关键词: 高频T-R层序, 层序类型, 层序演化, 孔隙结构, 沧东凹陷

Abstract: The shale in deep water areas of a continental lacustrine basin represented by the Member 2 of Kongdian Formation of Cangdong sag in Bohai Bay Basin boasts good shale oil exploration and development potential. However, the relationship between macroscopic distribution of lithofacies and microscopic characteristics of reservoirs is still unclear. Based on high frequency transgressive-regressive (T-R) sequence division of shale, using the methods such as core observation description, rock thin section identification, quantitative analysis of minerals by X-ray diffraction, elemental and organic geochemical testing, logging curve, field emission scanning electron microscopy analysis, nitrogen adsorption experiment, high pressure mercury injection experiment, and focused ion beam scanning electron microscope analysis, this paper analyzes the lithofacies filling evolution of different types of sequences and reservoir characteristics of different lithofacies, and explores the effect of high frequency sequence on reservoir development. The research results show that organic-rich shale section of the Member 2 of Kongdian Formation in Cangdong sag can be divided into 11 high frequency T-R sequences (5th-order sequence SQ①-SQ⑪, of which SQ⑨ develops gravity flow sandstone). According to the symmetry of lacustrine T-R cycles in high-frequency sequences, the sequences can be divided into asymmetric type dominated by T cycle, asymmetric and nearly symmetric types dominated by R cycle. Shale reservoirs mainly develop micropores such as intergranular pores, intra-granular pores, organic matter pores, and microcracks, among which inorganic pores such as intergranular pores and intra-granular pores are the main ones. The lamellar felsic shale facies and lamellar hybrid shale facies with higher content of felsic minerals and more laminated structures develop more macropores with better connectivity, while the massive hybrid shale facies and lamellar/massive carbonate shale facies with higher carbonate mineral content and more massive structures develop mesopores with general connectivity. Affected by mineral content and sedimentary structure, the shale reservoirs demonstrate the obvious characteristic of being controlled by facies, and the high frequency sequence can be regarded as a response to the changes in sedimentary environment in continuous time series, thus influencing the sedimentation of shale in the center of lacustrine basin. The significantly different reservoir characteristics correspond to high-frequency sequences at different evolutionary stages. Specifically, the middle-upper sequence at the rapid lacustrine transgression stage and bottom sequence at the oscillating lacustrine transgression stage are the high quality reservoir development section, and only local sequences at the stable high stage develop high quality reservoirs. These results are expected to provide theoretical support for the division of high-frequency shale sequences in deep water areas of a continental lacustrine basin as well as evaluation and prediction of high-quality reservoirs.

Key words: high frequency T-R sequence, sequence type, sequence evolution, pore structure, Cangdong sag