Rheological Structure And Pore Structure Characteristics Of Lower Paleozoic Shale In Eastern Sichuan Basin

The study area is connected across the steep detachment fold belt in Eastern Sichuan and block-basement fault-fold belt in Western Hunan-Hubei. Besides, it is the main area of the Paleozoic shale exploration and development. Carrying out the study of shale rheological structure and nano pore characteristics is not only important in structure research, but also has great influence to exploration and development. Using new geological investigation, core, well and seismic data, combined with X-ray diffraction, the characteristics of the two shale formations have been researched. With the help of thin sections, scanning electron microscopy, argon ion polishing field emission microscopy experiments, this article analyzes shale rheological structure in different scales and its influencing factors. Argon ion polishing field emission electron microscopy and porosity and permeability testing have been used to figure out nano pore characteristics.Some achievements are as below:1) The study area connects the steep detachment fold belt in Eastern Sichuan and block-basement fault-fold belt in Western Hunan-Hubei.It was layered decollement deformation, with Jialingjiang Formation detachment, Longmaxi Formation detachment, Longwangmiao gypsum detachment and the base detachment. Tectonic evolution can be roughly split into 2 stages. Late Jurassic to early Paleogene was the deformation development stage. During this period, stratigraphic uplifted rapidly from southeast to northwest until Late Cretaceous. The second stage is late Paleogene with stratigraphic rapid uplift again to the earth.2) Lower Cambrian Shuijingtuo shale is widely distributed in Western Hunan-Hubei, and missing in Eastern Sichuan at the same time. Compared to the east, Shuijingtuo formation in Western Hunan and Hubei is thinner in the west, while Liaojiacao is the sedimentary center. Dark grey mudstone is the main lithology of Shuijingtuo, with high content of quartz mineral and low content of clay mineral. Longmaxi formation is alos widely distributed. From the southwest to the northeast, the formation get thicker. Dark grey mudstone, carbon mudstone and silty mudstone are the main lithology, with low content quartz and high clay mineral relatively.3) Rheological characteristics of shale can be divided into 2 categories: rheid fold and structural fracture. Cores of Shuijingtuo shale show high angle fractured, while its nano scale shows strong mica sliding. Rheological intensity of Longmaxi shale is lower than Shuijingtuo formation. In core scale, bed-parallel fracture, low angle detachment joint or no significant deformation. In nano scale, there are lightly sliding of mica and interlayer joint. Through comparison, there is a certain rule about rheological structure. The rheological intensity in deep layer is larger than the shallow in the same place, such as Shuijingtuo formation> Longmaxi formation of EY-1. And when it comes to different parts of the same horizon, the rheological intensity generally reduces from east to west. This difference is related to the mineral contents and structure position. The content of brittle minerals, buried depth, tectonic stress, and its role in tectonic evolution are directly related to shale rheological strength. The higher brittle mineral content is, the more fracture develop. Deep buried, high tectonic stress, as well as being detachment layer, all does help to rheological deformation. In brittle deformation zone or low rheological intensity, micro fracture develops. In contrast, with high rheological strength, shale would develop fold.4) There developed various kinds of pores, including organic matter, inorganic holes and micro fractures. Among them, the pore of organic and clay minerals have contributed to the main porosity. Shale reservoir average porosity is very low, with 2.13% of Shuijingtuo formation and 3.57-5.33% of Longmaxi formation. On the whole, the total porosity decreases with the increase of the rheological strength. The relationship between them is negative. The pore of organic matter and clay minerals are easy to be affected by the change of rheology. With the changes in temperature and pressure, buried depth and tectonic stress, rheological strength of shale gradually increases, also the nano pore. At first the pores was roughly circular macropore, and gradually transformed into flat mesopore, ultimately became round micropores or even disappear.