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A simple, new porosity/permeability-depth profile was developed from available laboratory measurements on Triassic sedimentary red beds (sandstone) from parts of the South Georgia Rift (SGR) basin in order to investigate the feasibility for long-term CO2 storage. The study locations were: Sumter, Berkeley, Dunbarton, Clubhouse Crossroad-3 (CC-3) and Norris Lightsey wells.As expected, both porosity and permeability show changes with depth at the regional scale that was much greater than at local scale. The significant changes in porosity and permeability with depth suggest a highly compacted, deformed basin, and potentially, a history of uplift and erosion. The permeability is generally low both at shallow (less than 1826 ft/556.56 m) and deeper depths (greater than 1826 ft/556.56 m). Both porosity and permeability follow the normal trend, decreasing linearly with depth for most parts of the study locations with the exception of the Norris Lightsey well. A petrophysical study on a suite of well logs penetrating the Norris Lightsey red beds at depths sampled by the core-derived laboratory measurements shows an abnormal shift (by 50%) in the acoustic travel time and/or in the sonic-derived P-wave velocity that indicates possible faulting or fracturing at depth. The departure of the Norris Lightsey's porosities and permeabilities from the normal compaction trend may be a consequence of the existence of a fault/fracture controlled abnormal pressure condition at depth. The linear and nonlinear behaviors of the porosity/permeability distribution throughout the basin imply the composition of the SGR red beds, and by extension analog/similar Triassic-Jurassic formations within the Eastern North American Margin has been altered by compaction, uplift, erosion and possible faulting that have shaped the evolution of these Triassic formations following the major phase of rifting.

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The lack of the permeability log data necessary to assess reservoir injectivity as well as aid in the correlation and interpretation of existing porosity and resistivity logs for reservoir quality characterization for potential CO2 storage in the heterogenous and complex South Georgia Rift (SGR) basin provides the motivation for this study. The focus was on the Triassic-Jurassic red beds buried, entrenched beneath the Cretaceous-Cenozoic Coastal Plain sediments. Moreover, the significant cost typically between $10 M to $100M associated with drilling and logging for in situ permeability coupled with the limited resolution of existing core data further makes this work necessary. The purpose is to relate, use the interpretation of the predicted permeability distribution to assess feasibility for safe and long-term CO2 sequestration. This study also intends to establish the impacts of active and passive tectonism that has shaped and/or re-shaped the evolution of the basin on the present-day permeability. A methodology was applied that utilizes the pore space and geohydraulic properties of the reservoir from existing laboratory and well data to produce a newly derived permeability log. It shows a non-uniform distribution with depths possibly due to geologic changes in the confined and heterogeneous red beds. The derived log displays characteristics consistent with observations from the porosity and resistivity logs. The interpretation of these logs provides evidence for the presence of low permeable, tightly cemented and compacted red beds. We conclude that the low permeability aided by the low resistivity depicted in the red beds suggests increased confining stress and reduced injectivity, and that the uncharacteristically low permeability reflects a deformed basin shaped with episodes of uplift and erosion.

The primary mechanism for Coal Bed Methane (CBM) recovery in a typical biogenic, low-rank, shallow and subbituminous coal like that of the Powder River Basin (PRB) of Wyoming is by dewatering or depressurization of the coal beds. In this case history, cross-well seismic experiments were designed to image changes in seismic signatures emanating from the dewatering process. Quantitative estimation of these changes is vital to the technical viability of the CBM reservoir process optimization and monitoring. The pre-and post-CBM production cross-well seismic surveys were executed in July, 2002 and August, 2003 respectively at the PRB's Big George coal beds, Wyoming.Following 2D tomography of the acquired timelapse data sets, the resulting tomograms manifest rocks with different seismic velocities and petrophysical properties. Consequently, we identified 4 discrete subsurface layers: shaly-sand, sandy-shale, the low-velocity coal-bearing formation and sandstone. This result is consistent with the qualitative interpretation of the gamma ray logs in the 2 observation wells that were used for the surveys. Moreover, comparison between the two travel times-inverted tomograms yields P-wave velocity changes of 5% to 7% due to gas production. These observed changes are corroborated by our theoretically derived velocity models, underscoring the effectiveness and applicability of cross-well seismic experiments in imaging the CBM production process.

and Bill Clendenin for their contributions to this study. We also thank Professor Manika Prasad of the Colorado School of Mines in Golden, Colorado, for access to their Rock Physics Laboratory facilities. The thin sections that we used were provided by Dr.James Rine of the Weatherford Laboratories. We acknowledge the helpful comments from our anonymous reviewers.