DIAGENETIC HISTORY OF THE ELLENBURGER GROUP IN THE FORT WORTH BASIN, TEXAS

The Ellenburger Group in the subsurface of the Fort Worth Basin, Texas has undergone several episodes of diagenesis, which included various phases of dolomitization. Identifying the order of the diagenetic processes is important for hydrocarbon exploration and production in the Fort Worth Basin. Core samples used in this study were divided into five different lithologies: dolostone, dolostone breccia, dolomitic mudstone, dolomitic ooid-grainstone and dolomitic wackestone-packstone. Five types of dolomites were also identified using petrography (polarizing microscopy, reflected light microscopy and cathodoluminescence microscopy) and the Dolomite-rock Classification Scheme of Sibley and Gregg (1987). Dolomite type 1 is characterized by the very fine crystalline dolomite and the preservation of primary sedimentary structures. It is interpreted as forming shortly after deposition of micrite. Karstification developed vugs, cavities and collapse breccias and freshwater resulted in extensive calcification followed by silicification. Burial of the Ellenburger developed fractures, dissolution seams and stylolites that were later filled or replaced by dolomite types 2 and 3. Dolomite type 2 is found within dissolution seams, which may have been conduits for fluids. Dolomite type 3 is localized along crystal boundaries, cleavage planes, and fractures within calcite and formed from late burial fluid. Type 4 dolomite replaces earlier dolomites and is commonly found in dolostones. This is interpreted as a late stage diagenetic dolomite formed during burial. Dolomite type 5 is a cementing dolomite found in veins and vugs. It is characterized by saddle dolomite that formed at slightly elevated temperatures. A paragenetic sequence that summarizes the events recorded in the core samples was constructed base on the petrographic observations. The resulting paragenesis represents the evolution of a carbonate platform and similar trends and timings of diagenesis have been reported from the Permian Basin. Difference in the paragenetic sequence may be caused by local geologic parameters.