Oligocene Stratigraphy and Depositional Systems in the Deepwater Gulf of Mexico

Zarra, Larry¹ (1) Chevron, Houston, TX

Deep basin components of Frio and Vicksburg depositional systems exhibit significant variability across the deepwater Gulf of Mexico. Thick slope bypass zones in Port Isabel give way to separate sand poor sequences, and sand rich turbidites in Alaminos Canyon. Mudstones and marls dominate depositional systems to the east, in Keathley Canyon, Walker Ridge, and western Atwater Valley. Oligocene strata are starved farther to the east.

Post-depositional structuring and variable sand content complicated initial stratigraphic interpretations. Correlations were facilitated by developing an integrated sequence stratigraphic interpretation. Key components incorporate biostratigraphic data, revised chronostratigraphic models, and detailed mapping of local sequence boundaries. Lower slope reservoir sands and uncharged reservoir quality sandstones are almost entirely limited to the 29.4-32 Ma sequence. The older 32-33.7 Ma sequence (Vicksburg) is thick but sand poor. The younger 23.7-29.4 Ma sequences are alternately thin, starved, sand poor, or absent.

Depositional setting of the 29.4-32 Ma sequence includes weakly confined toe-of-slope turbidite fairways and adjacent thin-bedded overbank deposits. Characteristic turbidite lithofacies associations were interpreted from conventional cores. Core control was calibrated to OBMI (Oil Based Microresistivity Imager) logs, and extrapolated away from the cored interval.

Detailed mapping of sequences and turbidite sand packages within sequences enabled construction of horizon relative volumes that display distinct depositional features. Reservoir architectural styles include high to low sinuosity channels, anastomosing channel complexes, and discrete distributary lobes. While many of the seismic images of depositional features are un-calibrated, some images are calibrated to well data and conventional core.