The recent worldwide unconventional shale oil and gas boom raises questions about controls on deposition and evolution of porosity, permeability, and kerogen of organic-rich, fine-grained rocks. Environmental and depositional factors such as upwelling, nutrient supply, preservation, and paleo-oxygenation ultimately contribute to the organic richness and hydrocarbon potential of the source rock. In fine-grained rocks such as mudstones, geochemical investigations have proved useful in identifying productive source intervals and most attractive hydraulic fracturing intervals for unconventional plays. Most studies on Paleozoic and Mesozoic producing source rocks compare redox-sensitive metals and major elements, TOC, isotope geochemistry and mineralogy to identify sweet spots and probable producing zones. Geochemical and sedimentological studies of modern anoxic basins offer analogues for ancient basins and have the potential to elucidate answers to long-standing questions. However, a direct comparison of Pleistocene anoxic basins (e.g., Cariaco Basin, offshore Venezuela, the Walvis and Mid-Cape Basins, offshore southwest Africa. and Porcupine Basin, offshore Ireland) to Mesozoic source rocks, such as the Upper Jurassic Haynesville and Upper Cretaceous Eagle Ford Formations of Texas, the Vaca Muerta Formation of Argentina, and the Mississippian Lime/Chert has not been conducted to date. Geochemical and mineralogical variations, sedimentary structures, organic-matter accumulation, sedimentation rates, and bioturbation amongst other factors have been suggested as controls that are favorable to production intervals in oil/gas shale reservoirs. This study explores the controls on deposition and preservation of organic matter, and favorable lithology for hydrofracturing as pertinent factors to determine the best unconventional play strategies.
Exploration in unconventional mudrocks raises questions about controls on the deposition of organic-rich fine-grained rocks, such as upwelling, nutrient supply, preservation, paleo-oxygenation, etc. -- factors that ultimately contribute to the organic richness and hydrocarbon-production potential of shale oil/gas reservoirs. In fine-grained rocks such as mudstones, geochemical investigations have proved useful in identifying productive source intervals for unconventional plays. Most studies on productive Paleozoic and Mesozoic source rocks examine minor and major trace elements, total organic carbon (TOC), various isotopes, porosity, thermal maturation, and mineralogy to identify sweet spots and probable producing zones. Geochemical and sedimentological studies of modern anoxic basins offer analogues and proxies for ancient basins and have the potential to elucidate answers to long-standing questions regarding the origin and evolution of these unconventional reservoirs (e.g., Milliken et al., 2014). Our study compares Holocene anoxic basins (e.g., Cariaco Basin, offshore Venezuela, the Walvis and Mid-Cape Basins, offshore southwest Africa. and Porcupine Basin, offshore Ireland) to Mesozoic source rocks, such as the Upper Jurassic Haynesville and Upper Cretaceous Eagle Ford Formations of Texas, as well as the Mississippian Lime/Chert (e.g., Dupont and Grammer, 2017; Hammes et al., 2011; Hammes and Frebourg, 2012; Hammes et al., 2016; Vanden Berg et al., 2017, 2018). In this study, cores from IODP expedition 165 in the Cariaco Basin and IODP 307 in the Porcupine Basin (Fig. 1) were analyzed for sedimentologic structures, mineralogy, porosity evolution, and geochemistry to evaluate the influence and controls of paleogeography, sea-level fluctuations, up-welling and terrigenous input on these lithologic and sedimentary parameters. In the future cores from ODP Leg 175 from the Walvis and Mid-Cape Basins will be added to this study.
