EAGLE FORD GROUP DEPOSITION AND CORRELATION IN SOUTH AND WEST TEXAS, INSIGHTS FROM OUTCROPS AND CORES

The Late Cretaceous Eagle Ford Group is a prolific unconventional oil and gas source rock play. Southwest of the San Marcos Arch Eagle Ford strata are carbonate-rich consisting predominantly of carbonate-rich (up to 40 wt % calcium) organic-rich mudstone beds interbedded with foraminiferal packstone and grainstone beds. The packstone and grainstone beds commonly contain hummocky and wavy cross-stratification (HCS and WCS) suggesting deposition occurred within storm wave base (10’s of m’s water depth). Rare Chondrites on the top of HCS and WCS beds indicates the storms allowed oxygen to episodically reach the sea floor. However, geochemical tracers (Mo, V) and absence of bioturbation in carbonate-rich mudstone indicate that deposition of these facies occurred in anoxic and/or euxinic conditions. A hand-held gamma-ray scintillometer that measures the total gamma rays and their component parts (U, Th, K) provides unique signatures that allow correlations with abundant subsurface logs. Numerous widespread ash beds within the Eagle Ford Group also are providing a geochronologic framework to correlate this unit from the surface to the subsurface. The positive carbon isotope fluctuation associated with Ocean Anoxia Event 2 (OAE2), near the Cenomanian-Turonian boundary also provides another avenue for correlation from the outcrops to the subsurface. However, a regionally extensive unconformity near the Cenomanian-Turonian boundary suggests caution in using this as a stand-alone proxy.