Research On The Characteristics And Diagenesis Of Sandstone In The Upper Paleozoic Reservoir In Ordos Basin

Using theories of sedimentology and geochemistry, the petrologic characteristics, composition and formation mechanism of authigenic minerals, type and evolution of pore, digenesis and digenetic evolution of the upper Paleozoic sandstones reservoir of Ordos Basin were systematically studied through impregnated thin sections, cathodoluminescence, scanning electron microscopy, whole rock and clay mineral X-ray diffraction, isotope, and inclusion temperature.The upper Paleozoic sandstone reservoirs in Ordos basin presents low porosity of average porosity of 6.33% and permeability of average permeability 1.559×10^-3μm² derived from sandstone cores. The compositional maturity (Q1 / (F1 + R1) index) and feldspar content of sandstone reservoirs increase and reduce respectively which shows good complementarity from upper Shiqianfeng Formation to lower Taiyuan and Benxi Formations, primarily because of the diagenetic process of acid fluid dissolution of feldspars and other soluble components and the dissolved feldspar is mainly K-feldspar under microscopic and X diffraction analysis. The porosity of upper Paleozoic sandstone reservoirs are mainly dissolution porosity of feldspar, mold porosity and intragranular dissolved porosity. The porosity of thin sections shows a decreasing trend from the upper Shiqianfeng Formation to the lower Shangshihezi Formation, and the porosity constitute mainly primary porosity and pore reduction is mainly caused by the compaction and relatively small by the formation of coal. The porosity of thin sections increases from Xiashihezi Formation to Taiyuan Formation and secondary porosity is much larger than the original porosity in total porosity, especially in Taiyuan Formation, their porosity is almost entirely composed of secondary porosity, but the porosity of 0 to 1% account for 21.35% of the total porosity, while the average permeability is the Taiyuan Formation is the lowest of the Upper Paleozoike, only 0.35×10^-3μm², which makes the connectivity and reservoir quality of the Taiyuan formation poorer.The different Formation of the upper Paleozoic sandstone reservoirs has undergone the different compaction and cementation. The compaction of the Shihezi Formations and Shiqianfeng Formations is stronger although at relatively shallow burial depth, while the cementation of the low Shihezi, Shanxi and Taiyuan Formations are weaker, because the earlier cementation occurred to makes intergranular pore volume of sandstones higher than the overlying strata.The average content of authigenic minerals in of the upper Paleozoic of Ordos Basin is 10.8%. The authigenic minerals of sandstones are made of high silica cement and kaolinite cements and low calcite cement. The composition of authigenic minerals have obvious specificity that the Shiqianfeng Formation and Shihezi Formation show higher levels of chlorite while the Xiashihezi, Shanxi and Benxi Formations of higher authigenic kaolinite and silica cement. The lower Shanxi and Taiyuan Formation have high dolomite content and the former is also of a high authigenic illite content.The kaolinite content of Shiqianfeng to Shanxi Formation shows a growing trend, and the feldspar content is on the contrary. The trends of authigenic kaolinite display the Benxi– Taiyuan Formation cycles and the Shanxi - Shiqianfeng Formation cycles, which represents two types of changing diagenetic fluids. Silica cement is often accompanied with kaolinite, which is the product of feldspar dissolution, sometimes, the part of the precipitation of silica cement are related to alteration of volcanic materials on the same period. The siliceous cement homogenization temperature of inclusions of quartz in sandstones vary mainly between 90℃ 100℃, which indicate that precipitation temperature of authigenic quartz is lower and that precipitation time is earlier. The sandstones of Shiqianfeng and Shihezi Formations show high tuffaceous materials, which is important volcanic source of cement. The average content of calcite cement, with a very negativeδ(13)^C value and high homogenization temperature (150℃or so), is 2%, which is affected by organic acids during the formation. The precipitation of calcite cement occured later, and calcite often occupy space of feldspar dissolution.δ(13)^C values of dolomite cements are significantly higher than calcite, and close to the seawater value, and so do theδ(18)^O. the calcite precipitation temperature is lower and earlier than most of the precipitation of calcite and authigenic quartz, while dolomite cements donot occupy feldspar solution space. Kaolinite shows a positive impact on reservoir, silica and dolomite cements remain neutral in the evolution of the reservoir. However the calcite cements are the negative influence on reservoir.Dissolution of the upper Paleozoic of Ordos Basin was the main diagenesis and the dissolution fluid was mainly from the acid fluid of coal formation. The dissolving time started from the same period of the early burial diagenesis to deep burial of the late diagenetic stage which created the unique diagenetic conditions.Based on the study of the characteristics of sandstone reservoir, the formation mechanism of authigenic mineral, the impact of diagenesis on the reservoir, diagenetic history and diagenetic sequences and porosity evolution model were summarized for the different of the Paleozoic sandstone reservoir in Ordos Basin, which provides theoretical guidance for the reservoir quality prediction.