Upper Albian rudist buildups of the Edwards Formation in central Texas: A GPR-assisted reservoir analog study

摘要

Albian rudist-associated shallow water carbonates form important hydrocarbon reservoirs in the Middle East, Mexico, and the United States. Outcrops of rudist-bearing shallow water carbonates are common across parts of central Texas, USA, and in places display lateral and vertical relationships that form important analogs for the subsurface distribution and heterogeneity of hydrocarbon reservoirs and seals. In particular, the walls and spillways of several lakes in central Texas provide ideal conditions for the investigation by ground-penetrating radar (GPR) of the three-dimensional geometries of platform interior rudist lithofacies belts. This study integrates outcrop studies and GPR surveys to clarify lateral and vertical facies relations of rudist-bearing lithofacies in the Cretaceous Edwards Formation (Fredericksburg Group) that are partially exposed in spillways and gullies near Lake Georgetown and Belton Lake in central Texas, USA. GPR data illuminated four types of mounds that have distinct three-dimensional geometries, lithofacies, and biota and that record both seaward to leeward changes and vertical changes from below wavebase to within wavebase. The lowermost mounds are high relief circular caprinid mounds that mark the aggradational part of the Edwards lithostrati-graphic unit at our study locality. Well cemented, low relief toucasid-foraminiferal mounds and grainstones cap these caprinid-dominated buildups. Weakly cemented, circular, flat-topped radiolitid-caprinid-toucasid mounds developed above the toucasid mounds and low relief, often sucrosic-dolomitized mounds constitute the most shallow water rudist accumulations. Stromatolitic dolomites record final filling of accommodation space on the Edwards platform. GPR provides an unparalleled view of the three-dimensional growth, vertical accretion, and progradation of these mounds and lithofacies through time, and the geoelectrical signatures provide insight on porosity and permeability of the lithofacies in outcrop.