Characteristics Of The Cambrian Gypsum-salt Rocks In Sichuan Basin

Sichuan Basin is located in the western Yangtze Paraplatform, north of the Qinling fold belt, west of Songpan-Ganzi fold belt; It is within the Upper Yangtze Paraplatform activity of deep faults which crossed by North East and North West formed the diamond-shaped formation of tectonic-sedimentary basin. In the Cambrian period, in its unique structure-sedimentary evolution process, the development of marine strata have good prospects for oil and gas exploration, as a high-quality gypsum cap rock in oil and gas accumulation has played a crucial role. This paper, using sedimentology, sequence stratigraphy, petroleum geology, petrology, geochemistry, geophysics, and tectonics theory and methods, and full use of drilling core, outcrop, well logging, logging, etc. Researching the characteristics of Cambrian sequence stratigraphy in Sichuan Basin and establishing third-order sequence stratigraphic framework, on this basis, Targeted the Sichuan Basin Cambrian "characteristics of gypsum-salt rocks" at the scientific objectives, in-depth study characteristics of Cambrian sequence stratigraphic,characteristics of gypsum rocks'development, genetic mechanism and development pattern, lithologic assemble partition, distribution of gypsum rock and distribution and contrast of gypsum rock in the sequence stratigraphic framework, and according to the results of previous studies, combined with regional tectonic background, and then analyze and evaluate the characteristics of gypsum rocks in the study area.According to identification of sequence boundaries and analysis of sequence stratigraphic of the outcrop and well sections, and signs of the sedimentary in the sequence division, paleontologic evidence, signs of geophysical and geochemical, and so on, identifing 14 third-order sequences in the Cambrian, while, Lower Cambrian including six third-order sequences, four third-order sequences in the Middle Cambrian, and four third-order sequences in Upper Cambrian. The 14 third-order sequences constitute two super-sequences, SS1 and SS2, the super-sequences SS1 contains∈ SQ1~∈SQ6 six third-order sequences, while the super-sequences SS2 contains∈SQ7~∈SQ14 eight third-order sequences, viewing the nature of the sequence,∈SQ1,∈SQ6,∈SQ9,∈SQ12, this four third-order sequences were typeⅠsequence in the 14 third-order sequences, and the nature of other third-order sequence boundaries were not the same performance in different areas, in some areas were typeⅠ, other regions were typeⅡ, which relating to the regional sedimentary and tectonic background. Meanwhile, the 14 third-order sequences are not fully developed in every region and each section. In this paper, in contrast to the Cambrian sequence stratigraphy of Sichuan Basin, conducted a detailed discussion of each 14 third-order sequence and characteristics of its system tract development, and established sequence stratigraphic framework, which reveals characteristics of the sequence development and time-space evolution.In this paper, With gypsum, dolomite and clay mineral as three end members, gypsum-salt rock could be divided into three divisions of muddy dolomitic gypsum rock, gypseous dolostone and gypseous sandstone and ten subdivisions of gypsum-salt rock; they have three kinds of compositions and six kinds of structures, and with the different components they have different sealing capacities; gypsum-rocks all developed in the climate that the cycle turning dry and shallow upward.By analyzing more than 20 drillings and 3 sections of the lower-middle Cambrian in Sichuan basin, we found out that the sections containing gypsum-salt rocks consist mainly of carbonates (dolostone, calcareous dolostone, muddy dolostone, mudstone, marlstone, dolomitic marlstone etc.), sulphate (gypsum, dolomitic gypsum etc.)and so on, and they are interbeded with fine grained stones like mudstones, silty mudstones and so on. Gypsum-salt rocks occurrence in carbonate-sulphate buildings. According to the characteristics of the basic composition and structure of the sections that containing gypsum-salt rock in different places, the section of the lower-middle Cambrian in Sichuan basin could be divided into four categories:①Dingshan pattern, in the lower part of this pattern, grey oolitic limestone, microcrystalline dolostone, up to muddy dolostone and gypsum constitute a cycle.②Gongshen pattern, this pattern consists of grey marlstone, limestone in the lower part, microcrystalline dolostone in the middle part, gypseous dolostone in the upper-middle part and gypsum-rock in the top and they form a cycle.③Jianshen pattern, this pattern consists sandy dolostone in the bottom, huge thick layer of gypsum-rock in the middle part, and calcareous dolostone in the top.④Fandian pattern, characterized by the development of salt dissolved breccia.Researched Cambrian evaporite formation conditions of the Sichuan Basin, with characteristic of lithofacies palaeogeography and features of sections development to explore the formation mechanism of gypsum rock, and integrated Cambrian sedimentary characteristics of gypsum rock found that there are four centers in the distributions of gypsum-salt rock of Cambrian regionally, and they present'bovine eye'style, the evaporation lagoon facies of'bovine eye'style that distributed as girdle band is the most favorable development zone for gypsum-salt rocks; Established the contrast of gypsum rocks within three levels of Sequence stratigraphic framework, defined that the gypsum rocks develop in the sea level dropped during high-water system; By researching, we found out the regional distributions, lithology composition partitionings and variations of thickness of the Cambrian gypsum-salt rock, of Lower Cambrian gypsum rocks of the distribution mainly concentrated in the Southern District and Enshi-Youyang area; Middle Cambrian gypsum rocks in the overall distribution in southern Sichuan-northern Guizhou region and Wanxian-Zhong County town area, and two gypsum rock distribution regions have been linked. Through the gypsum rock distribution and comparison within a sequence stratigraphic framework, demonstrating the good blocking of gypsum rock in southeast of Sichuan basin.