Diagenesis And Its Influence On The Pore Characteristics Of Wuerxun-Beier Depressions In Hailaer Basin

Hailaer Basin, a continental rift basin, was formed multiphasically during Mesozoic and Cenozoic, which is located in Hulunbeier of Inner Mongolia Autonomous Region, with E 115°30′～120°00′, N 46°00′～49°40′. Beier depression and Wuerxun depression are bigger in the basin, having been explored and being most potential for further exploration.The diagenesis of this two depressions is described and analysed in detail, and its effect on the porosity character is also discussed in this paper. It not only deepens the understanding of the diagenetic types and means, but also enriches the theories of diagenesis, and provides the foundation for the practical exploration.The petrological character is the basic of the diagenetic study. Through core description and lab polarizering microscope analysis of 38 wells in Beier and Wuerxun depressions, it is found that the rock types in Hailaer Basin transits from lava to normal sedimentary rocks with the volcanism weakening gradually. The rock is classified into six types including lava, pyroclastic lava, pyroclastic, sedimentary volcanicclastic, volcaniclastic sedimentary and normal sedimentary rocks with the sedimentary process stronger. Volcaniclastic sedimentary rock and sedimentary rock take the major part, and volcaniclastic sedimentary rock ( mainly tuff sandstone ) occupied above 40%, of which the content of quartz is mostly between 20%～40%, and debris has the same content as feldspar.Observations from polarization microscope and scanning electron microscope show that the diagenetic types of volcaniclastic-sedimentary rock in Beier and Wuerxun depressions consist of compaction and pressolution, cementation and welding, replacement, recrystallization, forming of neogenic mineral, dissolution, devitrification and so on. Among these, the cement minerals are calcite, quartz, feldspar and adelforsit and clay, and tuff, sand and shale are also considered as kinds of cements. The types of welding are consists of hot tuff, lava, and magma fragments.The minerals in replacement are calcite, dawsonite, dolomite and so on. Recrystallizations are with the types of calcite, chlorite, quatz and feldspar and so on. Devitrification is consists of recrystallization and alternation of hyaline substance, which is a comprehensive diagenesis. The typical diagenesis in this area is weldition, tuffaceous cement, and devitrification. The main sequence of diagenesis in this area is in following order: weldition–compaction- tuffaceous cementation-tuffaceous and feldspar alteration-clay minerals cementation -clay minerals alternation and chlorite cementation- pressolution and quartz cementation-calcite replacement -secondary enlargement of quartz and feldspar, recrystallization- dissolution of calcite and forming of secondary pore- dissolution of quartz and feldspar, quartz intergrowth and adelforsit cementation.The typical diagenetic paragenetic relationships are altered muscovite and chlorite, mix-layered montmorillonite and illitic, and mix-layered montmorillonite and chlorite, adelforsit and authigenetic feldspar. The stage of diagenesis of Xingan group and Tongbomiao formation in Beier depression and Wuerxun depressions is among Phase A in late diagenetic stage. Nantun and Damoguaihe formation below 1600m is among Phase A in late diagenetic stage, above 1600m is Phase B in early diagenetic stage.According to the porosity classification of clastic reservoir rocks and recognition criteria of porosity, the porosity type is divided into four, which are intragranular porosity, intergranular porosity, inter-fillings porosity, and fracture porosity, through the comprehensive study of the description , polarizering microscope and scanning electronic microscope, combined with the volcanic character in the studied area,. The common porosity types in this area are primary intragranular porosity, enlarged intergranular porosity, reduced intergranular porosity, secondary dissolution intragranular porosity, tuffaceous fillings intragranular porosity, clay cements intragranular porosity and chemical cements intragranular porosity. When the tuffaceous fillings are dissoluted, the secondary intergranular porosity is formed.Through the observation of cast thin section and the porosity character analysis and examination of typical wells, the secondary porosity zone appears to great scale among Phase A in late diagenetic stage.The main reason can be explained as followings, mass of CO2 is charged during the transition from organic matter to hydrocarbon, which makes the fluid in the porosity become acidic, and the dissolution of acidic fluid is the main reason for the formation of massive secondary porosity. At the depth of 2300m~2500m of Su 33 in Wuerxun depression, the dissolution of acidic fluid to the tuffaceous fillings, carbonate and detrital grain, had the secondary porosity developed, and the surface area percent of porosity is between 10%~15% mainly.