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The Discussion of Mechanism of Tight Sandstone's Formation by ShanXi Formation of the Eastern Part of the Ordos Basin


In order to search the formation mechanism of tight sandstone, the paper take Shanxi formation of the eastern part of Ordos basin as example, combined with previous research results. Based on the core observation, conventional thin section, cathodoluminescence, scanning electron microscope experiments to analyzed. By means of quartz precipitation power analysis of the diagenetic environment siliceous mineral deposition rate, inclusion temperature measurement calibrated silica precipitation from diagenetic stage. At the same time, with the aid of basin simulation, hydrocarbon source rock thermal evolution, hydrocarbon generation, we simulate the generation of organic acids and restore ancient diagenetic environment, and introduced a compact sandstone from the various stages of diagenetic process. Research has shown that the organic matter type in Shanxi Formation in eastern area of Ordos Basin is dominated by type III kerogen and coal source rock. Into the early diagenetic stage, when Ro is less than 0.5, the main types of organic fluid is humic acid, when Ro is greater than 0.5, the type III kerogen can release than type I and type II kerogen more organic acid fluid and result in coal measure source rock pore type is acidic. Organic acids dissoluted to generated SiO2, quartz overgrowth provides a source of silica sources. It concluded that the inclusions starts at uniform temperature 70~90, and focused on the 110~150, we find that quartz overgrowth began in the early diagenesis stage and extensively developed in mesogenetic stage A. Through the calculation of quartz precipitation kinetics model, it combined with the simulation of the stratigraphic burial history made the following analysis, Shanxi formation bury began to in 280~260 Ma, and the original porosity is about 34%. At the end of the early diagenetic stage, quartz sandstone due to the early compaction and siliceous cementation function is more intense, resulting in the original reservoir porosity decreased to 15.47%. Late diagenetic stage A, compaction and siliceous cementation cause reservoir porosity is further reduced to 10.94%, late diagenetic stage B, compaction and siliceous cementation cause reservoir porosity is further reduced to 8.23%, late diagenetic stage C, porosity is further reduced to 5.84%, the densification of the reservoi, the result was silimar to the average porosity of the horizon measured nearly 5.4%. Key words: Tight Sandstone, Organic acid, Quarzt overgrowth, porosity