The study area is on the Chardzhou structural step in southwestern Uzbekistan along the border with Turkenistan. The section consists of Jurassic, Cretaceous, and Paleogene clastics, carbonates, and evaporites. Values of geothermal gradient for the various units are (degrees C/100 m): clays of the Paleogene - 5.5; sandy-clayey sediments of the Upper Cretaceous - 3.4; clays of the Turonian - 4.5; sandy-clayey sediments of the Albian - 3.3; clays of the Albian - 4; sandy-clayey sediments of the Neocomian-Aptian - 3.2; clays of the Neocomian - 3.7; halite-anhydrits deposits of the Upper Jurassic - 1.6; and carbonates of the Callovian-Oxfordian - 3.
Particularly significant are the times in the geologic history when the main thermally low-conducting units were deposited - thick clay members in the Paleogene, Turonian, Albian, Neocomian, and clastic Jurassic.
Temperatures have increased differently in different segments of geologic time. For example, clastics of the Lower-Middle Jurassic entered the 60 degree C zone (first desorption of hydrocarbons from organic matter) at the end of accumulation of the Neocomian clays. From then until the end of deposition of the Albian clays, temperature increased at the rate of 1.2 degrees C per million years (m. y). The most intensive increase in temperature (3.1 degrees C per m. y.) took place in the next segment of time up through deposition of the Turonian clays. The rate up through deposition of the Paleogene clays was 0.48 degrees C per m. y., and then up to the present it has been 0.23 degrees C per m. y. The largely humic character of the organic matter there led to the hydrocarbons being largely gas.
The limestones of the Callovian-Oxfordian began to produce liquid and gaseous hydrocarbons at the time of accumulation of the Albian sediments. The process reached of maximum intensity for liquid hydrocarbons in Late Cretaceous time.
The Neocomian sediments produced hydrocarbons at the end of the Cretaceous where organic matter was not less than 0.5 percent. At this time in a large part of the area they were either in the early gas window or partly in the oil window.
The Albian rocks have been subjected to less harsh temperatures than those that prevailed for the humic organics. In spite of this less subsidence, the amount of hydrocarbons generated by them is greater than for the Neocomian.
The sediments of the Turonian entered the early gas window only recently and also in only a limited area. Therefore, the scale of hydrocarbon generation has not been great.
Taken from Akramkhodzhayev, Pashkovskiy, Kirshin, and
Nabikhanov, 1981; digested in Petroleum Geology, vol. 19, no. 10.
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