Two Models of the Middle Devonian Petroleum System in the Volgograd Region: the Pros and Cons
Leonid Anissimov1 and Stanislav Chizhov2
1 LUKOIL-VolgogradNIPImorneft, Russia
2 LUKOIL-Niznevolzhskneft, Russia
Russian geological idea is always a challenge for the dominating biogenic origin of oil to maintain the permanent struggle between Hephaestus and Poseidon in geology. The last demonstration of it was on the conference “Earth Degassing” that was held in Moscow, 2002.
Nevertheless now some Russian geologists are going to conclusion about polygenetic origin of petroleum. Source rock concept is the main point in the organic theory of oil origin. Two outstanding technologies have been elaborated last two decades: basin modeling and rock evaluation. They have created the quantitative approach to characterize the petroleum origin and migration during the basin evolution and to predict undiscovered resources. Desorption process is a week branch in this theory.
Polygenetic concept in the petroleum origin can give a consensus between evidences on the high concentration of reduced carbon in sedimentary rocks as a source of oil and difficulties on the realization of expulsion processes to overcome electrostatic forces of absorption. “Reactor” approach may be productive to transfer on the coarse scale the processes from geodynamic models to hydrodynamic models to understand the mechanisms of expulsion. Volume of water involving in organic reactions and petroleum generation as well as in the expulsion process is the most important parameter in these models so it means an yield of gas, oil and nonhydrocarbon components. Collision, subduction and spreading have been taken into consideration as principal geodynamic models.
Collision when two continental plates collide head-on with forming mountain range. It is impossible to create great voids in such conditions. Significant increasing pressure and moderate increasing temperature in sedimentary rocks give moderate yield of non-hydrocarbon gases. Materials on the collision zones of Scytian and Anatolian Plates prove this thesis.
Subduction where the oceanic plate pushes beneath the continental plate with some portions of sea water. Magma rise and hydrothermal activity are accompanied these processes. Increasing pressure and temperature in the sedimentary section when water-rich rocks dive deeper provoke extraction processes especially in zones with great content of water. These zone produce oil and gas and their ratio depends on the distribution of noncompacted (hydrous pyrolysis) and compacted (anhydrous pyrolysis) zones.
Spreading as a consequence of the extension of crust creates a deep valley. Surface water can move into opening a space with decreasing pressure and temperature in the geological rock sequence. Dynamics of this geological process can be presented as follows:
- Fissure and fault zone in the crust (rifting);
- Penetration of surface water to the great depth;
- Landsliding, lictric faulting and forming graben system;
- Increasing temperature and hydrous pyrolysis of rocks in graben zone.
Relative low temperature yields mainly oil.
The Middle Devonian sediments of the Volgograd Region are a transgressive formation that overlain Precambrian basement rocks, Silurian and Lower Devonian sediments. They have been taken into consideration because of two rift events occurred before and after their sedimentation. On other hand, the geographic extent of this petroleum system is separated from the petroleum systems of the upper stratigraphic levels, it means the independence of the essential elements (source rocks, reservoir rocks, seal rocks, and overburden rocks) and processes (trap formation and generation-migration-accumulation of petroleum) from other petroleum systems.
Rifting and post-rift history is a special interest in the Volgograd Region because these events control the processes of hydrocarbon accumulations and distribution of oil and gas fields. Two rift systems have been postulated on the eastern slope of the Voronezh massif: Precambrian Pachelmsky and Late Devonian Mammon rift zones. The Pachelmsky rift system (aulacogen) has developed along the trend of the former orogen filled by the great Ripheian rock sequence, 2000 m in thick. Evolution of one branch of this rift system has been transformed in the intracratonic Umet depression. The Late Devonian Mammon rift system stretches parallel to the Dnepr-Donetsk aulacogen (fig.1). This rift intersects the central part of the Voronezh massif and is opened to the Umet depression.
History of the Mammon rifting was short and was finished by dispersion of rifting process. Crossing two rift systems provoked. subsidence and extensional movement associated with normal faults, magmatic activity, increasing basement fluid flow, reef construction and other related phenomena during the Late Devonian period.
