Igneous intrusion effect on the geochemistry of Late Miocene Onnagawa mudstone: implication on the atypical hydrocarbon generation of Akita Basin, Japan

This study investigated the physio-chemical changes in organic matter, from micro-to macromolecular scale, due to rapid heating from igneous intrusion, particularly if intrusion emplacement occurred syn-contemporaneously with the organic matter deposition. The Neogene siliceous mudstone of the Onnagawa Formation in the Akita Basin is a suitable interval for this study, as subvolcanic and volcanic rocks were found within the formation. Hence, eight cutting samples of the formation from the Yabase well were subjected to Rock-Eval pyrolysis analysis, while three samples near the intrusion body and one unintruded sample were then selected for further biomarker analysis. Generally, kerogen, represented by Tmax, and biomarker proxies respond to the extreme thermal stress from intrusion. While the background Tmax is very low in our well (Tmax<430 °C), this parameter is significantly elevated in the intrusion zone, accompanied by an increasing production index (PI) alongside with reduced TOC and hydrogen index (HI). In contrast, slight changes, including reversal trend, in several maturity-related saturate biomarker ratios, such as C₂₃ tricyclic terpanes (TT) βα/αα, C₂₄ tetracyclic terpanes/C₂₃ TT, Ts/(Ts + Tm), C₂₉ and C₃₀ moretanes/hopanes, C₃₁–C₃₅ homohopanes (HH) 22S/(22S + 22R), and C₂₉ steranes 20S/(20S + 20R) were observed near the intrusion proximity. These phenomena are related to three major factors, which are kerogen-related generation, failed isomerization, and thermal degradation. Rapid heating from intrusion is insufficient to exhibit isomerization reaction, while kerogen, which is more thermally resistant, absorbs the heat and generates bitumen, including more immature isomers. Furthermore, this generated bitumen correlates with the produced oils in the area, which also inherit low maturity characteristics. Thus, it becomes evident that igneous intrusion could activate the atypical petroleum system in one basin, despite the source rock being deposited concurrently with or immediately after the intrusion event.