Tectonic Evolution And Its Control On Coal Source Rocks In Northern Depression Belt Of Qiongdongnan Basin

Based on the theory of plate tectonics, this paper analyzed the structural characteristics and tectonic evolution in the northern depression, with the deep analyzing of regional tectonic background of the Qiongdongnan basin, and the northern depression belt was divided into different configurations block; At the same time, using the method of fault throw and fault activity rate and balanced section for quantitative analysis of the syndepositional faults in the study area; Finally delved into the coal accumulationg model in different sags, and combining the characteristics of the Yacheng group sedimentary facies, in the northern depression belt, summarized how the tectonic evolution controlled the Yacheng group hydrocarbon source rock. And on this basis the paper predict the distribution of Yacheng formation source rocks. Major research results as follows:Firstly, the northern depression zone mainly developed three sets of fault: NE, NW and near WE trending. According to the characteristics of the seismic profiles and the fault in the northern depression belt tectonic styles can be divided into simple half graben, half graben, graben, and strike-slip structure. Influenced by regional tectonic events, the northern depression tectonic evolution through four stages which include fault depression period, off depression period, regional thermal subsidence period, accelerated subsidence period, and than formed the structural sequence of Depression following the above faults.Secondly, in the northern depression the fractures which control the depression are F3, F5, F6.The F5 fault respectively controls the formation and evolution of the Yabei and Songxi sag.And the F6 fault controls the Songdong sag. Through the quantitative analysis of the fault activity, this paper believes that fault activity is consistent with the regional evolution, in fault depression period the fault activity is stronger, the Oligocene period is the main activities of the fault, and fault activities tend to be stable after the Oligocene.Thirdly, in the early Yacheng group of north depression, the major coal deposit system mainly braided river delta-submarine fan system, deposition in Yacheng group, the late major tidal flat-lagoon system. Among them, the braided river deltas and tidal flat environment most conducive to coal play a role. By predicting coal distribution within the different sections that the Yacheng coal is mainly developed in half-graben gentle slope with braided river deltas and tidal flats, followed by the steep slope with a fan deltas and tidal flats.Fourthly, Tectonic activities control the coal source rocks of Yacheng group gathered themselve together. Tectonic activitie in depression period and off depression period is relatively strong, and formed the main hydrocarbon source rocks in the northern depression belt. In regional thermal subsidence period, the formation of hydrocarbon source rocks is only in the local place. Coal accumulating in Yacheng group sedimentary period due to the effect of differences in tectonic activities under the control of sedimentary facies belt.Fifth, Through combining the sedimentary and tectonic analysis, this paper forecasted the distribution of coal source rocks in Yacheng formation, believed that delta system in north of Songdong sag and large loose gentle slope belt in Yanan sag, loose gentle slope belt in the east and north of Yabei sag, the northwest of Yabei sag, the transition of Songxi and Songdong sag are favorable for Yacheng group source rock distribution area.