A robust play assessment of the Mesaverde basin-centered gas accumulation in the Piceance Basin has contributed significantly to our understanding of the quality of that potential opportunity. Because of the unconventional nature of the play, the assessment required a nontraditional approach. Ultimately, an assessment based on well performance will provide the clearest indication of the recoverable resource and economic viability of the play. Unfortunately, performance-based approaches are not reliable until a high operational efficiency has been achieved and the number of wells with adequate production histories is sufficient to elucidate the primary controls on resource density. We have used a staged approach that starts with a multipronged volumetric assessment and allows systematic transition into a performance-based assessment as data permit. Although the analysis is performed on a play (regional) scale, the calculations are more similar to a multiple-segment prospect assessment than to a traditional play assessment. Initially, the play is assessed as a small number of large segments, each of which has a full probabilistic analysis. As the level of understanding increases, the play is subdivided geographically and stratigraphically into progressively smaller segments to better honor the observed regional trends within the geologic controls. Concurrently, the entire play is subdivided into a continuous framework, or grid, of well drainage areas. Deterministic calculation of recoverable hydrocarbon volume per well drainage area documents the geographic variation of resource density within the probabilistic analysis segments. This map provides a direct link for calibrating volumetric assessment calculations with well performance data as they become available.

The utility of a play assessment for exploration and development is enhanced when the results are tightly coupled to the primary geologic controls. To this end, several key observations from regional studies of the Piceance Basin have been incorporated into the assessment. For example, reservoir geometry is a critical factor in the production of gas and water within the basin. Accordingly, the stratigraphic section has been subdivided in eight assessment intervals mostly based on depositional facies and resultant sandstone geometry. Likewise, the top of the continuous gas within the basin is an important control on resource density. Although sandstones above this surface may contain some gas, they tend to produce large volumes of water (transitional zone of Hoak and Decker, 1995). This surface is complex and appears to cut across stratigraphic boundaries. We have used several alternative approaches to best map this effective vertical limit on the play. In our most likely interpretation, the top of the continuous gas mapped from log data was used to truncate the top of the assessed interval within the Mesaverde Group.

To accommodate the complexity of geologic inputs, the volumetric assessment was implemented using an automated GIS (geographic information system)-based work process. For each of the eight assessment subdivisions within the Mesaverde Group, maps of gross interval thickness, interval net-to-gross ratio, porosity, and formation volume factor were represented as grids in GIS. Data constraints for these maps were variable, but included approximately 400 key wells from across the basin and limited two-dimensional seismic coverage. Geographically, the basin was divided into 11 subregional polygons based on height of the gas column, reservoir facies, high structural dip and faulting (areas of high risk for water production), and formation pressure gradient. Most input parameters for volumetric calculations were extracted by processing the appropriate grids. Using this process, any updates to the input data sets were automatically incorporated into the analysis on the next run. Some key assessment parameters, including gas saturation, gas recovery efficiency, and condensate yield, were worked in considerable detail but at only a small number of locations. The available constraints did not warrant a continuously varying representation using grids, so these parameters were instead assigned by stratigraphic interval within each subregional polygon. With the exception of the polygon area, all input parameters were entered as distributions to represent the inherent uncertainty in the analysis.

Based on the assessment inputs, different volumetric calculations were performed to address different business questions. A Monte Carlo simulation was performed for each segment to estimate both the mean potential and the range of possible outcomes. Sensitivity analyses indicate that the calculated range is strongly dependent on the granularity of analysis polygons. The grid-based deterministic calculation captured geographic variation of resource density within each subregional polygon. Deterministic results were scaled in units of billion cubic feet per well drainage area to facilitate comparison to available well performance data. Such maps are useful in optimizing exploration and development strategies within the basin by identifying areas with the greatest resource potential. For both approaches, scenario analysis was used to evaluate the sensitivity of the results to alternative interpretations. Scenarios included different geologic concepts, such as alternative top of gas interpretations, as well as development strategies, such as excluding selected stratigraphic assessment intervals or limiting the total drill depth. Filtering the results by drill depth can be a key factor in economic analysis.