Indigirka-Zyryanka downwarp is located on the Kolyma regional high in the Russian Far Eastern Region between longitudes 144 and 156 degrees east and latutudes 64 and 68 degrees north. This feature began its development in the Late Jurassic and up until the late Miocene was combined with the Momo downwarp as a single Momo-Zyryanka depression. These became separated in the late Miocene by the Ilin'-Tas anticlinorium.
The Indigirka-Zyryanka downwarp is a foredeep with a thrust belt on its southwest and a platform terrane on its northeast. Width of the thrust zone does not exceed 50 km, and within it are 3-4 thrust slices. The basin fill consists of 8 km of Upper Jurassic Oxfordian, Kimmeridgian, and Volgian deposits, 5 km of Lower and Upper Cretaceous, and 2.5-3 km of Cenozoic. Thicknesses increase toward the southwest.
In the frontal part of the thrust zone sandstones of the Cretaceous and Paleogene have satisfactory reservoir properties. Porosity ranges from 12 to 18 percent. Porosity and permeability increase toward the platform flank where the sedimentary cover thins. The main seals are silt-clay members of the Lower Cretaceous to Paleogene Buorkemyus Formation at 600 m, upper Paleogene El'gandin Formation at 60 m, and the Neogene Darkylakh Formation at 30 m.
Alazey high is located on the northeast margin of Indigirka-Zyryanka downwarp. In its vicinity gravity surveys indicate a zone of local highs expressed on the Upper Jurassic and Lower Cretaceous sediments and related to uplifted basement blocks. It is possible that this block structure developed in Late Jurassic time during a rift stage of formation of the downwarp. In addition, pinch out traps may have formed in the Upper Jurassic and Lower Cretaceous as well as in overlying Aptian- Albian sediments over toward Alazey high. In the thrust belt the main traps may be associated with frontal anticlines, which are not less than 5 km wide and a few kilometers long. These formed in late Miocene and Pliocene time.
Shale and siltstone of the Upper Jurassic Bastakh Series and clays of the Buorkemyus and El'gandin Fromations are recognized as source beds.
In the outer, platformal part of the downwarp the Upper Jurassic
Bastakh Series entered the oil window in Volgian time, was entirely
within it at Albian time, and had passed through it late in the Neogene.
In the inner, thrust zone part of the downwarp this series entered and
passed through the oil window in Early Cretaceous time. This was
before traps had formed in the overlying section, and consequently
this oil was lost. Conditions were more favorable for generation and
accumulation of hydrocarbons for the Cretaceous and Paleogene
sediments. The most downwarped part of the platform flank of the
Indigirka-Zyryanka downwarp is very favorable for exploration in the
2.0-3.5 km depth interval where paleo-temperatures were
115-120 degrees C. (Taken from Syundyukob and Gayduk, 1990;
digested in Petroleum Geology, Vol.30, No. 1, 1996; one map and
three paleo-temperature diagrams)
An added note: I once saw a postcard depicting the Tectonic Map
of the Soviet Union. It showed the Mid-Atlantic Ridge curving through
the Arctic and headed toward the Indigirka-Zyryanka downwarp.
Copyright 2001 James Clarke. You are encouraged to print out this
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