Diagenetic History of Cotton Valley Limestone at Teague Townsite Field, Freestone County, Texas: ABSTRACT

Abstract

The Cotton Valley lime was deposited during a regressive phase of the Late Jurassic, in a shallow sea with an extensive platform. Mild salt tectonism has modified depositional and diagenetic environments through time. The Cotton Valley lime is composed of thick, massive oolitic, finely crystalline, micritic limestones which rim the west flank of the East Texas basin. The Cotton Valley lime at Teague Townsite field represents deposition within a shallow sublittoral marine sandbar environment and its associated laterally equivalent facies. Included within the sequence are as many as nine local shoaling upward cycles. Petrographically observed diagenetic features include products of neomorphism, compaction and pressure-solution, cementation, leaching, and replacement intergr nular porosity was occluded early by rim and pore-filling cements. Intragranular porosity was produced as a result of early meteoric leaching, a feature abundant in porous zones, and occasionally accompanied by equant cement. Compaction is expressed mainly as pressure-solution features and stylolitization. It is inferred that early mineralogic stabilization and meteoric cementation armored grains in porous intervals against burial End_Page 1472------------------------------ effects. Hydrothermal baroque dolomite is most abundant in porous intervals, and occurs as void-filling cement precipitated in interparticle porosity, fracture porosity, along stylolites, and also as a replacement mineral after anhydrite. During diagenetic dissolution and reprecipitation events, certain elements substitute for Ca2+ in the CaCO3 lattice, in proportion to the composition of the diagenetic fluids. The behavior of substituting cations in this study is largely indicative of diagenesis by subsurface fluids. It also implies that much of the observed diagenetic alteration occurred in a closed diagenetic system. During diagenesis the isotopic content of a carbonate will re-equilibrate with that of surrounding pore fluids. The depletion of O18 i plies precipitation at elevated temperatures or from subsurface fluids, while C13 consistency reflects original carbon composition. Values from this study coincide with those for other Jurassic carbonates and late dolomites. Petrography and geochemistry indicate that diagenesis occurred in the marine phreatic, mixed phreatic, meteoric phreatic, and subsurface diagenetic environments. End_of_Article - Last_Page 1473------------