Petroleum Habitat of Magadan Basin, Sea of Okhotsk,
Part V, Oil-Gas Generation and Expulsion
Internet Geology News Letter No. 167, September 16, 2002

The scale of generation of oil and gas was calculated for the present stage. Maximum values of oil generation are found in the western part of Zav'yalov downwarp at about 5 million tons per square kilometer. In Motykley downwarp it does not exceed 4 million tons per square kilometer. Maximum generation of gas is recorded in Zav'yalov and Motykley downwarps at 8 million tons oil equivalent per square kilometer.

A calculation was made for the Paleocene-middle Oligocene complex for the beginning of the middle Miocene. At that time this complex was in the oil window, and generation of oil and gas reached maximum in the central part of Zav'yalov downwarp and the western part of Motykley downwarp at 6 and 10 million tons per square kilometer, respectively.

Assessment of potential cannot be based on values of generation of micro-oil because the amount that is expelled to reservoirs ranges from a fraction of a percent to 80-90 percent. The rest remains in the rock. Practically 100 percent of the generated gas is expelled.

A quantitative estimate was made of the amount of oil that has been expelled at the modern stage of the basin and also at the beginning of the middle Miocene. At the present stage a large part of the shelf is characterized by a density of oil expulsion of 3-4 million tons per square kilometer. At the beginning of the middle Miocene when the main source beds were Paleocene-lower Oligocene, density of oil expulsion in the central part of Zav'yalov downwarp was more than 4 million tons per square kilometer.

In the modern stage a large part of the Magadan basin is in the oil-gas kitchen. Reconstructions show that the main downwarps of the basin were already in the oil-gas kitchen at the beginning of the middle Miocene. Most of the oil-gas kitchens of Magadan basin came into existance 60-65 million years ago in the Early Paleogene. During the first stage of about 40 million years 8 percent of the total micro-oil for the entire history of the region was generated. Expulsion was at 15,000 tons per square kilometer per million years.

During the next 10 million years (late Oligocene-middle Miocene) oil generation was at maximum, but expulsion remained low. The following middle-late Miocene stage of 8.75 million years was marked by decrease in generation of oil in the Paleocene-lower Oligocene and an increase in the upper Oligocene-lower Miocene sediments. Expulsion potential of the Paleocene-lower Oligocene was 900,000 tons per square kilometer. Expulsion potential of the upper Oligocene-lower Miocene at 2,670,000 tons per square kilometer secured practically 100 percent expulsion of the micro-oil from these source beds.

The late Miocene-Quaternery stage of 6.3 million years length is char- acterized by cessation of oil formation in the Paleocene-lower Oligocene sediments. Generation of oil reached maximum at this time in the upper Oligocene-middle Miocene complex. Sediments of the middle-upper Miocene complex had just begun the process of oil-gas generation; their product is not significant.

Magadan basin has all the necessary elements of a petroleum system: generation, expulsion, migration, and accumulation. Potential is high.

Taken from O. A. Krovushkina, 2001; digested in Petroleum Geology, Vol. 37, No. 1, 2003, in preparation, two maps, one seismic section, one chart showing range in petroleum generation and expulsion.
Copyright 2002, James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at: http://geocities.com/internetgeology/
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