--> Abstract: Extending Diagenetic Trends in the Eocene Wilcox Formation with Data from Deep to Ultradeep Sandstones of the Northern Texas Gulf of Mexico Coast, by Shirley P. Dutton and Robert G. Loucks; #90078 (2008)

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Extending Diagenetic Trends in the Eocene Wilcox Formation with Data from Deep to Ultradeep Sandstones of the Northern Texas Gulf of Mexico Coast

Shirley P. Dutton and Robert G. Loucks
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Deep hydrocarbon exploration in the Gulf of Mexico has resulted in new rock material from deep (4.5 to 6.0 km) to ultradeep (>6.0 km) sandstones within the GOM Tertiary siliciclastic wedge. Wilcox samples from depths as great as 6.7 km and temperatures of 225°C allowed us to extend diagenetic trends that had been derived previously from shallow rock data (<4.5 km). To evaluate diagenetic controls on deep-reservoir quality, we conducted petrographic analysis of 164 Wilcox thin sections from the northern Texas Gulf Coast.

Most macropores in Wilcox sandstones below 140°C are secondary pores generated by shallow to deep feldspar dissolution. Dissolution of some potassium feldspar (K-spar) during deep burial is indicated by (1) petrographic evidence of quartz cementation before feldspar dissolution and (2) progressive decrease in K-spar content with increasing temperature above 100°C. K-spar destruction in the Wilcox Formation is complete below 155°C. Kaolinite persists to 180°C. Average volume of secondary pores does not decline significantly with increasing temperature, even below 155°C (where K-spar dissolution is complete). Some rearrangement of fragments of partly dissolved feldspar occurs within secondary pores, but compaction of secondary pores formed during deep burial was not observed in these well-cemented sandstones. The deepest and hottest samples (>200°C) show some occlusion of secondary pores by quartz and ankerite cement.

Authigenic ankerite and calcite are abundant in the Wilcox Formation (averaging 3.4% and 2.7%, respectively). Calcite and ankerite coexist at temperatures between 90°C to 200°C, but only ankerite is present at higher temperatures. Calcite may dissolve once feldspar buffering ends. Pore types change from a mix of primary, secondary, and micropores at shallower depths to predominantly secondary pores and micropores in deeper sandstones.

 

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas