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Organic-rich shale deposited in a marine-continental transitional facies was well developed in the upper Yangtze area during the Late Permian. It is considered to contain a large amount of shale hydrocarbon resources. This paper takes the Upper Permian Longtan shale in the central Guizhou uplift as an example to characterize the transitional shale reservoir. Based on organic geochemistry data, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM) observation, the geochemical characteristics of this transitional shale were systematically studied and the shale gas potential was investigated. The results indicate that the Longtan shale has fair to good hydrocarbon potential with high total organic carbon (TOC) and contains type III gas-prone organic matter. Its high thermal maturity favors the generation of gas (Ro, 1.10%–2.74%). XRD analyses show that the mineral components of the transitional shale are dominated by high clay content (58.7 wt%). The clay minerals are dominated by illite (44%) and mixed illite/smectite (43%). SEM observation shows that intra-particle pores, inter-particle pores and microfracture were well developed in the Longtan shale, which can provide reservoir space for the accumulation of shale gas. Organic matter pores appear rare due to its organic type. In summary, the Longtan shale has high organic content, high maturation, and high gas content, all of which demonstrate its great potential as a shale-gas resource. But the high clay content of the Longtan shale may be a challenge to shale-gas production. [ABSTRACT FROM AUTHOR]

Based on the sedimentary and tectonic background of the Termit Basin, this paper focuses on the Upper Cretaceous Yogou Formation and uses organic geochemistry, logging, oil testing and seismic data to analyze the primary control factors of the hydrocarbon accumulation and establish corresponding model in order to predict favorable exploration target zones of hydrocarbon reservoirs. This study demonstrates that the Upper Cretaceous Yogou Formation is a self-generation and self-accumulation type reservoir. The Yogou Formation hydrocarbon reservoirs in the Koulele area are controlled by four factors: (1) the source rock is controlled by a wide range of YS1-YS2 marine shale, (2) the sandstone reservoir is controlled by the YS3 underwater distributary channel and storm dunes, (3) migration of hydrocarbons is controlled by faults and the regional monocline structure, and (4) the accumulation of hydrocarbons is controlled by lateral seal. The structures in the western Koulele area are primarily reverse fault-blocks with large throws, and the structures in the east are dominantly fault-blocks with small throws (co-rotating and reverse) and a fault-nose. In the western Koulele area, where the facies are dominated by storm dunes on a larger scale, it is easier to form lithologic reservoirs of sandstone lens. In the eastern Koulele area, high-quality channel sandstone reservoirs, fault-blocks with small throws, and the monocline structure benefit for the formation of updip pinch out lithologic traps, fault lithologic reservoirs and fault-nose structural reservoirs. Future exploration targets should be focused in the western storm dunes zone and eastern distributary channel sand zone with small fault-blocks. [ABSTRACT FROM AUTHOR]

The conception of special geologic bodies has been applied in this study to define and illustrate economic oil and gas reservoirs in sandbodies, which is helpful for hydrocarbon forecast and exploration. Using the comprehensive analysis of drilling, logging, seismic, seismic attribute extraction, and logging constrained inversion in 3D data volume, the special geologic bodies of the Shahejie Formation (P s ) in northern Qikou sag, which have good hydrocarbon potential, are identified and described under the constraint of sequence stratigraphic framework. The dimensions, geometry, and spatial distribution features of the special geologic bodies, as well as their inner architectures and sequence-structure patterns, are also ascertained. The geologic bodies evolved from relative centralization in the middle part of the lacustrine basin in the early stage of P s Formation, to several dispersive isolated parts in the later stage. This shows a small cycle interval (~2.5 Ma) at the end of the whole transgressive depositional process of P s1 (~7.5 Ma) during the expansion of the lake and relative lake level rise, with the control of sediments supply and fault-related subsidence, which effect the evolution of the sedimentary system. According to the relevance and regularity of the geologic bodies' development, different types of potential profitable reservoir traps, including the lithologic lenticular traps, lithologic updip pinchout traps, and structural-lithologic composite traps, can be preserved in the study area. In addition, the internal architecture models provide scientific basis for further hydrocarbon exploration in the frontier basin without enough data. [ABSTRACT FROM AUTHOR]