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Basin-scale sequence stratigraphy and distribution of depositional and mechanical units in the middle and upper Williams Fork Formation, Piceance Basin, Colorado
Wiechman, Michele Leigh
Wiechman, Michele Leigh
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2013
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Abstract
The Piceance Basin, northwest Colorado, is home to one of the most important basin-centered tight-gas accumulations in North America. The production in this basin is from the Cretaceous Mesaverde Group, of which a key formation is the Williams Fork Formation. The Williams Fork consists of fluvial sandstones with very low permeability (<0.1 md) and heterogeneous reservoirs that require production with 10-20 acre well spacing and expensive hydraulic fracturing. In many cases, the quality and lateral connectivity of these reservoirs are controlled by depositional environment, fracture networks, or both. However, predicting the properties and connectivity of the reservoirs is difficult because of abrupt facies changes geographically and stratigraphically. While new completion and stimulation technologies have helped make production economical, a regional sequence-stratigraphic context allows for better understanding of detailed lithofacies within the Williams Fork and disentangles the geologic controls on tight-gas. However, it is difficult to apply sequence-stratigraphy in the predominantly nonmarine fluvial strata within the Williams Fork. This study focuses on potential stratigraphic and mechanical controls on tight-gas sandstone reservoirs by building a regional sequence-stratigraphic framework integrating outcrop and subsurface data. Key questions addressed in this study are: (1) What was the basin-scale configuration and connection of depositional systems within a sequence-stratigraphic framework? And, what were the type of depositional systems and their orientation? (2) Do the regional-scale sedimentation trends help predict higher-than-average permeability zones? and (3) Can sedimentation patterns be used to define trends in potential mechanical units that affect fracture development? If so, in which units and their distribution in the basin? Detailed stratigraphic profiles (12 new and 1 previously published), outcrop gamma ray profiles (10), two cores, detrital mineral compositional changes, detailed facies (27) and facies associations (8) descriptions and interpretations, and paleocurrent data were collected from the middle and upper Williams Fork Formation. The integration of 13 stratigraphic profiles and 154 well logs allowed key stratigraphic stacking patterns and sequence-stratigraphic surface identified in outcrop to be correlated into the subsurface throughout the basin. Twenty-seven facies define fluvial cycles within the middle and upper Williams and help correlate seven sequence-stratigraphic surfaces. The changes in accommodation shown through facies stacking cycles show an overall decrease in accommodation through the middle and upper Williams Fork. The decrease in accommodation is mainly controlled by tectonic influences in the West in older sequences. Four regional depositional sequence sets, and seven sequence stratigraphic surfaces were identified within the middle and upper Williams Fork. Three sequence boundaries were confirmed to be low order, regionally extensive based on four major detrital mineral composition changes identified in core. Correlations and Isopach maps show westward thinning in all the sequences. This thinning constrains the timing of the Douglas Creek Arch (DCA) and Uinta Mountain Uplift (UMU), as well as a younger phase of motion on the Uncompahgre Uplift (UU). Sequence A identified thinning on the DCA and the development of thick drainage zones to the east, just off structure. Thinning on the east can be attributed to syn-depositional movement on the DCA and the UU. Thinning on the DCA is more pronounced in Sequence B. While portions of Sequence B are removed by a large, regional unconformity related to the Ohio Creek and Wasatch Formation there is still evidence of thinning within this sequence approaching the DCA. Sequence B also sees an increase of drainage areas near the DCA and the UU which would form during active uplift. Isopach maps of the upper Williams Fork Formation (Sequence C and D) not only show thinning of the interval to the west but a shift in the depositional center is identified by thickening trends to the west. Production from the Williams Fork is highly dependent on reservoir heterogeneity. Understanding depositional facies and their connectivity provides an insight into reservoir heterogeneity. Based on this study five facies associations have been identified as having the best reservoir potential based on their internal heterogeneity and lateral extent, from the best to the worst: (1) high-sinuosity, meandering fluvial, (2) isolated, low-sinuosity anastomosed fluvial, (3)tidally influenced fluvial channels, (4) regressive marine shoreline, and (5) transgressive marine shoreline barrier system. Natural fractures are important components of production from fluvial sandstones within the Williams Fork Formation. This study, in conjunction with Lee (2013) identified fourteen facies and six facies associations that are naturally fractured. These associations were (1) high-sinuosity, meandering fluvial, (2) isolated, low-sinuosity anastomosed fluvial, (3) undifferentiated floodplain, (4) tidally influenced fluvial, (5) estuarine systems, and (6) transgressive marine shorelines. These fractures seem to be controlled by facies composition and bedding character. Fracture and fracture swarm spacing is relatively proportional to bedding thickness, with thicker beds showing higher fracture spacing then thinner beds. Sandstones with high fractures concentrations are also found to have cementation trends that increase the sandstones brittleness.
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