Heterogeneity and influencing factors of marine gravity flow tight sandstone under abnormally high pressure:A case study from the Miocene Huangliu Formation reservoirs in LD10 area,Yinggehai Basin,South China Sea

The characteristics of reservoir heterogeneity of the marine gravity flow tight sandstone from the Miocene Huangliu Formation under abnormally high pressure setting at LD10 area in Yinggehai Basin are studied,and the influencing factors on reservoir heterogeneity are discussed,based on modular formation dynamics test,thin sections,XRD analysis of clay minerals,scanning electron microscopy,measurement of pore throat image,porosity and permeability,and high pressure Hg injection,as well as the stimulation of burial thermal history.The aim is to elucidate characteristics of the heterogeneity and the evolution process of heterogeneity of the reservoir,and predict the favorable reservoirs distribution.(1)The heterogeneity of the reservoir is mainly controlled by the cement heterogeneity,pore throat heterogeneity,quality of the reservoir heterogeneity,and the diagenesis under an abnormally high pressure setting.(2)The differences in pore-throat structure caused by diagenetic evolution affected the intergranular material heterogeneity and the pore throat heterogeneity,and finally controlled the heterogeneity of reservoir quality.(3)Compared with the reservoir under normal pressure,abnormally high pressure restrains strength of the compaction and cementation and enhances the dissolution of the reservoir to some extent,and abnormally high pressure thus weakening the heterogeneity of the reservoir to a certain degree.The favorable reservoirs are mainly distributed in the gravity flow sand body under the strong overpressure zone in the middle and lower part of Huangliu Formation.

Sedimentary Characteristics and Model of Gravity Flow Depositional System for the First Member of Upper Miocene Huangliu Formation in Dongfang Area, Yinggehai Basin, Northwestern South China Sea

The gravity flow deposit were mainly developed in the lowstand systems tract（LST） of the first member of Upper Miocene Huangliu Formation（Ehl1） in Dongfang area, Yinggehai Basin, has become a valuable target for gas exploration and production. The gravity flow sedimentary characteristics of lithofacies associations, sedimentary texture, seismic facies and logging facies were described in detail on the basis of integrated analysis of cores, logging and seismic data. The sedimentary microfacies types composed of neritic sandbar, continental shelf mud, main channel, bifurcated or cross-cutting distributary channel, overspill, and natural levee are revealed under the constraint of high resolution sequence stratigraphic framework in the Ehl1. The gravity flow deposit system in the LST is divided into three evolution stages corresponding to periods of three parasequence sets. The gravity flow deposit was induced in the early LST, expanded rapidly in the middle LST and decreased slightly in the late LST. But its developing scale decreased sharply in the transgression systems tract（TST） and finally vanished in the highstand systems tract（HST）. This spatial evolution rule is constrained by the integrated function of sediments supply of the Vietnam Blue River in the LST, the development of local gradient change in sea floor（micro-topography, i.e., flexure slope break）, and the fall in relative sea level. On the basics of the deep study of the coupling relationship among the three main control factors, the sedimentary model is established as an optimal component of ＂source-channel-sink＂ for shallow marine turbidite submarine fan.

Predicting Turbidite Channel in Deep-Water Canyon Based on Grey Relational Analysis-Support Vector Machine Model:A Case Study of the Lingshui Depression in Qiongdongnan Basin,South China Sea

The turbidite channel of South China Sea has been highly concerned.Influenced by the complex fault and the rapid phase change of lithofacies,predicting the channel through conventional seismic attributes is not accurate enough.In response to this disadvantage,this study used a method combining grey relational analysis(GRA)and support vectormachine(SVM)and established a set of prediction technical procedures suitable for reservoirs with complex geological conditions.In the case study of the Huangliu Formation in Qiongdongnan Basin,South China Sea,this study first dimensionalized the conventional seismic attributes of Gas Layer Group I and then used the GRA method to obtain the main relational factors.A higher relational degree indicates a higher probability of responding to the attributes of the turbidite channel.This study then accumulated the optimized attributes with the highest relational factors to obtain a first-order accumulated sequence,which was used as the input training sample of the SVM model,thus successfully constructing the SVM turbidite channel model.Drilling results prove that the GRA-SVMmethod has a high drilling coincidence rate.Utilizing the core and logging data and taking full use of the advantages of seismic inversion in predicting the sand boundary of water channels,this study divides the sedimentary microfacies of the Huangliu Formation in the Lingshui 17-2 Gas Field.This comprehensive study has shown that the GRA-SVM method has high accuracy for predicting turbidite channels and can be used as a superior turbidite channel prediction method under complex geological conditions.

The effect of thermal fluid derived from mud diapir on sandstone reservoirs in the Yinggehai Basin, South China Sea

The underground thermal fluid is one of the significant factors controlling the formation and quality of reservoirs.The Huangliu Formation(N_(1)h)in YF1 area of the central depression belt in the Yinggehai Basin,South China Sea,is characterized by intense thermal fluid activities related to mud diapir and large-scale shallow-water gravity flow deposits.The multi-episodic invasion of high-temperature and CO_(2)-rich thermal fluid into the formation induces complex water-rock reaction during diagenetic pro-cess,providing a preferable investigation opportunity for revealing how thermal fluid affects reservoir quality.This study characterizes the reservoirs in the formation through core and thin section analyses as well as physical property test.The reservoirs are dominated by fine-grained sandstone characterized by medium porosity(15.2-21.3%)and lower permeability(0.56-15.75mD).Based on an analysis of casting thin section,cathode luminescence(CL),scanning electron microscope(SEM),carbon and oxygen isotope,inclusion test,and electron-probe microanalysis(EPMA),we systematically investigate the diagenetic patterns and pore evolution process for the reservoirs in the formation.The episodic invasion of thermal fluid occurred approximately 0.4 Ma ago plays an important role in controlling reservoir development:The CO_(2)-rich formation water induces massive late-stage dissolution,resulting in a higher proportion of dissolved pores(38.7-46.4%),which improves the porosity of reservoirs at a depth of 2600-3100 m.Nevertheless,the late dissolution together with carbonate cementation occurred in closed diagenetic system blocks most seepage channels.Furthermore,the relatively high level clay mineral transformation in the YF1 area leads to a higher content of authigenic illite(44-62%)in the formation.Massive authigenic illite severely blocks the pore throats in fine sandstone,reducing permeability.This study offers an insight to the understanding of mud diapir-derived thermal fluid affecting and controlling the quality of reservoirs in some areas of the Yinggehai Basin.