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The AAPG/Datapages Combined Publications Database

Oklahoma City Geological Society


The Shale Shaker
Vol. 54 (2004), No. 4. (January/February), Pages 101-109

Sequence Stratigraphy and Reservoir Characterization of the Upper Morrow Sandstone, Texas County, Oklahoma, (Part 1)

Erin E. Van Evera


Upper Morrowan valley-fill sandstones are major oil and gas reservoirs on the northwestern shelf of the Anadarko basin. Three major lithofacies assemblages were recognized in cores and extrapolated from wire line log responses: marine, fluvial and estuarine. The primary marine lithofacies are dark fossiliferous shale and bioclastic sandstone. Fluvial lithofacies are characteristic of a braided stream/point bar channel sequence. The complex exhibits sedimentary structures such as trough cross bedding containing stacked fining upward sequences, low angle cross beds, fine to coarse-grained sandstones, with interbedded or laminated silty, shaly and coaly intervals. Estuarine lithofacies consist of interbedded fine to medium-grained sandstones and shales, with abundant trace fossils or burrows.

Incised valleys developed in response to major drops in relative sea level. Lowstand systems tract (LST) deposits were not commonly preserved and are limited to a few thin clay-clast conglomerates. Subsequent sea-level rises resulted in valley filling fluvial and estuarine facies of the transgressive systems tract (TST). As sea-level continued to rise, sediment deposition shifted landward. Therefore, deposition of marine silt and mud represents the highstand systems tract (HST) sediment assemblage.

Reservoir quality is found to be controlled by the sequence stratigraphic framework coupled with compositional and textural parameters. Braided stream/point bar channel sequences (F2, F3) deposited during the TST contain better reservoirs. Average porosity and permeability are 12% and 50.6 md respectively. Marine sandstones (Ml) contain abundant skeletal grains and carbonate cement that occluded porosity. Fine-grained estuarine sandstones are typically poor quality reservoirs due to high detrital clay content and the affects of bioturbation that destroyed primary porosity.

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