Genesis and Development of Porosity in Carbonate Reservoirs─Fluid Inclusion Evidence

Fluid inclusions in minerals filled in pores of reservoir rocks can be used as a good indicator of pore genesis and development so as to shed light on oil generation, migration and accumulation. The relationship between pore evolution and oil generation has been established based on fluid inclusion studies on the Lower Ordovician carbonate reservoir strata in the Ordos Basin, Northwest China. Seven stages of porosity developrnent can be recognized, i. e., the penecontemporaneous, the early and middle-late diagenetic, the supergene, the early, middle and late re-burying stages. The dissolution pores and fissures formed in the supergene and middle-late reburying stages and the strtJctural fractures formed in the late re-burying stage constitute the major traps of oil and gas. The major phase of oil migration and accumulation took place between Late Jurassic and Cretaceous, corresponding to the middle and late re-burying stages. The generation and accumulation of oil can be closely-related to Yenshanian tectonics.