Often one of the most basic challenges of using outcrops to better understand the subsurface is simply terminology. These terminology challenges include: 1) provincial outcrop nomenclature; 2) the fundamentally different way stratigraphic units are commonly defined in the subsurface (by surfaces) versus outcrops (by lithology); and 3) local variations in member boundaries and/or member nomenclature used by different researchers.
In the subsurface of South Texas, the Eagle Ford Group is commonly divided into an organic-rich Lower Eagle Formation and a carbonate-rich Upper Eagle Ford Formation. In contrast, the coeval strata that crop out nearby in West Texas are traditionally referred to as the Boquillas Formation. Adding further complexity to the nomenclature issue is that previous workers defined anywhere from two to five informal members to the Boquillas Formation in West Texas, but the names, as well as the stratigraphic intervals defined by these members, all differ. In order to highlight the distinct lithologic units defined by previous worker, but also provide a clear and easy to use nomenclature that conveys superposition, a simple five-fold succession of informal lithostratigraphic units, termed A to E from the base up was proposed. Unit A consists of foram grainstone interbedded with dark gray mudstone, whereas unit B is dominated by organic-rich mudstone. Unit C consists of white foram packstone interbedded with gray mudstone, while unit D is dominated by yellow-ochre colored bioturbated skeletal wackestone/packstone. Unit E consists of thin-bedded skeletal grainstone interbedded with mudstone and bentonite beds.
Clearly, the first step in effectively using Boquillas outcrops of West Texas for comparison to the Eagle Ford in the subsurface of South Texas, is to refer to these strata as the Eagle Ford Group, as is done elsewhere in the state. By incorporating information from a handheld gamma ray spectrometer, as well as geochemical data, to the outcrop lithologic information, the Lower and Upper Eagle Ford formations as defined in South Texas can also be defined in the outcrops of West Texas. The Lower Eagle Ford Formation contains units A&B, while the Upper Eagle Ford Formation contains units C, D, and E. Similar to the subsurface of south Texas, the base of the Upper Eagle Ford Formation in the outcrops of West Texas is marked by a drop in total GR values driven by a drop in U-content, a drop in TOC content, and the onset of a positive d13C excursion. With the inclusion of geochemical and geophysical log data from a research borehole, a clay-rich high-density and low resistivity marker zone also is defined at the base of the Upper Eagle Ford Formation in the West Texas outcrops, similar to the base of the Upper Eagle Ford Formation in the subsurface of South Texas.
Using the geochemical, petro-physical, and lithologic information in the West Texas outcrops and research borehole four distinct allo-stratigraphic members (depositional sequences) more suitable for regional subsurface correlations can also be defined. The Lower (Lozier Canyon) Member of the Lower Eagle Ford Formation can be characterized as organic-rich (high resistivity) mudstone. In outcrop, it consists of unit A and the lower portions of unit B. The Upper (Antonio Creek) Member of the Lower Eagle Ford Formation can be characterized as uranium- and bentonite-rich (high GR) mudstone, with lower (1-2%) TOC content. In outcrop, this member consists of the upper portions of unit B. The Lower (Scott Ranch) Member of the Upper Eagle Ford Formation is a uranium-poor (low GR) interval consisting of interbedded limestone and mudstone. A diagnostic feature of this member, which includes unit C in outcrop, is the presence of a distinct positive d13C excursion at its base, the peak of which is taken as the geochemical proxy for the base of Turonian. The Upper (Langtry) Member of the Upper Eagle Ford Formation is bentonite-rich bioturbated mudstone with high GR and low resistivity values at its interpreted maximum flooding surface (mfs). In outcrop this member consists of units D and E. All four members can in turn be readily correlated into the subsurface of South Texas. With this stratigraphic framework in place, researchers can now use the West Texas Eagle Ford Group outcrops to more readily study specific portions of the Eagle Ford Group in the subsurface of South Texas, and use this information to better understand the facies, distributions, thickness variations, and reservoir properties of specific reservoir zones.