Oil-gas source beds of Paleocene-early Oligocene age consist of siltstone and argillite. Microscopic examination of kerogen indicates a predominance of humic organic matter - up to 95 percent. Sapropelic material in some samples is up to 15 percent. Concentration of organic carbon in individual samples is more than 5 percent. Average generating potential is 1.55 kg hydrocarbons per ton of rock. In a large part of Magadan basin the sediments of this complex collected under shallow-water marine conditions. There were several sources of detritus at this time, leading to formation of coarse-grained clastics with mixed types of organic matter.
The oil-gas source beds of late Oligocene-middle Miocene age consist of argillite and siltstone. The kerogen in Magaden well 1 is a humic-sapropelic mix. Average content of organic carbon is 1.12 percent. Generating potential of the clastic-siliceous beds is 1.37-1.64 kg hydrocarbons per ton of rock. This complex was deposited in a marine basin with a deficit of clastics, leading to chemical-biogenic types of sediment. Sand-clay material was deposited only in direct proximity to areas of supply of detritus - the continental side of the shelf.
The oil-gas source beds of middle-late Miocene age include opokas (a siliceous earth) and thick beds (25-100 m) of opoka-like clays and siltstone. The kerogen is higher in sapropelic components. Content of organic carbon generally ranges from 0.55 to 2.23 percent but with maximum values of 3.3-6.3 percent in offshore Magadan well 1. Generating potential is 0.96-1.37 kg hydrocarbons per ton of rock. Deposition was in relatively deep water with considerable deficit of clastics, leading to collection of biochemical siliceous sediments practically everywhere.
Only the lower part of the upper Miocene-Quaternary complex, that is, the upper Miocene=Pliocene diatomite member, has an oil-source potential. Its thickness from drilling is 410 m. It consists of diatomites, diatomaceous clays, and tuff-diatomite. This member is present nearly everywhere in Magadan basin. Kerogen from cores ranges from 10 to 20 percent sapropelic components. Organic carbon content is 0.60-1.07 percent, and average generating potential is 0.68-1.24 kg hydrocarbons per ton of rock. In most of the basin the upper Miocene-Pliocene sediments were deposited in an open marine basin with deficit of clastics. With increase in share of clastic material there is an increase in the humic component of the disseminated organic matter.
Reservoirs in Magadan basin have no clear stratigraphic association, Pore reservoirs occur predominantly in deltaic systems. The extensive siliceous deposits of the basin make fracture-pore reservoirs.
Regional seals in Magadan basin are in the upper-Oligocene
clastic-siliceous sediments, which are also oil-source rocks.
Zonal seals may be present in other parts of the sections.
Continued in next News Letter
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
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