Quantitative research of diagenesis: its effect on pore evolution of the Fuyu oil reservoir in the north Qijia region

Abstract From observing cores of 18 wells, identifying 175 ordinary thin sections and 61 thin casting sections, energy spectrum analyses of 37 samples, homogenization temperature measurement of fluid inclusions of 11 samples, we determine the types of diagenesis and pores of the Fuyu oil reservoir in the north Qijia region. We classified the pores and measured their plane porosity using CIA 2000, the software of rock image analysis, calculated the effect of different kinds of diagenesis on porosity, studied the controlling actions of diagenesis to pore evolution quantitatively, combined with burial history, thermal history and their diagenetic environments. Our results show that mechanical compaction and carbonate cementation are the major destructive diagenesed, developed during early diagenesis stages. The reduction in porosity by mechanical compaction and carbonate cementation are about 25% and 8%, while the destructive intensity of siliceous cementation and clay mineral cementation is relatively much smaller, i.e., the reduction of porosity is about 2% and 0.2% Dissolution is constructive diagenesis, the increment of porosity is about 6%. There are four diagenesis evolution stages, during which the porosity reduced from 30%∼38% to 2%∼20%. Mechanical compaction and early cementation are the main diageneses in the early diagenesis stages, when porosity was reduced to 2%∼10%. Dissolution is the main diagenesis of an A I substage of the middle diagenesis stage, when porosity increased 1%∼8%. The dissolution of the A II substage of the middle diagenesis stage affected by late cementation, raised porosity 1%∼5%. The porosity varied slightly during the middle stage B.