2003 Annual Meeting
Controls on Prolific Gas Production from Low-Permeability Sandstone Reservoirs in Basin-Centered Regions: Implications from the Rocky Mountain Region for Resource Assessment, Prospect Appraisal, and Risk Analysis
Keith W. Shanley1, Robert C. Cluff2, Lee T. Shannon3, and John W. Robinson4
Low-permeability, basin-centered gas systems (LPBCG) are projected by some researches to be a major source of natural gas supply for at least the next 2 decades. Recent resource estimates for the Greater Green River Basin alone suggest in-place resources of between approximately 3,000 and 5,000 Tcf gas and a “currently recoverable” resource of between 17 and 100 Tcf. Resource projections for adjacent Rocky Mountain basins are equally impressive. It has been widely suggested that LPBCG systems deserve consideration as a unique type of petroleum system. We suggest that, although these systems have a number of unusual attributes that distinguish them from more conventional oil and gas systems, their uniqueness reflects the low-permeability structure of the reservoir rock and neither implies, nor requires, a paradigm shift in terms of hydrocarbon systems. Examination of the Greater Green River Basin (more than 50 fields examined) indicates that more than 90% of the currently recoverable resource occurs in low-permeability, conventional traps. We conclude that LPBCG accumulations are best viewed as an end member within well-understood petroleum systems and should be evaluated in a manner similar to, and consistent with, the manner in which conventional hydrocarbon systems are evaluated.
The key distinction between conventional hydrocarbon systems and LPBCG systems lies in an understanding of pore geometry and multiphase permeability—in particular, effective permeability—at varying degrees of water saturation. Successful exploitation of gas resources within these LPBCG systems requires a focused, deliberate effort that fully integrates the unique petrophysical nature of these reservoirs with all elements of petroleum systems analysis, particularly an understanding of trap-related elements. Failure to recognize these essential elements of low-permeability reservoirs has led to a misunderstanding of the risks associated with basin-centered gas plays and a significant overestimation of available resource levels.