Characteristics of Mixed Sedimentation in the Meber 4 of Shahejie Formation of Paleogene at Leijia Area of Western Sag of Liaohe Oilfield

Key words: 
• mixed sedimentation /
• carbonate rocks /
• terrigenous clastics /
• the Meber 4 of Shahejie Formation /
• Leijia area

Abstract: Mixed rocks is composed of lacustrine carbonates and terrigenous clastics, which is neither typical carbonate rocks nor classic clastic rocks. As a special type of rock, it develops extensively in the Meber 4 of Shahejie Formation of Paleogene in Leijia area of Western Sag of Liaohe Oilfield. Based on detailed observations of cores and thin sections combined with comprehensive analyses of testing data, the mixed sedimentation of Leijia area is studied carefully, such as geological setting, petrological characteristic and depositional environment, etc. The characteristics of mixed sedimentation are textural mixed and interbedded mixed. The former is between terrigenous fragment constituent and carbonate constituent in one layer. The latter is between terrigenous fragment and carbonate of lacustrine facies in high frequency. Various tectonic settings develop in different mineral matters in textural mixed rocks of Leijia area. Mixed rocks that mostly consist of intrabasinal composition develops in the west gently slope belt, which the lithologies are mainly composed of argillaceous dolomite, argillaceous limestone, dolomitebearing sandstone, limestonebearing sandstone and so on. Mixed rocks mainly composed of terrigenous components develops in the east steep slope belt and the middle part, which the lithologies are mainly composed of dolomitic sandstone, dolomitic mudstone, calcareous mudstone, calcareous sandstone and so on. Carbonated volcanic breccia distributes in local region, which the new type of mixed deposit is discovered for the first time by this paper. Mixed sedimentary action types include blended mixed deposits and complex mixed deposits I, salutatory mixed deposits are developing in the local region. Through the study of dolomite genesis on theory, the penecontemporaneous dolomization is pointed out in this area and the transgression and palaeoclimate have exerted an important effect on the formation of the dolostones. At last, the geological conditions and sedimentary environment for the formation of mixed deposits are discussed.