Fracture Cementation and Its Effects from Permeability Directionality in Tensleep Sandstones, Wyoming

Peigui Yin and Shaochang Wo
EORI, University of Wyoming, Laramie, WY

Four types of natural fractures were observed in Wyoming Tensleep sandstones: gouge-filled, mineral-filled, partially-open, and open fractures. Gouge-filled and mineral-filled fractures are pervasive in most of the highly-permeable Tensleep sandstones. The majority of these filled fractures is normal to an anticline axis, and formed as a response to horizontal compression during folding. These fractures are potentially rooted deeper than those parallel to the structure axis, which were formed as a result of the bending of involved stratigraphic layers. Therefore, the axis-normal fractures had more opportunities to communicate with the deep geothermal fluids than the axis-parallel fractures. Geothermal fluids were responsible for the cementation of fractures and nearby permeable sandstone matrix. These filled fractures and their related cemented zones display much lower permeabilities than those of the associated sandstone matrix, and act as baffles or barriers in the subsurface flow regime.

Fractures in Tensleep oil reservoirs have a significant impact on water-flooding recovery. In this study, a simulation model based on observed outcrops and subsurface drilling cores was constructed to evaluate the effect of fracture cementation on local permeability alternation and sweep efficiency. The simulation model is a dual permeability/porosity system, representing those of the matrix and fracture respectively. The effective fracture permeability and porosity were estimated as functions of fracture width, spacing and the fraction of fracture filling. With different settings in fracture filling, from totally open to completely cemented, the simulation shows that the reservoir preferential permeability direction is largely aligned with the fracture orientation regardless of fractures being baffles or conduits.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas