Effects of thermal cracking of hydrocarbons on the homogenization temperature of fluid inclusions from the Niigata oil and gas fields, Japan

Abstract

The changes of the homogenization temperature (Th), when hydrocarbons in inclusions were cracked thermally, were experimentally evaluated. In the experiments, the hydrocarbon-inclusions in quartz were heated in an autoclave, and the Th between before and after heating were compared. In the experiments, the Th decreased after heating. The amount of decrease became greater as the heating temperature or the duration time or both increased. The experiments also revealed that the samples with higher Th showed a greater decrease after heating. Because the experiments have been performed in a closed system, mass transfer between inclusions and the enclosing medium was impossible. No volumetric changes in the hydrocarbon inclusions were observed after heating. Therefore, the temperature depression after heating should be caused by the pyrolysis of the hydrocarbons in the inclusions. In the Shin-Kumoide SK-1D well of the Niigata oil and gas field in Japan, the Th of hydrocarbon inclusions decreases from 73 to 30 °C between 4040 and 4810 m in depth successively. This temperature range is lower than the present formation temperature by from 60 to 140 °C, and shows a trend opposite to the normal geothermal gradient. Therefore, it should reflect the degree of the thermal cracking after trapping rather than the trapping temperature.