Abstract: Sequence Stratigraphic Analysis and Regional Correlation of the Lower Guadalupian San Andres Formation, Northwest Shelf, Permian Basin

Troy B. Beserra, Steven L. Dorobek

The San Andres Formation on the Northwest Shelf of the Permian Basin is bounded by the underlying upper Leonardian Yeso Formation and the overlying upper Guadalupian Grayburg Formation. The main objective of this study was to develop a sequence stratigraphic framework for the San Andres Formation between surface exposures and oil fields on this part of the Northwest Shelf.

Studies in the Guadalupe Mountains have described the entire San Andres Formation as two third-order depositional sequences. Outcrops of the lower San Andres Formation in southeastern New Mexico record the transgressive systems tract (TST) of the initial San Andres sequence and can be correlated to the Guadalupe Mountains.

Parasequences (1-10m thick) immediately above the Yeso/San Andres sequence boundary are characterized by basal peloidal (dolo)mudstone overlain by either sandstone/siltstone, peloidal (dolo)mudstone/wackestone with leached evaporites, or a brecciated interval. A 10-15 m thick interval of skeletal-peloidal packstone/grainstone is stratigraphically in the middle of the initial San Andres TST. This subtidal interval can be correlated to the San Andres platform margin and suggests a period of extensive flooding of the Northwest Shelf. Parasequences in the upper part of the lower San Andres TST (approx. 75 m above the San Andres/Yeso contact) consist of basal skeletal-peloidal (dolo)packstone/wackestone that grade upward into skeletal-peloidal (dolo)packstone/grainstone. The maximum floodi g surface (mfs) of the lower San Andres TST, can be identified by a gamma-ray "kick" approximately 155-160 m above the Yeso/San Andres sequence boundary. In outcrop, the mfs is represented by a thin dolomudstone bed.

The highstand systems tract (HST) of the lower San Andres sequence is well-exposed locally, but its contact with the mfs is not exposed anywhere. Parasequences in the HST consist of peloidal (dolo)mudstone/wackestone that grade upward into oolitic-pisolitic-intraclastic (dolo)grainstone/packstone

or skeletal (dolo)grainstone/packstone. Parasequences in the HST are thinner than those in the TST and have less diverse faunas. It is difficult, however, to correlate precisely the initial San Andres HST across this part of the Northwest Shelf because of limited stratigraphic control. The second third-order San Andres sequence recognized in the Guadalupe Mountains is barely preserved in the field area due to post-San Andres erosion.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994