Geologic Characterization of Caddo Limestone Reservoir, Curry Unit, Stephens County, North-Central Texas: ABSTRACT

Abstract

Three classic carbonate models (carbonate bank, oolitic shoal, and tidal flat) are applied to interpret the depositional environments and diagenesis of the Caddo limestone (Middle Pennsylvanian). The study area is located in west-central Stephens County, approximately 4 mi southwest of Breckenridge, Texas. Phylloid algal mounds developed on a lower Caddo ramp, along the transition of the Concho platform and Fort Worth basin. Within the area defined as the Curry unit, phylloid algal mounds and intermound facies aggraded to sea level. This led to the development of an areally restricted carbonate bank. Some phylloid algal buildups underwent subaerial exposure, and fresh water leached unstable carbonate minerals, creating minor secondary porosity (<10%). An oolitic shoal (0-9.2 m thick) overlies and partially intergrades with the phylloid algal bank. Ooids were deposited along the margin of the preexisting bank, whereas fine-grained oolitic and peloidal packstone were deposited on the bank. The bank margin oolite exhibits oomoldic porosity in excess of 15%. The uppermost 6.1-24.4 m of the Caddo represents intertidal deposition. Sediment, primarily algal remains and carbonate mud, was deposited on a preexisting surface of ooids and peloids. Fenestral fabric. rhizoliths, brecciated zones, and small-scale channels are common features. A wide variety of poremore » types are present in the intertidal zone; intraskeletal, framework, biomoldic, and vuggy pores are most important. Throughout this zone, porosity exceeds 10% and permeability averages 12 md. The Curry unit has produced approximately 28 million bbl of oil (primary and secondary) from the upper Caddo. Depositional environments and diagenesis of the Caddo are complex. A greater understanding of these complexities may enable geoscientists to evaluate remaining future potential within the Caddo and in other similar carbonate accumulations.« less