Extensive regional geological and geophysical studies in the last twenty years on the north shelf of the Sea of Okhotsk have elucidated the geology of the sedimentary basins, and each of the oil-gas complexes has been assessed.
Magadan sedimentary basin has a clearly expressed two-stage structure controlled by systems of faults of different times of inception and kinematics. The lower structural stage consists of asymmetrical, rarely double-sided grabens of east-west and northeast trend accompanied by intra- and inter-basin horst highs. These developed along left-lateral listric faults of Early Paleogene age. These faults extend 80-150 km and have displacements of 3-6 km. The planes of the faults form the steep south and southwest borders of the rift downwarps. Normal faults have controlled the depocenters within the downwarps. These downwarps are 150-350 km long and 20-90 km wide.
The upper structural stage consists of post-rift basin structures and accompanying syn-sedimentary highs above syn-rift horsts, as well as fault-fold zones. Internal structure of the post-rift depressions becomes more simple upward in the section from weakly asymmetrical features to saucer-like swales completely discordant to earlier structure.
Three stages are recognized in the histroy of development of Magadan basin. A pre-Cenozoic stage was the time of establishment of the main structural features of the heterogeneous basement. Then in the Paleocene-early Oligocene stage the rift structures developed, and the main oil-source beds and lower reservoirs were deposited. Litho-stratigraphic traps formed at this time. The following late Oligocene-Quaternary stage corresponds with the post-rift downwarping of the basin, strong synchronous spreading in TINRO depression in the east of the study area, and complex transpression movements on the flanks of the Okhotsk ring. This stage marked the final accumulation of oil-source beds and reservoirs, formation of regional seals, and developemnt of the main structural traps.
The sedimentary section of Magadan basin is subdivided by regional
structural-stratigraphic discordances into five main sedimentary
complexes: Paleocene-Eocene, lower Oligocene, upper Oligocene-
middle Miocene, middle-upper Miocene, and upper Miocene-Quaternary.
The two lower complexes are combined into the single Paleocene-lower
Oligocene mega-complex. Litho-facies and age of these sediments
have been determined by drilling and seismic surveys as well as by
inter-regional stratigraphic correlation of Cenozoic sediments of the
northern and western parts of the Sea of Okhotsk region. Thickness
of the sedimentary pile ranges from a few hundred meters on highs
along the margin of the study area to 10-12 km in depocenters.
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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