Temporal and spatial distribution of Ordovician-Silurian boundary black graptolitic shales on the Lower Yangtze Platform

During the end-Ordovician-early Silurian interval, two sequences of dark shales and cherts, the Wufeng and Kaochiapien formations, were deposited in Jiangsu and Anhui provinces on the Lower Yangtze Platform. Both sequences have great potential as petroleum source beds. Using previous biostratigraphic schemes for the area, combined with new material from outcrops and a borehole in the Nanjing Hills, this study recognized 11 graptolite biozones over four stages from the Dicellograptus complexus Biozone (Katian, Upper Ordovician) to the Coronograptus gregarius Biozone (early Aeronian, Silurian). Our comprehensive study of both the graptolite biostratigraphy and hydrocarbon source-bed features of a borehole in the Nanjing Hills showed that the Rhuddanian Parakidograptus acuminatus and Cystograptus vesiculosus biozones are the favorable intervals for shale gas occurrence.