Sequence Stratigraphic Analyses of the Upper Cretaceous Deposits, Rio Grande Embayment, Using Well Logs, Cores and Outcrops

S. E. R. Mahmoud
University of Texas at Dallas, Richardson, TX

Differences between lithologic character and faunal content between updip and downdip rock units of the Upper Cretaceous Navarro Group in the Rio Grande Embayment, northwestern Gulf of Mexico Basin, are known to create difficulties in traditional lithostratigraphic correlations. Lithological boundaries can be time transgressive and may have no relationship to major stratal hiatuses. The present study is an attempt to solve problems of stratigraphic correlation in the Upper Cretaceous strata, South Texas, using sequence stratigraphic principles applied to cores, outcrops and wireline log information.

Results obtained from the present study indicate the deposition of San Miguel sand units of San Miguel Formation as short-term regressive pulses within a major transgressive episode. These sand units were deposited as strand plain units in wave-dominated deltaic systems of good reservoir quality. High reworking and cleaning of these sand units by longshore currents created good reservoir quality of these sand units. Moreover, downdip Olmos sand units belonging to the Olmos Formation occur as thin, clean sandstone units of better reservoir quality than updip equivalents. Downdip Olmos and Escondido formations were interpreted to belong to a transgressive systems tract. Updip Olmos, a coal-bearing sandstone and shale sequence of coastal-plain and deltaic origin, is of more argillaceous content and less reservoir quality and belongs to a deltaic highstand systems tract.

AAPG Search and Discovery Article #90905©2001 AAPG Southwest Section Meeting, Dallas, Texas