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, especially if intrusion emplacement occurred syn-contemporaneously with the organic matter deposition. The Neogene siliceous mudstone of Onnagawa Formation in the Akita Basin is a suitable interval for this study as extrusive and intrusive volcanic rocks were found within the formation. Hence, eight cuttings 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 background Tmax is very low in our well (Tmax<430ºC), the parameter is extremely elevated in the intrusion zone followed by increasing production index (PI) together with reduced TOC and hydrogen index (HI). In contrast, slight changes, including reversal trend, in several maturity-related saturate biomarker ratios, such as C 23 TT βα/αα, Ts/(Ts+Tm), C 29 and C 30 moretanes/hopanes, C 31 -C 35 HH 22S/(22S+22R), and 20S/(20S+20R) were observed close to 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 are more thermally resistant, absorb the heat and generate bitumen, including immature isomers. Furthermore, this generated bitumen is correlative with produced oils in the area, which also inherited low maturity characteristics. Yet, it becomes clear that igneous intrusion could activate the atypical petroleum system in one basin, despite the source rock being deposited concurrently with the intrusion event. • Igneous intrusion affected the geochemical characteristics of recently deposited organic matter. • Macro- and micromolecular proxies responded to extreme thermal stress with various sensitivities. • Bitumen related to igneous intrusion inherits distinct characteristics which applicable to atypical petroleum system studies.