Fluvial architecture and connectivity of the Williams Fork Formation: combining outcrops analogs and reservoir modeling for stratigraphic reservoir characterization
Matthew Pranter, J. Alexandra Sloan, Alicia Hewlett, James Gilman, Hai Zui Meng, and Rex Cole
The Upper Cretaceous Williams Fork Formation in the Piceance Basin, Colorado, is a succession of fluvial channel sandstones, crevasse splays, floodplain mudstones, and paludal coals that was deposited by meandering- and braided-river systems within coastal- and alluvial-plain settings. Williams Fork Formation outcrops on the basin margin serve as proximal reservoir analogs because the strata dip into the Piceance Basin where they form natural-gas reservoirs. Field descriptions, global-positioning-system traverses, and a combination of high-resolution aerial light detection and ranging data (LiDAR), digital orthophotography, and ground-based photomosaics were used to map and document the abundance, stratigraphic position, and dimensions of single-story and multistory channel bodies and crevasse splays. The mean thickness and apparent width of the 688 measured sandstone bodies are 12.1 ft (3.7 m) and 364.9 ft (111.2 m), respectively. Outcrop data show that most of the fluvial sandstone bodies are smaller than the distance between wells at 10-ac spacing (660 ft [201 m]). Given the lower net-to-gross ratio (N:G) of some Williams Fork intervals, this underscores the importance of representative sandstone-body statistics to aid in subsurface correlation and mapping and to constrain 3-D reservoir models. Three-dimensional static and dynamic reservoir models of the Williams Fork Formation that are constrained to both outcrop and subsurface data show how sandstone-body connectivity is sensitive to fluvial deposit type and width and varies with N:G and well spacing. Static connectivity analyses of 3-D outcrop models of architectural elements show that relatively wide sandstone bodies enhance connectivity for a given N:G and well spacing. For a 20-ac well spacing, static connectivity can range from approximately 35 to 75% and 45 to 80% for a N:G of 10 and 15%, respectively, depending on sandstone-body width. At Mamm Creek Field, static connectivity of pay sandstones within the higher N:G middle Williams Fork Formation is 12 to 18% greater than the lower Williams Fork Formation (lower N:G). Based on highly constrained subsurface 3-D reservoir models using object-based and multiple-point geostatistical methods, static and dynamic connectivity also vary due to mudstone plugs (channel-abandonment fill) and crevasse splays. Future research will focus on the impact of crevasse splays and mudstone drapes on reservoir connectivity and storage.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012