A reassessment of gas resources in selected Upper Cretaceous biogenic gas accumulations in southeastern Alberta and southwestern Saskatchewan, Canada

Abstract

Abstract Tens of thousands of production wells drilled during the past 100 years reveal that biogenic gas accumulations in the Upper Cretaceous succession in southeast Alberta and southwest Saskatchewan are a regionally pervasive gas field with mixed reservoirs and varying resource density across the region. Historical pool boundaries have disappeared gradually as a result of in-fill drilling. This suggests that previous resource assessments that used methods based on feature counting, such as the number of pools and individual pools sizes, may have significantly underestimated the resource potential of this field because the resource potential occurring between the pool boundaries were largely ignored and the areal extent of the field is still growing geographically. This study used available historical production data and employed a well performance-based method to re-assess the natural gas potential of this giant gas field. Three major production intervals, Medicine Hat, Milk River and Second White Speckled formations in Upper Cretaceous successions, were assessed. The geographical locations of 86 561 production wells with production from one of these three intervals were used to define the play boundaries. More than ten thousand production wells with historical records were collected and analyzed. All wells with comingled production from more than one zone were excluded to eliminate the impact from mixed contributions from multiple intervals. The remaining 2783 production wells with production from a single formation were used in the Estimated Ultimate Reserve (EUR) calculation for each of the three intervals. The estimated total technically recoverable natural gas resource in the three stratigraphic intervals vary from 30.2 to 73.3 TCF (P90 to P10) with a median of 43.6 and a mean of 50.1 TCF. The total inferred resources obtained for this study are much larger than those obtained previously for this field.