EFFECTS OF BIOGENIC POROSITY ON THE PETROPHYSICAL PROPERTIES OF THE KARSTIC LOWER CRETACEOUS GLEN ROSE LIMESTONE, BEXAR COUNTY, TEXAS

The combined Edwards and Trinity aquifer system is contained within the Lower Cretaceous (Aptian–Albian) karstic strata of south-central Texas and provides the primary source of freshwater for San Antonio and the surrounding area. The middle Trinity aquifer is contained within the Lower Glen Rose Limestone (GRL), which has been subdivided into six hydrostratigraphic units (HSUs) with distinct lithologic and hydrologic properties. These HSUs were first identified via core examination from the Camp Stanley Storage Activity (CSSA) in northern Bexar County, Texas and correlated to associated gamma-ray, spontaneous potential, and resistivity logs. The middle Trinity aquifer is a telogenetic karst and fluid flow is directed primarily through solution-enhanced faults and fractures and pervasive Thalassinoides networks because matrix porosity and permeability are both low. Meteoric water infiltrates the Trinity aquifer through the vertically-oriented faults and fractures of the Balcones fault zone and is transported laterally through a wide-spread biogenic pore network. This study used two 7.62 cm diameter cores and well logs from groundwater monitoring wells CS-MW9-CC and CS-MW5-LGR recovered from the CSSA to characterize the effect these large-scale Thalassinoides networks have on the petrophysical properties of four HSUs (Honey Creek, Rust, Doeppenschmidt, and Twin Sisters HSUs).Resistance values greater than 300 Ω-m are associated with interconnected Thalassinoides networks and values greater than 650 Ω-m are associated with solution enhancement biogenic pores. These values indicate that pervasive biogenic pore networks are a major fluid flow component and are also likely the starting point for karstic development along faults. Additionally, these high resistivity kicks are cyclical and are identified in muddy confining units, even when no changes in lithology or karstic development are identified. Natural gamma-ray values are inversely correlated to resistivity logs and display m-scale cyclicity. This study shows that resistivity logs can be used to identify interconnected Thalassinoides networks within subsurface GRL strata and when coupled with natural gamma-logs, the lateral distribution of these networks can be correlated.