Formation Environment and Development Models for the Lower Cambrian Source Rocks of the Southern North China Plate,...

In this article, the lower Cambrian marine shale of the southern part of the Ordos basin is taken as an example to investigate the formation environment and development models of source rocks with high organic abundance in the ancient strata of the southern North China plate. The lower Cambrian shale source rocks had a thickness of 10-80 m, with high total organic carbon contents (an average of 2.68% and maximum of 9.39%) and favorable organic matter types-mainly sapropelinite macerals-and yet low S1, S2, and hydrogen index, which indicate that the source rocks have relatively high thermal maturity. Therefore, it is demonstrated that marine shale of the lower Cambrian proves to be excellent gas source rocks at the southern part of the Ordos basin and are presently distributed mainly in the south part of the North China plate. During the early Cambrian period, paleoproductivity was relatively high, as indicated by the rich C₂₇ sterane contents, high sterane/hopane ratios, and high tricyclic terpane/hopane ratios in biomarkers and high P and Ba values in trace elements. The tectonic setting promoted the formation of highly abundant source rocks with favorable preservation conditions, as suggested by the relatively high V/Cr ratios and low Zr/Rb and Pr/Ph ratios. Thus, the paleoproductivity and preservation conditions were the main factors controlling the occurrence of highly abundant source rocks. The formation of highly abundant source rocks was under the joint effects of the tectonic setting and the undercompensation environments of the passive continental marginal slope of the Paleo-Qinling Ocean and deepwater bays. The paleo-geomorphologic troughs and/or fault depressions (because of tectonic movement) and deepwater bay environments together influenced the development scale of highly abundant source rocks, whereas the deepwater shelf environment of the continental marginal slope ensured the high quality of the source rocks.