Sequence Stratigraphic Control from Middle Albian (Cretaceous) Inner Platform Rudist Mound Geometries, West-Central Texas: Insights from Outcrops and Implications for Reservoir Connectivity
The Edwards Formation of the Middle Albian Fredericksburg Group of west-central Texas is a carbonate succession that is regionally divisible into a southern inner platform facies belt developed on the westward extension of the Concho Arch, and a northern open platform facies belt of the western extension of the East Texas Basin. A well - developed sequence boundary is recognized regionally that separates the underlying colloquial “Walnut - Comanche Peak” interval from the overlying Edwards Formation. Rudist accumulations are well - developed in the Edwards in both the inner and open platform facies belts. Inner platform rudist accumulations display distinct differences in external morphology and internal lithofacies architecture that are closely tied to the systems tracts of the Edwards sequence. Reservoir rock properties are closely tied to these lithofacies, thus distinct types of flow unit architecture and rock property heterogeneity are partitioned among systems tracts in the Edwards sequence.
At the focus outcrops south of Sweetwater, Texas the thin Edwards LST marl is abruptly overlain by an extensive 3m thick unit of ooid and skeletal grainstones and coarse rudist rudstone of the TST. The fine ooid and skeletal facies have moderate rock quality with porosity between 10% and 17%, while the rudist rudstones have up to 30% porosity. Although the external morphology of this rudist package is tabular, and would be modeled as an extensive tabular flow unit, the internal back-stepping sigmoids of highly variable lithofacies/rock properties would significantly reduce connectivity within the build-up facies. Accurate representation of this complexity would be a challenge.
The overlying Edwards HST contains coarse rudist grainstone - rudstone facies in discrete build-ups surrounded by peloid, mollusk, and foram grain-dominated packstones to wackestones. Although rudist build-ups have relatively good rock properties, the surrounding packstones and wackestones have very low porosities. This produces good within - mound communication but very poor inter - mound connectivity. Thus, these lithofacies would be modeled as discrete, high quality rudist patches with intervening poor quality inter-patch areas. The discrete nature of these lithofacies makes modeling HST heterogeneity and connectivity easier than for the internally complex TST sigmoids.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009