scholarly journals Fluid-rock interaction and dissolution of feldspar in the Upper Triassic Xujiahe tight sandstone, western Sichuan Basin, China

2018 ◽  
Vol 10 (1) ◽  
pp. 234-249
Author(s):  
Sibing Liu ◽  
Anqing Chen ◽  
Zhongmin Shen ◽  
Zhengxiang Lv ◽  
Xiaoxing Zhang
Keyword(s):  
Upper Triassic ◽  
Formation Water ◽  
Tight Sandstone ◽  
Secondary Porosity ◽  
Rock Interaction ◽  
Rock System ◽  
Western Sichuan ◽  
Western Sichuan Basin ◽  
Feldspar Dissolution ◽  
Low K

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.

Diagenesis of tight sandstone reservoirs of Xujiahe Formation (Upper Triassic), the Xinchang Gas Field, western Sichuan Basin, China

2019 ◽  
Vol 55 (6) ◽  
pp. 4604-4624
Author(s):  
Yi‐Jiang Zhong ◽  
Ke‐Ke Huang ◽  
Li‐Ming Ye ◽  
Ye‐Fang Lan ◽  
Lei Liu
Keyword(s):  
Sichuan Basin ◽  
Upper Triassic ◽  
Tight Sandstone ◽  
Gas Field ◽  
Xujiahe Formation ◽  
Western Sichuan ◽  
Sandstone Reservoirs ◽  
Western Sichuan Basin

scholarly journals Conventionally trapped natural gas accumulations in the Jurassic tight sandstone reservoirs: A case study from the Center of the Western Sichuan Basin, SW China

2017 ◽  
Vol 36 (5) ◽  
pp. 1022-1039 ◽  
Author(s):  
Yingchun Guo ◽  
Licai Song ◽  
Xinxin Fang ◽  
Kaixun Zhang
Keyword(s):  
Sichuan Basin ◽  
Low Permeability ◽  
Gas Migration ◽  
Tight Sandstone ◽  
Long Distance ◽  
Western Sichuan ◽  
Reservoir Rocks ◽  
Gas Source ◽  
Sandstone Reservoirs ◽  
Western Sichuan Basin

Tight gas accumulations, commonly characterized by low permeability, low porosity, and complicated pore structure, are widely distributed in the Sichuan Basin. Recent exploration in the Chengdu Sag, Western Sichuan Basin has proven that Jurassic tight-sandstone reservoirs attach significant gas potential. However, long distance migration between source and reservoir intervals entangles understanding of the tight-gas accumulation mechanism. It is unclear whether producible gas in Jurassic intervals is either from “simple sweet-spots in a continuous accumulation” or “conventionally trapped accumulations in low-permeability reservoir rocks”. To identify the regionally active gas system and characterize the charging pattern, a geochemical study was performed by interpreting the gas molecular and carbon isotope compositions in Jurassic and conducting gas–source correlations as well as gas migration distance calculation with the relationship among δ13C1 vs. Ro vs. H (burial depth). Research results indicate that the Jurassic tight gases in Majing-Shifang areas are coal-derived dry gases generated by the primary cracking of kerogen. Gas/source correlation and gas migration distance calculation reveal that gases are mainly sourced from the Upper Triassic humic source rocks (T3 x5, the fifth member of the Xujiahe Formation). Gas accumulations in the Jurassic Penglaizhen Formation were formed with an original vertical migration of about 2–3 km and then a long-distance lateral migration within tight sand layers, which is verified by the decreasing δ13C1 and the general increasing iC4/ nC4 in the Penglaizhen Formation. The Jurassic tight-sandstone reservoirs in Majing-Shifang areas occur in low-porosity and low-permeability reservoir rocks in conventional lithological traps, which are not continuous-type gas accumulations or basin-centered gas systems. The faults in Majing area serve as dominant vertical conducting pathway and the relatively permeable intervals within Jurassic and microfractures play an important role in the development of the conventionally trapped natural gas accumulations.

scholarly journals The Origin of Quartz Cement in the Upper Triassic Second Member of the Xujiahe Formation Sandstones, Western Sichuan Basin, China

Water ◽  
2021 ◽  
Vol 13 (14) ◽  
pp. 1890
Author(s):  
Jie Ren ◽  
Zhengxiang Lv ◽  
Honghui Wang ◽  
Jianmeng Wu ◽  
Shunli Zhang
Keyword(s):  
Sichuan Basin ◽  
Secondary Ion Mass Spectrometry ◽  
Trace Element Composition ◽  
Upper Triassic ◽  
Al2o3 Content ◽  
Xujiahe Formation ◽  
Western Sichuan ◽  
Rock Fragments ◽  
Quartz Cement ◽  
Western Sichuan Basin

High-precision in situ δ18O values obtained using secondary ion mass spectrometry (SIMS) for μm-size quartz cement are applied to constrain the origin of the silica in the deep-buried Upper Triassic second member of Xujiahe Formation tight sandstones, western Sichuan Basin, China. Petrographic, cathodoluminescence (CL), and fluid inclusion data from the quartz cements in the Xu2 sandstones indicate three distinct, separate quartz precipitation phases (referred to as Q1, Q2, and Q3). The Q1 quartz cement was formed at temperatures of approximately 56–85 °C and attained the highest δ18O values (ranging from 18.3 to 19.05‰ Vienna Standard Mean Ocean Water (VSMOW)). The Q2 quartz cement was generated at temperatures of approximately 90–125 °C, accompanying the main phase of hydrocarbon fluid inclusions, with the highest Al2O3 content and high δ18O values (ranging from 15 to 17.99‰ VSMOW). The Q3 quartz cement was formed at temperatures of approximately 130–175 °C, with the lowest δ18O values (ranging from 12.79 to 15.47‰ VSMOW). A portion of the Q2 and Q3 quartz cement has a relatively high K2O content. The dissolution of feldspar and volcanic rock fragments was likely the most important source of silica for the Q1 quartz cement. The variations in δ18O(water) and trace element composition from the Q2 quartz cement to the Q3 quartz cement suggest that hydrocarbon emplacement and water-rock interactions greatly altered the chemistry of the pore fluid. Feldspar dissolution by organic acids, clay mineral reactions (illitization and chloritization of smectite), and pressure dissolution were the main sources of silica for the Q2 and Q3 quartz cements, while transformation of the clay minerals in the external shale unit was a limited silica source.

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