Formation Mechanism Of Palaeokarst Reservoir Of The Majiagou Formation In Yichuan-Huanglong Area

The reserves and production of the palaeokarst reservoir in Ordos Basin continued to grow rapidly.It is urgent to develop new exploration fields with the deepening of exploration and pressure of increasing production.The Yichuan-Huanglong area in Ordos Basin has favorable prospect for exploration,which is of great significance for the exploration of Huanglong gasfield to research the main controlling factors and forming mechanism of reservoir.Based on the core observation,casting thin section identification,cathodoluminescence(CL),CT Scanning and geochemical analysis tests,combined with drilling and logging data,auther thoroughly studied the basic characteristics,diagenesis and diagenetic evolution,main controlling factors and forming mechanism of palaeokarst reservoirs of Majiagou Formation in Yichuan-Huanglong area.The main results and new recognitions as follows:(1)The powder crystal gypsodolomite,powder-aplitic crystal dolomite and powder crystal doloarenite mainly develop in the study area of Majiagou Formation;the gypsum mold pore,intergranular hole,solution pore,hole and fissure are the most important storage space.The correlation between porosity and permeability of reservoir is extremely poor,and pore throat structure has kinds of types.It shows that the overall physical property of the reservoir that the porosity and permeability are from low to extra-low.(2)According to the influence on physical property and petrologic characteristics,reservoir diagenesis is divided into constructive and destructive type.The constructive diagenesis of reservoir mainly includes dolomitization,degypsumization,multi-stage karstification and recrystallization,the destructive diagenesis of reservoir mainly includes compaction and pressure solution,cementation,multi-stage filling and dedolomitization.Based on diagenesis features and petrologic characteristics,the diagenesis stage of reservoir is classified as 6 types,combined with the diagenesis evolution and pore characteristics,the diagenetic sequence of reservoir was established in Yichuan-Huanglong area.(3)Author analysed the cause of the filling in Majiagou Formation of study area by trace element,cathodoluminescence(CL)and carbon and oxygen isotope experimental methods.It suggested that the vadose-silt was formed in the meteoric diagenetic environment,and it was the pieces of dissolution and broken of bedrock;Calcite filling has obvious period.The early-stage calcite filling was formed in the meteoric diagenetic environment,which was filled after vadose-silt,and the later-stage calcite filling occurred in the buried diagenetic environment.(4)The favorable development of reservoir in study area is affected by sedimentary micro facies,karstification,filling and dedolomitization,the anhydrite-bearing concretion dolomite flat,grain bank and dolomite flat micro facies in upward shallowing sedimentary sequence are the favorable foundation of reservoir;the plane distribution of reservoirs is controlled by the palaeokarst geomorphology,which platform margin slope is conducive to the formation of the reservoir;the vertical distribution is controlled by karst cycle,favorable reservoirs distribute in the vertical permeating belt of the supergene;the calcite-dominated filling in buried diagenetic environment severely destroyed porosity;the dedolomitization in supergene stage generated reservoir densification.(5)The supergene-weathering karstification is the most important factor in the formation of dissolved pore reservoir in the study area;the dedolomitization in late-supergene stage and the coarse grained calcite filling in deep-buried diagenetic stage thoroughly destoryed the original storage space,which were the direct factor of ultra-low physical property of the weathering crust reservoir in this area.