Shiyou shiyan dizhi (Jul 2021)
Dissolution experiments and geological implications of tight sandstones in the Xujiahe Formation of Upper Triassic, Sichuan Basin
Abstract
The Upper Triassic Xujiahe Formation in the Sichuan Basin is a tight sandstone reservoir with developed dissolution pores, but its formation mechanism is still unclear. In this paper, three samples, including feldspathic lithic, lithic arkose and feldspathic quartz sandstones of the Xujiahe Formation, were taken as examples. Some reaction fluid similar to the organic acid component of the formation was prepared. Five kinds of temperature and pressure conditions were applied in the corrosion simulation experiments, and four understandings were obtained. Firstly, with the increase of temperature and pressure, the concentrations of K+ and Na+ ions in the reaction solution increased, the concentrations of Ca2+ and Mg2+ ions remained stable, but the concentration of Al3+ ions decreased significantly. Secondly, the reaction produced a small amount of quartz and a large number of kaolinite and other new minerals, and the dissolution of feldspar particles and carbonate cement produced a large number of dissolution pores, the porosity was increased and the pore structure was then improved. Thirdly, with the same temperature and pressure, the dissolution rate of feldspathic lithic sandstone was relatively higher than that of feldspathic quartz sandstone. With higher temperature and pressure (e.g. 180℃, 53 MPa), the dissolution rate of sandstone samples were significantly increased. Lastly, the results of dissolution experiments provided a reference for the reconstructing of burial sequence, diagenesis as well as pore evolution of the Xujiahe Formation and the distribution of favorable reservoirs can be predicted. The rapid dissolution of feldspar under higher temperature pressure is one of the genetic mechanisms of effective reservoir of deep clastic rock.
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