Determination of trapping pressure and temperature of petroleum inclusions using PVT simulation software—a case study of Lower Ordovician carbonates from the Lunnan Low Uplift, Tarim Basin

Abstract

We report a case study carried out on Lower Ordovician reservoir carbonates of the Lunnan Low Uplift, Tarim Basin, to determine trapping pressures and temperatures of petroleum inclusions from the intersection of isochores of coeval petroleum and aqueous fluid inclusions using PVT SIM software. There are four types of petroleum inclusions from this sequence. Their trapping pressures and temperatures are: Type I 80–105 °C, 199–230 bar; Type II 135–145 °C, 235–250 bar; Type III 155–165 °C, 344–360 bar, and Type IV 220–252 °C, 345–390 bar, respectively, which are consistently higher than their homogenization temperature and saturation pressures by about 5–15 °C and 25–70 bar. To confirm the reliability of the PVT simulation approach, fluid inclusions were synthesized in calcite under controlled temperature and pressure conditions and studied using the same methods. The calculated trapping pressure and temperature of the synthetic petroleum inclusions closely match the actual pressure and temperature applied in the experiment. By reference to the burial and geothermal history of the Tarim host rocks, it was inferred that Types I, II and III petroleum inclusions represent fluids associated with three distinct phases of petroleum emplacement in the Lower Ordovician strata of this area, occurring during late Silurian to early Devonian, Cretaceous to early Tertiary, and late Tertiary, respectively. Type IV petroleum inclusions occur only in some fine calcite veins from few samples, and are believed to be related to an anomalous thermal event in this area. On the basis of the trapping pressures and trapping temperatures of the petroleum inclusions, three main phases of the petroleum-filling of the Lower Ordovician reservoirs can be delineated. Phase I and phase III petroleum events were related to petroleum migration and accumulation from the Lunnan Low Uplift or nearby area. The petroleum of the phase II petroleum event appears to have originated from Middle and Upper Ordovician source rocks in the northern slope area of the Manjiaer Depression with a long distance migration.