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

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.

Characteristics of Tight Gas Reservoirs in the Xujiahe Formation in the Western Sichuan Depression: A Systematic Review

“…The predominant silica cement in tight gas reservoirs in the Xujiahe Formation is quartz cement. Feldspar dissolution, clay mineral transformation, and pressure dissolution are the primary sources of siliceous material for quartz cement [37]. The main expression of siliceous cementation is the secondary enlargement of quartz.…”

Effects of Diagenetic Alterations on Hydrocarbon Reservoirs and Water Aquifers

“…In the voluminous literature on quartz cementation in sandstone reservoirs, the source of silica needed for quartz cementation is uncertain, and the issue is strongly debated among researchers. The paper by Ren et al [14] used high-precision secondary ion mass spectrometry (SIMS) for micrometer-sized quartz cement in order to obtain oxygen isotopes. The obtained δ 18 O values coupled with petrographic, cathodoluminescence (CL), and fluid inclusion data were used to constrain the origin of the silica in quartz cement in the deep-buried Upper Triassic tight sandstones in the western Sichuan Basin, China.…”