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Gravity-flow deposits are important hydrocarbon exploration targets in most basins. In this study, an integration of geological, geophysical, and geochemical analysis is used to investigate the gravity-flow deposits and their petroleum prospects in the Oligocene Dongying Formation (E 3 d), Bozhong Subbasin, Bohai Bay Basin. Mass-transport deposits (MTDs) and channel-lobe complexes are the main gravity-flow depositional units. The mass-transport deposits (MTDs) show hummocky and chaotic reflections with poor continuity and variable amplitude. The channels display linear geomorphology with weak root-mean-square (RMS) amplitude, and the lobes are dominated by high-continuous and high-amplitude reflection at the terminal channels. The formation of gravity-flow deposits is primarily controlled by high sedimentation rate during a relative lowstand of lake level. The E 3 d 3 source rock has TOC contents in the range of 0.6–3.1%, Rock-Eval S 2 values in the range of 0.7–19.8 mg HC/g rock, and Hydrogen indices (HI) in the range of 100–600 mg/g TOC. The E 3 d 2 L source rock has TOC contents ranging from 0.5 to 1.9%, Rock-Eval S 2 values from 0.4 to 7.8 mg HC/g rock, and HI from 40 to 480 mg/g TOC. Both E 3 d 3 and E 3 d 2 L source rocks have good hydrocarbon-generation potential. The E 3 d 3 and E 3 d 2 L source rocks have entered the oil-generation peak window with the vitrinite reflectance (R o ) higher than 1.0%. Oils generated from the underlying source rocks shortly migrated to the overlying gravity-flow lithologic traps. The gravity-flow reservoirs could serve as potential and effective hydrocarbon exploration targets. [ABSTRACT FROM AUTHOR]

In the Upper Cretaceous Balcones Igneous Province in South and Central Texas, numerous small volcanoes developed during Austin Chalk and Anacacho deposition that are similar to the recent Surtsey volcano offshore of Iceland. The volcanoes formed topographic highs on the deep-water (500–900 ft [150–275 m]), earlier-drowned Comanche Platform. The Elaine field in South Texas is associated with one of these relict volcanic highs and integration of a 3D-seismic dataset and cores permitted a detailed analysis of the deeper-water carbonate depositional systems of the field and associated carbonate reservoir development. This reservoir description extends the understanding of petroleum systems associated with relict, open-marine, deep-water, volcanic highs. In the shallow water at the top of the volcanic topographic highs, a carbonate factory developed that produced coarser-grained carbonate sediments that sourced the deeper-water flanks and toe-of-slope of the mound. Finer-grained carbonate chalk was deposited in the open-marine, deep-water environment adjacent to the volcanic mound. The coarser-grained carbonate sediments sourced from the mound top were transported downslope by gravity-flow processes that formed debrites and hyperconcentrated density-flow deposits. The gravity-flow deposits are composed of a mixture of shallow- and open-marine, deep-water biota indicating they are not shallow-water deposits, but are deeper-water mound-flank and toe-of-slope deposits. In the Anacacho Elaine field, the coarser-grained carbonates (i.e., skeletal packstones and grainstones) associated with the volcanic mound are the oil-producing reservoirs. The Elaine volcanic mound itself is tight and not productive. The Anacacho chalk immediately above the mound are also tight and form the top seal for the Elaine field. Associated with the relict volcanic mounds in the Balcones Igneous Province are a spectrum of carbonate plays. The relict volcano can be a reservoir, but not all the relict volcanic mounds are porous. The coarser-grain carbonates on the top, flanks, and at the base of the volcanic mounds are reservoirs in the area of the Balcones Igneous Province. The chalks deposited penecontemporaneously on the open-marine, deep-water platform can also be tight carbonate reservoirs. • Integration of 3D-seismic data and cores to define volcanic mound paleogeomorphology • Carbonate lithofacies associated with volcanic mounds. • Recognition of depositional environments associated with volcanic mounds. • Hydrocarbon reservoirs associated with volcanic mounds in open-marine settings. [ABSTRACT FROM AUTHOR]

Understanding the flow processes that form gravity-flow deposits is vital for modeling and predicting sandstone bodies in the subsurface, which is of great significance for conventional and unconventional oil and gas exploration and development in lacustrine basins. This study analyses gravity-flow deposits of the Late Triassic Chang 7 oil member of the Yanchang Formation in the Ordos Basin, using a combination of well-log analysis, core observations, thin-section analysis, and laboratory measurements. The sedimentary facies, formation mechanisms, distribution patterns, and depositional models of gravity-flow deposits are investigated. Thirteen facies and eight bed-types are recognized in the gravity-flow deposits. Bed types represent deposits of sandy slides, sandy slumps, debrites, high-density turbidites, hybrid event beds, transitional flow deposits, surge-like low-density turbidites, and quasi-steady low-density turbidites. Gravity-flow deposits, particularly those caused by sediment failure, are composed of slides, slumps, debrites, high-density turbidites, hybrid event beds, transitional flow deposits, and surge-like low-density turbidites. These deposits form as isolated lenticular sand bodies, with retrogradational internal stacking patterns. In contrast, gravity-flow deposits caused by flooding rivers are composed of high-density turbidites, hybrid event beds, transitional flow deposits, and quasi-steady low-density turbidites. These deposits exhibit elongate morphologies with progradation stacking patterns internally. The collision between the North China Block and the South China Block, which occurred during the closing of the Qinling Ocean, lead to frequent volcanic eruptions and earthquakes. These activities promoted the development of gravity-flow deposits associated with sediment failure from delta fronts in both the northeastern and southwestern parts of the basin. Concurrently, extremely humid climatic conditions promoted increased fluvial drainage, leading to enhanced hyperpycnal flows into the deep-lacustrine basin depocenters in the Late Triassic. This study emphasized that detailed facies analysis and distribution pattern analysis are the fundamental way to identify the formation mechanisms of gravity flow deposits in lacustrine depression basins. • Thirteen facies and eight bed-types are recognized amongst the gravity-flow deposits in the Late Triassic Chang 7 oil member of the Yanchang Formation in the Ordos Basin, which are composed of sandy slide, sandy slump, muddy debrites, sandy debrites, hybrid event beds, transitional flow deposits, surge-like low-density turbidite, quasi-steady low-density turbidite. • Gravity-flow deposits in the late Triassic Chang 7 oil member of the Yanchang Formation in the Ordos Basin are due to both sediment failure and flooding river. • The collision between the north China block and the south China block during the closing of Qinling Ocean, and extremely humid environment in the Late Triassic promotes the development of gravity-flow deposits in the Ordos Basin. [ABSTRACT FROM AUTHOR]