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Overpressure Genesis of the Tight Gas Sandstone Reserviors of the Southwestern Ordos Basin in the Upper Paleozoic

Abstract

In recent years, with the discovering of natural gas in the first member of Shanxi formation and the eighth member of Shihezi formation of the upper Paleozoic, the southwestern area of Ordos Basin becomes a new exploration area of tight sandstone gas, covering an area of 20000 km2. Now in the exploration wells, 5 commercial gas flow wells have been found. The former scholars believe that Shihezi formation and Shanxi formation of the upper Paleozoic in Ordos basin have a widespread overpressure in ancient times through the fluid-inclusion analysis, and the main mechanisms for generating the overpressure is disequilibrium compaction. Aiming at this question, we have made a discussed study, and we suppose that in this area the main reason for generating the overpressure is not disequilibrium but gas generation. The reasons for this point of view are as follows: 1. The deposition rate of the target layer is low. The disequilibrium compaction is mainly developed in the Cenozoic sedimentary basins, which had a higher deposition rate and filled with fine-grained lithology. As we calculated in this area, the target layers in the upper Paleozoic all have a low deposition rate, as well as its overlying strata. The highest deposition rate of the upper Paleozoic occurrs in the member of Shiqianfeng formation, which is about 45.2m/Ma, and the average is around 27.01m/Ma. As for the overlying strata from the Cretaceous to the Permian, the deposition rate in Triassic appears a tendency of increase, but the largest value is only 42.96 m/Ma. So we consider the low deposition rate is not enough to cause the disequilibrium compaction. 2. The logging response characteristics are not obvious. Generally, the disequilibrium compaction will make the porosity values significantly greater than the porosity values in the normal compaction curve, and make the density values smaller than that in the normal compaction curve. By making the AC-Depth curve and the Density-Depth curve of random wells in this area, we find no obviously abnormal tendency of these two logging curves in the overpressure section, and more similar to the logging response characteristics of overpressure caused by hydrocarbon generation that is summarized by Hunt. 3. According to the recent studies, overpressure has a good positive correlation with the hydrocarbon generation in this area, with the increase of hydrocarbon generation intensity, the pressure coefficient showed a trend of obvious increase.