The Study On Sedimentary Characteristics And Diagenesis Of Upper Ordovician Baota Formation In Sichuan Basin

In this paper,the Baota Formation of Upper Ordovician in Sichuan Basin is studied by using field,indoor,macro and micro methods,structural geology is used to study the tectonic background of the basin,stratigraphy is used as the basis for the division of strata,and sedimentary theory is used as the guide.The following results have been achieved:（1）.According to the percentage of developed thickness of lithology,Baota formation can be divided into bioclastic microcrystalline limestone（59.9%）,microcrystalline limestone containing debris（30.4%）,Microcrystalline cut limestone（8.8%）and recrystallized limestone（0.6%）.In terms of geochemical characteristics.Theδ¹³C value is between-0.95～2.76‰,the average value is 1.25‰,theδ¹⁸O value is between-11.42～-5.52‰,and the average value is-8.24‰;（2）.The study of diagenesis shows that compaction is the most developed type of diagenesis in the formation of Baota Formation,which widely developed in the whole region.In the process of rupture and filling,two stages of filling process are seen in"plaque"and fracture by cathodic luminescence test.Electron probe test shows that bright red is the fluid filling process involved in atmospheric fresh water:Fe²⁺/Mn²⁺<7,while dark red is the deep fluid filling process Fe²⁺/Mn²⁺>7.Among other diagenesis,recrystallization is developed in the northern section,the development degree of dissolution is weak in the whole area.According to the influence of diagenesis on reservoir,the constructive diagenesis can be divided into dissolution and rupture action,and destructive diagenesis has recrystallization,compaction and filling.According to the geochemistry data to judge the diagenesis environment and the diagenesis action type research,finally divides the study area diagenesis sequence.（3）.Through the field observation of 13 sections around and inside the basin and the indoor identification of the corresponding thin sections,the sedimentary facies marker of the Baota Formation of the Upper Ordovician were studied in detail,It can be divided into two sedimentary facies:middle gentle slope and inner gentle slope.The middle gentle slope facies are mainly distributed in the east of the basin.The inner gentle slope facies are widely developed in the basin.In the sub phase,intertank marine subfacies are mainly developed in the upper part of Baota formation,while the granular beach and tidal flat subfacies are mainly developed in the lower part of Baota formation.The paleosalt index calculated by carbon and oxygen isotopes Z>120,indicating that the salinity of diagenetic environment is high,which basically accords with the standard of normal marine carbonate rock,and the calculated value of paleotemperature is between 40.13～70.91℃.It reflects the warm and hot climate.In the trace element test,the bottom sample of Baota formation:Sr/Ba<1,and the upper and middle:Sr/Ba>1.The results show that from the beginning of the Middle Ordovician,the basin began the second sea level rise and fall cycle,to the Baota Formation sedimentary period,the original platform sediments have been submerged,transformed into gentle slope sedimentary environment;（4）The favorable distribution areas of Ordovician reservoirs in Sichuan Basin are divided into three categories,I favorable exploration area is located in the south of the basin,with an area of 3.2×10⁴km²,porosity mean 1.45%,and II favorable exploration area is located in the northern part of the basin,with an area of 1.1×10⁴km²,porosity mean 1.28%,III favorable exploration area is located in the eastern part of the basin with an area of 0.4×10⁴km²,porosity mean 1.21%.