Re-Os Geochronometer Constraints on the Timing of Petroleum Generation and Migration in the Northern Longmen Shan Thrust Belt, Southwest China

Abstract

Bitumen within Neoproterozoic to Lower Paleozoic units along the northern Longmen Shan trust belt (NLSTB), Southwest China, indicate the existence of oil migration and paleo-oil reservoirs. The bitumen predominantly occurs along the thrust fault plane and fracture system in the lower Cambrian strata. Here we use bitumen Re-Os geochronology to constrain the timing of hydrocarbon generation, to identify source units and to determine the age of fault activity of the NLSTB that is associated with contemporaneous bitumen precipitation. The bitumen Re and Os abundances range between 283.3–563.3 ppb and 4058.2–15347.3 ppt, respectively, which are significantly higher than that of oil from present day oil seeps (7.7–9.6 ppb and 90.3–127.2 ppt, respectively). The bitumen Re and Os values are significantly elevated from those of the average continental crust and previously reported bitumen samples from both hydrocarbon and metalliferous systems, and also the majority of marine and lacustrine organic-rich sedimentary rocks. The bitumen 187Re/188Os values are high (∼ 230.7 to 718.4), and possess radiogenic 187Os/188Os compositions (∼ 2.79 and 3.48). The bitumen Re-Os isotope data define two positive correlations, which yield a Re-Os date of 158 ± 77 Ma, with an initial 187Os/188Os value of 1.85 ± 0.61, and a Re-Os date of 199.7 ± 2.6 Ma, with an initial 187Os/188Os value of 2.06 ± 0.01. The two ages are in agreement with burial history and fluid inclusion analysis in the NLSTB and adjacent Sichuan Basin, suggesting two stages petroleum generation and migration process. Moreover, the Re-Os ages are coeval with two intense thrust fault movements of the NLSTB during the Mesozoic, which also provides a new direct absolute age evidence for the Late Mesozoic deformation of the NLSTB. Therefore, our bitumen Re-Os geochronology provides a new direct absolute age evidence for the timing of tectonism and coupled petroleum migration in the NLSTB. It also provides a new approach to determine the direct absolute timing of fault activity. Additionally the contrast between the 187Os/188Os compositions of the bitumen and present day oil seeps supports the available organic geochemistry indicating hydrocarbons were derived from various source rocks, for example, Cambrian and Sinian shale formations (bitumen) and Permian strata (present day oil seeps).

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016