High Temperature And High-Pressure Fluid Activities And Natural Gas Accumulation Mechanism In The Ledong Area Of The Yinggehai Basin

The Yinggehai Basin is characterized by high temperature and high-pressure.Abundant oil and gas resources have been discovered in the Ledong Area.Currently,the exploration focus is on the Huangliu and Meishan Formations gas reservoirs.However,under the background of high temperature and high-pressure fluid activities,the diagenetic evolution of sandstone reservoirs is complex,and the formation mechanism of high-quality reservoirs is unclear.How fluid activities,overpressure formation and evolution affect the migration and accumulation process of hydrocarbon rich gas,as well as the differential enrichment mechanism of natural gas,are currently difficult issues.This paper comprehensively utilizes core,drilling and logging,seismic data,and experimental data to focus on analyzing the Huangliu and Meishan Formations gas reservoirs,especially the gas reservoirs located in the Ledong Slope Zone.Finally,determine the composition and charging period of high temperature and high-pressure fluids,and restore the paleopressure and pressure evolution.Evaluate reservoir characteristics,classify and predict reservoir types,restore diagenetic evolution and pore evolution,and determine the formation mechanism of high-quality reservoirs under the high temperature and high-pressure fluid activities.Restoring the opening and closing characteristics of mudstone interlayer,divide gas reservoir types and clarify the dynamic reservoir formation process of natural gas.Analyze the characteristics of fault-reservoir-cap combination,determine the controlling factors of gas saturation,reveal the mechanism and main controlling factors of natural gas enrichment,and establish the reservoir forming model.The results show that the genesis of high-temperature and high-pressure fluids in the Ledong Area of the Yinggehai Basin is related to the overpressure caused by undercompaction,the overpressure caused by fluid expansion,and the late deep thermal fluid activities.There are mainly three stages of large-scale fluid charging,with the first two stages of high-pressure hydrocarbon fluids and the third stage of CO₂ rich high-pressure thermal fluid.The reservoirs of the Huangliu and Meishan Formations are divided into I to IV types,and the reservoir quality is a comprehensive effect of fluid activities and diagenetic evolution,with two stages of dissolution and three stages of carbonate cementation.High-quality reservoirs are developed in the sand bodies with coarse grain size,undeveloped carbonate cements,strong dissolution by acidic fluids,and near fluid migration channels.According to the relative magnitude of reservoir fluid pressure and hydraulic fracture pressure of mudstone interlayer,the evolution of the opening and closing characteristics of mudstone interlayer can be divided into three types,corresponding to the three types of gas reservoirs:early opening and late sealing,continuous sealing,and early sealing and late opening.The evolution type is controlled by the sedimentation rate of the Yinggehai Formation,the evolution process of the source rocks,the positional relationship between the reservoirs and the source rocks,and the diapiric activity.There are significant differences in the migration and accumulation processes of fluids in different types of gas reservoirs.For“early opening and late sealing”gas reservoirs,microfractures in mudstone interlayer are opened during hydrocarbon fluid charging,and CH₄ content and gas saturation in the shallow gas formations are high.“Continuous sealing”gas reservoirs have continuous sealing of mudstone interlayer microfractures,and there is a high CO₂ content and high gas saturation in the deep gas formations adjacent to the faults.“Early sealing and late opening”gas reservoirs are affected by diapir structural activities,and microfractures in mudstone interlayer are opened during CO₂ rich high-pressure thermal fluid charging,resulting in a high CO₂content in the deep gas formations.The diagenetic evolution,temperature and pressure evolution,and fluid charging of gas reservoirs in the Huangliu and Meishan Formations have a good configuration relationship.Organic acids were injected into the Huangliu and Formations gas reservoirs in the Ledong Slope Zone at 6～4Ma,and the first stage of dissolution occurred in the reservoirs.After 5Ma,the source rock began to generate a large amount of hydrocarbon,and the reservoirs entered the Mesodiagenetic stage A,forming an undercompaction overpressure.From 2.2Ma to 0.9Ma,the diapir entered the puncture stage,with high-pressure hydrocarbon fluids charging,fluid expansion overpressure occurring,and some microfractures in mudstone interlayer opening.The reservoir entered the Mesodiagenetic stage B,with high-quality reservoirs widely developed.From 0.9Ma to now,CO₂ rich high-pressure thermal fluid charging,and the secondary dissolution occurred,microfractures in mudstone interlayer were sealed,and the gas reservoirs were finally formed.The main controlling factors for natural gas enrichment in the Huangliu and Meishan Formations gas reservoirs are that effective reservoirs of a certain scale and early hydrocarbon rich fluids can be preserved.Two types of reservoir forming models are developed:（1）microfractures in mudstone interlayer serve as gas transmission channels between gas formations,and hydrocarbon rich gas is enriched in the shallow gas formations,while the high CO₂ content of the deep gas formations is high;（2）microfractures in mudstone interlayer continue sealing,and hydrocarbon rich gas accumulates in the deep gas formations,and gas saturation in the shallow gas formations is low.