Formation time and fluid source of calcite veins of Triassic Qinglong Formation in Huangqiao area, Subei Basin

Calcite veins in the reservoir record crucial information about fluid activity, which has important indicator for hydrocarbon accumulation process. A multi approaches consisting of thin section observation, cathodoluminescence, U-Pb isotope dating, REE, C, O and Sr isotopes of calcite veins from the Triassic Qinglong carbonate reservoirs in Huangqiao area of Subei Basin is used to analyze the source of calcite vein forming fluid. Four stages of calcite veins were successively formed at 115.30±0.42 Ma, 97.03±0.43 Ma, 85.29±0.25 Ma and 45.5±19.0 Ma, respectively. The first and second stage of calcite veins were derived from deep hydrothermal fluids and the mixed fluids of atmospheric fresh water and seawater respectively. The vein-forming fluid of the third stage of calcite veins was the mixed fluid of the same stratum water and deep shell source hydrothermal fluid. The fourth stage of calcite veins was derived from the same layer water mixed with the deep mantle-derived hydrothermal fluid. The formation time of the four stages of the Triassic Qinglong Formation carbonate reservoir in the Huangqiao area of Subei Basin corresponds to the sub-tectonic activities at different periods, which implies that fluid evolution was controlled by multi stages of tectonic movements. Importantly, three stages of deep hydrothermal fluid in the reservoir is the result of deep fault opening caused by tectonic activities, indicating multi-stages of oil and gas charges in the Qinglong Formation of the Triassic in the Huangqiao area.