The Controls on Gas Production from Low-Permeability Reservoirs in the Rocky Mountain Region: Implications for Exploration and Resource Assessment
Shanley, Keith W.1, Robert M. Cluff1, John W. Robinson2
1 The Discovery Group, Inc, Denver, CO
2 Consultant, Littleton, CO
Gas fields producing from low-permeability sandstone reservoirs of Late Cretaceous and Early Tertiary age in the Greater Green River Basin (GGRB) of southwest Wyoming occur in conventional traps, not as part of a continuous-type gas accumulation or basin-center gas system where productivity is governed by the occurrence of ‘sweet-spots’. Examination of gas fields with an expected-ultimate-recovery (EUR) in excess of 50 BCF shows that 38% of the gas fields involve structural traps accounting for 50% of the gas production from the GGRB, 40% of the gas fields involve stratigraphic traps contributing 30% of the gas production, and 22% of the fields occur in combination traps contributing 20% of the gas production. Production and petrophysical data suggests that the basin is neither regionally gas-saturated, nor is it near irreducible water saturation; water production is both common and widespread. Examination of approximately 7800 gas wells in the GGRB indicates that almost 70% are producing measurable formation water (water in excess of ‘water of condensation’). These wells account for slightly more than 50% of the gas production from the GGRB.
Virtually all resource assessments in the GGRB have assumed a widespread, continuous-type resource distribution resulting in very large assessed volumes. We suggest that resource volumes in the GGRB are likely to have been overstated and that the GGRB, as well as other basins similar to the GGRB, should be evaluated in a manner similar to, and consistent with conventional hydrocarbon systems. Low permeability gas systems such as those found in the GGRB do not require a paradigm-shift in terms of hydrocarbon systems as some have suggested. Considerable potential remains for natural gas discoveries and subsequent production in the GGRB, however, the risks associated with finding and producing these resources has generally been under-appreciated.