ABSTRACT: Application of Outcrop Information in the Optimization of Subsurface Reservoir Characterization for Reserve Growth

TYLER, NOEL, and EDGAR H. GUEVARA, Bureau of Economic Geology, University of Texas at Austin, Austin, TX, and NANETTE KUICH, Mobil Exploration and Producing U.S., Inc., Houston, TX

The Cretaceous Ferron Formation, central Utah, is considered an excellent analog for fluvially dominated deltaic reservoirs deposited under conditions of high rates of subsidence and accommodation development. Strongly progradational early genetic sequences that are succeeded by aggradational and, finally, retrogradational genetic sequences provide an excellent opportunity to contrast architectural style and permeability structure in seaward- versus landward-stepping deltaic sandstones. Outcrop characterization of geometric and dimensional parameters supplemented by high-resolution permeability data provides an excellent data set to constrain subsurface interpretation of analogous reservoirs.

Lake Creek field, north of Houston, has produced 120 bcf of gas from distributary-channel, mouth-bar, and delta-front reservoirs of the Eocene lower Wilcox Group. Eighteen gas-condensate reservoirs, ranging in depth from 9,200 to 14,500 ft, have moderate porosity (12 to 20 %) and low permeability. One of these reservoirs, the 300-ft-thick G Sandstone, is composed of a multistoried reservoir suite separated by flooding surfaces. Four landward-stepping parasequences comprise genetically related depositional facies showing spatial distributions and dimensions comparable to the Ferron. Mud-rich flooding surfaces provide primary barriers to vertical gas flow and effectively vertically compartmentalize the G Sandstone. Lateral heterogeneity, a response to deep incision by distributary chann ls into mouth-bar and delta-front sandstones, is also pronounced.

Delta-front facies are the most laterally persistent (on the order of several miles), mouth-bar facies have intermediate continuities (maximum lateral extents of 1 to 2 mi), and channel deposits are the most areally restricted (less than 6,000 ft in strike section). Outcrop characterization of analogous deltaic deposits in the Ferron has demonstrated that mud-clast-rich zones compose the fringes of the channel sandstones and that these zones are low-permeability barriers that effectively areally segment the reservoir.

Low permeability and corresponding limited drainage radii in laterally extensive delta-front facies of the G Sandstone may locally result in unrecovered gas reserves at wide completion spacings. In contrast, in the more proximal facies infill drilling and recompletion opportunities result from the complex style of vertical and lateral heterogeneity. Optimized recovery efforts can target facies-based heterogeneity where, for example, barriers or baffles to gas flow along facies boundaries adversely affect hydrocarbon recovery, as is suggested by a recent infill well targeting channel-mouth-bar facies in the G-4 parasequence that flowed 1.7 MMCFD after fracture stimulation. Permeability analyses indicate that these channel-mouth-bar and adjacent distributary-channel sandstones are poten ial "sweet spots" and may represent the best opportunity for incremental gas reserves in similar stratigraphically complex parts of the deltaic reservoir system.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)