The Characteristics Of Authigenic Minerals And Their Influences On Reservoir Properties In Chang-8 Oil-bearing Formation Of Triassic In Jiyuan-Huaqing Area, Ordos Basin

The main rock types of chang-8 reservoir sandstone in Jiyuan-Huaqing area are feldspathic litharenite and lithic arkose, which account for 92.95% of total sample. Enrichment of feldspar and lithic is the main feature of petrogenetic minerals in the research area, which is characterized by low composition maturity and high textural maturity.Chang-8 oil-bearing Formation can be divided into chang-81 and chang-82 sub- oil-bearing Formation. Based on the researches by means of thin-section, scanning electron microscope, cathodoluminescence and X-ray diffraction, the contents, types and distributions of authigenic minerals of chang-8 oil-bearing Formation in Jiyuan-Huaqing area were analyzed, and then the formation mechanisms and the influences of authigenic minerals on physical properties of the reservoir during the diagensis process were investigated. The main types of authigenic minerals in the sandstone reservoir include authigenic clay minerals, authigenic carbonate minerals and authigenic siliceous minerals.Authigenic minerals of chang-8 oil-bearing Formation in Jiyuan-Huaqing area mainly contain chlorite and kaolinite. The average content of the authigenic chlorite mainly present in the form of pore-lining cement(chlorite rim) is 2.38%. In the horizontal distribution, the content of authigenic chlorite in Huaqing area is slightly higher than that in Jiyuan area, and in the vertical distribution, the content of authigenic chlorite in chang-81 is slightly higher than that of chang-82. The material sources of chlorite are from the areas with rich provenance of debris, especially with abundant intermediate/basic volcanic rock fragments and femic dark minerals. The preservation of reservoir porosity chlorite has a positive effect. The study area has a relatively high kaolinite content, with the average content of 0.78%. In scanning electron microscopy, single crystal of kaolinite is six-party plate, and aggregates show the page-like. Authigenic kaolinite fills in the original intergranular pores and secondary pores, and often has intergranular pores. Authigenic kaolinite has significant strata exclusive on the vertical distribution. Chang-7 source rocks generated organic acids on the dissolution of feldspar about chang-81. And it also has a significant regional specialization of distribution in the plane. Authigenic kaolinite is the product of dissolution of feldspar and other aluminum silicate in study area.The content of Authigenic carbonate minerals of chang-8 reservoir sandstone in Jiyuan-Huaqing area is the highest, the average content of 5.83%, accounting for 51.59% of total authigenic minerals. Main types of authigenic carbonate minerals include calcite categories (mostly iron calcite), iron dolomite, often filling in intreranular dissolution pores or intergranular dissolution pores. Chang-82 has a higher content of authigenic carbonate minerals than that of chang-81 in vertical distribution; and Jiyuan area has a slightly higher content of authigenic carbonate minerals than that in Huaqing area of distribution in the plane. Dissolution of feldspar is the most important material source of authigenic carbonate minerals in the study area.The average content of authigenic siliceous of chang-8 oil-bearing Formation in Jiyuan-Huaqing area is 1.71%, and the authigenic siliceous mainly presents in the form of quartz overgrowths and authigenic quartz crystals existing in the surface of the detrital quartz grains and authigenic carbonate minerals and on the walls of intergranular pore and in the dissolution pores. The contents of authigenic siliceous have no substantial difference in the vertical and horizontal distribution. Like the carbonate cements, dissolution of feldspar is also the most important material provenance of authigenic siliceous in Jiyuan-Huaqing area.Among the reservoir spaces of chang-8 oil-bearing Formation in Jiyuan-Huaqing area, the primary pores (mainly intergranular pores) is 57.98% of the total area percrent of pore, and the secondary porosity accounts for 40.42%. The sdudy area is in a low porosity and permeability. The average porosity in chang-81 is very close to that in chang-82, however, the average permeability in chang-81 is more than double times than that in chang-82. Change rate of porosity and permeability, particularly permeability in the horizontal (different areas) is less than that in the vertical (chang-81 and chang-82).There is a good correlation between the porosity and permeability in Jiyuan-Huaqing area, and obtaining the same permeability has a little effect on porosity in different layers or regions.Authigenic minerals in the study area on the impact of reservoir properties, early diagenetic stage of precipitation of chlorite on the pore development of positive, the positive effect of porosity greater than the negative effect; In the absence of the presence of authigenic kaolinite, the cementation of reservoir sandstones reservoir properties will lead to greater negative impact; Porosity reduction caused by the study area is mainly authigenic carbonate minerals, reservoir properties on the largest negative impact; Authigenic silica minerals in general the impact on the reservoir properties is neutral. Organic acid is in the reservoir sandstone dissolution and the formation of secondary porosity major dissolution media.