--> Abstract: The Importance of Surface Geology to Subsurface Production - A Partial Answer to Willy-Nilly Drilling in Mature Basins (Examples from the Uinta Basin, Utah, USA), by S. R. Bereskin, R. L. Bruhn, B. A. Marin, M. C. Walters, and K. K. Reinschmidt; #90937 (1998).

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Abstract: The Importance of Surface Geology to Subsurface Production - A Partial Answer to Willy-Nilly Drilling in Mature Basins (Examples from the Uinta Basin, Utah, USA)

BERESKIN, S. ROBERT, TerraTek, Inc,; R. L. BRUHN, University of Utah, Dept. of Geology; B. A. MARIN and M. C. WALTERS; TerraTek, Inc.; and K. K. REINSCHMIDT, Equitable Resources, Inc.

Summary

In portions of many Rocky Mountain basins, including the Uinta Basin, development drilling has historically occurred in economic flurries most commonly when hydrocarbon prices attain temporary highs. Nonetheless, some basins remain difficult economically because of constant low-to-modest rates of return, especially from primary production activity. When reservoirs are pursued with a stratigraphic incentive, such as fluvio-deltaic sandstones, structural complications are literally ignored, especially in purportedly “structureless” basins. Drilling commonly occurs on traditional state or federal spacing, and seemingly unaccountable oilfield triumphs and disasters shortly follow. Detailed structural analyses in portions of the “structurally simple” Uinta Basin have revealed that exploration and development efforts involving primary and secondary production might result in fewer disappointments if more attention were paid to details of surface structure.

As a Uinta Basin example, the Duchesne Fault Zone extends >40 miles in an approximately east-west orientation and likely separates the over-pressured, deep drilling to the north from the normally pressured, comparatively shallow activity to the south. Exploration efforts and development drilling over the last 15 years have largely ignored this contrast. The most recent activity involves the pursuit of fluvio-deltaic sandstones of the Green River Formation (Eocene) for purposes of secondary recovery. Wells have been religiously spotted on 40 acre spacing limited mostly by the isopachous extent of such sandstones. Well distributions literally straddle all known surface traces of the fault zone. From a primary recovery standpoint, this activity has produced a series of inexplicably prolific, as well as disappointing wells. Furthermore, secondary recovery efforts may, in some cases, encounter catastrophic problems where fluvio-deltaic targets do not structurally connect.

Preliminary results from our study indicate that unusually good primary production has occurred or may occur in the following structural settings:

^bull on upthrown blocks along faults of even modest displacement (<10 ft; topographic expression can be subtle);

^bull on small anticlinal structures with almost imperceptible closure;

^bull on local steepening of beds trapped against observably sealed fault planes or fractures;

^bull on or near the fault traces themselves where traditional sandstones are more intensely fractured (1.8 ft average fracture spacing); and

^bull in select portions of the basin in proximity to buried faults.

Additionally, trends of mapped surface structures (Fig. 1, Fig. 2a) parallel those of natural fractures recognized at depth on imaging logs (Fig. 2b, Fig 2c).

Obviously, mapping of surface faults and corresponding displacements will clearly affect the success or failure of secondary efforts. Furthermore, recognition of surface fault patterns may account for reservoir pressure compartmentalization and perhaps the over-pressured gradients locally observed in the fault zone. Finally, a fundamental understanding of surface structural character is pertinent to intelligently defining horizontal or deviated exploration ventures.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah