EXTENDING FELDSPAR DIAGENETIC TRENDS WITH DATA FROM DEEP TO ULTRADEEP EOCENE WILCOX SANDSTONES OF THE NORTHERN TEXAS GULF OF MEXICO COAST

Timing of feldspar dissolution during burial diagenesis remains controversial—whether it occurs only in the shallow subsurface or whether some feldspar dissolves after deep burial. A related debate is whether secondary pores generated in the deep subsurface by feldspar dissolution undergo progressive mechanical compaction. Deep hydrocarbon exploration in the Gulf of Mexico has resulted in acquisition of new rock material from deep (4.5 to 6.0 km) to ultradeep (>6.0 km) sandstones within the GOM Tertiary siliciclastic wedge. The availability of new data from deep samples of the Eocene Wilcox Formation allowed us to address these questions and extend feldspar trends that were derived from shallow rock data (<4.5 km). By combining the new data with older data, we have a petrographic database of 164 point-counted Lower Wilcox thin sections from the northern Texas Gulf Coast.

Wilcox sandstones in the study area are mostly lithic arkoses and feldspathic litharenites. Potassium feldspar (K-spar) abundance decreases with increasing temperature below 100°C in the Wilcox Formation. K-spar destruction in the Wilcox is complete below 155°C, but kaolinite persists to 180°C. Dissolution of some K-spar during deep burial is indicated by (1) petrographic evidence of quartz cementation before some feldspar dissolution and (2) progressive decrease in K-spar content with increasing temperature. Porosity and thin-section data from Wilcox sandstones do not support the hypothesis that mechanical compaction of secondary pores continues during deep burial. Most pores in the Wilcox sandstone below 140°C are secondary pores generated by shallow to deep feldspar dissolution. The average volume of secondary pores does not decline significantly with increasing temperature, even below 155°C (where K-spar dissolution is complete). Petrographic evidence indicates that 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.