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Shale oil has been discovered and produced in the Cretaceous Qingshankou Formation (K 2 qn) of the Gulong sag in the Songliao Basin (the Gulong shale oil). However, the selection of favorable layers for shale oil development remains uncertain due to significant variations in oil production of different intervals in the Formation (Q 1 –Q 9). In this study, the geochemical features of shale, the content and chemical features of extracted oils in different shale intervals are analyzed in detail, and the favorable intervals and mechanism for shale oil accumulation are also addressed. It is shown that the Gulong shale has total oil contents of 200–400 mg/g TOC with considerable contents of light hydrocarbons in extracted oils. In the shale, there is a "W" trend for the evolution of chemical and carbon isotopic compositions (δ13C) of shale oil with depth. It is deduced that this evolution trend of shale oil is governed by the variation of organic matter precursor associated with the deposition environment. The distributions of δ13C of mud gas and individual n -alkanes with depth reveal that the Gulong shale oil can be separated as two independent petroleum systems at the Middle–Lower Q 7. Mass balance calculations imply Q 2 –Q 3 and Q 8 –Q 9 are two intervals with lower oil expulsion efficiency. Combined with the morphology of calcite cements and veins as well as the homogenization temperature of inclusions, an intralayer accumulation model of the Gulong shale oil was established. In addition, the statistical analysis of the oil content, lithological properties, and oil production capacity of different layers from 95 wells, demonstrates that Q 2 –Q 3 and Q 8 –Q 9 can be selected as the preferential target layers for future development of the Gulong shale oil. More importantly, this study may provide new insights into the enrichment of shale oil with tight shale type in China and around the world. • OM precursors govern the vertical "W" trend for group compositions and δ13C of Gulong shale oil. • The Q 2 –Q 3 and Q 8 –Q 9 layers have oil expulsion efficiency < 50% and are preferential target intervals. • The late micro-sealing by calcite veins or cements is responsible for the accumulation of Gulong shale oil. • Oil content and brittleness are the two key geological parameters determining Gulong shale oil production. [ABSTRACT FROM AUTHOR]

Highlights • Multiphase flow in the fractured medium was investigated using the method of "multiple interacting continua" (MINC). • The formation processes of calcite vein and its impact on the sealing efficiency of caprock were discussed. • The simulation results were validated using the field observations of the natural CO 2 gas fields. • The formation of calcite vein eventually prevented the CO 2 from leaking through the fracture at a long time scale. Abstract For the long-term CO 2 geological storage, the evolution of the caprock sealing efficiency has received increasing attention. In this paper, the Huangqiao CO 2 gas field in Jiangsu Province, China, where the calcite veins have been found in the lower part of mudstone caprock, is considered as a natural analogue site for CO 2 geological sequestration (CCS). To ascertain the dynamic formation process of calcite vein and its impact on the evolution of caprock sealing efficiency, a one-dimensional model was designed to represent the fractured reservoir-caprock system. Numerical simulations were performed using the multiphase reactive transport program TOUGHREACT. Sensitivity analyses of fracture permeability and calcite reaction rate were made. The simulation results illustrate that calcium bicarbonate decomposes to form calcium carbonate and release CO 2 from solution due to the pressure decay as the CO 2 -rich fluids migrate upwards along the fracture. The formation of calcite vein decreases the porosity and permeability significantly, which can enhance the integrity and sealing efficiency of caprock. Sensitivity analyses indicate that the formation of calcite vein is facilitated by the fast fluid flow, and calcite vein tends to form in the vicinity of reservoir-caprock interface under the slow-flow condition as the decrease of fracture permeability. Information currently available in Huangqiao CO 2 gas field and some other area describing the formation of calcite vein shows good agreement with our simulation results. The mudstone caprock with fractures and faults is able to keep CO 2 from leaking for a long time as the case of Huangqiao area. The methods and analyses presented here may be useful for CO 2 storage sites with similar conditions. [ABSTRACT FROM AUTHOR]