Abstract Geological complexities such as variable permeability and structure (folds and faults) exist to a greater or lesser extent in all subsurface environments. In order to identify safe and effective sites in which to inject CO 2 for sequestration, it is necessary to predict the effect of these heterogeneities on the short- and long-term distribution of CO 2 . Sequestration capacity , the volume fraction of the subsurface available for CO 2 storage, can be increased by geological heterogeneity. Numerical models demonstrate that in a homogeneous rock volume, CO 2 flowpaths are dominated by buoyancy, bypassing much of the rock volume. Flow through a more heterogeneous rock volume disperses the flow paths, contacting a larger percentage of the rock volume, and thereby increasing sequestration capacity. Sequestration effectiveness , how much CO 2 will be sequestered for how long in how much space, can also be enhanced by heterogeneity. A given volume of CO 2 distributed over a larger rock volume may decrease leakage risk by shortening the continuous column of buoyant gas acting on a capillary seal and inhibiting seal failure. However, where structural heterogeneity predominates over stratigraphic heterogeneity, large columns of CO 2 may accumulate below a sealing layer, increasing the risk of seal failure and leakage.

Abstract Depositional porosity and permeability distribution in Guadalupian-age grainstone-dominated parasequences of the central Guadalupe Mountains have been moderately to strongly modified by diagenesis. Early diagenetic features in the ramp crest and inner shelf crest can be related to inferred paleohydrology that developed during high-frequency (parasequence-scale) sea-level lowstands. In the San Andres example, leaching of carbonate grains and resulting precipitation of intergranular cement in a meteoric lens beneath an emergent bar crest produced a well-defined, highly porous but relatively low-permeability moldic zone in the thickest part of a grain-dominated parasequence. In the Grayburg example, preferential preservation of intergranular porosity and minor leaching along a paleo-water table produced a thin (less than 30 cm) stratiform zone of higher than average permeability. A zone of preferentially preserved intergranular porosity developed within an inferred meteoric lens beneath an emergent bar crest. Diagenetically influenced lateral and vertical changes in permeability distribution are predicted to be typical of intermittently exposed ramp crest and inner shelf crest environments. These examples demonstrate the efficacy of parasequence-scale mapping of depositional and diagenetic facies for improving prediction of permeability distribution in analogous strata.

Abstract Ochoan evaporites of the Castile and Salado formations in the Delaware Basin contain information about the final stages of basin evolution, specifically regarding the nature of the connection to the ocean and stability of local sea level. Core from the Gulf PDB-03 research well, Menton area, Loving County, Texas, was examined from near the base of the Castile Formation to the uppermost preserved halite in the Salado Formation (Figs. 1 and 2) to investigate evaporite depositional environments, especially with respect to paleodepth. A list of criteria for interpretation of paleodepth was developed (Table 1). These criteria are based on (1) comparison of observed evaporite fabrics with similar fabrics in modern and other ancient examples, and (2) interpretation of how observed evaporite processes would effect fabrics in different settings. Comparison of depth criteria with sequences of fabrics observed in the Castile and Salado formations shows systematic textural variation from the base to the top of individual depositional cycles as well as the entire sequence (Figs. 1 and 2).