Deep Cotton Valley Formation Diagenesis and Reservoir Quality, Viosca Knoll Area, Offshore Gulf Of Mexico

Andrew Thomas¹, David Balcer², Tom Himes², Linda Bonnell³, Jessy Jones⁴, and Larry O’Mahoney^1
1ChevronTexaco ETC, New Orleans; 
²ChevronTexaco Shelf BU, New Orleans
³Geocosm, Austin, Texas; 
⁴ChevronTexaco ETC, Houston

The Jurassic Cotton Valley Formation was penetrated in a deep shelf-margin growth-faulted position in VK Blk 251. The lithic arkoses cored in the subject well describe a series of three shallow water parasequences, each characterized by argillaceous transgressive rocks overlain by progressively coarser grained highstand sandstones. These shoreface sandstones contain hummocky cross stratification + bioturbation overlain by cross stratification or inclined lamination. Sequences are very micaceous both in the argillites as well as the coarse clastics. Each parasequence is approximately 20’-25’ thick. 

Intergranular, intragranular, and fracture porosity are contained within these lithic arkose sandstones. Openhole log porosity of Cotton Valley sandstones ranges from 1 - 14%, and core porosity ranges from 1.0 to 6.7% over the 60’ cored interval. Quartz cement is the main control on reservoir quality, and has progressed to variably advanced states in the reservoir sandstones. Gamma-ray patterns of reservoir sands often are funnel-shaped, and sandstones found at funnel tops are tight due to extensive quartz cementation. Chlorite cement is variably present as grain coatings, and acts to inhibit intergranular quartz cement. Chlorite coats average 30% throughout the cored interval, but range from 3% in the ultraclean reservoir sand tops to 90% in dirty gamma ray areas. The best reservoir quality is found in the center of the sands where the gamma ray is more active. Approximately half of the mica present in the sandstones is biotite and additionally the sands contain moderate amounts of metamorphic rock fragments, notably schist fragments. These biotite-containing grains are altering to form the chlorite coats that preserve porosity. 

Quartz crystals growing into the fractured zones show variably complete morphologies. The relationship observed in the fractures is most likely related to the axial orientation of the quartz seed crystal at the fracture margin, and those with c-axis orientations perpendicular to the fracture wall exhibit most dramatic crystal growth. 

Chlorite coatings are common in the VK 251 area, and stand in stark contrast to the North Louisiana Cotton Valley that contains quartz cemented quartzarenites. Examining sandstone composition across a strike section of the Cotton Valley depositional fairway, there is variance in composition that distinguishes the expanded Eastern GOM section from that of far updip or western shelf locations. Trends show that Cotton Valley compositions deposited near the east Texas Lone Oak Delta and the northern Louisiana Terryville Strandplain range from sublitharenite to subarkose to quartzarenite. Ultimate sandstone compositions are related both to fluvial composition and also to the extensive reworking along the Terryville Strandplain. Clay coatings are poorly developed in this area, and clay coating compositions are both ferroan chlorites as well as illite/smectite. Reservoir quality is related mainly to quartz cement volume, and reservoir viability commonly is lost below 12,000’. 

Sandstone composition changes to subarkose and lithic arkose in central Mississippi and further southeast along the Cotton Valley depositional trend. Ultimately in the Viosca Knoll area, the formation incorporates a greater degree of both di and trioctahedral mica along with schistose rock fragments. This sandstone composition change is controlled by proximal Appalachian drainages including the ancestral Mississippi River which fed the southeastern GOM. The biotite content influences the Cotton Valley diagenetic pathway to one favoring chlorite coating development and porosity preservation at depths greater than 20,000’. 

Detailed reservoir characterization and Touchstone modeling were performed recently to constrain reservoir quality uncertainty in deep Cotton Valley prospects and describe risk elements unique to this deep objective. Using the VK 251 20,500’ cored section as a calibration dataset, Touchstone modeling shows that reservoir quality will be preserved to even greater depths in sections with extensive chlorite coatings, while moderately coated samples lose porosity more rapidly due to quartz cementation. 

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