Characteristics of bound biomarkers released from pyrobitumens generated from the cracking of different types of crude oils

Abstract

Pyrobitumen has attracted significant attention from petroleum geologists and geochemists due to its close relationship with crude oil and natural gas. Generally, the analysis of free biomarkers in pyrobitumen is of limited use, because these biomarkers can be easily modified by biodegradation and thermal alteration. The utilization of bound biomarkers in pyrobitumen appears to be a viable solution to address these issues. However, the compositions and concentrations of bound biomarker moieties in pyrobitumen can also be influenced by external factors such as thermal stress during its formation and the original abundance of asphaltenes in oil. The goal of this study is to examine the evolutionary history of free and bound biomarkers during the formation process of pyrobitumen from different types of crude oil. This is beneficial for the interpretation of covalently bound biomarkers released from natural pyrobitumens in applications such as oil/gas–solid bitumen-source correlation and oil/gas migration studies. Anhydrous and hydrous pyrolysis experiments were conducted on three types of crude oil (condensate, black oil, and heavy oil) to simulate the formation process of pyrobitumen. The comparison of free biomarkers in the liquid residuals of the oil pyrolysis and the bound biomarkers released by the hydropyrolysis of the corresponding pyrobitumen products indicated that the differences between free and bound biomarkers vary with the types of original oils. At elevated thermal maturity (Easy %Ro ≥ ca. 1.0%), the higher the absolute abundance of the asphaltene fraction within a particular oil, the greater the similarity between the distribution of bound biomarkers (especially for steroids) released from pyrobitumen and the corresponding original oil. Thus, the use of pyrobitumen-bound biomarkers formed by heavy oil (asphaltene-rich) can obtain relatively accurate biogenic information on paleo-reservoirs. However, caution should be exercised when employing pyrobitumen-bound biomarkers in paleo-reservoirs formed by light oil (asphaltene-depleted).