Estimation of Thermal Maturation of Sedimentary Rocks in the Niigata Basin, Northeast Japan

Abstract

Present temperature gradients and some factors controlling the gradients in the Niigaga basin are discussed. Paleotemperature of sedimentary rocks in the Niigata basin is estimated from the present temperature gradients based on some assumptions.The distribution of temperature gradients in the Niigata basin shows that it is strongly affected by the depth of the "GREEN TUFF" formation (Oligocene-Early Miocene volcanic rocks). This is explained by a thermal model. After correction of the effects of the depth of the "GREEN TUFF" formation. The thermal potential values are expressed as in the values from zero to ten and both values are equivalent to the minimum and the maximum values of the initial temperature gradients in the Niigata basin. The values show thermal characteristics (thermal conductivity or heat flow) of the "GREEN TUFF" and underlying formations.Thermal potential values indicate that their distributive trend is perpendicular to basin trend. It presumably suggests that the differentiation of the thermal potential values has been made during and/or before the deposition of the "GREEN TUFF" formation. Thermal potential values accord with a distribution of oil and gas fields: Gas fields mainly locate in the high thermal potential (6-10) areas, oil fields mainly locate in the intermediate thermal potential (2-6) areas and no oil and gas fields locate in the low thermal potential (0-2) area.Paleotemperature is estimated from thermal potential values and the depth of the "GREEN TUFF" formation (burial history of the "GREEN TUFF" formation) on a assumption that thermal potential values have not been changed after deposition of the "GREEN TUFF" formation.