Fluvial Architecture and Static Connectivity of the Williams Fork Formation, Central Mamm Creek Field, Piceance Basin, Colorado

摘要

The reservoir architecture, reservoir quality, and associated sandstone-body connectivity of the Williams Fork Formation at Mamm Creek Field vary stratigraphically and with depositional setting. The fluvial and shallow-marine sandstones were deposited within alluvial-plain, coastal-plain, and shallow-marine settings. The stratigraphic interval consists of porous but low permeability (tight-gas) sandstones that form the main gas-producing reservoirs in the Piceance Basin.Fluvial sandstones that are observed and interpreted in the Williams Fork Formation at Mamm Creek Field primarily include single-story channel bodies (e.g., channel bars; point bars) and crevasse splays. These deposits can be isolated bodies but also form amalgamated multistory channel bodies and channel complexes.Stratigraphic analysis of the fluvial deposits shows a relatively low, but variable, net-to gross ratio (30-76%) for the lower Williams Fork Formation with numerous laterally continuous coal beds. The middle Williams Fork Formation exhibits a relatively higher net-to-gross ratio (50-80%), and the net-to-gross ratio of the upper Williams Fork Formation ranges from 15 to 60%. Horizontal variogram correlation lengths of the sandstones are relatively short (u3c800 ft; 244 m) with respect to the distances between wells (330 ft [100 m] in north-south direction, and 1,320 ft [402 m] in east-west direction) and do not vary significantly stratigraphically. Variogram polar plots of the sandstones and effective porosity values indicate preferential trends of continuity in the north northeast to south-southwest direction.Three-dimensional reservoir models are used to explore how the fluvial sandstone bodies, effective porosity, and pay are distributed and interconnected. Static sandstone-body connectivity is greater than 52% for irregular 10-ac [660 ft; 201 m] well densities and decreases by as much as 25% with lower well densities (i.e., 40- and 160- ac). The middle Williams Fork Formation exhibits static connectivity values that are as much as 15% higher as compared to the lower Williams Fork Formation. Considering only reservoir-quality sandstones and calculated pay, results illustrate a decrease in static connectivity by as much as 57 to 74%, respectively, as compared to scenarios that include all sandstones. The static sandstone-body connectivity results provide a high estimate of connectivity, whereas the pay-based results provide a “base-case” or lower estimate of connectivity.