Title: Geochemical characteristics and origins of the diagenetic fluids of the Permian Changxing Formation calcites in the Southeastern Sichuan Basin: Evidence from petrography, inclusions and Sr, C and O isotopes
Abstract: The Upper Permian Changxing Formation in the Southeastern Sichuan Basin exhibited complex fluid activities, and the petrographic features, fluid inclusions and stable isotopes (C, O, Sr isotopes) of calcites and host rocks from this formation were determined. The aim of the study is to determine the crystallization sequence, fluid trapping time and origins of diagenetic fluids of calcites in the Changxing Formation. The results show that four generations of calcite cements were formed in the Changxing Formation, and the corresponding four groups of primary inclusions were captured by these calcites. Group Ⅰ inclusions are characterized by high salinity (16.15–23.18 wt%, average 19.5 wt% NaCl equivalent) and low temperature (60–97 °C), and they were trapped in fine-grained calcite during from the Early Triassic to the early Middle Triassic (T1-T2). Inclusions of the other three groups, which were captured by medium-grained to ultra-coarse-grained calcites, are characterized by moderate salinity (average < 11 wt% NaCl equivalent) and relatively high temperature (101–171 °C). Their trapping times were from the Middle Triassic to Middle Jurassic (T2-J2). A study of the stable isotopes revealed the multiple sources of diagenetic fluids in the Changxing Formation. Nearly 60 percent of the samples showed relatively high 87Sr/86Sr values (0.707260–0.707608), which indicated the intrusion of strontium-rich fluid sourced from the Feixianguan Formation during the Early Triassic (T1). δ18OV-SMOW values of calcite-forming fluid (8.9–13.1‰) were influenced by fluid from the Feixianguan Formation and gas field water in the Changxing Formation. The geochemical features of the diagenetic fluids showed both authigenic and exogenetic characteristics in the Changxing Formation: (1) diagenetic fluids of the host rock are affected by the authigenic fluid (Changsingian fluid) and external fluid (strontium-rich fluid sourced from the Feixianguan Formation); and (2) diagenetic fluids of calcite are also dominated by authigenic fluid (dissolution of the host rock) and exogenetic fluid (petroleum in reservoir and strontium-rich fluid from the Feixianguan Formation). In addition, the strontium-rich fluid exerted a greater impact on the calcites than it did on the host rocks.
Publication Year: 2019
Publication Date: 2019-02-19
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 15
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