A conceptual diagram of upward gas migration in a shallow ground, hydrogeological structure model and two phase flow model of groundwater and methane were constructed in this study using results of the survey and the test which were performed for the shows of the natural gas in the surface adjacent to the fault and its fractured zone. Numerical simulation of the methane upward movement in the ground was conducted by using these models. The results showed that capillary pressure, methane saturation of around 0.02-0.03 at 100m deep, fault fracture zone, and trap structure caused by fault displacement could reproduce the behavior of the methane movement in the shallow ground.
Interpretation on radiolarian fossil data of Nakaseko and Sugano (1972) based on Kato and Obuse (2013) shows that oil reservoirs in the Amarume oil field, Yamagata Prefecture, belong to the lower part of the Kitamata Formation and the Kusanagi Formation. This result harmonizes with foraminiferal fossil assemblages in wells around the field. Oils produced in the field, Amarume oils, are mostly classified into light oil and slightly biodegraded. Medium oils in the southern part of the field are strongly biodegraded due to meteoric waters invaded from the south. The Amarume oils resemble oils in the Yurihara and Ayukawa fields in geochemical characteristics including relationship between API gravity and sulfur content, carbon isotope compositions of oils, light hydrocarbon compositions and biomarker maturity parameters, but are clearly distinguished from oils in the Kosei area, Akita Prefecture. Relative oil maturity among oils in the Amarume, Yurihara and Ayukawa fields varies depending on maturity parameters used. In addition to oil maturity, gas maturity and Bouguer anomaly map suggest that oil and gas generated in the center of depression near Sakata have migrated into the Amarume field.