Geochemical Characteristics and Oil–Gas Geological Significance of Calcite in Ultra-Deep Carboniferous Volcanic Reservoirs in the Central Junggar Basin, Northwestern China

Calcite that fills pores, vugs, and fractures in the ultra-deep Carboniferous volcanic reservoirs of the Junggar Basin has severely destroyed pre-existing reservoir space. To clarify its fluid sources and precipitation mechanisms, we integrate petrography, fluid inclusion microthermometry, C–O–Sr isotopes, and REE geochemistry for the reservoir-destroying main-stage calcite in the Penyijingxi and Fukang sags. Fluid inclusions record homogenization temperatures of 131–220 °C and salinities of 0.2–1.6 wt% NaCl in the Penyijingxi Sag, versus 85–134 °C and 0.2–13 wt% NaCl in the Fukang Sag. The main-stage calcite shares 87Sr/86Sr ratios of 0.7038–0.7048 in both sags; its δ13C and δ18O values are −4.42‰ to −1.42‰ and −19.57‰ to −7.13‰ (Penyijingxi) versus −11.43‰ to −2.35‰ and −23.00‰ to −10.88‰ (Fukang), which together define a linear array in carbon–oxygen isotopic space. REE patterns show a positive Eu anomaly in the Penyijingxi Sag and a pronounced negative Ce anomaly in the Fukang Sag. These data indicate that the initial fluids were hydrocarbon-rich fluids from carbonate strata (Penyijingxi) and organic-acid-rich oilfield brines (Fukang), modified by water–rock interaction with volcanic wall rocks. Calcite precipitation and reservoir space loss may have been broadly coeval with Indosinian tectonic activity, which could have breached the earlier paleo-hydrocarbon reservoirs; the resulting decrease in reservoir pressure then allowed the underlying bottom (edge) water to rise and encroach upward into (or migrate into) the pre-existing paleo-hydrocarbon reservoirs.