Controls on reservoir development in Devonian chert; Permian Basin, Texas

Cherts in the Thirty-one formation were deposited in deep-water as facies equivalent to shallow-water carbonate packstones. Translucent, nonporous chert displays chalcedony and quartz replacements of sponge spicules. Spicule fabric indicates that silica cement occurred before compaction. Porcellaneous, porous chert displays bimodal porosity with moldic porosity resulting from dissolution of spicules and carbonate rhombs, and intercrystalline matrix microporosity between 1 mu m quartz crystals. Although moldic pores contain druze quartz linings, they display pore sizes up to 300 mu m long and 50 mu m across. Delta ^>18O (sub SMOW) = 300/00 in the Three Bar field and 31.40/00 in the Block 31 field. These values are higher than typical for Lower Palaeozoic chert and suggest precipitation from original sea-water at low T, consistent with evidence for shallow silica diagenesis. Silica diagenesis is more abundant in cherts closer to carbonate packstones. That is, nonporous chert occurs adjacent to packstones and porous chert occurs removed from packstones. It is suggested that Mg solutions originating in the packstone act as a catalyst for silica precipitation.