandstones. Calcite replacement of anhydrite is commonly observed in the outcrops and shallow depths. The filled fractures and their related cemented zones display much lower permeabilities than those of the associated permeable sandstone matrix, and act as baffles or barriers in the subsurface flow regime. Whether the fractures are filled or open, the permeability directionality in the Tensleep sandstones is always parallel to the fracture orientation.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado