Geologic Controls on Reservoir Properties of Low-Permeability Sandstone, Frontier Formation, Moxa Arch, Southwestern Wyoming

Authors of this report summarize the results of integrated geologic characterization of the Frontier Formation along the Moxa Arch in southwestern Wyoming in four major areas of study: (1) stratigraphic and depositional systems, (2) diagenesis of reservoir sandstones, (3) disttribution of natural fractures, and (4) horizontal stress orientation. The authors describe the geological elements that were found to be important in characterizing these low-permeability sandstone reservoirs. Research for this report was conducted on behalf of the Gas Research Institute.ABSTRACTThe Upper Cretaceous Frontier Formation in Wyoming accounts for a significant portion of domestic tight gas production, reserves, and new completions. Tight reservoirs in the Frontier have produced more than 1 Tcf of gas, with most prolific production from the Moxa Arch in the westernGreen River Basin. Advanced technology will be needed to maximize gas recovery from the Frontier, and geologic characterization can contribute to understanding and efficiently developing these complex gas reservoirs. This report summarizes the results of integrated geologic characterization of the Frontier Formation along the Moxa Arch in southwestern Wyoming in four major areas of study: (1) stratigraphy and deposition systems, (2) diagenesis of reservoir sandstones, (3) distribution of natural fractures, and (4) horizontal stress orientation.Along the Moxa Arch, the Frontier Formation was deposited in fluvial and wave-dominated deltaic systems. Within these systems, strike-aligned shoreface sandstone and dip-oriented fluvial channel-fill sandstone form the most important reservoirs. The Frontier is divided into several sandstone-bearing intervals; of these intervals, the Second Frontier sandstone contains the most prolific gas reservoirs. The Second Frontier is composed of several sandstone benches, of which the First and Second Benches are most widespread. The First Bench comprises laterally discontinuous fluvialchannel-fill sandstones, whereas the Second Bench is a single progradational shoreface sandstone having good lateral continuity. The main depositional and stratigraphic controls on distribution and quality of Frontier reservoirs are sandstone continuity and detrital clay content. On the La Barge Platform, Second Bench upper shoreface sandstones have the lowest detrital clay content and occur at the top of the laterally continuous shoreface sequence. Most Frontier wells on the La Barge Platform have Second Bench perforations, although variable thickness and diagenetic modification influence the productivity of individual wells. The First Bench contains numerous discontinuous fluvial channel-fill sandstones, each composed internally of a complex arrangement of clay-rich and clay-free zones. Reservoir quality in the First Bench is highly variable, although it improves southward along the Moxa Arch.Diagenesis has overprinted depositional reservoir quality in these fine- to medium-grained litharenites and sublitharenites. Clean sandstones from all intervals contain an average of 1.6 percent primary intergranular porosity and 4.2 percent secondary porosity; microporosity averages 6.5 percent. Calcite, quartz, mixed-layer illite-smectite (MLIS), and illite are the most abundant cements. Authigenic MLIS clays consist of about 80 percent illite layers, suggesting that clays may be only moderately sensitive to fresh water. Low permeability in Frontier sandstones is caused by loss of porosity because of compaction, occlusion of pores by cements (particularly calcite and quartz), and lining of primary pores by fibrous illite and MLIS. Mean unstressed permeability to air is 0.19 md in upper shoreface sandstones (porosity = 14 percent), 0.13 md in fluvial channel-fill sandstones (porosity = 10 percent), and 0.08 md in lower shoreface sandstones (porosity = 13 percent). Although natural fractures are sparse in Frontier Formation core, production and hydraulic fracture-treatment results suggest that fractures are important reservoir elements in the Frontier. Fracture networks in outcrops have wide spacing and great lateral extent, which would tend to make them effective fluid conduits but difficult to intersect and detect with vertical wells. The direction of fracture strike can shift by 90 degrees between adjacent beds. Moreover, fractures commonly are in discrete, irregularly spaced swarms separated laterally by domains that have few fractures, rather than in regularly spaced, orthogonal fracture sets. Strikes of some fracture sets can be explained as resulting from regional tectonic extension directions. More challenging to predict are fracture orientation in a specific bed and the probability of encountering a dense cluster of fractures with hydraulic fractures or horizontal wells. Under typical reservoir conditions, the growth direction of hydraulic fractures parallels maximum horizontal stress (SHmax). The Green River Basin is in the Cordilleran east-west extension stressprovince but near the boundary with the east-northeast compressional midplate stress province. The boundaries between stress provinces are diffuse and may be characterized by transitional or inconsistent stress directions. Young north-striking normal faults along the western margin of the basin and borehole breakout data suggest east-west extension and approximately north-south-trending SHmax. In contrast, stress directions from some tests in the basin suggest an azimuth of 95" to 1 15' for the growth directions of hydraulic fractures in the Frontier Formation in the vicinity of the Moxa Arch. These directions are more consistent with the east-northeast SHmax direction of the nearby midplate stress province or growth of hydraulic fractures parallel to natural fractures. Geologic studies of the Frontier Formation have demonstrated that geological controls play a critical role in gas producibility. This report summarizes the geological elements that have been found to be important in characterizing these low-permeability sandstone reservoirs.