THE FORMATION MECHANISMS OF CAVITY AND CHARACTERISTICS OF CEMENTS AND FILLINGS OF SINIAN SYSTEM IN SICHUAN BASIN

Primary fenestral pores and secondary pore-fracture-cave system caused by meteoric water karstification,organic acid dissolution,TSR(thermal sulfate reduction),hydrothermal dissolution and tectonic fracturing are well developed in Upper Sinian Dengying formation.On the basis of geochemistry,petrography,detailed examination of the cores and thin sections,the following inclusions have been reached.Three types of cements including dolomite,calcite,and silica are common in Dengying Formation.They can be subdivided into eight subtypes.Several kinds of cements were mainly formed at the early diagenetic stage,such as fibrous or blade cements,columnar,radial,crescent cement and fine crystal dolomitic cement.Medium-coarse crystal dolomitic cement were formed at the late diagenetic stage.Further more,the silica material and calcite cements were formed at the late diagenetic stage in Yanshan tectonic uplifting movement.The growth of cements and fillings was correlated with regional structural position,and it is the most strongest in the slope zone.Upon carefully study,the source of fluids which had caused the deposition of cements and fillings in cavity,were proved to be external fluids,and they have regional characteristics.The characteristics of cements and fillings were closely related to adjacent rock.