Hydrocarbon yield evolution characteristics and geological significance in temperature-pressure controlled simulation experiment

Abstract

A semi-open system temperature-pressure controlled hydrocarbon generation and expulsion simulation experiment was carried out to explore the hydrocarbon generation and expulsion potential and mechanism of deep environment “coal measure” source rocks in the Qaidam Basin. A WYNN-3 high temperature and high pressure (HTHP) simulator and Middle Jurassic source rocks (III type organic matter, carbonaceous mudstone, and coal, R_O is 0.67% and 0.64%, respectively) of Well DMG1 in the northern margin of the Qaidam Basin were used during the investigation. The results demonstrated that: (1) The maximum total oil yields of carbonaceous mudstone and lignite, which respectively measured at 79.38 mg/g_TOC and 37.30 mg/g_TOC, revealed a “double peaks” evolution law as a whole. (2) In the lower evolution stages (T ≤ 300 °C, P ≤ 42.0 MPa), the expelled/discharged oil yields of the two types of source rocks were lower than those of the residual oil, and the hydrocarbon expulsion efficiencies were low. However, at 400 °C (51.0 MPa), they significantly increased reaching 76.84% and 83.72%, respectively. (3) The main group components of the discharged oil were resin and asphaltene, and the component yields were generally comparable to those of liquid hydrocarbons. The yields of expelled oil group components of carbonaceous mudstone were higher than those of coal. (4) The simulated gas was primarily composed of hydrocarbon gas and non-hydrocarbon gas (CO₂, N₂), and with the elevated thermal evolution, the yields of total hydrocarbon gas/gaseous hydrocarbon increased. The two types of source rocks had maximum hydrocarbon gas yield of 116.46 mL/g_TOC and 36.85 mL/g_TOC, respectively. (5) The vitrinite reflectance (R_O) increased as temperature and pressure conditions enhanced, and it exhibited good temperature consistency. The results of this temperature-pressure controlled simulation experiment showed that temperature was still the dominant factor in the thermal evolution of organic matter, fluid pressure had a “dual” control on the formation of type III organic hydrocarbon products, and “coal measures” source rocks still had a strong potential for hydrocarbon generation in the later stages of evolution. This research provided a certain data reference for the hydrocarbon generation and expulsion law of Jurassic deep “coal measures” source rocks in the northern margin of the Qaidam Basin.