Geochemical transformation of tuffaceous materials in tight sandstone and its significance for reservoir reconstruction: A case study from the Taiyuan and Shihezi Formation in Dingbei area, Ordos Basin

Abstract

Combined with thin sections and electronic probe microanalyzer (EPMA) analysis, we studied the effect of the chemical composition of tuffaceous materials on the quality of sandstone reservoir in the Dingbei area. Analysis of 100 thin sections from five drilled wells indicates that the tuffaceous materials in sandstones from the Taiyuan and Shihezi Formation are common due to the volcanic eruptions. There are three types of tuffaceous materials in the sandstone: intermediate-basic type, intermediate-acid type, and mixed type. Seven thin sections were selected to analyze the chemical elements of tuffaceous materials by EPMA, with 3–8 test points on each thin section. The results indicate a clear decline in most major elements except for SiO2 and Al2O3. The elemental differentiation of tuffaceous sandstone is largely controlled by the activity of the diagenetic system. Under the slow-moving fluid, the active elements such as K, Na, Ca, Mg, and Fe are lost, and Si and Al are enriched. The SiO2 contents of EPMA results are up to 42%. Under the strong-flow fluid, Si and Al can be transferred to precipitate the kaolinites. In the closed diagenetic system, the contents of active elements such as Na and K are high. Dissolution or alteration of tuffaceous materials with different types under the acid fluid is one of the reasons for the generation of heterogeneous reservoirs. The intermediate-acid tuffaceous materials with high SiO2 content experience transformation without dissolution. The mixed tuffaceous materials are mainly transformed into kaolinite to form intercrystalline pores. Due to a large number of soluble components in the intermediate-basic tuffaceous materials, a large number of secondary pores are developed.