Genetic Mechanism Of Deep High-quality Reservoir In The Palaeogene Huagang Formation Of Xihu Sag In The East China Sea Basin

The Xihu sag of the East China Sea Basin in the coastal waters of China is one of the sedimentary sags with abundant oil and gas resources and has great potential for exploration and development.The resources of tight sandstone gas in the Xihu sag are huge,accounting for 80%of the total natural resources in the whole sag.In recent years,with study enhanced,exploration of the Paleogene Oligocene Huagang Formation tight sandstone gas continues to make breakthroughs and leads to rapid growth of natural gas exploration reserves.At present,the Paleogene Oligocene Huagang Formation has become the most important layers of low permeability tight sandstone gas in the Xihu sag.With the exploration of the Huagang Formation tight sandstone gas reinforced,the genetic mechanism and distribution of the relatively high-quality reservoir under the background of low porosity and low permeability is not clear,which become key problems that restrict the economic and efficient development of the Paleogene deep tight sandstone oil and gas resources in the Xihu sag.This paper is based on the achievements of previous studies and guided by the theories of basin analysis,sedimentary petrology,petroleum geology,etc.In the research process of this paper,thin section identification,scanning electron microscopy analysis,electron microprobe analysis,fluid inclusion analysis,zircon U-Pb isotope dating analysis,laser carbon and oxygen isotope analysis and other testing techniques are used and combined with measured reservoir physical properties data and calculation results of pore evolution recovery of clastic rock reservoir to analyze reservoir lithology and physical characteristics of Palaeogene Huagang Formation in the Xihu sag in detail,and systematically summarize the diagenetic events,diagenetic sequences,identify diagenetic environment and establish the pore evolution model of the Huagang Formation.This paper comparatively analyzes the coupling relationships between the evolution of reservoir diagenesis-pore and hydrocarbon charging periods of the Huagang Formation in each tectonic zone of Xihu sag.Finally,this paper clarifies the range and depth limit of favorable diagenetic facies of the Huagang Formation,points out the spatial distribution characteristics of high-quality reservoir in each tectonic zone of Xihu sag,reveals impacts of deep high-quality reservoir and mechanism of secondary pore formation and preservation of the Huagang Formation in Xihu Sag.The main research achievements of this paper are as follows:(1)During the evolution of diagenesis,the Huagang Formation sandstone reservoir of Xihu sag has experienced a variety of diagenesis,such as compaction,carbonate cementation,siliceous cementation,authigenic clay mineral cementation,and dissolution.The compaction is the fundamental cause of reservoir densification,however the dissolution is the key to secondary pore development and formation of high-quality reservoir.The diagenesis environment evolution sequence of the Huagang Formation sandstone in the diagenetic process is:weak alkaline diagenetic environment(Syngenetic period?Stage A of early diagenesis)?weak acid diagenetic environment(Stage B of early diagenesis)?acid diagenetic environment(Stage A₁ of middle diagenesis)?acid-base transition zone(Stage A₂ of middle diagenesis)?alkaline diagenetic environment(Stage B of middle diagenesis).Through comparative analysis of longitudinal diagenetic stage division and diagenetic environment evolution of the Huagang Formation typical wells in each tectonic zone of the study area,it is shown that:in stage A1 of middle diagenesis,reservoir experienced relatively strong acid diagenetic environment which was beneficial to secondary dissolution pore development and was mainly corresponding to?and?types reservoir;In stage A2 of middle diagenesis,reservoir entered in acid-base transition zone which led to dissolution weakened,cementation enhanced and reservoir physical properties reduction and was mainly corresponding to?₁ and?₂ types reservoir.In stage B of middle diagenesis,reservoir experienced alkaline diagenetic environment and influenced by destructive compaction and cementation which led to reservoir physical property seriously drops and was mainly corresponding to?type reservoir.(2)Based on the analysis of diagenetic sequence and diagenetic environment evolution of the Huagang Formation in each tectonic zone of the study area and combined with classification evaluation of reservoir physical properties,this paper confirmed that most of acid diagenetic environment of middle diagenesis stage A1 and a few acid-base transition zone of middle diagenesis stage A2(mainly corresponding to better sorting medium-coarse sandstone and mixed accumulation glutenite)are the favorable diagenetic facies of the Huagang Formation high-quality reservoir(???₁ types reservoir).Due to the differences in burial-thermal evolution history,there are some differences in the lower limit depth of the favorable diagenetic facies in each tectonic zones of Xihu sag.The lower limit depth in central-north of central inversion tectonic zone is 3800m,however in western sub-depression,central-south of central inversion tectonic zone and Chunxiao Tianwaitian tectonic zone is 4700m,4400m and deeper than 3670m,respectively.The deep(>3500m)high-quality reservoir of the Huagang Formation mainly developed in western sub-depression,central-south and central-north of central inversion tectonic zone,and medium-shallow(<3500m)high-quality reservoir of the Huagang Formation is mainly developed in Chunxiao Tianwaitian tectonic zone.(3)This paper discussed the coupling relationship between reservoir densification and hydrocarbon charing of the Huagang Formation and revealed spatial distribution characteristics of high-quality reservoir.According to the relationship of reservoir diagenesis-pore evolution stage and hydrocarbon charging period,and in accordance with hydrocarbon accumulation efficiency and exploration potential,the sandstone reservoir of the Huagang Formation can be divided into three types:undensified reservoir,densifying reservoir,and densified reservoir.Densifying reservoir which has relatively large petroleum exploration potentials mainly correspond to deep high-quality reservoir of Huagang Formation.This type reservoir mainly distributed in the upper member of the Huagang Formation in central-north of central inversion tectonic zone and western sub-depression,and the lower member of the Huagang Formation in central-south of central inversion tectonic zone.Among them,better sorting medium-coarse sandstone and mixed accumulation glutenite which have relatively good physical property are the key exploration target of Huagang Formation deep high-quality reservoir.(4)Sedimentary provenance system,sedimentary facies,burial-thermal evolution history and diagenetic environment comprehensively influenced the high-quality reservoir development of the Huagang Formation.Sedimentary provenance system and sedimentary facies are innate factors of the Huagang Formation high-quality reservoir development,which directly control mineral composition and structural maturity of reservoir and indirectly affect the characteristics of diagenesis and physical properties.Burial-thermal evolution history and diagenetic environment are acquired factors that influence reservoir development.Burial-thermal evolution history directly controls the compaction strength and is the fundamental cause of reservoir densification.And the diagenetic environment under constraints of burial-thermal evolution history is the key to control dissolution and secondary pores formation of reservoir.Hence,diagenetic environment directly impacts the physical properties and spatial distribution of high quality reservoir of the Huagang Formation.In addition,the Huagang Formation reservoir space is mainly composed by secondary dissolution pore which is related to the organic acid dissolution of aluminosilicate,such as feldspar,basic volcanic rock,and so on.And abnormal high pressure is an important mechanism of development and conservation of the Huagang Formation deep high-quality reservoir.