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Preservation of Reservoir Quality by Chlorite Grain Coats in High-Temperature Wilcox Sandstones, Rio Grande Delta System, Western Gulf of Mexico


Upper Wilcox sandstones in far south Texas, deposited in the Rio Grande delta system, have retained anomalously high porosity (≥20%) and permeability (≥10 md) in some sandstones at temperatures >400°F (>204°C) and depths >15,000 ft (4.6 km). Reservoir quality was preserved where extensive, continuous chlorite coats inhibited quartz cementation. Petrographic analysis was conducted on Wilcox sandstones from Fandango field, Zapata County, Texas. Chlorite cement is present in all facies, but it is most abundant in coarser-grained upper-shoreface/wave-dominated-delta deposits and in transgressive deposits. Where chlorite coats are rare or discontinuous, quartz cement has filled most intergranular pores. Geometric-mean permeability in Fandango field is 0.33 md and ranges from 0.01 to 155 md. Several large fluvial systems carried sediment to deltas along the northwest Gulf of Mexico during the Late Paleocene and Early Eocene. Wilcox sandstones deposited in the Rio Grande delta contain a higher average volume of chlorite cement than Wilcox sandstones deposited in the other fluvial-deltaic systems farther north. Average volume of chlorite cement is 4.4% in Wilcox sandstones from the Rio Grande delta, compared with 2.2% in sandstones from the Colorado/Rosita delta system, 0.2% in the Houston delta system of the upper Texas coast, and 0.3% in the Holly Springs delta system in Louisiana. Different provenance is interpreted as the reason for the formation of greater volumes of chlorite cement in Rio Grande delta sandstones. Weathering of volcanic rock fragments and other iron-bearing minerals in the source area contributed iron to the Rio Grande fluvial system, which then transported the iron to the shallow-marine setting. Clay precursors formed where amorphous iron hydroxides carried in river water flocculated when mixed with seawater. Wilcox sandstones derived from the Rio Grande system were deposited in deepwater in the western Gulf of Mexico. Sand grains may have developed precursor chlorite rims in shallow-water environments before being carried down the slope by turbidity currents and deposited in deeper water. More extensive and continuous chlorite coats could have subsequently precipitated during burial diagenesis. Thus, the iron-rich source area for sediments in the Rio Grande fluvial-deltaic system might have led to the development of chlorite coats in deepwater sandstones in the western Gulf of Mexico and contributed to reservoir-quality preservation.