关键词: 柴达木盆地, 英雄岭页岩, 纹层结构, 水体分层, 源储组合

Abstract: A set of saline lake facies shale was deposited in Ⅳ-Ⅵ oil group of the upper member of Lower Ganchaigou Formation (E₃²) in Yingxiongling Structural Belt in Qaidam Basin, with high carbonate mineral content and well-developed laminae, in which great progress was made in the shale oil exploration. Therefore, the in-depth research should urgently be conducted on the laminated texture, depositional mechanism, and favorable lithofacies combination. The experimental results of core description, thin section identification, scanning electron microscopy and X-ray diffraction analysis show that the shale laminated texture in the study area is divided into three types, i.e., water stratified type laminated texture (Type Ⅰ) , seasonal type laminated texture (Type Ⅱ) and flood surging type laminated texture (Type Ⅲ). Type I shale shows a typical laminated texture deposited in seasonal stratified water body in saline lake, accounting for 70% of the total laminae, which is composed of clayey laminae and calcite laminae, with a single lamina thickness of 0.1-0.5 mm. In summer and autumn, the water body was stratified, and the upper water body was favorable for microbial proliferation. While in winter and spring, the water body stratification disappeared, which was beneficial to the deposition and the preservation of organic matters. In addition, Type Ⅰ laminated texture was featured by a low deposition rate, which was beneficial to the enrichment of organic matters, with an average TOC of 1.33%. Type Ⅱ laminated texture accounts for 20% of the total laminae, which is composed of clayey lamina and silty lamina, with a single lamina thickness of 0.5-1 mm. The deposition of Type Ⅱ shale was related to the seasonal variation of terrestrial inputs, showing rapid deposition rate, with an average TOC of 0.47%; The proportion of Type Ⅲ laminated texture is 10%, which is composed of clayey lamina and silty lamina, with a single lamina thickness of 0.5-2 mm, micro-scouring surface and lens-shaped siltstone veins developed, and an average TOC of 0.27%, indicating certain hydrodynamic conditions related to gravity flow and lake flow rather than non-hydrostatic environment, as well as rapid deposition rate. The shale in the study area was mainly developed in semi-deep lake, with the single sedimentary cycle including mudstone→shale→argillaceous dolomite→dolomite from bottom to top, and occasional sand gravity flow deposits. For Type Ⅰ laminated texture, the shale deposition sequence was superimposed on dolomite, with the average porosity of dolomite of 9.8%, forming good source rock and reservoir assemblage in micro scale, which was beneficial for shale oil accumulation, showing the favorable lithofacies combination of shale oil in Ganchaigou area. Macroscopically, this set of shale has high TOC, good physical properties, and high content of brittle minerals, which is an ideal interval for shale oil exploration and development in Qaidam Basin. The hydrocarbon generation potential of Type Ⅱ and Ⅲ shales is poor, which have low contribution to hydrocarbon accumulation.