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DOI:10.1306/07122423030
Igneous intrusion drives in-reservoir oil thermal cracking: Case from the Subei Basin, Eastern China
Ao Su123 , Honghan Chen1 , Yue-xing Feng34 , Jian-xin Zhao3 , and Qiang Liu5
1 China University of Geosciences (Wuhan), Hubei 430074, China
2 Yangtze University, Wuhan, Hubei 430100, China
3 Radiogenic Isotope Facility, School of Earth and Environmental Sciences, The University of Queensland, Brisbane, QLD 4072, Australia
4 Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Guangdong 519080, China
5 Bureau of Geophysical Prospecting INC., China National Petroleum Corporation, Hebei 072750, China
Ahead of Print Abstract
Highly fractured siliciclastic reservoirs that had experienced hydrocarbon accumulation failure proximal to igneous intrusive bodies were encountered in the Subei Basin, China. Within the Paleocene Funing Formation, pervasive reservoir pyrobitumen indicates a paleo oil accumulation. Publicly available burial history shows that the reservoirs reached a maximum burial temperature (~ 140 ℃) at present day, which is much cooler than the commonly presumed oil generation high temp limit (at least 160 ℃). This is interpreted as resulting from a scenario whereby igneous activity forced in-reservoir thermal cracking following initial oil charge. Petrographic textures indicate two distinct phases of oil inclusion trapping: an early phase synchronous with silica diagenesis, and a later phase occurring after silicification and carbonate cementation, and concurrently with fracturing. The older oil inclusions record the initial oils that served as precursors to the pyrobitumen, while the younger generation was likely formed during oil cracking. Fissure calcites containing primary oil inclusions yield a LA-MC-ICP-MS U-Pb age of 44.3 ± 1.5 Ma, coinciding with the published timing of volcanism. Homogenization temperatures from aqueous inclusions lie between 177.1 - 198.3 ℃, broadly consistent with clumped isotope temperature (195.5 ± 6.5 ℃) of fissure calcites and with bitumen Raman-derived temperatures (163.7 - 212.9 ℃). These observations suggest that rising temperatures that accompanied the mid-Eocene igneous intrusion resulted in the initial oils being in-situ thermally cracked into short-chain hydrocarbons and pyrobitumen. The resulting volumetric expansion, in conjunction with reduced permeability from pyrobitumen deposition, led to hard overpressures, consequent fracturing, and hydrocarbon leak-off.
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