Formation Mechanism Of Middle-deep Sandstone Reservoir Of Eocene In Zhu ? Depression,Pearl River Mouth Basin

Wenchang Formation and Enping Formation of the Eocene of Paleogene rich in oil and gas resources are the major objective horizons in Zhu ? Depression of Pearl River Mouth Basin.The influences of geological differences in structure,sedimentary environment and diagenesis on reservoir formation in different secondary sags have not been systematically studied,which restricts the exploration progress.Using thin section,scanning electron microscope and energy spectrum,cathodoluminescence,isotope,electron probe,fluid inclusion,X-ray diffraction and basin simulation,the basic geological characteristics,pore evolution characteristics,reservoir formation mechanism,hydrocarbon generation-reservoir formation-coupling characteristics,main controlling factors and reservoir development model of the Eocene middle-deep sandstone reservoirs are deeply analyzed.The final research findings can provide a scientific basis for the prediction of medium-deep sandstone reservoirs of Eocene in Zhu ? Depression and a reference for formation mechanism in the medium-deep,deep clastic reservoirs.The reservoir lithologies of Wenchang Formation mainly consists of lithic quartzite and arkosic quartzite,and those in Enping Formation are mainly lithic arkose and feldspathic litharenite.The reservoir space types in Wenchang Formation and Enping Formation mainly include residual primary pores,granular dissolved pores,cement dissolved pores and intercrystalline pores,and the granular dissolved pores is the most developed,and the reservoirs of the two formations in each sags have different pore combinations.The Eocene sandstone reservoirs in the study area have evolved to mesodiagenesis stage in most parts.Significant differences in the reservoir pore evolution characteristics in different secondary sags list as follows:(1)Porosity reduction rate is small in the formations such as Enping Formation in Huizhou and Lufeng sags which experienced early cementation and long-term shallow burial.(2)The period of cemented porosity reduction is closely related to the cement type.Calcite and anhydrite in Wenchang Formation of Lufeng Sag and Enping Formation of Huizhou Sag serious destroy the primary pores during shallow burial,while a lot of silica and kaolinite in Wenchang Formation of Huizhou Sag and Enping Formation of Enping Sag originates from feldspar dissolution and destroy the reservoir space later.In addition to the extensive feldspar dissolution,the intense dissolution of calcite and anhydrite occurred in some other areas.(4)The porosity increase due to dissolution in Enping Formation is mainly produced in the Miocene,while that of Wenchang Formation in Lufeng Sag mainly happened in the Miocene and Oligocene on the account of both dissolution of the cements and clastic grains.(5)In Huizhou Sag,the key porosity increase stage is in the Oligocene,and that of Panyu 4 Sag is the Miocene which is characterised by a typical late dissolution.(6)At present,the porosity of Wenchang Formation in Lufeng Sag,Huizhou Sag and Panyu 4 Sag is11.7%,13.2% and 12.7% respectively,and that of Enping Formation in Lufeng Sag,Huizhou Sag,Xijiang Main Sag and Enping Sag is 14.2%,16.6%,16.0% and12.9% respectively.There are three major coupling relationships among hydrocarbon generation,reservoir formation and reservoir formation in the Eocene of Zhu ? Depression:(1)The "early hydrocarbon generation,early preservation,medium improvement and continuous filling" type is represented by Wenchang Formation in well LF14-4 and Enping Formation in well LF14-4 in Lufeng Sag,in which high-yield reservoirs formed due to the good reservoir forming conditions.(2)The "early hydrocarbon generation,early densification,medium reservoir formation and late reservoir formation" type is represented by Wenchang Formation of well HZ25-7 in Huizhou Sag,which is characterized by the lack of early hydrocarbon filling,the rapid densification and the hydrocarbon accumulation followed by the dissolution of clastic particles owing to the organic acid fluid.(3)The "medium hydrocarbon generation,early preservation,medium improvement and late filling" type is characterized by the relatively late hydrocarbon generation and expulsion.In the prerequisite that high rigid particles and low geothermal field effectively inhibited the compaction and cementation and protected the intergranular volume,reservoir quality improved due to the porosity improvement resulting from the dissolution of clastic particles owing to the middle-stage organic acid.Good coupling relationship crated favorable conditions for high-yield reservoirs formedThe effective protection of intergranular volume is the key factor due to secondary dissolution pores,conservative diagenesis,early hydrocarbon filling,more rigid particles and long-term shallow burial.The high-quality source rocks in hydrocarbon generating sags provide the sufficient acid fluid relating to source-rock thermal evolution for the dissolution of Eocene sandstone reservoir,and the faults connecting the surface could be the channels for atmospheric acidcic water.The major reservoir formation mechanism is the feldspar dissolution owing to the acid fluid relating to source-rock thermal evolution and the cement dissolution owing to the atmospheric acidcic water.The porosity improvement due to the dissolution of the early cements which inhibited the mechanical compaction is the typical retention process.The early hydrocarbon filling,high content of rigid particles,long-term shallow burial slow down the process of compaction and diagenetic evolution,and inhibit the mechanical compaction and cementation,which is the main mechanism to protect intergranular volume.The lower limits of permeability of low-,middle-and high-yield reservoirs of Wenchang Formation are 2.16mD,3.57mD and 20.00mD respectively.Influenced by the pore structure differences,the same lower limit of permeability has different reservoir lower limits of porosity in different sags.The lower limits of low-yield porosity are lower in Xijiang Main Sag and Lufeng Sag,which are 10.2% and 9.5%respectively,and those of Huizhou Sag and Panyu 4 Sag are slightly higher,10.5%and 11.5% respectively.The lower limits of low-,middle-and high-yield reservois of Enping Formation are 2.27mD,5.60mD and 26.00mD respectively.The lower limits of low-yield porosity are lower in Xijiang Main Sag and Lufeng Sag,which are11.2% and 10.9% respectively,and those in Huizhou Sag and Enping Sag are 12.0%and 12.1% respectively.The development of the Eocene sandstone reservoirs is controlled by geological factors such as provenance conditions,sedimentary microfacies,diagenesis,geothermal field,early hydrocarbon filling and burial mode.The enrichment of rigid particles enhances the anti-compaction ability of sandstones.Sandstone developed in high-energy environments such as distributary channel and front mouth bar of braided-river delta is the basis for reservoir development.Low geothermal field,long-term shallow burial and early hydrocarbon filling are the important factors to protect intergranular volume;The dissolution of feldspars and early cements is the main constructive diagenesis to improve reservoir quality.The Eocene reservoir in the study area has five development modes:(1)Remote high energy + early cementation+ superimposed dissolution mode.(2)Rich in feldspar + early cementation +superimposed dissolution mode.(3)Rich in feldspar + intense dissolution mode.(4)Low geothermal field + favorable structure + intense dissolution mode.(5)Long-term shallow burial + rich in feldspar + intense dissolution mode.