Multiple isotopes (C-S-N-H) and bound biomarkers in asphaltenes: New constraints on the classification and genesis of reservoir bitumens from the northwestern Sichuan Basin, South China

Abstract

Reservoir bitumens in the northwestern Sichuan basin are significant for elucidating the sources and charging history of oil and gas, given their widespread occurrence in multiple strata in the area and abundant biomarkers. The debate regarding the origin and genesis of these bitumens has persisted for a long time, due to severe biodegradation and the development of multiple sets of high maturity source rocks with few biomarkers for oil-source correlation. To resolve these questions, asphaltenes, which are more resistant to biodegradation than free hydrocarbons, were systematically analyzed with the bulk multi-isotopes (C-S-N-H), bound molecules and the carbon isotopic compositions of bound individual n-alkanes. These characteristics were then compared to those of bulk bitumen and free hydrocarbons, leading to three main conclusions. In most samples, relatively abundant 25-norhopanes and 17-nortricylic terpanes were identified along with n-alkanes in free hydrocarbons, suggesting at least two oil charging events. In the samples having free n-alkanes, the carbon isotopic compositions of asphaltene-bound n-alkanes closely resemble those of corresponding free n-alkanes. Moreover, the bulk C-S-N isotopic compositions of asphaltene also approach those of corresponding bulk bitumen. These results suggest that the source of the oil charges occuring at different times are mostly the same for an individual sample, though different bitumen samples may have distinct sources. Free hydrocarbons, including n-alkanes and biomarkers, may have been produced by the secondary cracking of asphaltenes. Second, the integration of bulk C-S-N isotopic compositions of the asphaltenes and bitumens has enabled the studied samples to be classified into four groups. Source facies is the primary control of the distinct isotopic compositions with other factors like biodegradation, thermal maturity and migration having only minor influence. In combination with biomarkers, the organic matter and sedimentary environment of source rocks could be characterized for each group. A careful comparison of the bulk C-S-N isotopic compositions of asphaltenes and bitumens with those previously reported for source rocks (organic C and S and bulk N isotopes) suggests the main source rocks in the Upper Ediacarian-Lower Cambrian Formations, supporting the conclusions of previous studies. Furthermore, those source rocks in much younger formations, such as the Middle Permian as well as the Middle Devonian Formations, may also have contributed significantly to the widespread reservoir bitumens in the region. These findings highlight the usefulness of bulk C-S-N isotopic composition of asphaltenes for distinguishing oils with complex genesis and large gas exploration potential of the Upper Paleozoic source rocks in the region.