ABSTRACT The Cenomanian–Turonian Eagle Ford of South Texas is largely composed of two interbedded rock types: marls and limestones. The marls consist mainly of coccoliths with sand- and silt-size grains predominantly comprised of planktonic foraminifera with lesser amounts of inoceramid fragments and other carbonate grains. The limestones are recrystallized, and they contain calcified radiolarians and calcispheres, with almost all pore spaces having been filled with calcite cement. Most of the hydrocarbons in the Eagle Ford, regardless of thermal maturity, reside in the pore network of the marls. Economic production of hydrocarbons stored in these marls, which have nanodarcy permeabilities, can only be obtained by inducing and maintaining fractures with hydraulic stimulation. The interbedding of the marls with limestones form centimeter-scale brittle–ductile (or stiff-compliant) couplets that influence hydraulic fracturing over a range of scales, and at the smallest scale it may increase production by hosting complex near-wellbore fracture systems. Natural fractures that were already present may be open or cemented and reactivated during hydraulic stimulation and contribute to production. This can generate a hybrid fracture system with a larger drainage area and fracture surface area to allow for crossflow from the matrix to fractures. The Eagle Ford is a dual-porosity system, with the hydrocarbon stored in the marls feeds a network of progressively larger natural and induced fractures that carry those hydrocarbons to the wellbore. In most cases, the Eagle Ford will be most productive when the “right” mixture of marl and limestone are present. Too much limestone lowers the storage capacity of the system, and too much marl reduces the complexity of the fracture system. The distribution of the limestones is important: Even if the percentage of limestone in two sections is equal, hydraulic stimulation will produce a more complex fracture network when the limestone is present as a series of thin interbeds rather than as a single thick limestone. The interbedding of limestone and marl can be measured using limestone frequency—the number of limestone beds per unit thickness. Variation in production is observed in wells on the same pad completed with the same treatment but landed in zones of differing limestone frequency, with production in these wells increasing with limestone frequency. Also, in a multivariate analysis involving numerous engineering and geologic variables and over 1000 wells, all measures of interbedding reduced to a single factor, which we call limestone frequency, which positively correlated with production.

A chronostratigraphic framework was developed for the subsurface Eagle Ford of South Texas in conjunction with a log-based regional study that was extended across the San Marcos Arch and into East Texas using biostratigraphic and geochemical data to constrain log correlations of 12 horizons from 1729 wells in South and East Texas. Seven regional depositional episodes were identified by the study. The clayrich Maness Shale was deposited during the Early Cenomanian in East Texas and northern South Texas where it correlates to the base of the Lower Eagle Ford. After a fall in sea-level, East Texas was dominated by the thick siliciclastics of the Woodbine Group, whereas in South Texas deposition of the organic-rich EGFD100 marls of the Lower Eagle Ford began during the subsequent Lewisville transgression. A shift in depositional style to the limestones and organic-rich shales of the Eagle Ford Group occurred in East Texas during the Middle-Late Cenomanian EGFD200 and EGFD300 episodes produced by the continued rise in sea-level. Erosion along the Sabine Uplift shifted the focus of deposition in East Texas southward to the Harris delta and deposited the “clay wedge” of northern South Texas during the EGFD400 episode. The introduction of an oxygenated bottom-water mass onto the Texas shelf produced the considerable decrease in TOC preservation that marks the Lower/Upper Eagle Ford contact. This event coincided with the onset of Oceanic Anoxic Event 2 (OAE2) and the Cenomanian-Turonian Boundary sea-level high, which starved much of the Texas shelf of sediment. The only significant source of sediment was from the south; within the study area, the EGFD500 interval is essentially absent north of the San Marcos Arch. Deposition recommenced on much of the Texas shelf during the Late Turonian EGFD600 episode with the Sub-Clarksville delta of East Texas and the carbonate-rich Langtry Member of South Texas and eastern West Texas. Bottom-waters became oxygenated at approximately 90 Ma, initiating the transition from the Eagle Ford Group to the Austin Chalk.