## 5 Discussion  
The Hailar Basin is located in the western part of Northeast China. By the end of the Paleozoic, the region had completed its marine sedimentary evolution, forming a unified Paleozoic basement [9]. During the Early to Middle Jurassic, Northeast China was under a tectonic regime influenced by the flat subduction of the Paleo-Pacific Plate and the compression-extension effects caused by the closure of the Okhotsk Ocean. In the Late Jurassic, the region was primarily controlled by the Paleo-Pacific Plate, undergoing  
Figure 10: Thermal history of H7 in the Hongqi Depression. Figure 9: Thermal history of H6 in the Hongqi Depression. widespread intracontinental extension accompanied by extensive volcanic activity. The Hongqi Depression developed as a Mesozoic rifting controlled by basement faults [8]. The Tamulangou Formation was deposited during the initial rifting stage of the Hongqi Depression. The geochemical characteristics of siltstone mudstone and mudstone within the Tamulangou Formation indicate Sr/Ba values ranging from 0.25 to 0.49, suggesting a freshwater to brackish depositional environment. The Rb/Sr values range from 0.12 to 1.36, reflecting strong chemical weathering and a humid climate [14]. Under warm and humid climatic conditions, lacustrine lower aquatic organisms and higher plants thrived, leading to high paleo-productivity in the lake system, which provided abundant organic matter for hydrocarbon source rock formation. The V/Ni values range from 2.66 to 6.28, indicating that the Tamulangou Formation source rocks were deposited in a weakly oxidizing to weakly reducing environment [14]. As the depression continued to subside and the lacustrine basin deepened, the hydrocarbon source rock sequences were deposited in a brackish water environment under shore-shallow lake to semi-deep lake conditions. The weakly oxidizing to weakly reducing conditions facilitated the preservation of organic matter, resulting in source rocks with high content of organic matter of type III. The sedimentation rates of Hong 6 (Figure 11a) and Hong 7 (Figure 11b) wells were simulated using Petro-Mod software, and the sedimentation rate from the Tamulangou Formation to the present day was calculated. During the Late Jurassic to Early Cretaceous, the Hailar Basin was in a stable rifting stage, with continuous subsidence, increasing sedimentation rates, and a gradually rising geothermal gradient. During this period, the Hongqi Depression experienced stable rifting extension, characterized by rapid and large-scale subsidence and sedimentation, with a maximum sedimentation rate of 340 m/Ma (Figure 11b). Intense fault activity, accompanied by significant volcanic eruptions, led to a high geothermal regime, with a geothermal gradient of 40-65 \({}^{\circ}\)C/km and paleo-temperatures exceeding 140 \({}^{\circ}\)C (Figure 8). The heating rate reached 1.38-2.02 \({}^{\circ}\)C/Ma (Figure 8), bringing the source rocks to their maximum thermal evolution stage. This high thermal background is generally consistent with the Mesozoic thermal evolution observed in most northern Chinese basins. For instance, the Erlian Basin had a maximum Early Cretaceous paleo-geothermal gradient of 50-58 \({}^{\circ}\)C/km [55], while the Songliao Basin recorded an Early Cretaceous maximum paleo-geothermal gradient of 48-50 \({}^{\circ}\)C/km [61]. The tectonic setting of Northeast China during this time was influenced by the subduction of the Kula-Pacific Plate and the expansion and closure of the Okhotsk-Mohe-Mongolian Ocean. This resulted in an extensional tectonic environment, mantle upwelling, crustal thinning, and strong magmatic activity, all contributing to a high thermal regime [62; 63; 64; 65]. Since the Late Cretaceous, the Hongqi Depression has entered a sag development stage. Under the influence of an NW-SE compressional stress field, the strata experienced uplift and erosion, leading to a decrease in formation temperature and a weakening of hydrocarbon generation in the source rocks.