--> Abstract: Jurassic Cotton Valley Formation Reservoir Quality, Eastern Offshore Gulf of Mexico: Life Below 20,000 Feet, by Andrew R. Thomas, Dave Balcer, Tom Himes, Linda M. Bonnell, and Jessy Jones; #90039 (2005)

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Jurassic Cotton Valley Formation Reservoir Quality, Eastern Offshore Gulf of Mexico: Life Below 20,000 Feet

Andrew R. Thomas1, Dave Balcer2, Tom Himes2, Linda M. Bonnell3, and Jessy Jones4
1 ChevronTexaco, Bellaire, TX
2 ChevronTexaco, New Orleans, LA
3 GEOCOSM, Austin,TX
4 ChevronTexaco, Houston, TX

The Jurassic Cotton Valley Formation was penetrated in a shelf-margin growth-faulted position 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 - 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.

Subject area sandstone composition and diagenetic path stand in contrast to the quartzarenites found further west on trend. Fluvial-deltaic Cotton Valley sands deposited and reworked in East Texas and North Louisiana aggradational 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'. In contrast, progradational fluvial-deltaic 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 provenance 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'.

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. Chlorite coats vary up to 90% complete in select intervals.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005