ABSTRACT

Secondary leached porosity is common to dominant in near surface to deep subsurface lower Tertiary sandstone reservoirs along the Texas Gulf Coast. This secondary porosity is in the form of leached feldspar grains, volcanic rock fragments, carbonate cements, and carbonate-replaced grains. Leached porosity occurs in sandstones with compositions ranging from volcanic litharenite and lithic arkose to quartzose sublitharenite and quartzose subarkose.

A generalized diagenetic sequence indicates that leaching is a multi-staged phenomenon occurring at or near surface, at burial depths of 4000 to 6000 ft, and at burial depths of 7000 to 10,000 ft. Feldspar grains are dissolved during the first stage, whereas grains, cements, and replacement products are dissolved during the last two stages. Intensity of leaching in each stage varies in different formations and in different areas.

Plots of secondary porosity as a percent of total porosity versus burial depth show that secondary porosity is dominant beneath 10,000 ft, ranging from 50 to 100 percent of total porosity. Above 10,000 ft more than half the samples have secondary porosity as the dominant type. Similarly, individual plots for the Wilcox, Yegua, Vicksburgs, and Frio sandstones all demonstrate the predominance of secondary leached porosity.

Primary porosity is destroyed by compaction and cementation with increasing depth of burial. If this were the only porosity type, no deep, high-quality reservoirs would exist. Leaching, however, restores reservoir quality after primary porosity has been reduced. Most productive lower Tertiary sandstone reservoirs, especially deep reservoirs, along the Texas Gulf Coast exist only because of secondary leached porosity.