Lithofacies and paleogeographic study on late Cambrian hydrocarbon source rocks in Lower Yangtze region

Careful lithofacies and palaeogeographic studies have been devoted to the lower Cambrian of the Lower Yangtze region for the purpose of evaluation of source rock conditions upon the basis of data from outcrops, cores, rock microscopic analysis and organic geochemistry. There are six types of rocks in the lower Cambrian of Lower Yangtze region. They are the siliceous rock, siliceous shale, carbonaceous shale, gray mud shale, marl and dolomite. Stratigraphically, the Lower Cambrian can be subdivided into three sections. The three sections are only complete in the Anji-Shitai-Shanghai area. As observed, the first section is missing in all other areas, while the second section is lacking to different degrees from place to place. Five types of sedimentary facies have been identified i.e, the facies of shelf, slope, platform edge, and restricted platform. Hydrocarbon source rocks are mainly distributed in the Shitai-Anji, Yanzhou-Yancheng, and Dexing-Tonglu Basins, which are surrounded by continental facies. Therefore, the Shitai-Anji region is the most favored area for hydrocarbon source rocks deposition. There occurs a thick muddy sequence of 300~700 meters with 3%~9% of TOC on an average of 4.5~5.76%. The muddy sequence is 200~400 meters thick in the Dexing-Tongzi region, which contains 3%~9%TOC on an average 4.96%. In the Yanzhou-Yancheng sag, however, the muddy deposits are 50~200 meters in thickness with 2%~4% of TOC. The kerogens of the lower Cambrian source rocks of the Lower Yangtze region are all belonging to the type Ⅰ dominated by sapropel mainly from algae, with a small amount of amorphous sapropel. The lower Cambrian source rocks of the region are all overmatured, and lowered gradually from the Pannan basin towards the area surrounding the platform. Owing to the influence of volcanic activities, the Ro is over 4.0% in the areas of Jiujiang, Jingdezhen, Huangshan, Kaihua and Chunan; and in the basin areas to the south of Anqing-Fuyang- Nantong, and to the west of Huzhou-Suzhou, continental shelf and slope facies deeply buried dominate and the Ro varies between 3.0% and 4.0%; while the Ro in the Mesozoic-Ce-nozoic depressions of Qianshan, Nanling, Wuwei, Jintan, and North Jiangsu is above 3.0% due to the large burial depth in the later period. The Ro changes between 2.0% and 3.0% in other regions.