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Abstract: Jurassic Cotton Valley Reservoir Quality, Eastern
Offshore Gulf of Mexico: Chlorite Coatings and the
Porosity Preservation Story Found Below 20,000 ft
1 Chevron Energy Technology Company, Houston, Texas
2 Chevron GOM Business Unit, New Orleans, Louisiana
3 Geocosm, Austin, Texas
The Jurassic Cotton Valley formation was penetrated near 20,000 ft in a shelf-margin growth-faulted well location in Viosca Knoll Block 251. The sands cored in the subject well describe three shallow water parasequences, each having argillaceous transgressive rocks overlain by coarser-grained highstand sandstones.
Intergranular, intragranular and fracture porosity are contained within these sandstones. Log porosity ranges from 1% to 14% overall, and core porosity ranges from 1.0% to 6.7%. Although quartz is the main cement, chlorite grain coatings inhibit quartz cement when they are well developed. Late fractures are partly filled with quartz and carbonate cement, and unfilled fractures are thought to enhance reservoir deliverability.
Cotton Valley in the eastern Gulf of Mexico (GOM) composition and diagenetic path stand in contrast to the quartzarenites found farther west on trend. Fluvial-deltaic Cotton Valley sands deposited and reworked in East Texas and North Louisiana systems are highly quartz cemented and contain few clay coatings. Reservoir quality is driven by quartz cement volume and reservoir viability is lost below 15,000 ft. In contrast, progradational fluvialdeltaic sands in Viosca Knoll are lithic arkoses and contain muscovite and biotite both as discrete mica and as metamorphic rock fragments. This sandstone composition shift is related to proximal Appalachian drainages and minor strandplain reworking in the eastern GOM. Biotite dissolution influences the Cotton Valley diagenetic pathway to one favoring chlorite coatings and porosity preservation below 20,000 ft.
Reservoir quality uncertainty was constrained with Touchstone* modeling. With the subject well as calibration, chlorite coatings are shown to preserve reservoir quality to even greater depths.
EDITOR'S NOTE: *Touchstone is a software system for analysis of controls on reservoir quality in analog sandstones and for forward modeling of sandstone diagenesis and petrophysical properties.
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