Biostratigraphic correlation of graptolites from Late Ordovician to Early Silurian on the southwestern margin of the Yangtze region

Based on the data of graptolite biozonations of section outcrops and drilling cores in the southwestern margin of Yangtze region, the isochronous stratigraphic framework of the Wufeng-Longmaxi Formation black shales is established. By comparing the horizontal and verticalchanges of black shales' TOC, the thickness of depositions, and the thickness of high-quality shales, the relationship between the graptolite biozones and the depositional environments of high-quality shales is defined. The results show that the high-quality shales in Butuo, Jinyang, Ganluo, and Well Yunyongdi 3 are below graptolite biozones of WF2-LM4 while strata of LM5-LM6 are obviously thickened. In Daguan-Suijiang area, Well Yunyongdi 2, Well Xindi 2, and Well Suijiang B, the graptolite biozones and sedimentary environment are stable, but their exploration effects are obviously different. The highest level of high-quality shale is of the LM4 graptolite biozone, which may extend to the LM6 graptolite biozone gradually. Beside the Kangdian Oldland, high-quality Wufeng-Longmaxi black shales range from WF2 to LM4 graptolite biozones while away from the the Kangdian Oldland the black shales possess a higher horizon. High-quality shale of WF2-LM5 graptolite biozones well developed in Dajingping and Jiaodingshan areas. The thickness of the strata in the Jiaodingshan area of Hanyuan is obviously thinner, but it becomes thicker in the Dajingping area of Tianquan, indicating a special geological setting. This research results provide a basis for the favorable areas of shale gas exploration in the southwestern margin of the Yangtze region.