Application of Facies Analysis to Improve Gas Reserve Growth in Fluvial Frio Reservoirs, La Gloria Field, South Texas

W. A. Ambrose, M. L. W. Jackson, R. J. Finley

Geologically based infill-drilling strategies hold great potential for extension of domestic gas resources. Traditional gas-well drilling and development have often assumed homogeneous and continuous reservoirs; uniform gas-well spacing has resulted in numerous untapped reservoirs isolated from other productive sand bodies. Strategically located infill wells drilled into these undrained reservoirs may ultimately contact an additional 20% of original gas in place in Texas gas fields.

Tertiary formations in the Texas Gulf Coast commonly exhibit multiple fluvial and fluvial-deltaic reservoirs that contain vertical and horizontal permeability barriers. For example, the Frio La Gloria field (Jim Wells and Brooks Counties, Texas) contains isolated and compartmentalized reservoirs that can be related to the irregular distribution of heterogeneous facies. Net-sand and log-facies maps in areas of dense well spacing delineate relatively continuous pay defined by lenticular point-bar and channel-fill deposits 1,500-2,500 ft wide. These point-bar deposits are flanked laterally by sand-poor levee and splay facies that isolate the reservoirs into narrow, dip-elongate bands.

Volumetric methods of pay-continuity estimation, based on correlation of zones of high hydrocarbon saturation, may not realistically characterize the hydrocarbon potential of complex fluvial deposits in La Gloria field. However, facies boundaries superimposed on hydrocarbon-saturation cross sections can be used to subdivide the reservoirs into isolated compartments. Recognition of facies distribution in productive intervals can lead to identification of untapped areas and the optimum location of infill wells.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.