基礎試錐「三島」において発見された炭化水素包有物の産状およびマイクロサーモメトリーの検討

Abstract

Occurrences and microthermometric characteristics of hydrocarbon inclusions found in the MITI-Mishima well in Niigata Prefecture, Japan, have been studied and their implications are discussed. They occur locally in the lower part of the well, from the Miocene Lower Teradomari Formation through to the upper part of the Nanatani Formation, in which the main source rocks consist in the Niigata oil and gas fields.Occurrences and crystal shapes of quartz with hydrocarbon inclusions suggest that fractures were the main migration pathways of the hydrocarbons. Quartz crystals with hydrocarbon inclusions were found on extensional open fracture surfaces in cores. They also occur as euhedral or subhedral crystals in cuttings. Intergranular pore spaces in sandstone are rarely observed in this horizon. The occurences and crystal shapes of quartz indicate that they had precipitated in open spaces, and so the pathways of hydrocarbons mainly lie in the open-fractures of the source rock.The microthermometric characteristics of the inclusions suggest that the migrations of hydrocarbons occurred at least twice. The hydrocarbons in the inclusions from the MITI-Mishima well, commonly composed of vapor, liquid and bituminous solid, are classified into the vapor-homogenized type and the liquid-homogenized type. This is based on their behaviour when heated on the stage of the microscope. These two types of inclusions do not exist in the same grain, although they occur in the same range of depth in this well.The experiments on the estimation of the degree of thermal change of the hydrocarbon inclusions in a closed system, show that the homogenization temperature, which is initially 66.9°C, decreases to 45.9°C. This is after the inclusions have been heated to 370°C for 192 hours. Thus, the hydrocarbons in the inclusions are revealed to have cracked at this temperature.