Ground-penetrating radar imaging of Albian rudist buildups, central Texas

Abstract

Rudist buildups are important reservoirs in many Cretaceous fields in the Middle East, but they are generally near or below seismic resolution. The dimension, shape, and architecture of rudist buildups can be assessed using outcrop, although only partly so because of pseudo-2D observations of geobodies intersecting with the outcrop. We used ground-penetrating radar to enhance our understanding of the shape, dimension, and architecture of Albian rudist buildups in two outcrops in Texas. In the Lake Georgetown spillway, caprinid rudist buildups are 10–30 m wide and 2–7 m high. They are elliptical with an aspect ratio of as much as 1.7. They show no or very little flank development. The older buildup exposed in the Red Bluff Creek area displays 10- to 25-m-wide and 5- to 10-m-high in situ caprinid rudist mound core accumulations with as much as 100-m-wide reworked flanks in the shallower part of the depositional profile. Downdip along the depositional profile, caprinid buildups are 5–20 m wide and 3–7 m high with no flank debris. The buildups in the Lake Georgetown area have similar architecture and comparable size with the downdip buildups exposed in Red Bluff Creek. These buildups were compared with other outcropping Albian buildups in Texas that have different sizes, shapes, and stratigraphic architecture to provide dimensional data that could be used in subsurface reservoir modeling, either for calibrating variogram ranges or to build training images. The rudist buildups exposed in Texas are an order of magnitude smaller than those present in the subsurface in the Middle East, but they have comparable stratigraphic architecture. The size difference might be the result of subsurface buildups being mapped using well-log or core correlations or seismic extractions that cannot resolved at that scale of heterogeneity.