Abstract:

Water-rock-hydrocarbon interactions in the Middle Jurassic Shaximiao Formation sandstones, Western Sichuan, were studied to determine the relationship between authigenic mineral types and hydrocarbon generation through analyses of petrographic data, and scanning electron microscopy and cathode luminescence results.Formation timing of major authigenic minerals was also determined with analyses of inclusion temperature and isotope geo-temperature.Electron microprobe was employed to measure the chemical compositions of the minerals of different forming ages.Isotope analyses and formation water analyses were combined with X-ray diffraction to describe the evolving features of pore fluids of the formation.The result shows that the pore fluid in the Shaximiao Formation was originally characterized by high contents of Mg²⁺ and Fe²⁺, and high pH value.As burial depth increased and hydrocarbon entered, the fluid gradually turned acid and formed minerals such as chlorite, quartz, calcite, kaolinite, and etc, during the process.With authigenic chlorite consuming Mg²⁺, and Fe²⁺, feldspar providing Na⁺ and the charging of hydrocarbon providing acidic fluids, the pore fluids finally turned into this CaCl₂ type low-pH formation water with high content of Na⁺ and Ca²⁺.It should be noted that there are two factors that were favorable for the formation of good reservoirs:dissolution improved storage capacity of reservoirs and authgenic chlorite formed pore rim and helped preserving primary pores during the diagenetic process.

Key words: water-rock-hydrocarbon interaction, sandstone reservoir, the Shaximiao Formation, the Middle Jurassic, western Sichuan depression