The Chagan Depression in the Yingen-Ejinaqi Basin, located at the intersection of the Paleo-Asian Ocean and the Tethys Ocean domains is an important region to gain insights on terrestrial heat flow, lithospheric thermal structure and deep geodynamic processes. Here, we compute terrestrial heat flow values in the Chagan Depression using a large set of system steady-state temperature data from four representative wells and rock thermal conductivity. We also estimate the "thermal" lithospheric thickness, mantle heat flow, ratio of mantle heat flow to surface heat flow and Moho temperature to evaluate the regional tectonic framework and deep dynamics. The results show that the heat flow in the Chagan Depression ranges from 66.5 to 69.8 mW/ m 2 , with an average value of 68.3 ± 1.2 mW/m 2. The Chagan Depression is characterized by a thin "thermal" lithosphere, high mantle heat flow, and high Moho temperature, corresponding to the lithospheric thermal structure of "cold mantle and hot crust" type. We correlate the formation of the Yingen-Ejinaqi Basin to the Early Cretaceous and Cenozoic subduction of the western Pacific Plate and the Cenozoic multiple extrusions. Our results provide new insights into the thermal structure and dynamics of the lithospheric evolution in central China.
The Chagan depression has the greatest oil and gas exploration potential in the Yingen-Ejinaqi Basin, Inner Mongolia, northern China. In this paper, source rocks are evaluated and the thermal maturity evolution of typical wells and the primary source rocks is modeled using BasinMod 1D and Basinview software provided by Platte River Associates, Inc. The results show that the Chagan depression developed three sets of source rocks, including the Baiyingebi 1 and 2 Formations and Suhongtu 1 Formation. The Baiyingebi 2 Formation has been the most important source rock in the Chagan depression, dominated by the medium to high maturity source rocks with type II kerogen. This study also reveals that the thermal maturity evolution of source rocks is controlled by the paleogeothermal fields during the Yingen Formation depositional periods when the maximum maturity was reached. The Bayingebi 1 and 2 Formations both experienced the hydrocarbon generation peak and reached the medium to over mature stage. While the Suhongtu 1 Formation has only reached the low to medium mature stage with lower hydrocarbon generation potential. In addition, the source rocks thermal maturity evolution in the West sag is superior to that in the East sag. Therefore, the Bayingebi 2 Formation should be investigated for the oil exploration, and the Wuliji fault nose structural zone and central structural zone will remain the primary targets for the next exploration.
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