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41582-G8
Quantification of Fluvial Stratigraphic Architecture Using High-Resolution Laser Imaging (LIDAR), Williams Fork Formation, Colorado: Implications for 3-D Reservoir Modeling
This project evaluates fluvial channel-body (sandstone-body) dimensions and architecture using outcrop analogs of the Lower and Upper Williams Fork Formation in Coal and Plateau Creek Canyons, Piceance Basin, Colorado. The exposures within Coal and Plateau Creek Canyons are stratigraphically analogous to the undifferentiated interval of the Williams Fork Formation in the subsurface that forms petroleum reservoirs within the Piceance Basin. The sandstone-body characteristics and architecture are evaluated using high-resolution aerial LiDAR, digital orthophotographs, photomosaics, behind-outcrop cores, nearby well logs, and field measurements. Sandstone bodies in the Lower Williams Fork Formation were deposited within a meandering-stream system and are grouped into 3 major types: single-story channel-fill (Cf), multistory channel-fill (MCf), and crevasse-splay (CS) deposits. Exposed sandstone bodies (Cf, MCf and CS) have been mapped using aerial LiDAR, orthophotos, and land-based photomosaics. There are 633 sandstone bodies measured along a 5.7 mi (9.2 km) transect in the NE-trending and NW-trending segments of Coal Canyon. These consist of 109 (17%) channel-fill, 258 (41%) multi-channel, and 266 (42%) crevasse-splay sandstone bodies. Single-story channel-fill (Cf) sandstone bodies range in thickness from 4 to 21 ft (1.2 to 6.4 m). Multistory channel-fill (MCf) sandstone bodies range in thickness from 4.5 to 32.5 ft (1.3 to 10 m), and crevasse-splay (CS) sandstone bodies range in thickness from 0.5 to 6.5 ft (0.1 to 2 m).
Three-dimensional architectural-element models constrained to outcrop-derived sandstone-body dimensional statistics show that connectivity is controlled largely by sandstone content. The Lower Williams Fork Formation is a relatively low net-to-gross ratio (< 50% sandstone) succession of approximately 700 feet of fluvial channel-fill sandstone bodies, crevasse splays, floodplain mudstones, and subordinate coal interpreted to have been deposited in a highly sinuous meandering river system in a coastal-plain setting. Through this project, we have developed two computer programs, ECONN and WCONN to evaluate the static connectivity of fluvial sandstone bodies of 3-D reservoir models. These programs determine the number, volume, net-to-gross ratio, and other parameters for sandstone bodies by considering the entire reservoir model domain or based on different well patterns (e.g., 10-acre well spacing). Static connectivity of these high-sinuosity meandering-fluvial sandstones is most sensitive to sandstone-body widths; orientation and objects used to simulate Lower Williams Fork Formation sandstones in boolean models have a less pronounced effect on static connectivity. Connectivity analyses reveal an S-curve relationship between net-to-gross ratio and connectivity at 160-acre well spacing. As well density increases, the S-curve relationship diminishes and connectivity increases linearly with increasing net-to-gross ratio. At dense well spacings, there are enough wells to intersect a high proportion of sandstones, regardless of sand content. Even at 10% net-to-gross, 10-acre spacing connectivity is typically above 60% and can be as high as 85%. Infill drilling scenarios increasingly intersect a higher percentage of crevasse splays; recent work in avulsion stratigraphy suggests that presence of crevasse splays can enhance static connectivity.
The Upper Williams Fork Formation in Plateau Creek Canyon is characterized by sheet-like sandstone bodies and associated mudrocks that include planar-to-trough cross-stratified sandstone, convolute-to-wavy-laminated siltstone, and calcareous, fissile, silt-dominated shale. These sandstones bodies are interpreted to have been deposited in a low-sinuosity sand-dominated, braided alluvial plain setting. Based on 113 sandstone-body measurements from aerial LiDAR data, fluvial sandstone bodies include single channels to multiple-story-channel complexes with dimensions ranging from 0.30 – 16.5 m (1.0 - 54 ft) in thickness (or composite thickness) and 62 - 782 m (204 - 2565 ft) in apparent width. The sandstone bodies have an average apparent width-to-thickness ratio (W:T) of 34:1. Between-well correlation of the sandstone bodies at a reservoir scale is tenuous. Amalgamation of sandstone bodies is common in this high net-to-gross system; however, thin laterally extensive interbedded mudrocks exist that could compartmentalize sandstone bodies at the reservoir scale.
These data and results have been utilized by industry sponsors of the project and other institutions to improve their 3-D fluvial characterization and modeling efforts of analogous subsurface petroleum reservoirs. The principal investigator, associated student researchers, and faculty collaborators were awarded best poster for a presentation of these results at the 2006 Rocky Mountain Section AAPG Convention. Based on the results, this research will likely expand to address additional aspects of fluvial sandstone-body geometry, architecture, and prediction.
