Diagenesis of Strawn Limestone, Lovington East Field, Lea County, New Mexico
Gray, Terry Lee
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Oil producing Strawn bioherms in Lovington East Field represent mud mounds where lime mud was baffled by blastoids, crinoids, and cryptostome bryozoans. Abundant aragonitic molluscs, fusulines, and ostracods also inhabited the bioherm core. Bioherms were flanked by aprons of grainstone which consist predominantly of echinoderm components and siliceous sponge spicules with some ooids. Relatively soon after deposition unstable Strawn carbonates became subaerially exposed by an eustatic drop of sea level. Development and retention of effective porosity reflects interaction of lithofacies and biofacies with vadose-phreatic groundwater dynamics. All primary porosity of marginal grainstone lithofacies was rapidly occluded by precipitation of epitaxial cement on monocrystalline echinoderm components and wells drilled into bioherm flanks are nonproductive. Development of secondary porosity was initiated in bioherm cores by solution of aragonitic mollusk shells. Because lime mud matrix was incompletely lithified, molds of aragonitic fossils partially collapsed, forming crumbly fractures which became enlarged into solution channels. Interconnected vugs were formed along solution channels by solution "stoping." Owing to continuous flow of fresh ground water and greater efficiency of solution-precipitation reactions, the paleophreatic zone stabilized long before the superjacent vadose zone where diagenesis was related to rainfall and downward infiltration of water. As carbonate mineralogy of the paleovadose zone slowly stabilized by solution and solution-precipitation, Ca and C0^~ ions were transported down to the phreatic zone and precipitated in unfilled voids as coarsely crystalline ferroan calcite cement which occluded all effective porosity. Large voids within the vadose zone remained unfilled because, after stabilization, there was no superjacent unstable zone with which it could engage in a donor-receptor relationship as it had with the subjacent phreatic zone.