Hydrocarbon phase limit and conversion process in the deep formation from Qikou Sag of Bohai Bay rift lacustrine basin, China

Abstract

It is an important research direction for the prediction of hydrocarbon phase in the reservoir during the deep exploration of rift basins in eastern China. The typical lacustrine crude oil of Shahejie Formation in Qikou Sag was used for oil cracking to gas simulation experiment by the gold tube in this paper. Then, the kinetic parameters obtained from the experimental data were studied during the cracking reactions and the hydrocarbon phase conversion process under geological conditions. The results showed that the activation energy of the oil cracking to gas from the BH-28 lacustrine crude oil ranged from 250 kJ/mol to 270 kJ/mol with an average of EO = 255.47 kJ/mol, and the frequency factor was about A = 1014 s−1. It is higher than that of typical marine oil EO = 246.97 kJ/mol. Using these kinetic parameters, the simulated cracking process of the lacustrine oil of Shahejie Formation was far different from that of marine oil from western China under the same geological heating rate (3 °C/Ma). And the simulated oil cracking degree in Well Niudong-1 is 73.9%, almost the same with the conversion cracking degree (60%–75%) by concentrations of 3,4-dimethyldiamantane. The kinetics of oil cracking gas could be used to investigate the oil cracking degree easily, then to get the theoretical separate oil phase depth limit. By using this model, oil in Qikou Sag, as a theoretical separate phase destruction, occurs above 5700 m depth limit (RO is about 2.0%, reservoir temperature = 209 °C, with cracking transition C of 62.5%), and condensate gas destruction occurs above 6700 m depth limit (RO is about 3.2%, reservoir temperature = 240 °C, with C to 99%) under the geological conditions from Qikou Sag. Actually, the hydrocarbon phase is affected by many factors. For example, migration and changes of temperature and pressure conditions have great influence on phase conversion behavior. The injection of gas, originated from kerogen cracking gas and oil cracking gas in deeper, could increase GOR and the real separate phase depth would reduce.

Keywords

• Deep strata
• Hydrocarbon phase
• Oil cracking gas
• Rift basin
• Kinetics