Formation evaluation in heterogeneous carbonate reservoirs is complicated by the presence of multiple pore types. Acoustic properties, Archie cementation exponents, and permeability transforms differ according to pore type. This study evaluates the various models for estimating vuggy porosity and the Archie cementation exponent from wireline logs, illustrates the difficulty of log interpretation applied to complex pore types, and presents generalized models developed to resolve some discrepancies within various models.

This report summarizes geologic, engineering, and production information on 24 low-permeability, gas-bearing sandstone reservoirs in 13 basins in the United States. The information compiled in this report comes from publicly available sources: Gas Research Institute and Department of Energy project reports, applications by gas producers to state regulatory agencies for tight formation designation, and published literature.

Discovery of volcanic glass detritus in fluvial deposits of the middle Frio Formation (Seeligson and Stratton fields) of South Texas has resulted in a new classification of reservoir types (type I and type II) within this formation. Type I reservoirs consist of sandstones that do not contain volcanic glass and have an average permeability of 36 md (geometric mean).