Formation and significance of the oil shales from the North Qiantang Basin

The thick-bedded successions of the Late Jurassic-Early Cretaceous oil shales and gypsum have been recently found in the Changsheshan area,North Qiantang Basin.These oil shales,together with those recognized in the Changliangshan and Shengli River areas,constitute the largest Mesozoic marine oil shale zone,North Qiantang oil shale zone in China.Sedimentary facies and palaeogeographic reconstruction have disclosed that during the Late Jurassic-Early Cretaceous,the North Qiantang Basin was once a giant and relatively closed gulf in which the sedimentary facies associations were developed,including the fluvial-delta,gulf(tidal flat-lagoon) and shallow sea-continental shelf facies associations.The palaeoclimatic approaches have revealed that the Shengli River area once lay in the moist tropic-subtropic zones during the deposition of the oil shales.The injection recharge of plentiful fresh water and high productivity are considered as the controlling factors for the formation of the oil shales.The oil shales were dominantly aligned around the restricted gulf-lagoon areas,whereas the gypsum beds occurred in the closed environments under the xerothermic climatic conditions.The Late Jurassic-Early Cretaceous oil shales and their overlying gypsum beds are interpreted as the important source rocks and regional seal rocks in the North Qiantang Basin.The discovery of the oil shale-gypsum sedimentary associations is of important implications for the improvement of our knowledge of the hydrocarbon accumulation and preservation in the whole Qiangtang Basin.