Abstract
Temperature and time were generally considered as the two key influences on the thermal evolution of both source organic matters and secondary hydrocarbon products (i.e., oil, gas, bitumen and solid bitumen). And for the hydrocarbons, other factors such as the types of source organic matters, the alteration reactions and fluid pressure evolution can also influence their degree of thermal maturation. As a consolidated product of bitumen, the traditional view is that the solid bitumen originated from the same source would exhibit similar maturity after experiencing identical paleo-temperature. However, things need to be more thoughts, such as the impact of system open or not on the solid bitumen thermal evolution and maturation degree. In this regard, we compared the solid bitumens within both reservoir pores and inclusions in deep carbonate reservoirs of lower Cambrian Longwangmiao Formation (Є1l) and the fourth member of the Sinian Dengying Formation (Z2dn4) in the Moxi area in the central Sichuan Basin. And by analyses of the Raman spectroscopy and bitumen reflectance, our results show a significant difference of maturities for these homologous solid bitumens after experienced the same highest geological temperature. Specifically, based on petrological observations, three types of solid bitumen occurrences (Bit. A, Bit. B, and Bit. C) were identified. Bit. A fills the early fine-grained dolomite dissolution pores with small and dispersed morphology, which eRo-Raman (equivalent vitrinite reflectance obtained by Raman) values show a narrow range from 2.60% to 2.80%. Bit. B with larger, blocky morphology, distributes within pores between the early recrystallized dolomites or the subsequent euhedral dolomites, including the bitumen filling in the fracture as stylolite. Their eRo-Raman values mainly concentrate in the range of 1.80%–1.90%. Bit. C refers to the bitumen inclusions hosted in various diagenetic minerals, with eRo-Raman values range from 2.70% to 2.87%. Since these solid bitumens originated from the same source rocks and have experienced the same highest geological temperature, their heterogeneous eRo-Raman values, therefore, suggest that whether the system open or not matters the thermal evolution of solid bitumen. Our results suggest that caution should be applied when using maturity to study the thermal evolution history in an open system, and hydrocarbons trapped in a closed system (e.g., fluid inclusion) can serve as a more credible recorder of thermal evolution.
Paper Information
Zhang, H., Chen, Y., Wang, M. and Song, Y., 2024. Thermal evolution differences of reservoir bitumen and their implications: A case study of the Moxi area in the central Sichuan basin. Marine and Petroleum Geology, 169, p.107039. https://doi.org/10.1016/j.marpetgeo.2024.107039
