Pb, Sr, and Nd isotopic features in organic matter from China and their implications for petroleum generation and migration

To better understand processes and sources of crude oil and bitumen generation from kerogen rocks, a comprehensive study of Plp, Sr, and Nd isotopic systematics was undertaken for bitumen, crude oil, and kerogen from two large oil fields (Karamay and Liaohe) and two paleo-oil deposits (Tarim and Guizhou) in China. The bitumen samples from the Karamay oil field have present-day Nd-143/Nd-144 ratios pointing to a depleted mantle origin (0.5126-0.5130). They define a Rb-Sr isochron age of 286 +/- 12 Ma. Pb isotopic compositions for bitumen and crude oil from Karamay, Liaohe, and Tarim all show features of crust-mantle mixing. They are evidently different from Pb isotopic compositions of kerogen that occurred in the same strata of the various oil fields, which mostly show crustal signatures. Sr isotopic compositions in crude oil from Liaohe are consistent with those of carbonate sediments and volcanic rocks in island arc environments (0.7065-0.7100). Radiometric isotope systems can be preserved in the bitumen with low vitrinite reflectance R-O (<0.5%) and therefore allow us to date petroleum generation. Both isotopic data and the geologic setting of the oil fields indicate that petroleum generation was associated with fluid action from deep sources, including the mantle, lower crust, and buried strata in rift basins and buried foreland basins at craton margins or subduction zones. Copyright (C) 2001 Elsevier Science Ltd.