To explain the morphology of the tectonic structures in the extensional parts of a basin the conception of gravitational slides has been taken into consideration. Presence of normal faults in the narrow stratigraphy interval and the bow shape of fault patterns prove that the normal faults around the depressions are commonly listric and have been recognized as segments of gravitational slides. Tail parts of these segments in the central part of depression may be stressed, forming the compression-induced structures. On the basis of the proposed conception and actual location of rift trends and depressions, the formation of intracratonical basins on the east plunge of the Voronezhsky massif is as follows:
Prior to the Precambrian period, on the spot of the Umet depression there was formed a meridionally stretched rift zone with the axial line passing approximately over recent Dono Medveditsky dislocations. During the Proterozoic and Paleozoic the rift zone was filled with terrigenous and carbonate rocks and in the Middle Devonian Period there appeared conditions for reef formation in the Late Devonian.
At the beginning of the Frasnian Age a zone of maximal subsidence shifts to the east. The subsidence is followed by faults formation that faulted strata of the Terrigeneous Devonian. Faults are absent in the overlaid Carbonate Devonian
Subsidence of the Mammon depression was preceded by weakened zone occurrence as a result of tectonic stresses spacing due to intensive bowing of the Dneprovsko-Donetsky paleorift. At the border of Zhivetsky and Frasnian Ages on the south-east area of the Voronezh massif (in a zone of the future reef trough), fault activation followed by submarine explosive volcanism with the formation of tuff, tuffite and tuff sandstone. Initial stages are expressed as explosion of the alkaline – basaline composition while final stages – explosion of trachyliparite and liparite composition.
Fault opening, being especially intensive in the south-east area of the Mamon depression, peaked at the Late – Frasnian Age that resulted in alkaline – basaline lava flow. It is found that lava flow occurred under continental conditions in two phases giving covers of the Petinsky and Yevlano-Livensky Age.
Effusive volcanism is connected with periods of the most active uplift in the Upper – Devonian arch zone of the Voronezh massif. Westward, within Volgograd region, effusive rocks are found on the Khoper monocline. Diabases have been discovered by well 166-Terkinskaya at the interfaces between the Semiluksky horizon and nonconformably occurred deposits of the Voronezhsky horizon in the interval 1218-1228 m, whereas rocks of the crystalline basement in the same well are observed at the depth of 1546 m. The mineralogical composition and structure of diabases refer to dike formations.
Basalts are observed in some wells on the Khoper area. They have a composite structure and the mineralogical composition varying through the section. Amygdules irregular or round-oval in shape, often oriented and having inclusions of calcite and crystallized basalt glass are the characteristic feature of basalts. Up the section basalt gradually transfers to the flinty-carbonate rock with an admixture of crystallized basalt glass products. Some authors suppose that basalt formation is accounted for by submarine flow of the basic magma and it occurs as a sill. Meridional and crossing faults are a special feature of the zone. This presents favorable conditions for intensive volcanic activity followed by flow of the magma through faults.
The origin of the Umet trough in Frasnian Period was accompanied by recharge of subsurface fluids enriched by carbon dioxide as post-volcanic processes. This is correlated with recent models of M. Hovland (1990) who suggested that carbonate reefs form due to fluid seepage. Morphology and internal structure of elongated reef bodies with the width of 1 km proves the seepage-associated reef formation model. Numerous reefs along the margin of Umet depression formed favorable traps for oil accumulations and ancient dislocations provided hydrocarbon migration from deep source beds.
Numerous oil shows and small oil accumulations have been discovered in the Middle Devonian sediments around the Devonian post-rift depression (fig.2). These are light oils (810-830 kg/m3) with low content of sulfur (0.1-0.2 %) and low gas content. It has been postulated that source rocks of these oils are Devonian shales, the migration pathways are very short. This is the first model of the Middle Devonian petroleum system. Good correlation of some geochemical parameters between oils and organic matter in rocks is the principal supporting point for this model.
On the contrary, there are some facts that demand to accept other model. Along with unsaturated oils there are some gas-condensate accumulations in the same strata and under the same thermodynamic conditions. Some parameters such as paraffin content from 2% up to 35%, low content of sulfur in oils in the rocks enriched by sulfate are difficult to explain on the basis of in-situ model. These facts demand to engage the external sources of hydrocarbons and accept the possibility of petroleum or some key components migration from the crust. Geodynamic situation and petroleum parameter distribution are favorable to accept the second model of oil origin in the Middle Devonian sediments.