w„,L.hon -international Coooetation on Investigation and Research of Manne Natural Resource and Environmgf ABOUT INVESTIGATION OF THE TEKTONOSPHERE DEEP STRUCTURE AND THE PREDICTION OF OIL-AND-GAS PRESENCE IN THE DELTAS OF THE RED AND MEKONG RIVERS V.M Nikiforov R.G Kulinich, N.G Shkabarnya, I.V Dmitriev V.lirichev Pacific Oceanological Institute, Vladivostok, Russia, nikiforovv@mail.ni Introduction The importance of the deep factors in the formation of oil-and-gas deposits is recognized by most researchers, regardless of their views on the origin of hydrocarbons: Organic or inorganic The spatial relationship between the oil-andgas areas and deep faults, weakness zones in the Earth's crust, heat flow anomalies can be considered as established fact [1] A large amount of electromagnetic soundings in the Far East of Russia, in the transition zone from continent to marginal seas where several large industrial oil-and-gas bearing basins are located, has revealed the geoelectric criteria prediction of oil-and-gas presence, taking into account the fluid regime of deep horizons of the crust and lilhosphere In addition, as revealed in recent years, the concentration of oil-and-gas deposits is controlled by zones of anomalous conductivity in the crust and subcrustal lilhosphere [2] Detection of such zones using electromagnetic soundings, tracking them over large areas, the study of relations with the structural plan, tectonics, material constitution contributes to a more efficient prediction and conducting oil and gas exploration Such an approach may be useful in studying the resources of the EastVietnam Sea, characterized by the fact that the patterns of distribution of hydrocarbon deposits have specifics, significantly different from the classical ones Geoelectric section of the tectonosphere in the transition zone from continent lo the marginal sea (for the example of the Russian Far East) The concepts of normal and abnormal differentiation of the electrical resistivity are important for understanding the overall tectonosphere structure, its petrology, fluid dynamics and direction of the underlying processes, ways of the deep substance transportation There are different approaches to constructing models of the electrical resistivity distribution in the depth [3] Important role in this series is the standard gradient-section model the assumption L.L Vanyan [4] The standard (normal, planetary) model was obtained as a result of the interpretation of magnetic-variation curve constructed from the data analysis worldwide network of magnetic observatories and magnetotelluric sounding for all the earth shields According to this model, the electrical resistivity of crustal rocks and upper mantle decreases steadily from 200,000 Ohmm at a depth of about 20 km to 20 Ohmmal a depth of 300 km V.M Nikiforov H$i Ifaio khoa hoc - Hop tac Qu6c ik troDR dieu tra, pfihiep ctru tai nguyen vi nl6i tnianR bien The standard model is deep sequence of electrical resistivity values of the dry rocks under conditions of temperatures and typical pressiu-es for areas with normal heat flow equal to 45 mW/m^ The increase in heat flow or fluid saturation of deep rocks lowers electrical resistivity relative to a standard incision The increase in heat flow or fluid saturation of deep rocks reduces the electrical resistivity level relative to the standard modeL Experimental data analysis on the results of deep electromagnetic soundings in the transition zone fi'om continent to marginal seas of the Russian Far East, showed a significant difference between the regional deep geoelectric model and the standard model Instead of a monotonic decrease with depth, the electrical resistance changes abruptly, forming three main layers The parameters of these layers beneath the continent and the marginal sea are different (Fig 1) ^^m^!^-' V jilin KO I~>(1()\ ll' + -.-FSiiit y I! V / A A y^ + -f +- Sfl-150()hmi ,v 5-20 Ohmm A 48,7 A 2IHI A 2.10 300 A 20-50 Ohmm ^ A ^ ^ norniiit pUncl Lfcoclccirk section {by I.J Vany.tn) CD'' 65.0 SI.2 07,5 Fig Geoelectric section of transmit zone from continent to marginal sea - Earth's crust; - subcrust lilhosphere; - asthenosphere, - upper mantle; - borders of tectonosphere layers; - anisotropic electric conductivity zones in subcrust lilhosphere identifiable with the largest shearing systems of region; anisotropic electric conductivity zones in lower part of Earth's crust identifiable with hydration of crust basic composition rocks Thickness of the upper layer, which is relatively high-resistivity (1000 ohm-m), beneath the continent is 35-40 kin, and under the marginal seas bottom is reduced to 20-35 km Beneath it lies the subcrustal lilhosphere formation that differs from the standard model in electrical resistivity significant reduction up to 80-150 ohmm The bottom of this layer is located beneath the continent at a depth of 150 km and beneath a marginal sea at a depth of 80-100 km Below it the layer of lowresistivity (5-20 ohmm), usually identified with the asthenospheric layer, is bedded Below the asthenospheric layer the section closes to the standard model Low levels of electrical resistivity of the asthenospheric layer beneath the continent where the heat flow does not exceed 45 mW/m^ can not be explained by V.M Nikiforov Workshop- "International Cooperation on Investifiation and Reseait:h of Marine Natui^ Resourt:e and Environnv ,|- the phenomenon of the dry rocks melting [4] This circumstance forces suggest that the upper mantle rocks melting occurs ig the presence of fluid, which lowers the melting point of the ultrabasic rocks to 1200 C, achievable at a depth of 150 km, even in conditions of normal heat flow Relatively low electrical resistivity values in the subcrustal lithosphere can't be associated with the melting of rocks A possible reason for the decrease of the resistivity level is the saturation of lithospheric rocks with graphite, could be explained by following: in the vertical migrations of deep gases, including H2, H2O, CO, CO2 at a temperature below 700 °C, the Boudoir reaction proceeds: 2CO = C (graphite) -1- COj Decreasing temperature [5] should cause the start of the process nf hydrocarbons formation under the scheme: CO -I- H2 = CP -I- HC -I- CO2 + H2O, where CP - compression products, HC - hydrocarbons Water which is formed by chemical reactions at temperatures below 500 °C interacts with the host rocks: Enstatite-I- Water = Antigorite -i- Forsterite; Antigorite -I- Water = Talc + Forsterite; Forsterite + Water = Serpentinite -i- Brucite Hydrous minerals: talc, antigorite, serpentinite - occupy a greater volume than the original anhydrous minerals Because of this the healing of deep gas migration channels occurs A regional cover, which prevents the promotion of fluids in the crust, IS established This provides a high level of electrical resistance mJ^T.\ !*" " T " " 8f==1^^ !