Fault structures and their geoelectric parameters in the epicentral zone of the 27 September 2003 Chuya earthquake (Gorny Altai) from resistivity data

This paper presents an integrated measurement technique based on DC methods (vertical electrical sounding, electrical resistivity tomography) which was used to identify faults and determine their geoelectric parameters in the western part of the Chuya basin. New information on the structure of the Chagan River valley located in the zone of the disastrous 27 September 2003 Chuya earthquake has been obtained from the results of these methods. Geoelectric cross-sections of the sedimentary sequence and the upper part of the basement were obtained from VES data, showing the block structure of the study area. Electrical resistivity tomography sections confirm the presence of a major fault between basement blocks of different heights and indicate the presence of faults bounding the valley on its right side and in the southwestern part.     

Early Mesozoic lamprophyres in Gorny Altai: petrology and age boundaries

The Mesozoic Chuya dike complex was recognized by R.V. Obolenskaya based on the similar mineral composition of dikes and their age characteristics. Lamprophyres occur along the large Terekta–Tolbonur and Kurai–Kobda shear zones. The Chuya complex was studied by the example of two areas, South Chuya and Yustyd, with different levels of erosional truncation. The dikes of the first area are localized in the South Chuya Ridge, where they cut Cambrian–Ordovician metamorphic rocks, and the dikes of the Yustyd area occur in the Devonian terrigenous blackshale deposits of the Yustyd trough. The dikes of these areas differ in structures, textures, the degree of carbonatization, and mineral composition. The performed studies of rocks and minerals confirmed that the dikes of both areas belong to the same complex. They helped to establish the regularities of the lithologic composition of the entire complex and its local areas and to substantiate the recognition of areas not only from their geologic position but also by the composition, structures, and textures of rocks and their mineral composition. Geochronological data show two stages of the complex formation: 236–234 and 250–242 Ma. The results of studies also demonstrate that the lamprophyres and coeval syenites of the Tarkhata massif are fractionates of the same parental melt and can be united into a hypabyssal-plutonic complex. Comparison with other Permo-Triassic lamprophyre complexes showed that the wide variations of the composition of the Chuya rocks and its trend as well as the geochemical anomalies are specific features of complexes of high-K lamprophyres.     

Uranium in saline lakes of Northwestern Mongolia

Analysis of major- and trace-element compositions of water in hypersaline soda closed basin lakes of Northwestern Mongolia and Chuya basin (Gorny Altai) shows high enrichment in ²³⁸U (up to 1 mg/l). Proceeding from new data, uranium accumulation in water has been attributed to (i) location of the lakes and their watersheds in potential provinces of U-bearing rocks and (ii) uranium complexing with carbonate in presence of carbonate (bicarbonate) anions. Among the explored hypersaline soda lakes of the area, the greatest uranium resources are stored in Lake Hyargas Nuur (about 6000 ton).     

Upgrading of the inverse slope method for quantitative interpretation of earth resistivity measurements

The present study is an attempt to upgrade the previously used semi-analytical inverse slope method. The upgrade method was applied to the interpretation of VES’s measured with different electrode arrays in two and three layers laboratory tank model. Then the method was applied on VES’s measured at different sites of different geologic conditions. The study showed that the inverse slope method is a quick method for obtaining depths to interfaces. The method can be implemented on a hand-held calculator or more efficiently on a small microcomputer where the data can be processed at a faster rate than that of conventional data acquisition systems. This allows the method to be potent tools for infield data analysis and more cost effective to assess the success of a sounding on-the-spot and modify the experimental set-up as appropriate. The comparison between the results obtained from this method and the traditional digital computerized methods indicated that the geoelectrical models derived from inverse slope method have a good correlation with the actual model. Also, when applying the method on VES’s measured by using Schlumberger and Wenner arrays at areas of different geologic setting, gave results in comfortable with the actual lithology obtained from the lithologic data of the neighboring wells. The results obtained from the lab experiment and field survey indicated that the inverse slope method has the potential to be applied on VES data collected with any electrode array, the advantage that is not available with all the traditional interpretation methods available nowadays.     

Paleoproterozoic granitoids of the Chuya and Kutima complexes (southern Siberian craton): age,petrogenesis, and geodynamic setting

Detailed geochemical, isotope, and geochronological studies were carried out for the granitoids of the Chuya and Kutima complexes in the Baikal marginal salient of the Siberian craton basement. The obtained results indicate that the granitoids of both complexes are confined to the same tectonic structure (Akitkan fold belt) and are of similar absolute age. U–Pb zircon dating of the Kutima granites yielded an age of 2019±16 Ma, which nearly coincides with the age of 2020±12 Ma obtained earlier for the granitoids of the Chuya complex. Despite the close ages, the granitoids of these complexes differ considerably in geochemical characteristics. The granitoids of the Chuya complex correspond in composition to calcic and calc-alkalic peraluminous trondhjemites, and the granites of the Kutima complex, to calc-alkalic and alkali-calcic peraluminous granites. The granites of the Chuya complex are similar to rocks of the tonalite–trondhjemite–granodiorite (TTG) series and are close in CaO, Sr, and Ba contents to I-type granites. The granites of the Kutima complex are similar in contents of major oxides to oxidized A-type granites. Study of the Nd isotope composition of the Chuya and Kutima granitoids showed their close positive values of εNd(T) (+ 1.9 to + 3.5), which indicates that both rocks formed from sources with a short crustal history. Based on petrogeochemical data, it has been established that the Chuya granitoids might have been formed through the melting of a metabasitic source, whereas the Kutima granites, through the melting of a crustal source of quartz–feldspathic composition. Estimation of the PT-conditions of granitoid melt crystallization shows that the Chuya granitoids formed at 735–776 °C (zircon saturation temperature) and > 10 kbar and the Kutima granites, at 819–920 °C and > 10 kbar. It is assumed that the granitoids of both complexes formed in thickened continental crust within an accretionary orogen.     

