Petrological and organic geochemical characteristics of oil sands from the Middle Jurassic Yan’an Formation in the southern Ordos Basin, China

Abstract

An integrated petrological and geochemical analysis of surface and drilled oil sands from the Middle Jurassic Yan’an Formation in the southern Ordos Basin was conducted to investigate the petrophysical properties, organic matter origin/type, and thermal maturity and their relation to environmental conditions during sediment deposition. Petrographic analysis (thin section, scanning electron microscopy (SEM), and porosity and permeability determination) of the oil sand was performed to establish the relationships between porosity and permeability and bulk density, reservoir quality index (RQI), normalized porosity index (NPI), and flow zone indicator (FZI). Geochemical analyses include oil extraction, column chromatography, and gas chromatography–mass spectrometry (GC-MS). The strong correlation between RQI and permeability (R2 = 0.98) reveals that porosity has a good correlation with RQI. The positive correlations between porosity and permeability and NPI and RQI (R2 = 0.77 and R2 = 0.65, respectively) show that the studied rocks contain many large pores with pore-to-pore throat connection structures that enhance permeability, which is further supported by the relatively high connection coefficient between NPI and RQI (R2 = 0.65). The organic matter in the Yan’an Formation is mainly composed of oil-prone type I kerogen indicated by the recovery of a large amount of crude oil rich in saturated hydrocarbon fractions from the extracted oil. The biomarker signatures of the analyzed oil (nC13-nC35, Pr/n-C17 and Ph/n-C18, and low C27/C29 regular sterane) reveal predominantly land plant materials as organic input sources based on the average concentration of αααC27:C28:C29 sterane 20R of approximately 38%, 22%, and 40%, respectively, and on high values of tricyclic terpane/αβC30 hopane and regular sterane/αβC30 hopane. Deposition within a lacustrine paleoenvironment under anoxic conditions enhanced organic matter preservation in the area based on the low Pr/Ph ratios (average 0.74), low αβC31-22R-hopane/αβC30 hopane ratios, and high C26/C25 tricyclic terpane ratios, as well as the presence of gammacerane and low water salinity. Biomarker maturity parameters (e.g., C32 homohopane 22S/(22S + 22R), moretane/hopane, and C29 sterane 20S/(20S + 20R) ratios and CPI) show that the extracted oil sand entered the early oil window stage.