USING WELL LOGS TO ASSESS THE EFFECTS OF THALASSINOIDES-DOMINATED ICHNOFABRICS ON THE SUBSURFACE FLUID PATHWAYS OF THE TRINITY AQUIFER IN NORTHERN BEXAR COUNTY

The Trinity aquifer is contained within the Lower Cretaceous (Aptian–Albian) Trinity Group and is the primary source of freshwater for San Antonio and the surrounding area. The Lower Glen Rose Limestone (GRL) contains most of the middle Trinity aquifer, which has been subdivided into six hydrostratigraphic units (HSUs) with distinct lithologic and hydrologic properties. These HSUs were first identified in core and correlated to associated gamma-ray, spontaneous potential, and resistivity logs at the Camp Stanley Storage Activity (CSSA) in northern Bexar County. The middle Trinity aquifer is predominately marl and wackestone, and the matrix porosity is low. Therefore, this aquifer is classified as a telogenetic karst where fluid flow is directed through solution-enhanced faults and fractures and pervasive Thalassinoides networks. These biogenic pore networks facilitate the lateral movement of water away from vertically-oriented faults and fractures. The offsets caused by the Balcones fault zone provide the unique opportunity to compare the patterns in trace fossil abundance seen in outcrop directly to subsurface data. 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 the solution enhancement biogenic pores. The correlation between the amount of bioturbation and resistivity values indicate that these biogenic pore networks are a major fluid flow component and are the likely the starting point for karstic development along faults. Additionally, cyclical patterns are observed 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. 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.