Olistoliths are the exotic rock fragments in masses of chushed rock (olistostromes) produced by
gravity sliding associated with thrusting. The term was introduced by Flores in 1955 and applied
further in the Mediterraneanregion by Klemme in 1958. Olistostromes are now being played for oil
in Chechen and Dagestan in the East Cis-Caucasus.
The Maykop sediments of the North Caucasus are largely clays and are Oligocene-early Miocene
in age. They have been important source beds for hydrocarbon accumulation in both overlying
middle Miocene and underlying Eocene-Upper Cretaceous reservoirs. Significant oil and gas
discoveries have been made in sandstone and siltstone reservoirs of the Maykop itself in the West
and Central Cis-Caucasus. In spite of numerous shows the Maykop of the East Cis-Caucasus has
been regarded as only a seal for pools in the underlying Eocene and Cretaceous reservoirs. This
thinking has now been revised.
The Maykop Series in the Cis-Caucasus is subdivided into six formations with total thickness
of 1100-2200 m. The lowest-most of these are the Miatly and underlying Khadum Formations.
The Miatly carries some beds of siltstone and sandstone and is 300-900 m thick. The Khadum is
a limy clay with beds of marl and sandstone with total thickness of 50-400 m. Within the Maitly
Formation are recognized several complexly stratified olistolith units. They are commonly
conformable with bedding. Their total thickness in the Miatly is 100-300 m, which is 20-50 percent
of the total thickness of the formation. The olistoliths are largely carbonates from the section
below the Maykop. Total volume of these olistolith carbonates in Achisu field in Dagestan is
5.8 cubic kilometers, and total volume in the outer zone of the Dagestan wedge with an area
of 6,000 sq km in 150 cubic kilometers. The Maitly Formation is thus a transitional unit in the
Maykop Series from the complex structure of the underlying Eocene-Cretaceous to the relatively
simple structure of the upper Maykop. Its origin was in the Oligocene phase of folding, as
indicated by the abundance of olistoliths within it. The intensive deformation of the Eocene
and Mesozoic rocks was followed by decrease in tectonic activity and transgressive overlap by
the upper Maykop sediments. Plasticity of the clays of the upper Maykop has led subsequently to
crypto-diapirism at the crests of Miocene structures.
Following the Oligocene tectonism, hydrocarbons began active migration from Maykop source
beds into adjacent siltstones and overlying sandstones. Fault zones as well as fractured carbonate
olistoliths of the Maykop Series were traps for oil and gas. The Talgi oil seep is in an olistolith.
Commercial flows of oil have been recovered from the Maykop in numerous well that targeted
the Eocene-Cretaceous section. In spite of intensive shows, these were regarded as but chance
phenomena, and the wells were shut in, or the Maykop was closed off with heavy mud or cement.
Gigantic olistoliths commensurate in size with local structures constitute closed reservoirs
embedded in clays. They are similar in form to reef and bioherm traps. Their isolation is reflected
in anomalously high formation pressures (Petroleum Geology, vol 34, no. 2, p. 118, 2000
in press; largely from Sharfutdinov and Sharfutdinov, 1998).
Copyright 1999 James Clarke. You are encouraged to copy this News Letter and to forward
it to others.