5:15 PM - 7:15 PM
[SSS14-P10] Aosawa Fault as a Methane Migration Pathway: Its Spatial Continuity and Methane Origin
Keywords:Shonai-Heiya-Toen fault zone, Aosawa Fault, Microseepage, Geologic Methane, Cavity Ring-Down Spectroscopy
1. Introduction
The Shonai-Heiya-Toen fault zone consists of multiple reverse faults, and the development of anticline uplift and flexure zones on the Late Pleistocene terrace surface has been observed. The Aosawa Fault, which defines the boundary between the Shonai Plain and the western margin of the Dewa Mountains, is not recognized as an active fault because it does not displace the pyroclastic deposits of Mount Chokai, nor does it cause any detectable displacement of the terrace surface. Meanwhile, in this region, the Middle Miocene Kusanagi Formation, composed of siliceous shale, is widely distributed and serves as the petroleum source rock for the Higashi-Amarume Gas Field and Amarume Oil Field.
In this study, we focused on the role of faults as potential pathways for deep-seated gas migration. We aimed to detect microseepage from petroleum reservoirs by measuring near-surface methane concentrations along survey transects crossing these faults.
2. Measurement Methods
Methane measurements were conducted using cavity-enhanced absorption spectroscopy, a laser-based technique that has recently seen significant advancements in portability and precision. A portable analyzer was mounted on a vehicle, and methane concentrations near the ground surface were recorded while driving at low speed (approximately 20 km/h). Additionally, in certain areas, methane measurements were conducted on foot.
Previous surveys revealed a distinct increase in methane concentrations (up to approximately 20 ppm) at locations near the fault in the upstream area of the Aisawa River (Kitamata) and the Nakanomata-sawa River. Methane concentration anomalies were found to be limited to regions where the fault and the Aosawa Basalt Formation are distributed. In this study, we extended the survey area southward, conducting vehicle-based measurements along multiple rivers and roads. Furthermore, geochemical analyses were performed on collected gas samples to investigate the origin of the methane.
3. Measurement Results
-Confirmation of Methane Anomaly Reproducibility
The reproducibility of methane anomalies was confirmed in the upstream area of the Aisawa River (Kitamata) and the Nakanomata-sawa River, demonstrating continuous methane seepage along the Aosawa Fault.
-Southward Extension of the Survey Area and Detection of New Methane Anomalies
The survey area was extended southward, and new methane anomalies were detected at multiple locations along the valleys where the Aosawa Fault is expected to be present.
In particular, at the southernmost survey site near the Tachiyazawa River, a maximum methane concentration of approximately 23 ppm was detected.
-Analysis of Methane Origin
The hydrocarbon composition and carbon isotope ratio of the collected gas samples strongly suggest that the seeped methane originates from thermogenic processes.
4. Discussion and Conclusions
The results of this study indicate that the Aosawa Fault and the surrounding basaltic formations function as a methane migration pathway extending at least 33 km in the north-south direction. Additionally, since the seeped methane is of thermogenic origin, the Kusanagi Formation, buried deep enough in front of the Aosawa Fault or its branching faults to the west, is suggested to function as a petroleum kitchen for the Amarume Oil Field and the Higashi-Amarume Gas Field.
Furthermore, this study highlights that high-precision methane measurements utilizing laser spectroscopy techniques, such as cavity ring-down spectroscopy (CRDS), have great potential as a new tool for fault investigations.
The Shonai-Heiya-Toen fault zone consists of multiple reverse faults, and the development of anticline uplift and flexure zones on the Late Pleistocene terrace surface has been observed. The Aosawa Fault, which defines the boundary between the Shonai Plain and the western margin of the Dewa Mountains, is not recognized as an active fault because it does not displace the pyroclastic deposits of Mount Chokai, nor does it cause any detectable displacement of the terrace surface. Meanwhile, in this region, the Middle Miocene Kusanagi Formation, composed of siliceous shale, is widely distributed and serves as the petroleum source rock for the Higashi-Amarume Gas Field and Amarume Oil Field.
In this study, we focused on the role of faults as potential pathways for deep-seated gas migration. We aimed to detect microseepage from petroleum reservoirs by measuring near-surface methane concentrations along survey transects crossing these faults.
2. Measurement Methods
Methane measurements were conducted using cavity-enhanced absorption spectroscopy, a laser-based technique that has recently seen significant advancements in portability and precision. A portable analyzer was mounted on a vehicle, and methane concentrations near the ground surface were recorded while driving at low speed (approximately 20 km/h). Additionally, in certain areas, methane measurements were conducted on foot.
Previous surveys revealed a distinct increase in methane concentrations (up to approximately 20 ppm) at locations near the fault in the upstream area of the Aisawa River (Kitamata) and the Nakanomata-sawa River. Methane concentration anomalies were found to be limited to regions where the fault and the Aosawa Basalt Formation are distributed. In this study, we extended the survey area southward, conducting vehicle-based measurements along multiple rivers and roads. Furthermore, geochemical analyses were performed on collected gas samples to investigate the origin of the methane.
3. Measurement Results
-Confirmation of Methane Anomaly Reproducibility
The reproducibility of methane anomalies was confirmed in the upstream area of the Aisawa River (Kitamata) and the Nakanomata-sawa River, demonstrating continuous methane seepage along the Aosawa Fault.
-Southward Extension of the Survey Area and Detection of New Methane Anomalies
The survey area was extended southward, and new methane anomalies were detected at multiple locations along the valleys where the Aosawa Fault is expected to be present.
In particular, at the southernmost survey site near the Tachiyazawa River, a maximum methane concentration of approximately 23 ppm was detected.
-Analysis of Methane Origin
The hydrocarbon composition and carbon isotope ratio of the collected gas samples strongly suggest that the seeped methane originates from thermogenic processes.
4. Discussion and Conclusions
The results of this study indicate that the Aosawa Fault and the surrounding basaltic formations function as a methane migration pathway extending at least 33 km in the north-south direction. Additionally, since the seeped methane is of thermogenic origin, the Kusanagi Formation, buried deep enough in front of the Aosawa Fault or its branching faults to the west, is suggested to function as a petroleum kitchen for the Amarume Oil Field and the Higashi-Amarume Gas Field.
Furthermore, this study highlights that high-precision methane measurements utilizing laser spectroscopy techniques, such as cavity ring-down spectroscopy (CRDS), have great potential as a new tool for fault investigations.