Using clumped isotopes to determine the origin of the Middle Permian Qixia Formation dolostone, NW Sichuan Basin, China

Abstract

The dolostone reservoir in the Middle Permain Qixia Formation (P₂q) is a new region of natural gas exploration in the Sichuan Basin. The formation temperature, the δ¹⁸O of the dolomitization fluid, and the origins of the two types of dolostone in the northwestern Sichuan Basin are discussed, using new carbonate clumped isotope data and conventional petrology and geochemical analyses. The massive dolostone (MD) is composed of coarse anhedral dolomite, whereas the leopard dolostone (LD) is composed of euhedral and subhedral dolomite. A formation temperature of 56.7 ± 3.8 °C for the MD was determined using carbonate clumped isotopes. This is significantly higher than the burial temperature (about 45 °C) with coeval geothermal gradient. The δ¹⁸O_(VSMOW) of the dolomitization fluid is −4.0 ± 0.7‰, which is about 2 permil lower than that of the Middle Permian seawater. The Fe and Mn concentrations, ∑REE abundances, δ¹³C values, and ⁸⁷Sr/⁸⁶Sr ratios of the MD are higher, lower, lighter, and heavier, respectively, than those of the coexisting limestone. These findings indicate that the dolomitization fluid for MD was meteoric water modified-Middle Permian saewater. Thus, we conclude that the MD was formed by meteoric-water-modified seawater replacement in shallow burial condition, but it experienced abnormally high temperatures, resulting from a basaltic eruption at the end of the Middle Permian. A formation temperature of 65–72 °C was determined for the LD using clumped isotopes. This is also significantly higher than the burial temperature with coeval geothermal gradient. The δ¹⁸O_(VSMOW) of the dolomitization fluid is approximately +2.0‰, which is significantly higher than that of the Middle Permian seawater (−2.2‰ ~ −1.0‰). The trace element abundances of the LD are higher than those of the coexisting limestone, and the REE patterns of the LD are similar to that of hydrothermal dolostone in the southwestern part of the Sichuan Basin. Thus, we conclude that the LD was formed by hydrothermal replacement at shallow depths. During the hydrothermal replacement process, the pre-existing MD was also altered by the hydrothermal fluid and dissolution vugs were produced, which improved the reservoir's porosity and permeability.

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