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Eustatic Cycles, Shoreline Stacking, and Stratigraphic Traps: Atkinson Field, Live Oak and Karnes Counties, Texas

T. P. Bulling, W. M. Smith, J. A. Breyer

Atkinson field in south Texas produces gas from the updip pinch-out of a shoreline sand body deposited during a stillstand or minor regression within the early middle Eocene transgression of the Texas Gulf Coast. The sand body is elongate parallel to depositional strike and pinches out downdip into marine shales of the Reklaw Formation. The sand has a maximum thickness of 60 ft, extends 9 mi along strike, and reaches a width of 2 mi. Electric log patterns indicate interfingering between sand and shale on the updip edge of the sand body and a coarsening-upward sequence from shale to sand on the downdip edge of the sand body. Most logs from wells in the central part of the sand body have blocky patterns, indicating abrupt transitions with the overlying and underlying shales and no systematic variation in grain size. Many ancient shoreline sandstones have similar characteristics.

The producing sand in Atkinson field occurs in the regressive phase of a fourth-order cycle of change in relative sea level, within the transgressive phase of the third-order cycle that comprises the early middle Eocene advance and retreat of the sea in the Gulf Coast region. Other shoreline sand bodies occur at the same stratigraphic zone along depositional strike. Models of shoreline stacking patterns within third-order cycles indicate that similar sand bodies and traps should be present in younger fourth-order cycles higher on paleoslope.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.