The Characteristics Of Temperature And Pressure And Influence On The Reservoir In L Depression Of Qiongdongnan Basin

L depression is located in deep-water area of the Qiongdongnan Basin.Several gas fields have been discovered,which means that the sag is an important direction for increasing storage and production in the Qiongdongnan Basin.The L depression has typical high-temperature and overpressure environmental characteristics,and the type of the Miocene reservoir is mainly gravity flow sandstones,which includes Huangliu Formation canyon channel sands and Meishan Formation submarine fan sand body.The sand body has the characteristics of multiple sources of supply,reservoirs are stacked vertically in multiple stages,lateral changes are large,and the heterogeneity is strong.The overall evolution characteristics and main control factors are significantly different.The studies of previous on canyon channel sands and submarine fan sand bodies mainly focused on aspects of lithofacies,provenance characteristics,evolutionary rules,and sedimentary patterns.It is still unclear on the evolutionary characteristics and controlling factors of reservoirs in different temperature and pressure environments.Therefore,figuring out the influence of temperature and pressure on the reservoir is of great significance for defining the main controlling factors and forming mechanism of high-quality reservoirs,and guiding the evaluation of favorable lithologic traps and the later development.On the basis of collecting regional geological data and conducting various tests and analysis,the paper firstly defined the characteristics of the temperature and pressure in the study area.The temperature and pressure environment was divided into three types of temperature and pressure field which are high temperature and atmospheric pressure,high temperature and overpressure and high temperature and strong overpressure by a temperature gradient of 3.7°C/100m and a pressure coefficient of 1.27 and 1.8.Secondly,the characteristics of the fluid environment were determined,and it was clarified that the study area had three phases of hydrocarbon fluid charging and one stage of CO₂ fluid filling.The accumulation period was 1.9 Ma to 1 Ma,corresponding to the end of the Neogene Pliocene Yinggehai-Quaternary Pleistocene Ledong period.At the same time,the petrological characteristics of the Miocene reservoir of different sand body types and different temperature and pressure fields were defined,which is used to select experimental samples for compaction simulation experiments and organic acids and atmospheric water compaction and water-rock simulation experiments.The compacted experimental samples were made to casting thin sections,which is observed by polarizing microscope and tested surface porosity,and some of the samples were tested by scanning electron microscope,and the reaction liquids were tested ion concentration.Combined with the diagenesis type and reservoir physical properties under different temperature and pressure environments in the study area,it was known that overpressure could suppress the compaction.In this way,the original porosity of the reservoir is protected,and the physical properties of the reservoir in the high temperature and strong overpressure zone are significantly higher than those of the same depth under the normal compaction.And in the compaction simulation experiment,overpressure and strong overpressure protect the surface porosity of primary pores rate up to 1.55%and 6.74%.The dissolution of organic acids has a greater contribution to secondary pores.It can be observed under the microscope that feldspar particles and carbonate cement dissolution hole,and the surface porosity of organic acid physics simulation samples was improved by 0.96%-7.38%compared with normal compaction simulation experiments,effectively improving the physical properties of the reservoir.However,the dissolution of atmospheric water is weak,and the surface porosity of atmospheric water physical simulation samples is only 0.19%higher than that of normal compaction simulation samples.Therefore,overpressure and organic acid charging are the main influencing factors of pore evolution of reservoir.