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Sequences of wave‐enhanced sediment‐gravity flows (WESGFs) have been widely recognized in the marine shelf environment. In this study, we show observations of WESGF deposits in lacustrine settings using well core and thin section data from the Paleogene in the Jiyang sub‐basin, Bohai Bay basin, eastern China. The findings of this study include the following: 1) the sequence of WESGFs in the lacustrine basin is similar to that of marine; it consists of three units, MF1 unit: siltstone with basal erosion surface, MF2 unit: silt‐streaked claystone, and MF3 unit: silty‐mudstone; and 2) prodelta sand sheets are found in the lacustrine WESGF sequence and are classified as the MFd unit: clay‐streaked siltstone. However, because the system size and variability in hydrodynamic conditions are different between the lacustrine and marine basins, lacustrine WESGFs do appear to have three distinguishable features: 1) the sediment grain size and sand content are slightly higher than those of the marine WESGFs; 2) lacustrine WESGFs may contain prodelta sediments or sedimentary sequences of other types of gravity flows, such as hyperpycnal flows; and 3) the scale of the sedimentary structures for lacustrine WESGFs is smaller. The WESGFs found in the continental lacustrine basin provide a new model for sediment dispersal processes in lake environments and may be helpful to explain and predict the distribution of sandy reservoirs for oil and gas exploration. [ABSTRACT FROM AUTHOR]

Diagenesis significantly influences sandstone reservoir quality, so that a detailed understanding of diagenesis is essential for the identification and prediction of favorable reservoirs. Several studies have been conducted on individual sandstone reservoirs in the Bonan Sag, a typical half-graben, oil-producing sag in eastern China, but few have explicitly considered the potential impact of the asymmetric shape of the sag, which is steep in the north and gentle in the south. This research takes full consideration of the tectonic setting and distribution of source rocks in the sag to compare the style, magnitude and timing of diagenesis in the northern Steep Slope and Sag Zone (SSSZ) and the southern Gentle Slope Zone (GSZ) by means of drill core observations, thin section petrography, high resolution scanning electron microscopy, fluid inclusion microthermometry, as well as cathodoluminescence and carbonate stable isotope analysis. Petrographic observations identify the wide-spread occurrence of authigenic quartz and clay minerals in association with dissolved feldspar, as well as abundant carbonate cements occurring as several distinct generations due to evolving pore fluid composition. The SSSZ has variable carbonate mineralogy while ankerite dominates in the GSZ. The homogenization temperature of fluid inclusions in authigenic quartz in the SSSZ has a wider range (93°C–157.9 °C) and lower starting temperature (93 °C) compared with the GSZ (106.2 °C) and is characterized by more lighter carbonate δ13C and δ18O values (δ18O: −14.8‰~-11.3‰, δ13C: 1.1‰–2.7‰ for the SSSZ; δ18O: −11.9‰~-5.8‰, δ13C: 2.9‰–5.5‰ for the GSZ). The results indicate that (1) Diagenetic conditions in the Bonan Sag evolved from alkaline to acidic to alkaline in general, but with distinct late stage carbonate cementation in SSSZ vs GSZ; (2) the diagenetic processes as well as the nature of pore waters varied across different zones and also temporally. Acidic conditions in the SSSZ started earlier and persisted for longer than in the GSZ. (3) The regional differentiation of the diagenetic processes and pore fluid evolution result from the source rock distribution and burial histories which differ across the Bonan Sag because of the asymmetric shape of the half-graben. We propose a model of fluid evolution over space and time which may provide a new framework for understanding the diagenetic processes and the fluid evolution of oil-producing sags and also contribute to the prediction of favorable reservoirs. • The E s 3 in Bonan Sag went through an alkaline-acid-alkaline diagenetic conditions. • The structure of the sag leads to temporal and spatial differences in diagenesis. • Source rock distribution and burial histories are significant controls in diagenesis. • A temporal and spatial diagenesis evolution model was put forward. [ABSTRACT FROM AUTHOR]

The present research study was aimed to identifies the effect of the lithofacies (assemblages) on lacustrine shale oil accumulation, the shale oil layers of the Eocene Shahejie Formation in the Bohai Bay Basin, China, were characterized. The petrological analysis, included core, thin section and scanning electron microscopy observations and X-ray diffraction, indicated that the shale oil layers of the Shahejie Formation are dominated by carbonates, which are characterized by uniform or laminar distributions. The laminar carbonate components were formed through sedimentogenesis or diagenesis. Based on the genesis patterns and the petrology, five major lithofacies (assemblages) were identified in the formation: LF (lithofacies) 1 + 5 is characterized by thin lenticular laminae of crystalline carbonates with black shale; LF2+3 depicts laminae of micritic carbonates with massive lime mudstone; LF3 is a thick layer of massive lime mudstone; LF4 is massive claystone; and LF5 is black shale. The relationship between the lithofacies and the favorability for hosting shale oil reservoirs was investigated using microdrill sampling of the total organic content, reservoir quality analysis, and shale oil-bearing property. In LF1+5, the crystalline carbonates contain abundant interparticle pores, which can act as reservoir spaces, whereas the shale layers have a high potential for hydrocarbon generation. In addition, the hydrocarbon reserves can enter the crystalline carbonates of the assemblage after migrating a short distance. Consequently, LF1+5 is the most favorable assemblage for shale oil accumulation. In addition, LF4 and LF5 have high hydrocarbon generation potentials, and thus a relatively smaller amount of hydrocarbons could accumulate in these assemblages through clay mineral adsorption. Thus, LF4 and LF5 are also valuable sites for shale oil accumulation. • The lacustrine shale oil accumulation could be classified into two categories, "crystalline carbonate reservoir (CCR)" and "in-situ generation and in-situ reservoir (GR)" model. • In CCR model, the crystalline carbonate provides reservoir space and the adjacent shale generate hydrocarbon. • In GR model, the hydrocarbon in-situ is generated and accumulated in shale and mudstone. [ABSTRACT FROM AUTHOR]

Shale oil is attracting increasing attention in the petroleum industry as an essential target of unconventional oil and gas exploration. Recent exploration efforts have indicated that laminar carbonates are closely related to the favorable depositional strata of shale oil. This research focused on inter-bedded layers of laminar carbonates as well as mudstone and shale in the Dongying depression of the Bohai Bay Basin in eastern China, and attempted to reveal the genesis and evolution of the carbonates by means of drill core observations, thin sections, high resolution scanning electron microscopic observations, and fluorometric, cathodoluminescence and geochemical analyses. The results demonstrated that: (1) the carbonates could be categorized into two groups, crystalline carbonates and micritic carbonates; (2) the crystalline carbonates were well crystallized and greenish under fluorescent light; and (3) the micritic carbonates were not well crystallized and had biotic textures. It was discovered that the crystalline carbonates were mainly developed in the shale layer and were formed from the late-filling effects of the diagenetic cracks along the shale fissility planes; shale layers with crystalline carbonates are usually favorable for shale oil reserves. The micritic carbonates were formed from direct deposition controlled by biological effects in the stratified flows of the seasonal lake basins and are unfavorable for shale oil reserves. [ABSTRACT FROM AUTHOR]