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Abstract: Cements and In-Situ Widths of Natural Fractures, Almond Formation, Green River Basin, Wyoming

Thomas L. Dunn, Ronald C. Surdam

Natural fractures in the Mesaverde Group of the Green River Basin, Wyoming, potentially provide higher production rates and/or increased estimated ultimate recoveries. Therefore, fracture width and cementation history are key reservoir characteristics. Cementation history and widths at in-situ pressures were determined using Almond cores from the Echo Springs area. Samples were epoxy impregnated at an effective horizontal pressure of 2500psi. Polished thin sections were used to image the in-situ widths of fractures using scanning electron microscopy.

Initial quartz overgrowths were locally followed by sparse kaolinite cement. Patchy barite cement followed the kaolinite. Sparry calcite was last. Both barite and calcite prop open the fractures; quartz does not.

In-situ fracture widths (both open and mineralized) were measured along the length of the fracture traces. Average widths range from 42 to 192 microns. Apparent one-inch plug permeabilities calculated from open fracture widths range from 300md to 7 darcies. In-situ fracture permeabilities of this magnitude should provide high rates of production if the fractures are numerous, have extensive areal dimensions, and are not plugged with either natural cements or drilling and completion fluids. Borehole imaging of the sampled well indicates numerous fractures at spacing of 1 to 2 meters. Yet the production rates do not support fracture production. This study and borehole imaging indicates the presence of open fractures at in-situ conditions. Hence, the areal extent of the fractures is limi ed and/or the fracture apertures have been plugged during drilling and completion.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada