Carbon Isotope Kinetics Effect on the Natural Gas Accumulation: A Case Study of the Baimiao Area, Dongpu Depression, North China

Abstract

Carbon isotope kinetics is developed from hydrocarbon generation kinetics for studying the natural gas accumulation process. On the basis of the kerogen pyrolysis experiments conducted in a closed gold tube autoclave system, the hydrocarbon generation kinetic parameters and carbon isotope kinetic parameters of the Shahejie Formation source rocks in the Qianliyuan sag are obtained by the Kinetics software and Excel spreadsheet software. In combination with the thermal and burial histories of the Qianliyuan sag, the long-term cumulative curve and periodic cumulative curve starting from 28.6 Ma under geological conditions are established. In combination with the geochemical parameters of the Carboniferous–Permian gas source rocks, the gas source and natural gas accumulation process in the Baimiao area are revealed. The results show that (1) the oil–gas reservoirs in the Baimiao area are formed earlier, and their accumulation time occurred from 28.6 to 27–25.8 Ma, and (2) the natural gas accumulation process in the Baimiao area was revealed to progress from the Shahejie 1 Formation depositional period to the Dongying Formation depositional period, the coal-type gas generated by the Carboniferous–Permian source rocks was accumulated in the Paleogene traps along the Lanliao fault and its derived fault systems, and these traps were formed during the Shahejie 2 Formation depositional period. Approximately 28.6 Ma, some oil-type gas generated by the Paleogene source rocks in the Qianliyuan sag accumulated in the traps formed during the early stage of the structural wing. Other oil-type gas migrated to the central part of the structure along the Duzhai fault and along a series of secondary faults in the Baimiao area, which then mixed with coal-type gas that accumulated in the early stage. At the end of the Dongying Formation depositional period, hydrocarbon generation ceased and the oil-type gas accumulation process was blocked from 27 to 25.8 Ma, which terminated the coal-type gas accumulation process. This work may provide some insights into the natural gas accumulation process and distribution in the Dongpu depression.