Relationship between tight sandstone reservoir diagenetic process and hydrocanbon charging:A case study of Chang8 reservoir in Fuxian area,Ordos Basin
The time course of reservoir densification and hydrocarbon charging and their relationship play a key role in exploration evaluation.The diagenetic process and pore evolution of the sandstone were analyzed in detail by the cast thin section observation,X-ray diffraction,SEM,and nuclear magnetic resonance.Based on the existing oil-gas accumulation data,the evolution sequence of the sandstone burial,diagenesis,pore evolution,and oil-gas charging was established comprehensively.The results show that the reservoir types of Chang8 in Fuxian area were mainly feldspar sandstone and lithic feldspar sandstone,and the pore-throat relationship was complex,and it was a tight reservoir with low to extra-low porosity and extra-low to ultra-low permeability.In the early diagenetic stage,the Chang8 reservoir in stage A experienced mechanical compaction(pressure solution)(Φ2=21.6％),a small amount of cement was generated,and oil-gas were charged in stage I from 192.5 to 152.0 Ma(J1～K1).A large number of cements were generated in early diagenetic stage B(Φ3=6.9％).In the middle rock formation stage,the large-scale oil-gas charging occurred in stage Ⅱ from 152 to 126 Ma(K1)(main hydrocarbon accumulation stage).In the late stage of basin tectonic uplift,the oil-gas charging occurred in stage Ⅲ from 65.0 to 36.5 Ma(E1)(Φ3=10.8％).The diagenetic process leads to the selectivity of oil and gas charging and the difference of reservoir oil content,and the research results have important practical significance for finding the"sweet spot area"of Chang8 reservoir in Fuxian area.
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