Fracture Enhanced Permeability Within a Crystalline Porous Dolostone Matrix: a Reservoir Model in the Knox Formation, Maben Gas Field, Mississippi

Julia F. Gale¹, Michael S. Cameron², and Robert M. Reed^1
1Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, TX
²Devon Energy Corporation, Oklahoma City, OK

Fracture and matrix permeability constitute the hydrocarbon reservoir in the upper Knox Formation in the Maben Field, Mississippi. Two whole cores from boreholes were examined for reservoir characterization. Matrix porosities average 5-9 % with matrix permeabilities at the 1 md range. The dolostone consists of subhedral to euhedral microcrystalline dolomite with intercrystalline solution-enhanced pores. A ubiquitous fracture system has developed alongside this diagenetic fabric. Two sets of subvertical opening-mode (extension) fractures are apparent; an early F1 set and a later F2 set. Distinction is based on cross-cutting relationships and differences in aspect ratio. The F1 fractures are short and wide compared to the F2 fractures. Aperture sizes range from 0.05 to 3.3 mm and fracture heights range from about 1 cm to more than 20 cm. Fracture intensity studies suggest fractures follow power-law aperture size distributions. Most narrow fractures in both sets are filled with dolomite. Larger fractures may be lined with dolomite and dolomite bridges are common. Crack-seal relationships in the bridges show that dolomite is synkinematic in both fracture sets, and must therefore be of two different ages (D1 and D2). Gaps between bridges in the F1 fractures tend to be sealed with postkinematic calcite cement (C1), except in highly porous euhedral dolostones that lack calcite. Similar relationships apply to F2 fractures but by contrast more of these fractures remain open, because calcite postkinematic to F2 fractures (C2) was less widespread than C1 calcite. Thus, recognition of diagenetic changes that were concurrent with fracture development is fundamental to characterizing fracture openness and to predicting permeability enhancement.

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