ALBERT

Description: Many published sequence-stratigraphic frameworks lack a systematic and consistent designation for both depositional sequences and key surfaces, despite the original goal to provide a fully integrated stratigraphic architecture, including diagnostic age information. Based on a system in use for more than 10 yr at ExxonMobil, we recommend methodologies for a chronostratigraphic designation system (CDS) using more uniform and robust sequence-stratigraphic designations. After objectively defining important physical stratigraphic surfaces, biostratigraphic and other age-constraining information is used to designate surfaces and the packages of rocks they bound. This leads to the establishment of a sequence chronostratigraphic framework for a local area of investigation (outcrop section, field, region, or basin). Only after demonstrating clear well-documented ties to Phanerozoic global coastal onlaps or cycle charts are these sequences and associated surfaces considered as "global" entities and designated as such. Higher frequency sequences and surfaces are also accommodated in this CDS. Alternative designations for areas with limited or poor quality chronostratigraphic information are also discussed. The CDS has proven to have great use in all Phanerozoic strata, different tectonic settings, and depositional environments, especially when chronostratigraphic age constraints are robust. We have used this system at regional and basinal scales in many geographic locations to help reduce uncertainty in identifying and correlating reservoirs, sources, and seal rocks. Predicting the local distribution and quality of reservoirs as well as seals within a producing field and near field wildcats is also facilitated by this system. This system has demonstrated use in the correlation of outcrop sections within a basin or between basins. Rigorous use of the CDS proposed here will permit meaningful regional and/or interbasinal correlation that is difficult to carry out with the diverse systems currently in use. This uniform designation scheme will also facilitate communication within a company and between institutions, as well as among academic investigators.

Description: Pennsylvania is not only the birthplace of the modern petroleum industry but also the focus of the modern Marcellus Shale gas play. For more than 150 yr, Pennsylvania has experienced a rich history of oil and gas exploration and production, witnessed the advent of modern petroleum regulations, and now sits deep in the heart of the largest domestic shale gas play the United States has ever seen. Although a known source rock for decades, the Marcellus Shale was not considered a viable gas reservoir until Range Resources Corporation (Range) discovered the play with its completion of the Renz No. 1 well in Washington County in October 2004. Using horizontal drilling and hydraulic fracturing techniques used by operators working the Barnett Shale gas play, Range has gone on to complete hundreds of horizontal shale gas wells in Washington County alone. Other operators have followed suit in counties from one corner of the state to the other, and as of June 2011, the Commonwealth has issued nearly 6500 Marcellus Shale gas well permits. Based on publicly reported well completion and production data, an average Marcellus Shale gas well requires 2.9 million gal of water during the hydraulic fracturing process and produces 1.3 mmcf gas/day. Furthermore, the U.S. Energy Information Administration has estimated that as of mid-2011, daily Marcellus Shale gas production in Pennsylvania exceeds 2.8 bcf. Because of the level of drilling activity and production associated with the Marcellus play, Pennsylvania has become the nexus of shale gas production and water management issues.

Description: Predicting spatial distribution, dimension, and geometry of diagenetic geobodies, as well as heterogeneities within these bodies, is challenging in subsurface applications, and can impact the results of reservoir modeling. In this outcrop–based study, we generated a data set of the dimensions of fracture–related dolomite geobodies hosted in Ediacaran (Khufai Formation) limestones of the Oman Mountains that are up to several hundreds of meters long and a few tens of meters wide. The dolomite formed under burial conditions by fluids that interacted with siliciclastic layers, as demonstrated by the enriched Fe (up to 4.4%) and Mn (up to 0.8%) contents and $$^{87}\mathrm{Sr}/^{86}\mathrm{Sr}$$ ( $$\sim 0.710$$ ) signatures. Dolomitization probably occurred during the Hercynian Orogeny (or pre-Permian) because dolomitization predates some folding and pre-Permian rocks have seen intense deformation related to the Carboniferous Hercynian Orogeny. Moreover, dolomitization occurred between the onset and termination of bedding-parallel stylolitization and thus most likely before deep burial related to the Alpine Orogeny. Hence, dolomitization most likely occurred before deep burial related to the Alpine Orogeny and during or following the intense deformation related to the Carboniferous Hercynian Orogeny had affected pre–Permian rocks. The clumped–isotope signature yields a temperature of approximately 260°C (500°F), interpreted as the apparent equilibrium temperature obtained during uplift after deepest burial during the Late Cretaceous. Lateral transects across the dolomite bodies show that zebra dolomite textures are common throughout the body and that vugs are more common at the rim than the center of the bodies. Moreover, a weak geochemical trend exists with more depleted $$^{18}\mathrm{O}$$ , Fe, and Mn concentrations in the core than at the rim of the dolomite bodies. These results show that minor heterogeneities exist within the dolomite bodies investigated. These data contrast with previous studies, in which more significant variation is reported in width of the dolomitization halo and texture for larger dolomite bodies that formed in host rocks more permeable than the examples from the Oman Mountains.

Description: Understanding the distribution and geometry of reservoir geobodies is crucial for net-to-gross estimates and to model subsurface flow. This article focuses on the process of dolomitization and resulting geometry of diagenetic geobodies in an outcrop of Jurassic host rocks from northern Oman. Field and petrographic data show that a first phase of stratabound dolomite is crosscut by a second phase of fault-related dolomite. The stratabound dolomite geobodies are laterally continuous for at least several hundreds of meters (~1000 ft) and probably regionally and are one-half meter (1.6 ft) thick. Based on petrography and geochemistry, a process of seepage reflux of mesosaline or hypersaline fluids during the early stages of burial diagenesis is proposed for the formation of the stratabound dolomite. In contrast, the fault-related dolomite geobodies are trending along a fault that can be followed for at least 100 m (328 ft) and vary in width from a few tens of centimeters to as much as 10 m (~1–33 ft). Petrography, geochemistry, and high homogenization temperature of fluid inclusions all point to the formation of the dolomite along a normal fault under deep burial conditions during the Middle to Late Cretaceous. The high 87 Sr/ 86 Sr ratio in the dolomite and the high salinity measured in fluid inclusions indicate that the dolomitizing fluids are deep basinal brines that interacted with crystalline basement. The dolomitization styles have an impact on the dimension, texture, and geochemistry of the different dolomite geobodies, and a modified classification scheme (compared to the one from Jung and Aigner, 2012 ) is proposed to incorporate diagenetic geobodies in future reservoir modeling.