The completion of a recent Gas Research Institute study (GRI -00/0026) allowed for significant insight into the controlling mechanisms for gas production in Siberia Ridge Field, southwestern Wyoming. The purpose of the study was to characterize the Almond Formation (Cretaceous) in Siberia Ridge Field to better understand controls on productivity and to compile this information for use as an analogue in similar tight gas sandstone reservoirs. Of particular interest was the role that natural fractures play on productivity, because this reservoir's average porosity ranges from eight to ten percent and matrix permeability is in the micro-darcy range. The Siberia Ridge Reservoir Characterization (SRRC) study comprises full-field geoscience, petrophysical and engineering analyses and the results of a GRI cooperative research well. As an industry partner in this study, Amoco Production Co. committed to drilling and completing a well to test a "sweet spot" hypothesis as well as gather data utilizing a comprehensive log and well evaluation suite that is generally unavailable in developed areas. The Siberia Ridge Unit #5-2 well (Sec. 5, T21N - 94W) commenced drilling in late 1997 and was successfully completed in February 1998 for an IP of 2.56 MMcfd and 240 BWPD on a 32/64" choke. The well was directionally drilled to intersect a predicted system of natural fractures interpreted primarily from well data.

Gas production in Siberia Ridge Field is quite variable, with EURs averaging 1.8 BCF and ranging from less than 0.5 BCF to nearly 20 BCF. At the beginning of the study it was unknown whether the locations of productive sweet spots were controlled by increased natural fracturing, by better matrix quality, or by completion practices.

Geological facies analysis and petrophysical data were mapped along with production data to determine productive trends. Several horizons within the 3-D seismic survey were picked, and isopach derivative maps were made. Coherency analysis was run on several horizons to determine the location of any significant linear features. The combination of depositional, petrophysical, and structural data revealed that mapped areas of better petrophysical properties generally indicated better production, even though the range in reservoir quality is very small.

Fullbore Formation Microimager (FMI) and core data from three wellbores were used to characterize the natural fracture system. These fractures appear to be related to the regional extension fracture network and are ubiquitous in the wellbores studied. Natural fracture density was found to be mostly a function of wellbore depth, lithology, and deviation as well as a function of linear feature proximity. Rather than providing increased conductivity to natural gas in the reservoir intervals, natural fractures were found to provide increased relative permeability to water in the deep Almond. The presence of natural fractures is not thought to be a significant factor in gas production; rather, proper well completion practices were found to be critical to well performance.

The enhanced understanding of the Almond reservoir in the Siberia Ridge Field provided by this multidisciplinary study can be used by the operator to improve drilling, completion and production practices, ultimately impacting well economics by decreasing risk and increasing recoverable reserves.

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