Sequence stratigraphic analysis and distribution features of Lower Carboniferous organic-rich shales in northern Guangxi

In the Early Carboniferous Epoch, the Earth's system is highly disturbed. Multiple dark shale successions developed in the Lower Carboniferous of northern Guangxi. The Lower Carboniferous Series can be divided into three third-order sequences, SQ1-SQ3, by analyzing the sequence stratigraphy from several sections. The dark shales are primarily developed in SQ1 and SQ2, corresponding to shale-bearing lithostratigraphic units, such as the Luzhai, Yingtang, and Simen Formations (Fms). Moreover, three stratigraphic properties of the Luzhai Fm and its evolutionary relationship with the Yingtang and Simen Fms are discussed, and the sequence stratigraphic frameworks in three directions are established. The distribution ranges of dark shale are restored under the sequence stratigraphic framework, and the total organic carbon (TOC) values are evaluated by sampling in dark shales. The results indicate that basin and delta facies are favorable for forming organic-rich shales, followed by shelf facies, and the quality of ramp facies shales is relatively poor. According to the statistics of the TOC values in different genetic units (systems tracts), transgression is favorable for forming organic-rich shales, and the quality of dark shales developed in the transgressive system tract of SQ1 (TST1) is the best.