Abstract
A fluid inclusion petrographic and microthermometric study was performed on the sandstones gathered from the Yanchang Formation, Jiyuan area of the Ordos Basin. Four types of fluid inclusions in quartz can be recognized based on the location they entrapped. The petrographic characteristics indicate that fluid inclusions in quartz overgrowth and quartz fissuring-I were trapped earlier than that in quartz fissuring-IIa and fissuring-IIb. The homogenization temperature values of the earlier fluid inclusions aggregate around 80 to 90°C; exclusively, it is slightly higher in Chang 6 member, which approaches 95°C. The later fluid inclusions demonstrate high homogenization temperatures, which range from 100 to 115°C, and the temperatures are slightly higher in Chang 9 member. The calculated salinities show differences between each member, including their regression characteristics with burial depth. Combining with the vitrinite reflection data, the sequence and parameters of fluid inclusions indicate that the thermal history of the Yanchang formation mostly relied on burial. Salinity changes were associated with fluid-rock interaction or fluid interruption. Hydrocarbon contained fluid inclusions imply that hydrocarbon generation and migration occurred in the Early Cretaceous. The occurrence of late fluid inclusions implied that quartz cement is a reservoir porosity-loose factor.
Highlights
Fluid inclusions are micro-scale samples of fluids, which were trapped during the evolution of sedimentary basins, and can be used to identify the composition of fluids associated with diagenetic and post-diagenetic processes (e.g. Conliffe et al, 2017)
Fluid inclusions could be subdivided into four types based on the location that they were entrapped in the quartz and the sequence that they were entrapped
In the earlier quartz cementation, the fluid inclusion is developed with (i) quartz overgrowth (Figure 2) and (ii) quartz fissuring (Figure 3); and in the later quartz cementation, the fluid inclusions are associated with the fissuring mainly, including that (iii) developed inside of quartz (Figure 4(a) and (b)), and (iv) traversed the whole quartz grains including the overgrowth part (Figure 4(c) and (d))
Summary
Fluid inclusions are micro-scale samples of fluids, which were trapped during the evolution of sedimentary basins, and can be used to identify the composition of fluids associated with diagenetic and post-diagenetic processes (e.g. Conliffe et al, 2017). Fluid inclusions are micro-scale samples of fluids, which were trapped during the evolution of sedimentary basins, and can be used to identify the composition of fluids associated with diagenetic and post-diagenetic processes Conliffe et al, 2017). Fluid inclusions could be revised depending on the geological events they undergone after entrapment. The confined burial processes preferentially keep them original. These undisturbed fluid inclusions held the pore fluid where the inclusions were trapped throughout the geological history steadily. The geological information hidden in these undisturbed fluid inclusions can be used to reveal the geothermal and geochemical environments when the entrapment occurring The evolution processes of sedimentary rocks can be re-established by systematical research of fluid inclusions
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