Burial Cementation--Is It Important? A Case Study--Stuart City Trend, South-Central Texas: ABSTRACT

Abstract

The Stuart City trend is a shelf-edge buildup of Lower Cretaceous bioclastic and reefal carbonates that is currently buried to depths of between 3,300 and 5,000 m (11,000 and 16,000 ft). Compaction and cementation have generally reduced rock porosities to less than 9%. Sediments were cemented in the marine environment by finely crystalline bladed, isopachous cement and volumetrically important (14 volume %) coarse to very coarsely crystalline, fibrous to bladed, isopachous, Mg-calcite cement. These cements have been neomorphically altered End_Page 536------------------------------ to low Mg-calcites forming an unusual radiaxial texture observed in the coarse to very coarsely crystalline, bladed, calcite cement. Evidence of their marine origin consists of a relative 1 mole % Mg++ memory, a marine-like isotopic character (^dgr18O ^approx -2.5 and ^dgr13C ^approx +2.0), and early relative timing of precipitation. Diagenetic alteration of these carbonate sediments by the interaction with meteoric water in lenses that formed within topographic highs along the shelf margin changed the initial marine chemical, isotopic, and textural character of the sediments. Secondary porosity formation, mineral stabilization, aggrading neomorphism, and equant spar calcite cementation are the important products of meteoric diagenesis. The equant spar calcite cements make up approximately 16% by volume of the limestones studied. They are iron and manganese poor. The majority have a ^dgr13C composition which falls in the range of modern marine carbonates, i.e., 0.9 to +3.5^pmil. The ^dgr18O compositions range from -1.3 to -6.6^pmil relative to the PDB standard. Oxygen stable isotopic and petrographic data suggest that over 50% of the equant spar calcite cements were formed in a near-surface meteoric environment. A large percentage of the remaining equant spar calcite cements formed at shallow burial depths in a water-limited system where mass transfer was dominated by diffusional processes. Thermally induced ^dgr18O depletion of the equant spar calcites, indicating significant fluid flow, was of minimal importance. Pyrobitumen pore fillings and inclusions in the outer 1.0 mm-thick rims of the very coarsely crystalline, equant spar calcite cements indicate that only minor amounts of cementation have occurred since the introduction of hydrocarbons. Deep burial diagenesis (i.e., post-hydrocarbon migration) consisted of the precipitation of minor amounts of galena, fluorite, and Sr++-rich equant spar calcites. These diagenetic products can be directly related to the present-day formation water. The chemical, isotopic, and textural characteristics of the Stuart City trend limestones contain the imprints of their initial marine composition and shallow diagenetic alterations in a hydrodynamic system. Burial diagenesis has not significantly altered these limestones. Fault and fracture control on the movement of formation waters in this system determine the location and intensity of late stage diagenetic events. End_of_Article - Last_Page 537------------