Abstract
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.
Highlights
Quartz cement is the volumetrically most significant diagenetic mineral altering porosity in deep-buried sandstones, which occurs in mature, medium- to coarse-grained quartz arenites, typical of tectonically stable cratonic basins [1,2,3]
Authigenic quartz cements have been extensively studied, the pH, temperature, pressure, detrital composition, oil and gas emplacement, and chlorite coating play an important role in the precipitation of authigenic quartz, but the formation mechanism and accurate formation time of authigenic quartz are not fully understood [3,5,6,7]
The quartz cements in most sedimentary basins were formed at diagenetic temperatures ranging from 60–145 ◦ C, at burial depths greater than 2 km, and some scholars have hypothesized that temperaturecontrolled precipitation kinetics are the main control on quartz cement formation and that quartz precipitation exponentially increases with the temperature once the kinetic barriers are overcome above the cement threshold (70–80 ◦ C) [2,5,11,12,13]
Summary
Quartz cement is the volumetrically most significant diagenetic mineral altering porosity in deep-buried sandstones, which occurs in mature, medium- to coarse-grained quartz arenites, typical of tectonically stable cratonic basins [1,2,3]. Understanding the origin and distribution of quartz cement is of economic importance for reservoir quality prediction in oilfield sandstones [4]. Silica solutions are introduced into sand bodies from external sources through faults and fractures or by advection from deep underlying reservoirs or from adjacent shale units [3,7,8,9,10]. The quartz cements in most sedimentary basins were formed at diagenetic temperatures ranging from 60–145 ◦ C, at burial depths greater than 2 km, and some scholars have hypothesized that temperaturecontrolled precipitation kinetics are the main control on quartz cement formation and that quartz precipitation exponentially increases with the temperature once the kinetic barriers are overcome above the cement threshold (70–80 ◦ C) [2,5,11,12,13]
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