ガス炭素同位体組成からみた北蒲原地域における炭化水素の移動・集積

抄録

Molecular and isotope compositions of gas samples from oil and gas fields, and two exploration wells in the Kitakanbara area were analyzed. Most gases are classified into mixture of microbial and thermogenic gases. Maturity assessments of the reservoir gases were performed using ethane and propane carbon isotope compositions in two representative fields in the area, the Iwafuneoki and Higashiniigata fields. In both fields, vertical migration of thermogenic gas through a large fault system is inferred. In one exploration well, thermogenic gas migration to reservoir horizons was observed based on the carbon isotope composition of headspace gas, mud gas and drill stem test gas samples. On the other hand, no migration of thermogenic gas was detected in the other exploration well, despite hydrocarbons are deposited in the Higashiniigata field located in the updip of the well, suggesting that hydrocarbons passed through a deeper level below the total depth of the well. These results suggest that the hydrocarbons generally migrated laterally within the source horizons and did not migrate vertically into shallower reservoir horizons until the hydrocarbons reached the large fault systems.