Characteristics And Controlling Factors Of High-quality Reservoir From Nantun Formation In Aonaohai Area, Hailaer Basin

Volcaniclastic rocks are a kind of transitional rocks between lavas and sedimentary rocks. As the industry oil fields have found in them, scholars , both at home and abroad pay much attention to them. This paper take Aonaohai area Hailaer Basin for example, studying on the diagenesis of Volcaniclastic rocks, the reservoir characters and their controlling factors.Routine thin sections identification, observation of casting thin sections, observation of SEM, image analysis and X-ray diffraction were adopted on the study.Through the identification of thin sections, the rock types of nanyi formation are mainly of sediment-tuff and tuff sandstone, secondary of tuff, sandstone and ignimbrite. the rock types of naner formation is mainly of sandstone, with a small amount of tuff sandstone.Diagenesis of tuff and sediment-tuff include piezocrystallization, devitrification, alteration, erosion and dissolution, clay minerals, agglutination and metasomatism. Diagenesis of tuff sandstone are the same with sandstone, include compaction, erosion and dissolution, clay minerals, agglutination and metasomatism. The differences of diagenesis between them are: the concretion of tuff behave as piezocrystallization, the sediment-tuff and tuff sandstone behave as compaction, the same with sandstone; there are much glassiness matter in tuff and sediment-tuff, so devitrification is a special type of diagenesis of them; the siliceous cement of tuff and sediment-tuff are mainly of micro-crystalline quartz, and in tuff sanstone the siliceous cement are mainly of overgrowth quartz.According to the symbiosis of authigenic minerals in Aonaohai area, determine the diagenetic paragenesis is grain-coating clay minerals→piezocrystallization and compaction→devitrification→micritization carbonates→erosion and dissolution→authigenic chlorite→authigenic micro-crystalline quartz→authigenic illite→agglutination and metasomatism of carbonate minerals→erosion and dissolution.According to division principles of diagenetic facies, there are three diagenetic facies in volcaniclastic rocks in Aonaohai area. They are piezocrystallization and compaction plus grain-coating clay minerals diagenetic facies, erosion and dissolution plus grain-coating chlorite and grain-coating micro- crystalline quartz diagenetic facies, erosion and dissolution plus agglutination and metasomatism of carbonate minerals diagenetic facies. In term of reflectance of vitrinite, pyrolysis temperature of rocks and clay minerals association, the diagenetic stage of nantun formation is mainly of middle diagenetic A stage, and local part is still in early diagenetic stage.The pore types comprise primary intergranular porosity, corrosion intergranular porosity, corrosion intragranular porosity and corrosion gap filling porosity, the last three types are the main types. Through the assess of porosity structure, pyroclastic rock are generally centralized in medium (IIB) class ,and secondly of low (IIIA) class and low (IIIB) class .Statistic analysis of properties of reservoir indicate that the reservoir in Aonaohai area is mainly of special low-porosity and special low-permeability. By the image analysis of surface and pore space, there are more than sixty percent of pores are secondary pores. The intial porosity of nantun formation is 34% according to the result of Sneider reset mode.The chief controlling factors of high-quality reservoir of volcaniclastic rocks in aonaohai area comprise: grain-coating chlorite, grain-coating illite and grain-coating micro-crystalline quartz are wide distribution in this area, they can effectively restrain the overgrowth quartz, at the mean time they can increase the stand against the compaction ,so the primary pores were protected. The tufaceous matter can be dissolved easily in activity acid water, the sedimentary facies are mainly of fan-delta front, and it has a powerful hydrodynamic force, this is advantage for the forming of secondary pores and chlorite. Besides gap filling matter has a certain bad influence to the reservoir porosity.