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Re-defining Depositional Model of the Buda Formation Utilizing Outcrops From West Texas and Cores From South and Central Texas

Abstract

The Upper Cretaceous Buda Formation was exposed across Texas in an outcrop belt from west Texas to northern Fort Worth County, and extending continuously in the subsurface from south to central Texas. This research integrates dataset from outcrops, cores, and more than 200 wireline logs in west, south and central Texas to investigate the thickness distributions and depositional settings of the Buda Formation. 2D strike and dip cross sections constrained by cores indicate the thickness of the Buda Formation varies significantly from 30 to 172 feet through central to south Texas. Across the San Marcos Arch (SMA), the Buda Formation thins, but thickens to the northeast and southwest. The isochore map derived from the wireline logs shows considerable thickening between the pre-existing Stuart City and Sligo shelf margins in south Texas. Outcrops in west Texas are predominantly skeletal wackestone with rare packstone, whereas cores from central and south Texas are dominated by globigerinid wackestone and bivalve mudstone. Both outcrops and cores are extremely burrow mottled. One core in central Texas has a 20 feet interval interpreted as “False Buda” with rock types similar to the Buda Formation. Stable isotope analyses of the core sampled every 0.5 m show a negative excursion in δ13CPDB from +1.5 ‰ near the top of the Buda Formation to -0.5 ‰ within the basal part of the “False Buda”, and then back to +1.5 ‰ at the base of the Eagle Ford Group. The dramatic variations of the Buda Formation thickness along the dip direction are consistent with filling accommodation space across the paleotopographic profile of the pre-existing Lower Cretaceous reef trend. Meanwhile, facies variations from the outcrop and subsurface data record a transitional change of the depositional environments from the inner shelf to subtidal shallow marine. The abundance of bioturbation across the depositional profile suggests the deposition occurred along a well-oxygenated low relief ramp. The negative excursion of δ13CPDB during the Buda-“False Buda” contact zone suggests possible subaerial exposure or a rapid drowning event during its deposition. We propose the “False Buda” succession is likely the basal parasequence of the Eagle Ford Group according to our core analysis from central Texas. In general, the outcrop and core studies help to construct a more complete 3D depositional model for the Buda Formation, and provide more detailed insights into its reservoir characteristics.