Problem of the formation of solid bitumens in the Tengiz deposit

Results of long-term petrological and geochemical investigations of solid bitumens (SB) in the Tengiz deposit are presented. It has been established that they belong to the class of epiimpsonites (Jacob, 1989), higher kerites (Radchenko and Uspenskii, 1979), or adulgites (mature bitumens) (Ammosov et al., 1987). Facies features of the accumulation of sediments, as well as tectonic and paleotemperature history of the Tengiz deposit are given. The upper carbonate sequence (3950–4500 m), which includes the main SB and oil reserves, accumulated on a shallow-water shelf that was favorable for the formation of collectors, whereas the lower sequence accumulated in a deep-water shelf belongs to oil source rocks in terms of facies features. Paleotectonic evolution of the Tengiz deposit includes three major stages: first subsidence, inversion, and repeated (second) subsidence. Each of these stages played an essential role in the genesis of SB formed from oil of an ancient deposit during the first subsidence. In the course of inversion, the ancient oil was transformed into maltha and asphalt (weakly altered bitumens) that were altered into epiimpsonites during the repeated subsidence. This process was accompanied by the release of light low-boiling hydrocarbons that made a significant contribution to the formation of the present-day oil pool in the Tengiz deposit.Translated from Litologiya i Poleznye Iskopaemye, No. 2, 2005, pp. 205–214.Original Russian Text Copyright © 2005 by Karpov, Stepanov, Tverdova, Sheikina.