Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes

Abstract

A section of the ARCO Bodcaw No. 1 core from the Upper Smackover (Jurassic) of southern Arkansas was studied in detail in an effort to understand the control of porosity. Petrographic, isotopic, atomic absorption, and cathodoluminescence data on whole rocks and components were generated on 24 samples from a 115-foot length of core. The core is composed almost entirely of accretionary and cryptocrystalline grains and cement. Porosity is unrelated to grain type, size. or sorting on a sample-to-sample basis. The porous intervals are interpreted to have been stablized to calcite under fresh-water phreatic conditions. The porous intervals approach values of -5.0 per thousand delta 18 O, 1.5 mol percent Mg, and 300 ppm Sr and are further characterized by luminescent oolites and relatively slight solution compaction effects. The nonporous intervals represent relict vadose and subphreatic environments which stabilized after burial. The nonporous intervals approach values of -2.8 per thousand delta 18 O, 2.5 mol percent Mg, and 600 ppm Sr and are further characterized by nonluminescent grains and intense solution compaction. Data indicate that oolites were initially composed of a metastable mineralogy, cryptocrystalline grains were composed of a more stable mineralogy, and algal grains were composed of a mixture of mineralogies. Oolites in the vadose and subphreatic environments were not fully stabilized under fresh water conditions, but vadose grains carried with them a light delta 13 C soil gas signal after burial. These grains were fully stabilized under post-fresh-water, burial conditions. Observed porosity distribution is readily explained in terms of solution compaction and cementation. Grains with metastable mineralogy in relict vadose and subphreatic intervals were heavily solution-compacted at relatively shallow depths before being converted to stable calcite. Intense solution compaction destroyed pore space in these intervals. Dolomite cementation seems linked with the compaction process. The stablized mineralogy of the phreatic lens, together with associated meteoric cements, resisted solution compaction and thereby preserved pore space.