Diagenesis of Triassic Feixianguan Formation in Sichuan  Basin, Southwest China

Key words: 
• Sichuan Basin /
• Feixianguan Formation /
• dolomitization /
• secondary pores /
• TSR /
• C /
• O stable isotope /
• CO2 

Abstract: The proved reserves of sour gas pools were up to 1×1012 m3 from oolitic dolostone reservoirs of Feixianguan Formation in northeast Sichuan Basin，that are mainly distributed in the sedimentary sequence with evaporites. Remnant gypsum and anhydrite , their moidic pores or molds in the reservoirs indicate that the dolomitization was related to the evaporated seawater. Furthermore, in some diagenesis sequence of the reservoirs, the dolomitization extent increases with depth from ancient subaerial exposure surface downwards. This implies that the dolomitization of the oolitic limestone took place in the early stage of diagenesis，and was possibly influenced by mixing rain water and evaporated seawater. 
For δ13C and δ18O values between structural constituents of the oolitic dolomite in the reservoirs, the difference is great, so the values of intermix sample of reservoir rock are not suited for dolomitization research.
In the reservoirs , the most oolitic dolostone preserved the remainder of oolitic fabric or original rock framework , the various pores of the dolostone are almost secondary dissolved pores and not shrinkage pores due to dolomite crystals shrink. The replacement process of dolomitization is a volume replacement for volume, not a mole for mole. The relationship between bitumen and secondary dissolved pores indicates that late burial dissolution pores are dominant in the reservoirs . 
The ionization constant of H2CO3 is much higher than that of H2S in the solution ,whereas the mole of CO2 is much lower than that of H2S in the sour gas of Feixianguan Formation. The late period calcite cement ，filled in the dissolved pores and fractures of the oolitic dolomite reservoirs， is characterized by low δ13C (-18‰) and high homogenization temperature (130~180℃), so the CO2,related to thermochemical sulfate reduction in the reservoirs , causes mainly late burial dissolution.