STRATIGRAPHIC CORRELATION OF THE AUSTIN CHALK USING SPECTRAL GAMMA RAY DATA FROM OUTCROPS IN NORTH-CENTRAL TEXAS

The Austin Chalk is a rhythmically bedded sequence of chalk and marl that represents pelagic to hemipelagic carbonate deposition in the ancestral Gulf of Mexico during the Upper Cretaceous. The Austin differs from traditional chalk deposits due to its relatively high abundance of clay and volcanic ash. The outcrop trend of the chalk mirrors the subsurface trend of the Ouachita orogen from north-central Texas to west Texas. The Austin Chalk is heavily fractured and is deformed by the normal faults that are part of the Balcones Fault Zone.

Historically the Austin Chalk has been exploited as a conventional reservoir produced from natural porosity and permeability without large hydraulic stimulations. More recently, the Austin Chalk is being explored as a combination fractured and unconventional reservoir, relying on natural porosity and permeability combined with induced hydraulic fracturing to generate new fracture porosity to release hydrocarbons trapped in microscopic pores. In addition to its reservoir properties, much of the city of Dallas is built within the outcrop trend of the chalk. Understanding the properties and deformation features of the Austin Chalk is essential to the construction industry in north-central Texas.

Here, we present new spectral gamma ray data collected from various outcrops in north-central Texas using a RS-230 spectrometer. The presence of laterally extensive ash deposits and distinct lithofacies within the Austin Chalk allows for correlation between outcrops, and correlation from outcrop to subsurface. We use lithofacies descriptions and gamma ray profiles from the Getty 1 Lloyd Hurt well type core described by Loucks et al. (2020) as the basis of our correlation. Outcrops in north-central Texas are correlated to the subsurface using the USGS GC-2 core. We show that the Austin Chalk can be correlated throughout north-central Texas, and that lithofacies described from south Texas may extend into north-central Texas.