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LORENZ, JOHN C., Sandia National Labs, Albuquerque, NM 87185; and JAMES T. NEAL, Consultant, Prescott, AZ

Abstract: The Effects of Evaporite-Dissolution Fronts on Overlying Fractured Reservoirs

The local structures that influence reservoirs in strata overlying evaporite-dissolution fronts can be examined in outcrops at the southern margin of the Holbrook basin, east-central Arizona. The late Paleozoic and Mesozoic sandstones and limestones at the edge of this basin are draped over a dissolution front formed in underlying evaporitic strata, and the surface strata have been offset by approximately 100 in by the differential evaporite removal. This has created a draped monocline, called the Holbrook Anticline, that extends along a northwesterly trend for more than 100 km. Strata overlying the monocline have been flexed and stretched by offset and flexure of the strata during migration of the dissolution front. Stretching at the crest of the anticline has reactivated the regional fractures that are widespread in these strata. Whereas these fractures are typically mineralized and narrow, fractures at the crest of the anticline may have open, reactivated "apertures" up to several meters wide. Large wedges of rock between such fractures may locally be displaced downward as much as 15 in, forming small grabens. Many of these grabens have been enlarged to form sinkholes hundreds of meters across, locally filled with a talus breccia derived from collapse of adjacent and overlying strata. Buckling of the surficial strata is also present, typically associated with localized zones of compressional zones at the toe of the anticline.

Reactivated regional fractures, grabens, and sinkholes form a suite of structures that can be recognized in seismic records and logs in oil and gas reservoirs found overlying similar dissolution fronts in the deeper subsurface (e.g., Silo field, northeastern Colorado). Buckles have not been recognized in the subsurface but may be present. The features found in outcrops at the edge of the Holbrook basin can help to refine interpretations of reservoir characteristics in similar structural settings.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana