Re-Os Dating and Oil-Source Correlation of the Pyrobitumen in the Member 4,dengying Formation of the Kangjiadong Paleo-Reservoir,northern Margin of the Sichuan Basin

There are pervasive pore-filling bitumen found in the dolomite reservoir of the Member 4 , Sinian Dengying Formation. However, the high maturity of the source rocks and bitumen and the complex geological tectonic activities make it difficult to restore the history of hydrocarbon accumulation. In recent years, the Re-Os radioisotope system has been increasingly used for the dating of the oil and gas generation and oil-source correlation. The high-maturity bitumens (R-b of ca. 1. 4% similar to 2. 5%) from the Member 4, Sinian Dengying Formation in the Kangjiadong paleo-reservoir, Ningqiang, Shaanxi have defined Re-Os isochron ages of ca. 260Ma, 60 similar to 70Ma and 8 similar to 13Ma. The Re-Os ages of 60 similar to 70Ma and 8 similar to 13Ma may indicate the time of when the Re-Os system was closed at the end of the crude oil being thermally cracked and the pyrobitumen forming process due to the uplift and temperature decrease. The older Re-Os age of about 260Ma is consistent with the general view that the Permian is the main hydrocarbon generation period of the Precambrian and Cambrian source rocks. It shows that besides the thermal cracking of crude oil, the Re-Os age defined by highmaturity bitumen may also indicate the crude oil generation. Meanwhile, at 263 Ma, most of the Kangjiadong bitumens are consistent with the Os-187/Os-188 range of the Qiongzhusi Formation source rock samples rather than the lower Os-187/Os-188 ranges of the Member 3 of the Dengying Formation and the Maidiping Formation. The Qiongzhusi Formation may be the main source rock of the bitumen in the Kangjiadong paleo-reservoir. These understandings are important for the evaluation of natural gas exploration in the Dengying Formation in the northern margin of the Sichuan Basin.