Lower Cretaceous Lithology And Logging Responses In Wuerxun Depression In Hailaer Basin

Hailar basin is one of the key areas in the surrounding region of Daqing, and Wuerxun depression is the biggest in exploration area and higher in exploration degree in the basin. Through the core description and lab polarizering microscope analysis of 32 wells in Wuerxun depression, it is found that the rocks can be classified into five categories, including lava, pyroclastic rock, sedimentary pyroclastic rock, volcaniclastic sedimentary rock and normal sedimentary rock, showing the volcanism weaker and the sedimentary process stronger gradually. Normal sedimentary rock take a major part, and volcaniclastic sedimentary rock, pyroclastic rock, lava and sedimentary pyroclastic rock are in a shortening order.The diagenetic types of Nantun and Tongbomiao formations are obtained through lab polarizering microscope analysis. They include not only compaction and pressolution, cementation, dissolution, forming of neogenic mineral and recrystallization in normal sedimentary rock, but also tuffaceous cementation, welding, devitrification and alteration related to Volcanism. Dawsonite cementation is a special carbonate cementation in this area. There is also laumontite cementation, which would be products of special diagenesis rather than signs of beginning of metamorphism. There are three types of welding textures identified as weldings of magma or lava, magma fragment and hot ash. Devitrification and recrystallization were combined, showing special textures, sush as a complex cementation of quartzs and siliceous matter resulted from the devitrification of rhyolitic glass and recrystallization.The main order succession of diagenesis in this area is following: weldition, compaction, tuffaceous cementation-tuffaceous, feldspar and rock debris alteration-clay minerals cementation, initial quartz cementation-clay minerals alternation and chlorite cementation-pressure solution and siliceous cementation-calcite replacement-secondary enlargement of quartz and feldspar, recrystallization-dissolution of calcite and forming of secondary pore-strong dissolution of quartz and feldspar-appearance of autogenic feldspars-laumontite and epidote cementation. The typical diagenetic paragenetic relationships are altered muscovite and chlorite, quartz dissolution and crystallization montmorillonite, illitic and chlorite, laumontite, tuffaceous and autogenic feldspars. Combined with vitrinite reflectance, Nantun formation above 1600m is in early diagenetic B stage, while below 1600m is in late diagenetic A stage. Tongbomiao formation is also in late diagenetic A stage.Through the observation of cast thin section, it was found that the porosity type in the studied area can be divided into three types, which are primary porosity, secondary porosity and mix porosity. Secondary porosity is the major part in the three types. In secondary porosity, dissolution intragranular porosity and dissolution intergranular porosity are the major part, and dissolution inter-fillings porosity was the minor part. The diagenesis affected the porosity. Compaction and cementation may decrease the porosity and dissolution increase the porosity. The reservoirs can be divided into four categories according to reservoir quality index and permeability. Porosity structure is analyzed by the cast thin section and capillary pressure curve, and the average pore-throat ratio is calculated. The physical properties of reservoirs tend to decline gradually from typeâ… toâ…£, with decreasing in pore diameter, reducing in number and diameter of throat, and increasing in average pore-throat ratio. The lithology of reservoirs is added, showing that almost all the lithologies distribute in each kind of the reservoir. The lithology could not reflect the physical properties of reservoir completely. Inâ… a ndâ…¡r eservoirs, volcaniclastic sedimentary rock has the highest abundance, followed by normal sedimentary rock, pyroclastic and pyroclastic rock. In volcaniclastic sedimentary and normal sedimentary rocks, the coarse has better physical properties than the fine. The tuffaceous gravel sandstone and the conglomerate are maximum. The statistics suggests that the normal sedimentary and volcaniclastic sedimentary rocks are good in oil bearing, and the coarse has high grade and thickness in oil show.AC-TU, LLD-SP, GM-SP, NPHI-DEN, AC-K, and M-N crossplot are compiled by logging curves to identify lithology. Every crossplot has a certain ability to figure out the lithology, but the effect is not so good in identification of tuffaceous sandstone. The relationship between DEN, AC and compaction is discussed. DEN has increased and AC has decreased with compaction strengthing.