Petroleum charge history in the Lunnan Low Uplift, Tarim Basin, China – Evidence from oil-bearing fluid inclusions

Abstract

The petroleum charge history in the Lunnan Low Uplift (Tarim Basin, China) was investigated by comparing the geochemical characteristics of fluid inclusion (FI) oils from Ordovician and Triassic reservoirs with crude oils from stratigraphically different reservoirs. Quantitative Grain Fluorescence (QGF) and Total Scanning Fluorescence (TSF) were used to screen the reservoir samples so as to identify suitable samples for Molecular Composition of Inclusion (MCI) analysis. All of the samples contain C 30 n-propylcholestanes, diagnostic of marine source rocks. The Ordovician-hosted FI oils differ from the Triassic-hosted FI oils, having higher relative abundances of C 19 tricyclic terpane, C 24 tetracyclic terpane and rearranged hopanes (Ts and C 29 Ts), and higher Pr/Ph ratios, while the Triassic-hosted FI oils have lower Pr/Ph ratios and higher relative abundances of C 23 tricyclic terpane, arylisoprenoids, 25-norhopanes and C 35 homohopane. Several of the crude oils are similar to the Triassic-hosted FI oils. Based on the geochemical characteristics of the FI and crude oils combined with information on the burial history of the area, at least two petroleum charge episodes can be distinguished in the Lunnan Low Uplift. An early charge represented by the Ordovician-hosted FI oils was biodegraded, as evidenced by the presence of 25-norhopanes in the Triassic-hosted FI oils and crude oils but not in the Ordovician-hosted FI oils. Ordovician-hosted FI oils provide the only evidence for oil generated from Cambrian-Lower Ordovician source rocks. Triassic-hosted FI oils and crude oils in the Ordovician, Carboniferous and Triassic reservoirs are mixtures of biodegraded oil from an early charge episode, and more mature oil from later charge.