Reservoir Characteristics Of The Upper Devonian Sandstone And Its Natural Gas Accumulation Conditions In The Marsel Area

As a trading hub connecting Europe and Asia,conducting hydrocarbon exploration in the Marsel area is conducive to the construction of the Belt and Road in China.The Upper Devonian subsalt sandstone reservoir in the study area is rich in natural gas resources,but its sedimentary reservoir characteristics and accumulation conditions still remain unclear,thus seriously restricting the natural gas exploration in the Marsel area.Based on the seismic and logging data,an integration of core observation,petrographic thin section,cast thin section,petrophysical measurements and high pressure mercury intrusion test was utilized in this study to analyze sedimentary reservoir characteristics of the Upper Devonian subsalt sandstone reservoirs;to study the microscopic pore structure of reservoirs and their effects on reservoir petrophysical properties;to investigate the possibility of characterizing the complexity of pore structure using fractal dimensions.In addition,the accumulation conditions and main controlling factors of subsalt sandstone gas reservoirs were discussed.Finally,the favorable natural gas accumulation zones were predicted.The research results show that braided river delta is the most favorable sedimentary facies for subsalt sandstone reservoirs and the reservoirs are dominated by lithic quartz sandstone and feldspar lithic quartz sandstone.The reservoirs are currently at the B stage of the middle diagenetic phase,and they are characterized by overall dense but with high porosity-permeability layers locally developed.The pore types include residual intergranular pores,intragranular dissolved pores,intergranular dissolved pores and fractures.The reservoirs can be divided into four types according to the pore structure and petrophysical properties,and the petrophysical properties of reservoirs are mainly controlled by pore throat connectivity and pore throat radius.The fractal dimensions of small pore throats can be used to characterize the complexity of pore structure.The accumulation conditions of tight sandstone gas are good in the Marsel area and the distribution of subsalt tight sandstone gas reservoirs is mainly controlled by the source rocks,sedimentary facies and faults.It is proposed that the favorable zones for natural gas accumulation are mainly distributed in the middle and northern parts of the Marsel area.