ABSTRACT

The present study is an attempt to solve problems of stratigraphic correlation in the Upper Cretaceous strata, Rio Grande Embayment, using sequence stratigraphic principles applied to cores, outcrops and wireline log information.

Well-defined log signatures, supplemented by core and outcrop data, were observed and correlated between boreholes. Chronostratigraphically significant surfaces, especially transgressive surfaces, were correlated. Upward-coarsening sand units within the three Upper Cretaceous progradational wedges, San Miguel, Olmos and Escondido formations, encased in prodelta and shelf mudstones were laterally correlated through the study area. Observations indicate a basinward decrease in number of sand units and net sandstone thickness. Outcrop studies in Maverick County indicate deposition of the coal-bearing Olmos Formation in the lower deltaic plain whose upper part is eroded and unconformably overlain by transgressive units of the Escondido Formation. Cored intervals through reservoir-quality clean and mature sandstones from the San Miguel Formation confirm long reworking history by wave action and longshore currents forming strand plain deposits within a wave dominated deltaic setting. Cores sampled in the updip position indicate more tight and argillaceous sandstones with more fluvial influence and low reservoir potential. 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 more argillaceous and low reservoir quality and belongs to a highstand systems tract.

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