ABSTRACT

Original mineralogy of ooids affects both their diagenetic history and, therefore, the reservoir quality of ooid lithologies. Previous studies have shown that ancient ooid cortices were composed either of aragonite, Mg calcite, or calcite. This study, based on fabrics and geochemical analyses of Smackover oolites, suggests that both aragonite and calcite ooids were deposited on the Upper Jurassic Smackover shelf of southern Arkansas and northern Louisiana. Smackover ooids exhibit both a blocky calcite spar fabric and a radial calcite fabric. Grainstones with blocky calcite spar ooids and oomoldic porosity have a higher strontium content than those with a radial calcite fabric. The high strontium has a Jurassic marine isotopic signature supporting the interpretation that the blocky cal ite spar is an early diagenetic fabric after aragonite. Radial calcite ooids were originally calcitic.

The ooids accumulated in a series of linear bars parallel to shore. Ooids with original aragonite mineralogy are older and were deposited in the north. The younger, more southern bars consisted of calcite ooids. Aragonite ooids in the north were susceptible to dissolution and formation of moldic porosity during freshwater diagenesis. Moldic porosity is generally accompanied by low permeability. Calcite ooids in the south, however, were less susceptible to dissolution in meteoric waters; hence, preserved porosity is interparticle porosity and permeability is generally higher. Porosity types in the upper Smackover of Arkansas and Louisiana reflect both the original mineralogy of the ooids as well as their diagenetic history.