--> Abstract: Reservoir Quality and Pore-Type Evolution in Deep to Ultradeep Tertiary Sandstones of the Northern Texas Gulf of Mexico Coast; #90063 (2007)

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Reservoir Quality and Pore-Type Evolution in Deep to Ultradeep Tertiary Sandstones of the Northern Texas Gulf of Mexico Coast

 

Dutton, Shirley P.1, Robert G. Loucks2 (1) Bureau of Economic Geology, Austin, TX (2) Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin,

 

Exploration in Tertiary reservoirs of the Gulf of Mexico focuses increasingly on depths below 15,000 ft, where reservoir quality is a critical risk factor. To evaluate controls on deep-reservoir quality, we conducted petrographic analysis on deep Eocene to Pliocene sandstones of the northern Texas Gulf Coast; samples come from Texas coastal areas and Federal waters. With increasing burial depth, total volume of porosity decreases, and the proportion of different pore types changes. Mean porosimeter porosity of samples from 10,000 to 15,000 ft is 18.9%, compared with 12.6% in samples from 15,000 to 20,000 ft and 9.6% in samples from >20,000 ft. Thin-section point counts demonstrate a pronounced change from a mix of primary, secondary, and micropores (P36S24M40) at shallower depths to predominantly secondary pores and micropores in deeper sandstones (P5S20M75). Primary pores are lost mainly by quartz cementation, which begins significantly at ~9,000 ft in Wilcox sandstones and ~11,000 ft in Frio sandstones, and compaction. Secondary-pore volume decreases more slowly with depth than primary-pore volume. Below 15,000 ft, quartz cement begins to precipitate in secondary pores. Micropores appear most resistant to compaction and cementation at these depths.

 

Decrease in permeability with depth correlates mainly with loss of primary pores. Geometric mean permeability of samples from 10,000 to 15,000 ft is 8.6 md, compared with 0.4 md in samples from 15,000 to 20,000 ft and 0.1 md from >20,000 ft. At deeper burial depths, permeability is lower per porosity unit than at shallower burial depths because secondary pores and micropores dominate.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California