Open Geosciences (Jun 2018)
Fluid-rock interaction and dissolution of feldspar in the Upper Triassic Xujiahe tight sandstone, western Sichuan Basin, China
Abstract
Secondary porosity in the Upper Triassic Xujiahe tight sandstone of the western Sichuan Basin is mainly the product of feldspar dissolution. In the Xu-4 Member, the upper reservoir of the Xujiahe Formation, feldspars are dissolved to a significant extent and observations indicate that nearly all feldspars have been dissolved completely, with only 1.73% content of feldspar remaining. In the Xu-2 Member, the lower reservoir, feldspars are well preserved; the current content of feldspar is 12.54% on average, and the secondary porosity derived from feldspar dissolution is less than 1%. Kaolinite occurs almost exclusively in the Xu-4, but it is nearly absent in the Xu-2. The K+ content in the Xu-2 is 3.3 times higher than that in Xu-4. The K+/H+ ratio in the Xu-2 is also higher than that in the Xu-4. These differences between the two reservoirs can be attributed to their distinguishing fluid-rock systems. The low K+ content and relatively high δ18O in the Xu-4 formation water are the result of intensive fluid-rock interaction in an open fluid-rock system. The upper Xu-4 is close to the overlying coal-measures of the Xu-5 from which organic acid flowed into the Xu-4. Meanwhile, K+ contained in sandstone migrated out to the mudstones. The resulting low K+/H+ ratio in the formation water of the Xu-4 was responsible for almost all the feldspar dissolution and kaolinite formation. In contrast, due to the relatively closed fluid-rock system in the Xu-2, K+ did not migrate into adjacent rocks and acidic fluids did not invade, which led to K+-rich formation waters maintaining a high K+/H+ ratio. Hence, K-feldspar was well preserved and kaolinite was completely transformed into illite. Therefore, in contrast to the Xu-2 tight sandstone, the Xu-4 sandstone has relatively higher secondary porosity, which favours the formation of better quality reservoirs.
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