Sand-Body Architecture in the Lower Williams Fork Formation (Upper Cretaceous), Coal Canyon, Colorado, with Comparison to the Piceance Basin Subsurface

摘要

This paper summarizes the results of a sedimentologic and stratigraphic examination of lenticular sand bodies in the lower 700 ft of the Williams Fork Formation (Upper Cretaceous), as exposed in Coal Canyon, near Palisade, Colorado. The study interval is a low net-to-gross sequence (less than 50% sandstone) deposited in a lower coastal-plain setting. Fluvial systems were sinuous (meandering) to anastomosing, floodplains poorly drained, and peat-forming mires, marshes, swamps, and lakes were common. The lower Williams Fork includes the Cameo-Wheeler coal zone and is stratigraphically equivalent to part of the productive interval in gas fields in the southern Piceance Basin. Within the study area (-800 acres), 136 lenticular to channel-form sand bodies were mapped using GPS receivers. Stratigraphic control was defined by 15 measured sections. The 136 sand bodies can be grouped into five types: 1) narrow (Type A); 2) simple sinuous (Type B); 3) compound sinuous (Type C); 4) crevasse-channel (Type D); and (5) crevasse-splay (Type E). For the 136 sand bodies, the average sand-body thickness ranges from 0.5 to 29 ft (mean average = 9 ft), and the range in apparent sand-body width (defined as the linear distance between the two terminations of a sand body outcrop) is from 40 to 2,791 ft (average = 528 ft). Type C sand bodies are the thickest and the widest and have the best reservoir potential, followed, by the type B and A sand bodies. Type D and E sand bodies have the least potential. The paleocurrent values (N = 1,646) exhibit a unimodal distribution with a vector mean of 74.8° and a circular standard deviation of 56.1°. Paleocurrent values for the type A, B, and C sand bodies have weak bimodal distributions and vector means of 45°, 91°, and 72°, respectively. Type D and E sand bodies have polymodal distributions, with vector means of 44° and 46°, respectively. Williams Fork fluvial sands are being actively developed in gas fields east of Coal Canyon. Portions of Grand Valley, Parachute, Rulison, and Mamm Creek have been developed at 10-acre density. This dense well control allows detailed mapping of the fluvial sands. Stratigraphic datums that can be correlated with a high level of confidence (commonly coals) can be used to define intervals for mapping individual sand trends. Smoothed gamma-ray cross sections reveal gross stratigraphic architecture that is not as apparent in conventional gamma-ray cross sections. Ten-acre density is the equivalent of 660 ft between wells. The average apparent sand body width measured in outcrop (528 ft) and correlations between closely spaced wells (<600 ft), indicate that many sand bodies are penetrated in only one well. Therefore, 10-acre density may be required to produce a reasonable amount of the gas-in-place in the Williams Fork Formation.