Research On The Diagenesis Of Chang4+5 Reservoir In Zhidan-danba Area, Ordos Basin

Ordos basin is a suitable place for research on coexistence of multiple energy resource and coexisting mechanism that contains petroleum, natural gas and coal etc in the same basin. Choosing Chang 4+5 sandstones of Yanchang formation in Danba of Zhidan area is a keystone to analysis. Profound study on diagenetic environment, aqueous medium and pore types accomplished from diagenetic aspect. It has not only important theoretic significance for comprehensive evaluation, exploration and development of reservoir, but also practical economic meaning for study of low permeability reservoir.Based on fieldwork and observation of drill cores, using theories and methods of reservoir geology, geology of ore deposits, geology of petroleum and gas, geochemistry etc, research of the various aspects of diagenesis are conducted by polarizing microscope, scanningelectron microscope, Spectrum analysis, reflectivity ofvitrinite, and trace element analysis etc. Conclusions are show as below:1. Adoption of both major and trace elements, organic matter and carbonate content discrimination, diagenetic environmen of Chang₄₊₅ sandstones of Yanchang formation in Danba of Zhidan area is hypoxia-reduction environmentthe of humid swamps or delta, the pH value in the 7.81 to 8.03, and Eh is weakly alkaline,2.Reservoir are mainly composed of feldspar sandstone and siltstone. Maturity of reservoir constituent is low. Types of diagenesis include mechanical compaction, cementation, metasomatism and dissolution. Based on the comprehensive analysis of the mineral composition and distribution,the transformation of clay minerals, maturity of organic etc, these sandstones have reached the period of the late diagcnctic A stage, at the same time, the sequence of diagenes has been Determine. Influencing factors of diagenetic include construction, clastic components, aqueous media, and so on. 3.The type of oilfield water in the study area is mainly CaCl₂, the original oil water which is weakly acidic and Oxidative changes to mineral-rich acidic water through three stages. Fist, weakly acidic water changs to alkaline in the period of the early diagcnctic A stage. Second, strong alkaline water chang to weakly acidic in the period of the early diagcnctic B stage. Finally, weakly acidic in strong in the period of the late diagcnctic B.4.By double influence of both sedimentary and diagenesis the pores of Chang₄₊₅ reservoir being of abundant secondary dissolution pores are under the control of acidic diagenetic environment. On the basis of the statistics of sandstone compositions, it is indicated by the quantitative calculation that compaction is the dominant reason for the loss of the primary pores in the study area and next cementation. whereas the increasing ratio of pores caused by the dissolution is decisive. The pores evolution experienced three steps, includeing the loss of primary pores in the period of the early diagcnctic A stage, beginning of secondary pores formation in the period of the early diagcnctic B stage and formation of a large number secondary pores in the period of the late diagcnctic A stage.