Using monazite U-Pb geochronology of Precambrian shales to constrain fluid flow and hydrocarbon accumulation events of a petroliferous basin

Abstract

Obtaining precise ages of the Phanerozoic fluid flow events of petroliferous basins is essential for understanding the tectonothermal histories, petroleum- and ore-forming processes. Conventional isotopic dating methods are often compromised by inheritance from detrital constituents and/or subsequent thermal resetting. Monazite is a trace mineral widely present in low-grade metasedimentary rocks, and as an excellent U–Th–Pb geochronometer it has great potential in dating fluid flow activities related to basin evolution. We present results of an in-situ monazite U-Th-Pb geochronology study on shale samples from the Neoproterozoic (651 Ma) Datangpo Formation intersected by a drill-hole in the Sichuan Basin, a typical petroliferous basin in western South China. We recognized two fluid flow events at ca. 450 Ma and ca. 263 Ma, and relate the former to the far-field effect of the Proto-Pacific Ocean subduction and the latter to emplacement of the Emeishan large igneous province. We infer that the ca. 450 Ma fluid flow event induced the first stage of oil charging in the paleo oil pools in the Sichuan Basin, as also recorded by the bitumen samples from the Anyue gas field which yielded re-Os isotopic ages of 458–492 Ma, whereas the ca. 263 Ma event triggered the gas generation of the Paleozoic marine strata in the Sichuan Basin. The results and findings of this study have important implications to reconstructing the complex histories of fluid flow, oil charging and gas generation events of petroliferous basins.