The Bighole fault zone
Faults impact reservoir production performance in numerous structural settings. Developing cost effective strategies for oil recovery in faulted reservoirs requires increased understanding of how faults can create reservoir compartments or pathways for premature breakthrough of injectate. Integrated outcrop-to-simulation studies of shallow analogs to faults found in producing reservoirs can aid in answering the following questions:- Are individual faults found in a specific geologic/tectonic environment likely to act as seals, conduits, or conduit-seals ? What are the controls on and predictors for the distribution of permeability associated with individual faults and fault networks ?
- Are fault networks found in a specific geologic/tectonic environment likely to form linked systems that produce extensive conduits or seals?
- Can the answers to (1) and (2) and a knowledge of the geological evolution of the faulted reservoir be used to develop improved representations of faults in reservoir simulators?
The U.S. Dept. of Energy (DOE) is prepared to fund an interdisciplinary, outcrop-to-simulation project, but industry cost-share partners are required before the funds will be awarded.The first year of the 3-year DOE project is devoted to collecting surface and subsurface geological and hydrogeological data in an array of coreholes to be drilled through the Bighole fault ( Figure 1 ). With industry funding, more reservoir geoscience data will be collected and integrated with the drilling, testing, and geology funded by DOE and Utah State University. In addition, DOE may provide another level of supplementary funding because to recognize the industry/academic collaboration developed here.This proposal outlines Phase I of a 3-Phase plan to use the DOE project as a foundation for reservoir geoscience studies that would help maximize the value of the DOE project to participating petroleum industry sponsors.AN ANALOG FAULT EXPOSED IN OUTCROP
The Bighole fault (BHF) is one of a number of northeast and northwest-striking normal faults which cut aeolian, Navajo Sandstone and thin-bedded limestones in the Chimney Rock fault array of the northern San Rafael swell ( Figure 2 ) (Witkind,1988; Hood and Paterson,1984; Shipton and Cowie, in review; Krantz, 1988). The Chimney Rock array is one of many arrays across the San Rafael swell whose origins are not well understood (Witkind, 1991). These faults may have accommodated north-south extension during the development of the San Rafael swell, whereas faults east of the proposed study area may be due to salt movement (Witkind, 1991). The faults are 1 to 8 km long (several exceed 8 km trace lengths), form networks, and seldom have displacements in excess of 30 m. The faults likely span the seismic to sub-seismic scales. Furthermore, the faults appear analogous to reservoir-scale faults found in numerous petroleum provinces. The structure and character of the Bighole fault forms a representative example of faults exposed in the area.Similar faults may have contributed to oil and gas production from nearby reservoirs. A small abandoned oil field lies 6 km east of the proposed study area. Other abandoned and producing fields are also scattered across the edges of the San Rafael swell (Lane and Stanley, 1991). The Farnham Dome field, at the northern end of the swell, approximately 40 km north of the proposed study area, has produced over 4.76 BCF of natural gas from the Navajo Sandstone. Permeability measurements at 200 and 500 psi confining pressures show permeabilities to water are 4 - 9 md for 'typical' indurated Navajo Sandstone sandstone. More friable samples range from 79 - 460 md (Hood and Paterson, 1984). Several samples exhibited horizontal/vertical permeability ratios of ~1.6. The conceptual model for the permeability structure of the fault zone (Figure 1) is based on work by both Shipton and our Utah-based group.
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