Geology and hydrocarbon potential of the Siberian Platform (Russia). In 6 vol. Vol. 3
дата публикации: 1993
The Angara-Lena Region lies in the southern Siberian Platform and coincides areally with the "Irkutsk Amphitheatre" distinguished by E.Zyiss (Obruchev, 1927) at the end of the last century. The region includes two superorder structures, namely the Angara-Lena Step, situated in the southeast, and the Sayan-Yenisei Syneclise, together with a number of subordinate structures, in the northwest. To the southeast, south and southwest, the Angara-Lena region is bounded by the Sayan-Baikal mountain belt, and the west by the Yenisei Ridge. To the northwest, north and northeast it is contiguous with the Baykit, Tunguska and Nepa-Botuoba regions. The region covers an area of 370.000 km(2). Structurally, the Angara-Lena Region, at the level of the oil- and gas-bearing Vendian Complex, comprises a large, northwestwards dipping monocline, complicated by a variety of lower-order structures. Structural configuration varies both areally and vertically within the sedimentary section. Other first-order structures, in the Vendian horizons, have been recorded in the Sayan-Yenisei Syneclise. Sharply defined structures, often linear in character, have been mapped in the salt-bearing Cambrian section, mainly around the periphery of the area under review, but sometimes also in the inner zones of the area. Trapps, karst and other types of structures play an important role in the highly complex structural configuration of the Angara-Lena Region. The sedimentary cover of the Angara-Lena region is comprised of Riphean, Vendian, Paleozoic, Mesozoic and Cenozoic sediments. The Riphean sediments are not widely distributed and are mainly comprised of clastic rocks. The northeastern part of the region is an exception, where the Riphean is represented by carbonates. The Vendian section unconformably overlies the Riphean, and forms a continuous sheet, a lower clastic section is separated by a regional hiatus from an upper carbonate sequence. A sequence of transitional beds, dated as Vendian-Cambrian, passes upwards into a salt-carbonate Cambrian section. The overlying Ordovician, Silurian, Devonian, Permian, Carboniferous, Triassic and Jurassic sediments are characterised by rapid facies variations, and by the presence of numerous erosional unconformities in the section. Within the sedimentary cover of the Angara-Lena Region, three oil and gas bearing complexes (sequences) have been delineated, namely the Vendian clastic section, the Upper Vendian-Lower Cambrian carbonate section and the Cambrian salt-carbonate section. The oil and gas complexes are subdivided into reservoir and seal units which are of both regional and local significance. Gas accumulations, and hydrocarbon flows in individual wells have been established only in sandstone members of the Chora Formation. In the underlying Riphean sediments, hydrocarbon flows have not yet been obtained. The Upper Vendian-Lower Cambrian oil and gas complex comprises Vendian carbonates, together with the lower and middle subformations of the Cambrian Usolka Formation. The complex has three potential hydrocarbon reservoir units. Within the Cambrian salt-carbonate complex, a number of beds and subformations are considered as potential oil- and gas-bearing reservoir units. There is a number of hydrocarbon trap types, in the Angara-Lena Region, e.g. anticlinal and fault-bounded structures, together with lithological and stratigraphical traps. Three gas-condensate fields have been discovered in the region, namely the Bratsk, Atovo and Kovykta fields, in sandstones of the Upper Chora Subformation of the Vendian complex. The Bratsk and Atovo fields, which are ready for development, are in small anticlinal traps. The Kovykta field is the largest in East Siberia and is still under appraisal. The patterns of distribution of organic matter in the Riphean-Vendian paleobasins of the Angara-Lena region are examined. The present day areal and vertical distributions of organic matter and soluble bitumoid within the sedimentary section are illustrated. For the Riphean and Vendian-Cambrian clastic-carbonate sequences, the areal distribution of thermal maturation zones is also shown (catagenetic transformation zones). A quantitative evaluation of the volumes of liquid and gaseous hydrocarbons generated in Vendian-Riphean source rocks has been made using a volumetric-kinetic method. The results of these calculations are presented in map form, where areas with different intensities of liquid (103 tonnes/sq km) and gaseous (106 cu m/sq km) hydrocarbon generation are delineated. The patterns of variation in composition of hydrocarbon fluids, both areally and vertically within the sedimentary section, are also described. Numerous analytical data on the composition of free gases, oils and condensates and their separate fractions are tabulated.
страницы: 164 p.