Multiple charges to Sinian reservoirs in the middle Sichuan basin, SW China: Insight from the adsorbed/occluded hydrocarbons in solid bitumens

Abstract

Mild oxidative degradation was used to release the aliphatic hydrocarbons occluded inside the macromolecular structure of solid bitumens collected from Sinian carbonate reservoir, Sichuan basin, SW China. In contrast to the adsorbed aliphatic hydrocarbons extracted from the solid bitumens, the mild oxidative products showed unaltered n-alkanes distributions, which had been trapped within the macromolecular structure of the bitumens and protected from biodegradation processes. These observations are quite informative and suggest that at least, two phases of hydrocarbon charging to the Sinian reservoirs might have occurred. The occluded hydrocarbons seem to represent the first phase of hydrocarbon charge while the adsorbed and biodegraded hydrocarbons correspond to the second phase of hydrocarbon charge. However, terpane and sterane distributions in both fractions were preserved and similar, and the carbon isotope analysis results obtained in this work along with the previous geological information, corroborates the idea that Sinian reservoirs hydrocarbons have been sourced from the black shale of Lower Cambrian Qiongzhusi Formation. This study reveals the geochemical significance of adsorbed/occluded aliphatic hydrocarbons in understanding the charging history of oil into reservoirs.