Study On Tight Gas Enriching Mechanism Of The Shanxi Formation In The Eastern Zhoukou Depression Of The Southern North China Basin

The east of the Zhoukou depression,Southern North China Basin is a frontier area without discovered oil and gas fields.The Carboniferous to Permian is marine-continental transitional facies,and widely developing sandstone and mudstone interbeded sedimentation.There are basic geological conditions for pool formation with hydrocarbon self generation and self reserving.Dark mudstone and coal are developed in the Shanxi formation.Twelve wells met oil and gas shows in the research area.This indicates that the Shanxi Formation could become a promising exploration target for tight gas.Based on singular drilled well evaluation data,combining with the previous research results,comprehensively using the principles and methods of sedimentology,sequence stratigraphy,oil and gas geology,adopting thin section observation,scanning electron microscope（SEM）,X-ray diffraction,high pressure mercury injection,low-temperature nitrogen adsorption,rock-eval,total organic carbon（TOC）,main and trace elements and fluid inclusions and other experiments,this thesis studied pool accumulation conditions including tectonic evolution,sedimentary system distribution,distribution of source rocks,and restoration of burial and thermal history.The four achievements are as follows:1.The Shanxi Formation in the research area is divided into three fourth-order sequences,namely SQs1,SQs2,and SQs3 from bottom to top.Furthermore,four sequence boundaries and three maximum flooding surfaces are identified.Carbonate tidal flat-clastic tidal flat sedimentary system is developed in Shanxi Formation in the research area under the control of regression.The peat flats and mud flats in SQs1,SQs2 and SQs3are favorable hydrocarbon source layers.The sand flats and the mixed flats formed in the transgression periods of SQs2 and SQs3 constitute favorable hydrocarbon reservoirs.Hence,the self generation and self storage reservoir developed.2.The source rock of the Shanxi Formation in the research area are type II₂and type III kerogen at the high mature stage.The plants developed in the tidal flat sedimentary system are the main source of organic matter.The type of kerogen is affected by the sedimentary environment.Three types of kerogen,II₁,II₂ and III are developed in the south seaward side,while there is only ype III kerogen developed in the north landward side.The R_o is lower in the south while higher in the north due to the intrusive volcanic rocks.The R_o of source rock in the south is 1.52%on average,which is in the mature and high maturity stage.The R_o of source rock in the north is 2.90%on average,which is in the stage of high maturity and over maturity.The Shanxi Formation source rock is developed under the abundant oxygen,low paleoproductivity conditions.The main and trace elements in the marine-continental transitional facies are less affected by terrestrial inputs.The DOP（Degree of pyrite）,C_org/P_tot（Ratio of total organic carbon to total Phosphorus element）,Ba _XS（Barium element related to biology）,Si_ex（Silicon element related to biology,also known as excess of Silicon element）and P（Phosphorus element）element are low.Under the rise and fall circulation of sea level,dark mudstone and coal bearing strata interbeds organic rich deposition in the Shanxi Formation.This provides the organic matter for the Shanxi Formation.3.Ultra-low porosity,ultra-low permeability reservoirs are developed in the Shanxi Formation in the research area.The fractal dimension of reservoir pore structure is high,indicating that pore structure is complex and reservoir heterogeneity is strong.The sandstone reservoirs have higher quality in the sand flat.The correlations between fractal dimension and porosity,permeability,and pore structure parameters of sandstone reservoirs of the sand flat in the study area shows sandstone with larger porosity and permeability,lower fractal dimension,and stronger reservoir capacity.The correlations between fractal dimension and porosity,permeability,organic matter,minerals and clay minerals of the sandy mudstone reservoir of the mixed flat in the research area shows the fractal dimension of sandy mudstone has complex relationship with the porosity,permeability and other parameters.4.There are primary hydrocarbon accumulation model and secondary hydrocarbon accumulation model in the study area.The secondary hydrocarbon accumulation model has higher hydrocarbon generation and expulsion.In the primary hydrocarbon accumulation model,the hydrocarbon generation period of the Shanxi Formation source rocks is from Triassic to Jurassic,the hydrocarbon expulsion period is from Jurassic to Cretaceous,and the critical moment for reservoir formation is from Jurassic to Cretaceous.In the secondary hydrocarbon accumulation model,the first hydrocarbon generation and expulsion period of the Shanxi Formation source rocks is from the late Permian to the early Triassic,and the second generation and expulsion period is the Neogene.The Shanxi Formation entered the gas generation stage in the second hydrocarbon generation.The secondary hydrocarbon accumulation model is better for tight gas exploration.