--> --> Microfacies Analysis, Diagenesis, Paleoenvironments, and Reservoir Quality of Early Ordovician El Paso Group of the Southern Hueco Mountains, Hudspeth County, Texas

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Microfacies Analysis, Diagenesis, Paleoenvironments, and Reservoir Quality of Early Ordovician El Paso Group of the Southern Hueco Mountains, Hudspeth County, Texas

Abstract

The Early Ordovician El Paso Group overlies time-transgressive Cambro-Ordovician Bliss Sandstone, and underlies the Upper Ordovician Montoya Group. The rock samples collected from the Group are analyzed to identify and describe the microfacies, diagenesis, paleoenvironments of deposition, and hydrocarbon reservoir potential. The rocks of El Paso Group are classified into eleven major microfacies based on laboratory analyses of the rocks’ components. The major microfacies are (from common to least common), pelsparite, pelsparite/pemicrite, bioclastic wackestone/grainstone, packstone/grainstone, wackestone/packstone, algal grainstone, lithoclastic grainstone, nonlaminate pure micrite, grainstone/packstone, oolite, and fenestral, peloidal and laminate micrite. The main microfacies are further classified into several submicrofacies to more fully and adequately describe the rocks of El Paso Group. The variations from the major microfacies are attributed to sea level changes during early Ordovician time. The sea level changes resulted in a large scale mixture of carbonates with sandstone, indicating proximity to clastic source. The rocks of El Paso Group were deposited in a shelf lagoon with open and restricted circulation, subtidal and intertidal environments. Deposition in these environments is indicated by the presence of land-derived clastics (silt- and sand-size quartz), abundant peloids, pellets, ooids, and a variety of textures (mudstones to grainstones), algal structures, cross-bedding, and inter-fingering of clastics with limestones due to sedimentary offlap and onlap resulting in clastic progradation west and east of the Diablo platform (the clastic source). The upper formation was, in part, deposited in a supratidal environment, as shown by the presence of abundant dolomite. The microfacies also indicate deposition in a shallow water (shallow undathem). The presence of fragmented fossils indicate the presence of at least occasional high-energy environment that subjected the sediments to wave action and winnowing. The rocks were subjected to diagenetic processes such as dolomitization (supratidal and through meteoric mixing), compaction, bioturbation, silicification, micritization, stylolitization, recrystallization, dissolution, fracture filling, secondary porosity developments and cementation. From the permeability and porosity, and microfacies studies the rocks of Early Ordovician El Paso Group have good hydrocarbon reservoir qualities.