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Abstract The Lucaogou Formation in the Jimusar Sag of the eastern Junggar Basin is an important sedimentary stratum accumulating huge amounts of lacustrine tight oil in China, where organic-rich rocks are commonly observed. Focusing on the Lucaogou Formation, a precise analysis of the inorganic and organic petrology and the inorganic geochemistry characteristics was conducted. The paleoclimate and paleoenvironment during sedimentation of the Lucaogou Formation were established, and the key factors that were controlling the accumulation of organic matter during this time were identified. The results of this study suggest that during the sedimentation of the Lucaogou Formation, the paleoclimate periodically changed from a humid environment to an arid environment. As a result, the salinity of the water and the redox environment fluctuated. During the sedimentation period, the lake showed sufficient nutrient supplies and a high primary productivity. The interval studies in the Lucaogou Formation were divided into five sedimentary cycles, where the first, second, and fifth sedimentary cycles consisted of cyclical paleoclimate fluctuations varied from a humid environment to an arid environment and shifted back to a humid environment with levels of salinity from low to high and decreased again. The third and fourth cycles have cyclical fluctuations from a humid to an arid environment and corresponding salinity variation between low and high levels. During the period when organic-rich rocks in the Lucaogou Formation deposited in the Jimusar Sag, the paleoclimate and the water body were suitable for lower aquatic organisms to flourish. As a result, its paleoproductivity was high, especially during the early period of each cycle. A quiet deep water body is likely to form an anoxic environment at the bottom and is also good for accumulation and preservation of organisms. Fine-grained sediments were accumulated at a low deposition rate, with a low dilution of organic matter. Therefore, high paleoproductivity provided a sufficient volume of organisms in the studied area in a quiet deep water body with an anoxic environment and these were the key factors controlling formation of organic-rich rocks.

Two new types of sandbars are found in distributary channels of the modern Ganjiang Delta, Poyang Lake, China, respectively named as branching-point bar (BPB) and merging-point bar (MPB). BPB is defined as sandbar formed at the branching point of a channel where one channel branches into two. MPB is defined as sandbar formed at the merging point of two channels. BPB results from current velocity decrease at the branching point. BPB, arrowhead-shaped and growing towards upstream through upstream accretion, can be divided into bar head, middle and tail. Bar head refers to the lower part of the bar below average low water level, submerges under water whether in rainy seasons or dry seasons, and no plants grow on it. Bar middle refers to the middle part of the bar between average low and average high water levels, and sparse plants grow on it. Bar tail refers to the upper part of the bar above average high water level, and plants grow densely on it. From bar head to bar tail, sediment becomes finer and sorting degree becomes poorer; plant fossils increase in number, and silting-mud layers increase both in number and thickness. Vertically BPB shows a fining-upward sequence. BPB can be classified into 3 types: baby BPB with only bar head, child BPB with bar head and middle, and adult BPB with bar head, middle and tail. Generally from the lower delta plain to upper delta plain, BPB becomes more and more mature. If a channel is suddenly abandoned, BPB at different stages may be preserved. MPB is very similar to BPB in depositional characteristics, and main differences lie in its origin and growth direction. Discovery of BPB and MPB has important implications in finding remaining oil in oilfields. Previously, geologists considered point bars on convex bank of meandering channels or longitudinal bars and transverse bars in the middle of braided channels as favorable locations for finding remaining oil. Our study shows such locations as branching points and merging points of channels are also significant because BPB and MPB develop at these locations. Remaining oil has been found at branching points of channels in Pubei Oilfield, Songliao Basin, Northeast China.

Abstract The genesis of the fine crystalline dolomites that exhibit good to excellent reservoir properties in the upper fourth member of the Eocene Shahejie Formation (Es 4 s ) around the Sikou Sag, Bohai Bay Basin, is uncertain. This paper investigates the formation mechanisms of this fine crystalline dolomite using XRD, SEM, thin section analysis and geochemical data. The stratigraphy of the Sikou lacustrine carbonate is dominated by the repetition of metre-scale, high-frequency deposition cycles, and the amount of dolomite within a cycle increases upward from the cycle bottom. These dolomite crystals are 2–30 μm in length, subhedral to anhedral in shape and typically replace both grains and matrix. They also occur as rim cement and have thin lamellae within ooid cortices. Textural relations indicate that the dolomite predates equant sparry calcite cement and coarse calcite cement. The Sr concentrations of dolomites range from 900 to 1200 ppm. Dolomite δ18O values (−11.3 to −8.2 ‰ PDB) are depleted relative to calcite mudstone (−8.3 to −5.4 ‰ PDB) that precipitated from lake water, while δ13C values (0.06–1.74 ‰ PDB) are within the normal range of calcite mudstone values (−2.13 to 1.99 ‰ PDB). High 87Sr/86Sr values (0.710210–0.710844) indicate that amounts of Ca2+ and Mg2+ have been derived from the chemical weathering of Palaeozoic carbonate bedrocks. The high strontium concentration indicates that hypersaline conditions were maintained during the formation of the dolomites and that the dolomites were formed by the replacement of precursor calcite or by direct precipitation.