Patterns of Clay Minerals Transformation in Clay Gouge, with Examples from Revers Fault Rocks in Devonina Niqiuhe Formation in The Dayangshu Basin

The role of authigenic clay growth in clay gouge is increasingly recognized as a key to understanding the mechanics of berittle faulting and fault zone processes,including creep and seismogenesis,and providing new insights into the ongoing debate about the frictional strength of brittle fault(Haines and van der Pluijm,2012).However,neither the conditions nor the processes which

Clay mineral formation and transformation in non-marine environments and implications for Early Cretaceous palaeoclimatic evolution:The Weald Basin,Southeast England

Analyses of clay minerals within the Early Cretaceous Weald Basin,Southeast England reveal kaolinite,illite and chlorite as the main detrital clay minerals while glauconite and smectite are subordinates.A kaolinite-rich assemblage which characterized the sand-dominated Ashdown and Tunbridge Wells Sand formations and an illite-dominated assemblage associated mostly with the Wadhurst Clay and Weald Clay formations are recognized.Kaolinite was enriched in the Ashdown and Tunbridge Wells Sand formations during warm and humid climate with high precipitation that encouraged chemical weathering and leaching,while cold and dry conditions favoured the concentration of illite in the Wadhurst Clay and Weald Clay formations.Rainfall patterns associated with warm climate were drastically reduced during the drier climatic conditions.Most clay minerals are detrital in origin,with chlorite being more prominent than previously recognized.Contrary to previous studies and assumptions,this study revealed that authigenic clay minerals are present in the Hastings Beds,with vermiform and mica-replacive kaolinite being the most common,consistent with humid depositional environments.Isolated authigenic illite is also present,along with a chloritized grain,providing evidence for mesodiagenesis.The absence of dickite and occurrence of kaolinite,suggest that authigenic illite formed in relatively shallow burial conditions,indicating a maximum burial depth of2500 m-3000 m,about 1000 m deeper than previous estimates of 1500 m-2000 m.Authigenic clay minerals are absent in the Weald Clay Formation possibly because of hindered flow of meteoric water and limited growth space for authigenic minerals.This study is significant in:1)reinforcing multiple methods to facilitate a robust and balanced knowledge of formation and transformation of clay minerals;2)investigating detrital and authigenic clay mineral assemblages when assessing the palaeoenvironments of sedimentary basins.

Geochemistry of Pore Water and Associated Diagenetic Reactions in the Diapiric Area of Yinggehai Basin, Northwestern South China Sea

This study examined the geochemical features of pore water in the diapiric area of the Yinggehai Basin, northwestern South China Sea, and illuminated the origin and evolution of basin fluids. Pore water with low salinity occurs in marine sediments in the diapiric area even without meteoric water infiltration. The presence of low-salinity water within deep, overpressured compartments is assumed to be due to smectite-illite transformation. Howerver, in shallow portions（less than 2 000 m） of diapiric areas with normal pressure, pore water has a much wider variation and much lower salinity than that in the overpressured intervals. Its total dissolved solid（TDS） content is ~5 336 to 35 939 mg/L. Moreover, smectite and chlorite content sharply decreases as kaolinite and illite content increase in shallower intervals. The geochemical variation of pore water in diapiric structures indicates the expulsion of low-salinity, overpressured fluids along vertical faults. Strong injection of hot fluids from deep overpressured sediments results in rapid clay mineral transformation in shallow reservoirs. Consequently, fluid mixing due to fluid expulsion from deeper overpressured deposits leads to variation in salinity and ionic composition as well as some diagenetic reactions. This includes transformation of clay minerals caused by the higher temperatur of deeper hot fluids, e.g., the transfromation of smectite to illite and chlorite to kaolinite. Therefore, variations in salinity and ionic compositions in various pressured systems provide a clue to flow pathways and associated diagenetic reactions.

Shale pore characteristics of Shahejie Formation: Implication for pore evolution of shale oil reservoirs in Dongying sag, north China

The pore characteristics of shale reservoirs in the lower submember of Member 3 to upper submember of Member 4 of Shahejie Formation in Dongying sag are analyzed,influences of mineral content and organic matter content on porosity and pore size are also investigated,and through the diagenetic thermal simulation experiment,the main pore evolution is further discussed.The results show that the pore structure of shale reservoirs is complex,the micron-nanometer pores can storage liquid hydrocarbons,and the free-phase crude oil is mainly distributed in intergranular dissolution pores of calcite,recrystallized intergranular pores,intergranular shrinkage fractures of clay mineral which have large pore size.Framework minerals and organic matter content directly influence porosity and pore size of shale reservoirs,relationship between porosity and content of felsic mineral as well as content of organic matter content is linear and positive,while relationship between content of carbonated mineral is negative.At the buried depth from 2500 to 3500 m,concentration of organic acid from hydrocarbon generation and expulsion of organic matter,increasing range of pressure coefficient,are well corresponding to highporosity intervals;pore formation in shale oil reservoirs are almost controlled by diagenetic evolution of clay minerals;framework storage spaces formed by carbonate grain crystals as well as intergranular and intergranular dissolution pores of carbonate increases porosity of shale oil reservoirs;local increase of porosity at the depth of 3500e3800 m is mainly caused by coupling of hydrocarbon-generating overpressure and dissolution,and size,distribution and connectivity of pores are enhanced obviously.