Study On Sandstone Reservoir Characteristics Of Permian Longtan Formation,interaction With CO₂-rich Fluid And Petroleum Accumulation In Huangqiao Area,Subei Basin

The tectonic belt of Huangqiao area is located in the transitional part of the Mesozoicand Cenozoic residual basins of Lower Yangtze region and the Cenozoic uplift in southern Jiangsu,where the tectonic feature is complicated.This area has one of the largest CO2 gas fields on land in China,with the production of a small amount of condensate oil,which is a special reservoir for both CO2 and condensate.As an important CO2 production area in China,the questions attract much attention about the sandstone reformation by CO2-rich fluid in Permian Longtan Formation.Interaction between CO2-rich fluid and sandstone is the main factor for forming secondary porosity in sandstone reservoirs.To reveal the interaction characteristics of the CO2-rich fluid and its influences on sandstone reservoirs,systematic core description and micro-observation on typical coring wells are carried out,as well as in-situ observation and major elements analysis on EPMA.It is found that the reservoir of the Longtan sandstone is significantly different in reservoir characteristics.It not only has the characteristics of rock,mineralogy and reservoir property,but also develops different dissolution and alteration phenomenon.The results show that carbonate cements in sandstone near the fracture zone are almost completely dissolved with some displacement siderite left,especially in the upper wall where the CO2-rich fluid influx is active.Meanwhile,the dissolution of clastic grains such as k-feldspar is strong,and there is much kaolinite precipitation,quartz secondary enlargement and dawsonite which indicates effects of high concentration of CO2.It develops the combination of typical minerals related to the CO2-rich fluid including dawsonite,kaolinite,secondary quartz and siderite.In the area away from faults where the CO2-rich fluid has few effects,so the dissolution is weak and much calcite and siderite precipited on the contrary.As to the former,reservoir has better secondary porosity,while for the latter the reservoir is much more tight.Based on above all,the genesis mode related to interaction between CO2-rich fluid and sandstone,as well as its effects on the objective reservoir is set up,considering the deep faults play the main role in the sandstone reservoir improving.In addition,study on fluid inclusions is conducted to analyze oil and gas filling period of Huangqiao fields,as well as main controlling factors and the accumulation process of hydrocarbon.The widely developed fluid inclusions of Permian Longtan Formation reservoir distribute in quartz secondary enlargement edge and quartz crack,mainly including aqueous and oil inclusions.Most oil inclusions have two phases and can be divided into three filling phases according to its distribution and appearance,combined with microscopic temperature measurement.Period ? of organic inclusions have the homogenization temperature of 70?75?;period ? of organic inclusions have the homogenization temperature of 120?130?;period ? of organic inclusions have the homogenization temperature of 60?.Based on inclusions observation,tectonic evolution and diagenetic evolution analysis,we come to the conclusion that the oil and gas in sandstone reservoir of Permian Longtan Formation experience 3 major filling periods:the first period for 85?80 Ma,the second period for 49?41 Ma and the third period of about 25 Ma,predominately the second and the third period.The second and the third period experience oil and gas filling process accompanied by the accumulation of CO2 gas.Comprehensive analysis of oil/gas reservoir distribution and accumulation characteristics of Huangqiao field show that there several main control factors which affect the accumulation process.Construction is the main factor which controls the hydrocarbon accumulation process;the reservoir feature is the direct factor controlling the distribution of field;the effect of the CO2-rich fluid is the important factor affecting the accumulation of oil and gas.Based on above all,this paper puts forward the "double factor" mode of the reservoir feature and the CO2-rich fluid which control the high quality reservoir under the effect of construction.It is believed that the development of the high quality reservoir is not only restricted by the original sedimentary facies,the reservoir physical quality,but also the later impact of the CO2-rich fluid.