The Evolution of Carbonate Sediment Diagenesis and Pore Fluid in Dengying Formation, Central Sichuan Basin

Key words: 
• diagenesis /
• karstification /
• hydrothermal dolomite /
• evolution of pore space fluids

Abstract: Samples of core and formation water in Dengying Formation of central Sichuan Basin were studied for petrology and geochemistry in order to understand the evolution of carbonate sediment diagenesis and pore fluids. There are several shallow marine carbonate platforms in Precambian Dengying Formation on the Yang zi mass. It is known that dolomite constitutes an important part of Dengying Formation, which is a major gas reservoir in Sichuan basin. The carbonate sediments in deposition and diagenesis of Dengying stratum were widely influenced by pore space water and diagenetic alteration. Seawater cementation, early meteoric diagenesis(dissolution and precipitation), epigenetic karstification, structurally controlled hydrothermal dolomitization and MVT(Missipi Valley-Type mineralization),dissolution in production of C and H, pyrobitumen filling in oil pyrolysis occur in the Dengying stratum, in which Karstification and hydrothermal dolomitization play an important role in development of reservoir. The pore space fluids of Dengying strata were widely modified by dolomitization and karstification. Analysed present formation water data suggest this diagenesis exsert an influence on the formation fluids of evaporative seawater. Diagenesis has an influence on reservoir of Dengying Formation, in which karstification and hydrothermal dolomitization play an important role in increase of porosity and permeability.
Analyzed formation water data indicated the evaporated sea water in the Ca-Br and Mg-Br relation. Mg concentrations have been reduced on average compared with evaporated sea water. Ca concentrations have been increased on average compared with evaporated sea water. Some workers believe that this is dolomitization of limestone.