Transient fluid flow in the Binbei district of the Songliao Basin, China Evidence from apatite fission track thermochronology

The Songliao Basin is famous for the Daqing Oilfield, the biggest in China. However, no economic hydrocarbon reservoir has been found in the northeastern Binbei district. Its thermal history, which is of great importance for hydrocarbon generation and migration, is studied with apatite fission track （AFT） thermochronology. Samples with depositional ages of the late Cretaceous （108-73 Ma） are analyzed. The AFT ages of the samples from reservoir rock （depositional age >76.1 Ma） fall between the late Cretaceous （72±5 Ma） and the early Eocene （41±3 Ma） period, indicating their total annealing after deposition. In contrast, two samples from the main seals of the Qingshankou （depositional age > 89.3 Ma） and the Nenjiang Formation （depositional age >73.0 Ma） are not annealed or partially annealed （AFT ages of 97±9 Ma and 70±4 Ma, respectively）. Because the maximum burial temperature （<90 °C） evidenced by low vitrinite reflectance （Ro<0.7） is not high enough to account for the AFT total annealing （110-120 °C）, the transient thermal effect arising from the syntectonic fluid flow between the late Cretaceous and the early Eocene is proposed. Transient thermal effects from fluid flow explains the indicated temperature discrepancies between the AFT thermometer and the Ro thermometer because the transient thermal effect from the fluid flow with a temperature high enough （110-120 °C） to anneal the AFT thermometer does not last long enough （104-105yrs.） for an enhancement of the Ro （minimum 106-107 yrs. under the same temperature）. This indicates that dating thermal effect from fluid flow might be a new means to reconstruct the tectonic history. It also answers why the samples from the main seals are not annealed because the seals will prohibit fluid flow and supply good thermal insulation. The large-scale fluid flow in the Binbei district calls for a new idea to direct the hydrocarbon exploration.