Abstract

Cretaceous carbonate shelf systems, including patch reef complexes, are host to some of the world’s most prolific and complex oil and gas reservoirs. Patch reef complexes are still poorly understood due to their heterogeneous development and distribution throughout Cretaceous time; much of our understanding stems from outcrop studies where a clear link between external geometry and stratigraphic context is demonstrated. Such studies have been conducted in uppermost Albian reef complexes in central and south Texas, but little is known of the upper Aptian- lower Albian interval that is time-equivalent to productive patch reef complexes in the Maverick Basin. A possible outcrop analog for Maverick Basin patch reef systems is located in the Mural Limestone of southeastern Arizona.

The Mural Limestone is exposed in a number of folds and east-dipping fault blocks in the Mule Mountains and at the Paul Spur locality of southeastern Arizona, to the northeast and southeast of Bisbee, Arizona. The Mural Limestone represents a shelfal remnant of a south-facing carbonate ramp to rimmed shelf profile that prograded into the Chihuahua Trough during the Cretaceous; coeval exposures of coalesced patch reefs representing a low-relief shelf margin “bank” are located in Sonora, Mexico (Warzeski, 1983). The Mural Limestone is divided into two lithostratigraphic members with time-transgressive boundaries above and below: the lower member composed transgressive intercalated carbonates, siltstones, and shales (90 to 160m), and the upper member composed of transgressive and regressive massive (buildup-bearing) to bedded carbonates (50 to 75m). Results of biostratigraphic studies in Arizona and geochronologic studies in Sonora, Mexico determined that the Mural Limestone is time-equivalent to the Pine Island Shale member of the Pearsall Formation (Scott, 1987) and the lower Glen Rose Formation (Warzeski, 1987; Gonzalez-leon et. al., 2007) in central Texas. An important aspect of the sequence architecture that is currently missing from published literature is the recognition and description of the high frequency cycle- or parasequence-level depositional history of the patch reefs and associated shelf facies. Patch reef facies in the Mural Limestone are well documented (Scott, 1979), but their relation to surrounding shelf facies, including ooid “shoal” facies, has yet to be determined.

The purpose of this study is to document the detailed facies architecture and stratigraphic setting of patch reefs and their associated shelf facies within the transgressive upper Mural Limestone. The stratigraphic framework will be used to develop an outcrop analog model for similar shelf carbonates that are productive in the Maverick Basin. The model will allow for predictive understanding of Mural facies development within a sequence framework and provide a statistical understanding of patch reef dimensions and distributions for improved subsurface facies modeling.

This study will employ a combination of standard field techniques (section measuring, photomosaic tracing, and RTK GPS-guided mapping) and use of detailed digital outcrop characterization of patch reefs using ground-based LIDAR. Thin-section and core-plug porosity/permeability analysis will allow petro-physical characterization of key facies. Carbon isotope analysis of selected sections will constrain chronostratigraphy (Scholle and Arthur, 1980; others). Data integration will occur in a 3D geocellular modeling environment using Polyworks, GoCad, and/or Petrel.