The thermal history of sedimentary basins is a key factor for hydrocarbon accumulation and resource assessment, and is critical in the exploration of lithospheric tectono-thermal evolution. In this paper, the Cenozoic thermal histories of nearly 200 wells and the Mesozoic thermal histories of 15 wells are modeled based on the vitrinite reflectance and apatite fission track data in Bohai Bay Basin, North China. The results show that the basin experienced Early Cretaceous and Paleogene heat flow peaks, which reveals two strong rift tectonic movements that occurred in the Cretaceous and the Paleogene in the basin, respectively. The thermal evolution history in Bohai Bay Basin can be divided into five stages including（1） the low and stable heat flow stage from the Triassic to the Jurassic, with the heat flow of 53 to 58 m W/m2;（2） the first heat flow peak from the Early Cretaceous to the middle of the Late Cretaceous, with a maximum heat flow of 81 to 87 m W/m2;（3） the first post-rift thermal subsidence stage from the middle of the Late Cretaceous to the Paleocene, with the heat flow of 65 to 74 m W/m2 at the end of the Cretaceous;（4） the second heat flow peak from the Eocene to the Oligocene, with a maximum heat flow of 81 to 88 m W/m2; and（5） the second thermal subsidence stage from the Neogene to present, with an average heat flow of 64 m W/m2.
Paleo-pressure reconstruction in sedimentary basins is one of the most important aspects of hydrocarbon accumulation research.In view of the advantages and disadvantages of the current methods for paleo-pressure research,a new method to reconstruct the paleo-pressure is presented in this paper.According to the geological background,quantitative analyses of the factors that might control overpressure were first conducted to clarify the contributions of each mechanism during different geological periods.Pressure evolution was reconstructed by fluid-compaction modelling with constraints imposed by the paleo-pressures obtained from fluid inclusions or differential stress methods.Determining the mechanisms responsible for overpressures during geological history is the basic prerequisite for paleo-pressure research.Thus,quantitative studies were conducted of the contributions of disequilibrium compaction,gas charging,oil cracking,temperature reduction,and tectonic uplift and subsidence to overpressures.Three case studies of paleo-pressure reconstruction were performed for the Sinian strata in the Sichuan Basin,Ordovician strata in the north uplift in the Tarim Basin and the Permian strata in the Sulige Gas Field in the Ordos Basin,where these three study sites are normally pressured,weakly over-pressured and abnormally low pressured at present,respectively.The new method developed in this paper is very important for the practical reconstruction of the paleopressure in marine strata and ancient strata in deep basins.
The apatite(U-Th)/He thermochronometry has been used to study the tectono-thermal evolution of mountains and sedimentary basins for over ten years.The closure temperature of helium is important for the apatite(U-Th)/He thermochronometry and has been widely studied by thermal simulation experiments.In this paper,the apatite He closure temperature was studied by establishing the evolutionary pattern between apatite He ages and apatite burial depth based on examined apatite He ages of natural samples obtained from drillholes in the Tarim basin,China.The study showed that the apatite He closure temperature of natural samples in the Tarim basin is approximately 88±5℃,higher than the result(～75℃) obtained from the thermal simulation experiments.The high He closure temperature resulted from high effective uranium concentration,long-term radiation damage accumulation,and sufficient particle radii.This study is a reevaluation of the conventional apatite He closure temperature and has a great significance in studying the uplifting events in the late period of the basin-mountain tectonic evolution,of which the uplifting time and rates can be determined accurately.
The Bohai Bay Basin is a region where part of the North China Craton has been thinned and destroyed. It has experienced two periods of crustal thinning that occurred during the Cretaceous and Paleogene, but investigations of its Mesozoic and Cenozoic lithospheric thermal structure are limited. Therefore, in this study,the distributions of mantle heat flow, crustal heat flow, and Moho temperatures during the Meso-Cenozoic are calculated based on analyses of the thermal history of the Bohai Bay Basin. The results indicate that the ratio of mantle heat flow to surface heat flow peaked during the late stages of the early Cretaceous and during the middle to late Paleogene. The corresponding mantle heat flow was more than 65% of the surface heat flow. Moho temperatures reached three peaks: 900-1100℃ in the late stages of the early Cretaceous;820-900℃ in the middle to late Paleogene; and(in the Linqing Depression, Cangxian Uplift, and Jizhong Depression) 770-810℃ during the early Neogene. These results reveal that the Bohai Bay Basin experienced significant geological change during the Cretaceous, including the transformation of lithospheric thermal structure from "cold mantle and hot crust" before the Cretaceous to "hot mantle and cold crust" after the Cretaceous. The results also indicate that the basin experienced two large-scale rifting events.Therefore, this work may provide the thermal parameters for further investigations of the geodynamic evolution of eastern China.
