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Preservation of High Reservoir Quality by Early Quartz Cementation in the Historically Significant East Texas Field, Upper Cretaceous Woodbine Sandstone

Loucks, Robert 1; Reed, Robert M.1; Ambrose, William 1
1 Bureau of Economic Geology, University of Texas at Austin, Austin, TX.

East Texas field is a unique, giant, sandstone reservoir relative to its high oil-recovery rate of 77%. The exceptional recovery rate in the fluvial and deltaic systems of the upper Cretaceous Woodbine sandstones is partly the result of a well-preserved primary intergranular pore network. The sandstone, lithologically classified as a quartzarenite, contains abundant quartz overgrowths. A few samples range into the sublitharenite category, with chert and mud clasts comprising lithic fragments. Other cements such as siderite, calcite, and chlorite are only locally developed. Matrix within the reservoir sandstone is the major factor leading to occurrences of diminished reservoir quality in lower energy facies. Matrix-free sandstones can have up to several darcys of permeability, whereas matrix-rich sandstones generally are limited to a few tens of millidarcys to microdarcys. Petrographic and cathodoluminescence analyses indicate that two distinct stages of CL-zoned quartz overgrowth cements formed. The first stage may have been associated with pedogenesis and devitrification of volcanic rock fragments and/or ash beds within the sands. Because this early stage of quartz cementation inhibited compaction, it is an important factor in preserving intergranular pores and associated macropore throats. Percent abundance of primary pores ranges into the high thirties, and precement porosity also ranges into the high thirties, indicating the early onset of quartz cementation. Because reservoir quality is generally high (mean porosity of matrix-free sandstones is 26%, and mean permeability is ~2 darcys), it represents little risk when bypassed pay is being sought and secondary and tertiary recovery projects are being developed.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009