Abstract

To more efficiently explore the North Sumatra Basin for oil and gas, a better knowledge was sought of factors effecting generation, migration, and accumulation of hydrocarbon. Of particular interest are those factors determining oil versus gas occurrence. To accomplish this study, many samples of sediments over the basin were analyzed to determine amounts and type of organic matter, diagenetic changes, and degree of maturation. The results are combined in a geologic study; hydrocarbon occurrences and characteristics are described and considerations concerning hydrocarbon source and migration are described and discussed. It is concluded that the Tertiary source rocks of both gas and oil are deep within the basin and lower Middle Miocene or older. The deep-seated gas within the central basin shales did not migrate to any appreciable extent while the oil, found generally on the foreland or shallow edge of the basin, must have migrated as much as several thousand feet vertically. It was found that the shales of the central basin area were dewatered before they became mature while on the flanks dewatering and maturation largely overlapped. This prior dewatering, or attendant geopressuring, may have inhibited the generation or the migration of the larger oil molecules from the shale. Another possible explanation for oil versus gas accumulation is that the so-called oil of the basin perimeter may be actually condensate of the deep-seated gas which accumulated separately from most of the gas.