Case Studies Examining the Discovery Sequence and Gas Accumulations in Tight-Gas Sandstones Coleman, James

摘要

An examination of the geologic characteristics and discovery history of seven plays, which were originally classified by the U. S. Geological Survey (USGS) in 1995 as continuous-type gas sandstone plays, shows that these plays have a high degree of similarity with conventional discrete accumulations in terms of reservoir continuity, sand body geometry, and trapping configurations. The general decline in discovery size with increasing numbers of discoveries suggests a means to put limits on volumes of resources assessed in un-drilled areas of a particular play. Routine time-series analyses of conventional plays typically show a decline in field discovery size as each subsequent discovery within the play trend is announced. If gas accumulations in low-permeability sandstone plays occur in trap settings typical of discrete conventional accumulations, then modeling of the discovery sequences within plays may provide an effective way to constrain regional estimates of remaining recoverable resources. At the other extreme, if the play is regarded as a single homogeneous continuous entity (albeit, with some "sweet spots"), only the play boundary constrains the number of un-drilled sites that could contribute to remaining recoverable resources, and there should be no general decrease in discovery size. The seven continuous-type gas sandstone plays selected for this study had a sufficient number of observations to test whether discovery size correlates with sequence of discovery. These showed that discovery size tends to decline with sequence of discovery and in three of the seven the trend was statistically significant. The discovery size rank and sequence relationship was found to be similar to several well known conventional plays.