ABSTRACT During latest Devonian to Middle Mississippian parts of the Neoacadian and Ouachita orogenies, the Appalachian Basin and parts of the Illinois Basin were filled with clastic debris derived from the westward-prograding Borden-Grainger-Price-Pocono clastic wedge. This delta complex is overlain by the widespread shallow-water Newman–Greenbrier–Slade–St. Louis–Warsaw–Salem–Harrodsburg carbonate interval across sediment-starved surfaces, comprising the Floyds Knob bed or interval. The Middle Mississippian (late Osagean; early Viséan) Floyds Knob interval is less than a meter to several meters thick and is composed of multiple zones of pelletal glauconite, finely divided glauconitic shales, glauconitic carbonates, and locally derived carbonate mud mounds. The interval occurs across most of the Borden-Grainger delta platform, delta front, prodelta, and within the starved-basin area seaward of the delta complex, which was then filled with the Fort Payne Formation. This study reports herein the first occurrence of the Floyds Knob interval within the Fort Payne Formation. Glauconite deposition in this interval apparently occurred in mildly oxic to dysoxic, sediment-starved, shallow-marine settings and is believed to represent termination of major clastic influx in more proximal parts of the Neoacadian foreland basin during lowstand conditions. Moreover, these starved-basin conditions can be correlated with delta diversion following bulge migration during flexural loading–type relaxation. During these sediment-starved, lowstand conditions, glauconite was deposited across deltaic and basinal settings in central and distal parts of the Neoacadian foreland basin, as well as in eastern parts of the present-day Illinois intracratonic basin. The cessation of deltaic clastic sedimentation permitted development of carbonate mud mounds and associated glauconitic shales on and near reactivated structures in central parts of the Fort Payne starved basin and set the stage for the widespread deposition of thick, Meramecian–Chesterian carbonates throughout the basins during succeeding subtropical and lowstand conditions. Whether less-than-a-meter or tens-of-meters thick, the Floyds Knob interval is a widespread Middle Mississippian chronostratigraphic interval in the east-central United States that reflects a change in tectonic regime, which is recorded in the shift from predominantly clastic to carbonate sedimentation across a broad region. Aside from its correlative value, the unit demonstrates consequent sedimentary responses to the interplay among tectonism, paleoclimate, and paleogeography.

ABSTRACT Utilizing a basin-wide data set of three-dimensional seismic volumes and the application of principles of seismic stratigraphy and seismic geomorphology allowed identifying numerous depositional elements within the Vaca Muerta–Quintuco system, a set of clinoforms whose topsets belong to the Quintuco Formation, whereas the bottomsets and foresets belong to the Vaca Muerta Formation. Within the topsets, small circular geobodies clustered near the prograding shelf margin, averaging 200–800 m (656–2625 ft) in diameter and up to 75 m (246 ft) in height. These features comprise small carbonate buildups defining carbonate factories trending strike parallel. Identification of intervals where these geobodies are abundant is important because wells drilled through them have experienced either drilling mud admission or gas influx. In addition to these biogenic carbonate mounds, the topsets show elongated oolitic grainstone shoals oriented orthogonal to coeval shelf margins, in some cases measuring up to 22 km (14 mi) long and 5 km (3 mi) wide. The foresets (slope deposits) become progressively enriched in total organic carbon (TOC) and porosity downdip—key variables for a self-sourced unconventional reservoir. These deposits commonly comprise mudstone and marlstone, interbedded with limestones. In the lower foresets to toesets, strike-parallel, high–seismic-amplitude, and high-energy calcareous deposits are embedded in organic-rich mudstones. In some instances, these amplitude anomalies, drilled and cored by a few wells, show both well-defined linear geobodies along the toeset and evidence of bottom currents in cores (thicker limestone beds, ripples, bioturbation, and occasional centimeter-scale soft-sediment deformation). Identification of such geobodies is critical, as there is evidence from ongoing development drilling that these may act as hydraulic-fracture barriers and can also affect well performance as evidenced by increased water production. The bottomsets consist of low–amplitude-parallel, “railroad track” reflections that extend for tens of kilometers, characterizing the classic basin center Vaca Muerta play. Within these deposits, no major mappable geobodies are observed other than localized compressional ridges of mass-transport deposits (MTD) near the toesets. In some areas of the basin, the Vaca Muerta Formation was deposited directly on top of a preexisting non-marine paleo-aeolian dune topography which had a direct impact on the stratal geometries and the bottomset facies of the Vaca Muerta Formation. Acoustic impedance (AI) from seismic inversion show well-defined Vaca Muerta low-impedance seismic facies, which relate to the presence of predominantly fine-grained, organic-rich (~5%), porous (~11%) mudstones and marlstones. Within the high-TOC Vaca Muerta interval, most AI three-dimensional (3-D) volumes throughout the basin show an average of five to six seismic facies with discrete AI values that can be directly correlated to rock types, depending on their position within the clinoform (i.e., topset, foreset, bottomset). These seismic facies correlate to facies associations with distinct petrophysical and geomechanical properties at core/outcrop scale, as measured by lab studies. Understanding this relationship and its distribution in space is critical to predicting optimum horizontal well landing zones and sweet spots.

ABSTRACT This chapter analyzes the different lithofacies of the Vaca Muerta Formation using macroscopic observations, standard microscopy textural studies, scanning electron microscopy (SEM), and x-ray diffraction (XRD) techniques and presents them in a paleo-environmental and a sequence stratigraphic context. Microfacies-scale analyses in the Vaca Muerta Formation reveal a wide range of sedimentary and diagenetic processes that are essential for the understanding of this heterogeneous self-sourced unconventional reservoir. Seventeen of the more representative lithofacies in the Vaca Muerta Formation are described based on textures, grain type, and matrix composition. XRD data indicate that most of the mudstones microfacies show quartz–feldspar values above 40%, carbonate between 20% and 40%, clay lower than 35%, and total organic carbon (TOC) between 2% and 8%. Porosity of crushed rock samples shows common values between 10% and 16%. Porosity observed in SEM can be divided into interparticle, intraparticle, and organic-hosted type. Coccoliths are commonly found as calcareous fecal pellets that can make up to 35% of the rock volume. Pores within pellets range from hundreds of nanometers to more than 5 μm. Lateral and vertical distribution of lithofacies is controlled by the sedimentary environments and the systems tracts. Transgressive systems tracts (TST) show an enrichment in siliceous particles, the highest TOC values, and abundant organic-hosted porosity. Highstand systems tracts (HST) are carbonate rich, with lower TOC values and interparticle and intraparticle porosity. Comparison of the lithological-dependent characteristics from the Vaca Muerta Formation with other well-known unconventional plays shows that the Vaca Muerta Formation shares many characteristics with the Oxfordian–Tithonian Haynesville/Bossier Shale and the Cenomanian–Turonian Eagle Ford Shale, such as lithofacies, grains composition, mineralogical composition, diagenesis, percentage of TOC, porosity volume, and pore types.