Geochemistry and correlation of crude oils from reservoirs and source rocks in southern Biyang Sag, Nanxiang Basin, China

Abstract

The Paleogene Hetaoyuan Formation, a sequence of gray to dark calcareous mudstone and dolomitic mudstones, is the main source rock and reservoir in the Biyang Sag, Nanxiang Basin, eastern China. Organic geochemical analysis study performed on 57 source rock samples from the Hetaoyuan Formation demonstrates poor to abundant organic matter content, ranging from 0.24–5.4wt% (TOC content), characterized by oil prone Type II kerogen. Eleven crude oil samples from the Hetaoyuan Formation in southern Biyang Sag were analyzed using bulk properties and molecular methods to determine their maturity and origin. The results suggest that the crude oils exhibit different maturity and source organic matter.Three groups of oils were identified by integrated analysis of gas chromatography, gas chromatography–mass spectrometry, stable carbon isotope, physical properties and gross composition of crude oil. Group A oils, defined as low mature oil, are characterized by high density, viscosity, sulfur content and low saturated hydrocarbon content. This group of oils has relatively high values of Ph/n-C18, C24/C26 and gammacerane index, indicating that the original organic matter had more contribution from terrigenous plants and was deposited in saline and stratified water columns, under anoxic conditions. Group B oils, defined as normal maturity oil, are characterized by moderate density, viscosity, sulfur content and moderate saturated hydrocarbon content. This group of oils has relatively low values of Ph/n-C18, C24/C26 and gammacerane index and displays a slight predominance of C29 steranes, indicating that the original organic matter had more contribution from marine algae and was deposited under sub-oxic to anoxic conditions with low salinity. Group C oils, defined as light oil, are characterized by low density, viscosity and abundant saturated hydrocarbons. Representative source rock samples were selected for further correlation. The results of oil-source rock correlation suggest that crude oils from the Eh2 member of Hetaoyuan Formation are mixture derived from source rocks in the Eh2 member and upper layer of the Eh3 member, while oils from the upper and lower layer of the Eh3 member correlate with source rocks in the upper and lower layers of the Eh3 member, respectively.