Abstract The Eagle Ford Group crops out in a series of spectacular cut-bank exposures within Lozier Canyon region in Terrell County (west Texas). These outcrops provide an unparalleled opportunity to examine the Eagle Ford Group and gain valuable insights into explaining and predicting the vertical and lateral variability, as well as the thickness changes that can occur regionally within an unconventional source rock play. In the subsurface of south Texas, the Eagle Ford Group is typically divided into an organic-rich Lower Eagle Formation and a carbonate-rich Upper Eagle Ford Formation. Both formations are petrophysically distinct, especially on gamma ray (GR) and sonic logs. When geochemically analyzed, the basal portion of the Upper Eagle Ford Formation also contains a unique positive carbon isotope δ 13 C excursion interpreted as the Ocean Anoxic Event 2 (OAE2). The peak of this isotope excursion is the assigned proxy for the base of the Turonian Stage. Within the Eagle Ford outcrops of west Texas a vertical succession of five informal lithostratigraphic units, referred to as units A to E from the base up, are fairly obvious. Unit A consists of interbedded grainstones and carbonate mudstones. Unit B is dominated by organic-rich black carbonate mudstones. Unit C consists of packstone beds interbedded with light gray carbonate mudstones. Unit D consists of bioturbated marls, while Unit E consists of grainstones interbedded with carbonate mudstones and bentonites. By incorporating petrophysical and geochemical data, the Lower and Upper Eagle Formations from the subsurface of south Texas can also be defined in the Eagle Ford outcrops of west Texas. Our work suggests that outcrop units A and B represent the Lower Eagle Ford Formation, while outcrop units C, D, and E represent the Upper Eagle Ford Formation. Similar to the subsurface of south Texas, a distinct positive carbon isotope δ 13 C excursion also occurs in the basal portions of the Upper Eagle Ford Formation (unit C) in outcrop. More detailed analysis of the outcrop and subsurface data from the Eagle Ford Group in west Texas indicates that the five informal lithostratigraphic units can be further divided into a vertical succession of 16 subunits. This more detailed vertical facies succession was used to define four genetically related depositional sequences each with distinctive geochemical and petrophysical characteristics which make them particularly suitable for regional subsurface mapping. For nomenclature simplicity, these four sequences are herein termed the lower and upper (allo-) members of the Lower Eagle Ford Formation and the lower and upper (allo-) members of the Upper Eagle Ford Formation. The lower member of the Lower Eagle Ford Formation is an organic-rich, high-resistivity, uranium-poor mudstone-dominated sequence. A distinctive clay-rich, low-resistivity zone also marks its base. This sequence appears to be the primary unconventional reservoir interval in the subsurface of south Texas. The upper member of the Lower Eagle Ford Formation can be characterized as a uranium- and bentonite-rich, mudstone-dominated sequence. The lower member of the Upper Eagle Ford Formation is a uranium-poor interbedded mudstone and limestone succession characterized by an overall (low) blocky GR pattern, the presence of a distinctive positive carbon isotope δ 13 C excursion, and a clay-rich, low-resistivity zone at its base. The upper member of the Upper Eagle Formation is a bentonite-bearing, low-TOC interval that is more bioturbated toward its base and interbedded toward its top. It is characterized by the presence of a high GR, low resistivity, and low velocity mudstone at its interpreted maximum flooding surface. Regional correlations of the four defined Eagle Ford depositional sequences (allomembers) reveal that the unconformities at the base of each of the four sequences, as well as the one at the base of the overlying Austin Chalk, modify the thickness and distribution of underlying strata. Thus any attempt to explain and predict the distribution and thickness variations of any of the four sequences (allomembers), especially the organic-rich lower member of the Lower Eagle Ford Formation, is highly dependent on the recognition and regional mapping of these unconformities.

Abstract Drilling horizontal wells is the common mode of operation for field development in low-permeability unconventional reservoirs such as the Eagle Ford Shale. Assumptions are made regarding the homogeneity of the reservoir as wells are drilled away from the vertical pilot well. It is assumed that the reservoir characteristics remain uniform and also that the structure is constant based on the dip of the beds in the pilot hole wellbore. Making such assumptions can lead to wells being placed out of zone and in rocks with much different reservoir quality and stress magnitude than those in the pilot hole, which can adversely affect the production potential of the well. With the high cost of drilling and completing these wells, it is generally economically beneficial to do some evaluation of the lateral to ensure proper placement of the well and also the optimal placement of completion zones along the lateral. Lateral measurements and petrophysical interpretations can be used to define variations in reservoir quality (RQ) and completion quality (CQ) along the wellbore, which can then be used to optimize the completion design, for example, placing perforation clusters in similar rocks to increase production when compared to peer wells completed with a geometric design. The next step in integration is correlating pilot and lateral wellbore measurements with the structural component. This process is defined as geology quality (GQ). After taking together, RQ, CQ, and GQ, a comprehensive design of a wellbore-specific completion treatment can be achieved. This methodology of integrating data from many sources provides a better understanding of the variability and structural challenges of these complex reservoirs.

Abstract The objectives of this chapter are threefold: (1) to provide a historical perspective on considerations of pervasive tight-gas accumulations, (2) to provide some observations on the present understanding of these accumulations, and (3) to anticipate where the industry is headed in the future. From 1979 to about 1987, various workers (industry, government, and academe) discussed pervasive tight-gas accumulations and established important relationships for source rock, maturity, expulsion and migration, pressures, rock quality, and fluid content. Their main conclusion was that the hydrocarbons in these reservoir systems were dynamic and not static as in conventional structural and stratigraphic traps. The paradigm shift made by 1987 concluded that these accumulations were continually adjusting to existing conditions in both time and space. In more recent years, additional examples have been documented, and questions have arisen about the validity of the original model, noting the presence of more water in some systems than the model would predict. The close proximity of the mature, gas-generating, and gas-expelling source rock to the reservoirs is critical. The amount and richness of mature source rock has to be adequate for the volume of reservoir rock being charged. The proper combination of these circumstances produces more gas than can be contained under normal pressure. The quantity of this gas charge relative to available pore space in the reservoir system will dictate the reservoir pressure. Pervasive tight-gas accumulations have now been documented in more than 20 North American basins and are the targets for major ongoing exploration and development programs. The average reservoir porosity for these producing units is in the 8–9% range, with average in-situ permeabilities of hundredths of a millidarcy. We believe the industry will likely move forward in four directions: (1) revisit older mature basins, (2) expand into new basins, (3) move into carbonate reservoirs, and (4) continue to develop tighter and tighter rock. With continuing technology improvements (especially in drilling and completing) and robust gas prices, the industry will access vast new reserves farther down into the resource pyramid.