Characteristics of light hydrocarbons under the superimposed influence of biodegradation and subsequent thermal maturation

Recently, several large light oil and condensate reservoirs were discovered in the Tarim and Junggar basins (China). In these superimposed petroliferous basins, the early formed palaeo-oil reservoirs experienced complex tectonic movements, resulting in complex secondary alterations (e.g., biodegradation and water washing), known to alter the chemical composition of crude oils. Thus, crude oils previously modified by biodegradation exhibit significantly different generation potential, chemical composition, and timing of generation (in terms of light oils/condensates), during subsequent thermal alteration. To elucidate the influence of biodegradation on crude oil cracking (to form light hydrocarbons), we artificially matured four crude oils of the same origin (with varying degrees of biodegradation) to various maturity levels, using programmed-temperature gold tube closed-system pyrolysis. The yields, molecular parameters, and stable carbon isotope ratios of the light hydrocarbon products derived from the matured oils were analysed and compared. The preferential removal of n-alkanes and branched alkanes during biodegradation significantly reduced the generation potential of light hydrocarbons. Under the superimposed influence of biodegradation and subsequent thermal alteration, referred to as superimposed secondary alteration (SSA), a few hydrocarbon ratios used for oil–oil correlations were severely modified. Most maturity-related light hydrocarbon parameters of the pyrolysis products derived from the biodegraded oils achieved maximum values at a lower maturity (Easy%Ro) than that observed for non-biodegraded crude oil. At 0.5–1.4 Easy%Ro, SSA caused the depletion of 13C in most light hydrocarbons, due to an increased sourcing from the biodegradation-resistant fractions of the oil. Such carbon isotopic differences decreased with increasing maturity.