Formation of the Ordovician Reservoir in Yubei Area,Tarim Basin

The Ordovician carbonate rocks in Yubei area have undergone the Caledonian and Hercynian tectonic movements. Faults mainly developed in the eastern fault-folded belts and the western slope, and rarely developed in the central platform. This resulted in differences of the reservoir characteristics of the upper member of the Yingshan Formation, and the lower member of the Yingshan Formation to the Penglaiba Formation. The reservoirs of the upper Yingshan Formation are mainly controlled by the exposure and leaching of meteoric water during the Caledonian period, forming a karst reservoir. The lower member of the Yingshan Formation and the Penglaiba Formation are mainly controlled by the burial dolomitization, silicification, and fault-related hydrothermal alteration. The distribution characteristics of oxygen isotopes, carbon isotopes and the ⁸⁷Sr/⁸⁶Sr ratios further illustrate the main controlling factors of the two different reservoir types. The cave-filling calcite in the upper Yingshan Formation has δ¹⁸O values of -12——4 ‰VPDB and δ¹³C values of -3-+1 ‰VPDB, which are lower than those of the coeval marine calcites (δ¹⁸O_VPDBvalues of -9.5——7.5‰VPDB,δ¹³C values of -1.5-+0.5‰VPDB). This indicates that the cave-filling calcite is precipitated from meteoric fluids. The saddle dolomite cements in the lower member of the Yingshan Formation and the Penglaiba Formation have δ¹⁸O values of -10.7——5.9‰ VPDB, and the ⁸⁷Sr/⁸⁶Sr ratios are higher than seawater (>0.7094). This further support that the saddle dolomites are of hydrothermal origin, and that the dissolution vugs may be related to the hydrothermal events.