The method of estimating the effect of the temperature-time factor on processes of hydrocarbon generation proposed by Lopatin (1976) is based on studies of processes of maturation of kerogen as a reaction in which the rate doubles with each 10 degrees C increase in temperature. The relative maturity of organic matter in a volume of sedimentary rock is proportional to some function of temperature and time of residence in a particular temperature interval and is expressed by a time-temperature index (TTI) (Waples, D. W., 1980).
Amu-Dar'ya basin is located in western Uzbekistan and eastern Turkmenistan. The sedimentary fill, which rests on a Hercynian base, consists of Lower-Middle Jurassic clastics, Upper Jurassic carbonates and evaporites, Lower and Upper Cretaceous clastics, and Paleogene clastics and carbonates. Area of this basin is 370,000 sq km. The basin extends farther to the east into Afghanistan.
Using data from17 wells in various parts of the Amu-Dar'ya basin, temperature and degree of maturation of organic matter (vitrinite reflectance) were used to elucidate processes of oil-gas generation in the Lower-Middle Jurassic, Callovian-Oxfordian, and Lower Cretaceous sedimentary complexes.
Based on comparison of vitrinite reflectance and modern formation
temperatures the following parameters were calculated:
1. Temperature at the top and base of each oil-generating complex.
2. Vitrinite reflectance and corresponding degree of maturation of
the organic matter.
3. Time of entry of base (or top) of sediments into each type window
(condensate, oil, gas) and time of exit therefrom.
Three zones of modern generation of hydrocarbons are recognized for the Lower-Middle Jurassic. The first, and dominant, is a zone of condensate generation, which is present over most of the basin. A zone of oil generation extends along the northeast margin of the basin, and a zone of dry gas occurs in the south of the basin.
In the Callovian-Oxfordian carbonate complex the northern third of the basin is oil-prone, and the southern two-thirds is largely condensate-prone. Dry gas is present in three isolated areas in the south.
Conditions favorable for oil obtain over most of the basin for the Lower Cretaceous sedimentary complex. A zone of condensate generation extends along the southern border of the basin, and a small gas-prone zone is present in the southwest.
This systematic distribution of zones of oil, gas, and condensate generation in Amu-Dar'ya basin as reflected in the temperature-time indexes can be used in assessing exploration potential.
References
Lopatin, N. V.,1776, Historical-genetic analysis of oil formation
using a model of even, continuous subsidence of oil-source beds
(in Russian): Ivz. Akad. Nauk SSSR, Ser. Geol., no. 8.
Waples, D. W., 1980, Time and temperature in petroleum formation:
application of Lopatin's method to petroleum exploration: Bull. A. A. P. G.,
vol. 64, p. 916-920.
(Taken from Kuleshov and Ignatova, 1990; digested in Petroleum Geology,
vol. 32, no. 4, three tables of depth, temperature, TTI, reflectance, and
time of entry into the various windows; three maps of zones of
hydrocarbon generation.)
Copyright 2001 James Clarke. You are encouraged to print out this News
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