The Prediction Of Sweet-spot Reservoir In The Triassic Baikouquan Formation Of The Maxi Slope,Mahu Depression,Junggar Basin

The Triassic Baikouquan formation,located in Maxi slope of Mahu Sag,Junggar Basin,lies on the source area providing a good condition for oil and gas accumulation,and presents great potential for oil-gas exploration.However,the permeability and porosity of the Baikouquan formation is generally low,and the prediction of sweet spot with relatively high permeability and porosity is critical for oil-gas exploration and development in the study area.Based on cores,well logs and seismic data,the fault distribution,sedimentary facies and reservoir physical properties have been studied.The results show that the fault systems in the Maxi slope can be divided into three types: the first-level faults,which extend long longitudinally and link hydrocarbon source rocks,are the main channels for hydrocarbon migration;the second-level faults cut the first-level faults forming fault blocks and provide barriers for oil and gas accumulation;the third-level faults connect sandbodies and provide seepage channels for fluid flow.Fan-delta sedimentary systems are developed in the study area.The underwater distributary channels and mouth bars,with thick sandbodies and round grain shape being well-sorted,are favorable reservoirs.The diagenesis in the study area consists of compaction,metasomatism and dissolution.The compaction and metasomatism are the main causes for the low permeability and porosity.In the late period of diagenesis,the organic acid migrated from source rocks to reservoir rocks resulting in lots of dissolution pores,which greatly improve the permeability of the reservoir,and this is the main mechanism for sweet spot developed in low permeability reservoirs.The distribution of sweet spot is mainly affected by sedimentary facies,faults,palaeogeomorphology,and diagenesis.According to the prediction of sedimentary facies,diagenetic facies and seismic inversion,the sweet spots of Maxi slope are mainly distributed in the east of AH1-Ma18-Ma6 well blocks and the west of MZ1 well block.The areas of sweet spot in the first member and second member of Baikouquan formation are 266 km2 and 189 km2,respectively.