Origin and Formation of Pyrobitumen in Sinian-Cambrian Reservoirs of the Anyue Gas Field in the Sichuan Basin: Implications from Pyrolysis Experiments of Different Oil Fractions

摘要

Plenty of pyrobitumens were found throughout the Sinian Dengying and Cambrian Longwangmiao reservoirs of the Anyue gas field in the central Sichuan Basin. However, their genetic sources and formation processes still remain less constrained due to their strong heterogeneity in carbon isotopes and contents. In this study, an oil sample collected from the Devonian bituminous sandstones in the northwestern Sichuan Basin, along with its four fractions (i.e., saturates, aromatics, resins, asphaltenes (SARA)), was pyrolyzed in sealed gold tubes to investigate the generation and evolution process of pyrobitumen. Experimental results show that the four fractions generate pyrobitumen at different thermal maturity stages during oil cracking and the pyrobitumen yields of different oil fractions follow the following order: asphaltenes resins aromatics saturates. For the pyrobitumen generated from the same oil fraction, the stable carbon isotopes of pyrobitumen become only slightly heavier with increasing maturation. At the same level of thermal maturity, pyrobitumen from saturates is more enriched in ~(13)C than those from the other three oil fractions, and the maximum difference is over 2‰. Based on the experimental results, kinetic parameters for pyrobitumen generation from the four oil fractions were determined and then applied to investigate the origin and formation time of pyrobitumen in the Sinian Dengying and the Cambrian Longwangmiao reservoirs. The results show that the pyrobitumen in both the Sinian and the Cambrian reservoirs were mainly formed during the middle Jurassic to early Cretaceous. Although the pyrobitumen yields are significantly dependent on the SARA composition of oils, the change in pyrobitumen carbon isotopes is usually less than 1‰. This indicates that the carbon isotopes of pyrobitumen are mainly controlled by the initial carbon isotopes of SARA fractions that are closely related to their source rocks. Based on the carbon isotopes of pyrobitumen and potential source rocks, the precursor oils of pyrobitumen in the Cambrian Longwangmiao reservoirs are mainly derived from the Lower Cambrian Qiongzhusi source rock, while the precursor oils of pyrobitumen in the Sinian Dengying reservoirs are mixed oils sourced from both the Sinian and the Lower Cambrian source rocks.