Tectonics and geodynamics of Gorny Altai and adjacent structures of the Altai–Sayan folded area

Packages of Late Paleozoic tectonic nappes and associated major NE-trending strike-slip faults are widely developed in the Altai–Sayan folded area. Fragments of early deformational phases are preserved within the Late Paleozoic allochthons and autochthons. Caledonian fold-nappe and strike-slip structures, as well as accompanying metamorphism and granitization in the region, are typical of the EW-trending suture-shear zone separating the composite Kazakhstan–Baikal continent and Siberia. In the Gorny Altai region, the Late Paleozoic nappes envelop the autochthon, which contains a fragment of the Vendian–Cambrian Kuznetsk–Altai island arc with accretionary wedges of the Biya–Katun’ and Kurai zones. The fold-nappe deformations within the latter zones occurred during the Late Cambrian (Salairian) and can thus be considered Salairian orogenic phases. The Salairian fold-nappe structure is stratigraphically overlain by a thick (up to 15 km) well-stratified rock unit of the Anyui–Chuya zone, which is composed of Middle Cambrian–Early Ordovician fore-arc basin rocks unconformably overlain by Ordovician–Early Devonian carbonate-terrigenous passive-margin sequences. These rocks are crosscut by intrusions and overlain by a volcanosedimentary unit of the Devonian active margin. The top of the section is marked by Famennian–Visean molasse deposits onlapping onto Devonian rocks. The molasse deposits accumulated above a major unconformity reflects a major Late Paleozoic phase of folding, which is most pronounced in deformations at the edges of the autochthon, nearby the Kaim, Charysh–Terekta, and Teletskoe–Kurai fault nappe zones. Upper Carboniferous coal-bearing molasse deposits are preserved as tectonic wedges within the Charysh–Terekta and Teletskoe–Kurai fault nappe zones.Detrital zircon ages from Middle Cambrian–Early Ordovician rocks of the Anyui–Chuya fore-arc zone indicate that they were primarily derived from Upper Neoproterozoic–Cambrian igneous rocks of the Kuznetsk–Altai island arc or, to a lesser extent, from an Ordovician–Early Devonian passive margin. A minor age population is represented by Paleoproterozoic grains, which was probably sourced from the Siberian craton. Zircons from the Late Carboniferous molasse deposits have much wider age spectra, ranging from Middle Devonian–Early Carboniferous to Late Ordovician–Early Silurian, Cambrian–Early Ordovician, Mesoproterozoic, Early–Middle Proterozoic, and early Paleoproterozoic. These ages are consistent with the ages of igneous and metamorphic rocks of the composite Kazakhstan–Baikal continent, which includes the Tuva-Mongolian island arc with accreted Gondwanan blocks, and a Caledonian suture-shear zone in the north. Our results suggest that the Altai–Sayan region is represented by a complex aggregate of units of different geodynamic affinity. On the one hand, these are continental margin rocks of western Siberia, containing only remnants of oceanic crust embedded in accretionary structures. On the other hand, they are represented by the Kazakhstan–Baikal continent composed of fragments of Gondwanan continental blocks. In the Early–Middle Paleozoic, they were separated by the Ob’–Zaisan oceanic basin, whose fragments are preserved in the Caledonian suture-shear zone. The movements during the Late Paleozoic occurred along older, reactivated structures and produced the large intracontinental Central Asian orogen, which is interpreted to be a far-field effect of the colliding East European, Siberian, and Kazakhstan–Baikal continents.     

Hydrogeophysical investigations for assessing the groundwater potential in part of the Peshawar basin,Pakistan

A large number of valleys and basin systems are present in the northwestern part of the Himalayas in Pakistan which form significant aquifers in the region. Hydrogeophysical investigations in the western part of Nowshera District, a part of the intermontane Peshawar basin, were undertaken to help to determine the availability of groundwater resources in the region. Thirty vertical electrical resistivity soundings (VES) were acquired using a Schlumberger expanding array configuration with a maximum current electrode spacing (AB/2) of 150 m in delineating the groundwater potential in the study area. The results of the interpreted VES data using a combination of curve matching technique and computer iterative modeling methods suggest that the area is underlain by 3 to 5 geo-electric layers. The interpretation results showed that the geo-electrical succession consists of alluvium comprising of alternating layers of clay, silty clay, fine to coarse sands, sand with gravels and gravels of variable thickness. High subsurface resistivity values are correlated with gravel–sand units and low resistivity values with the presence of clays and silts. The modeled VES results were correlated with the pumping tests results and lithological logs of the existing wells. The pumping test suggests the transmissivity of the aquifer sediments is variable corresponding to different sediments within the area. The gravel–sand intervals having high resistivity value show high transmissivity values, whereas clay–silt sediments show low transmissivities. It is concluded that majority of the high resistive gravel–sand sediments belong to an alluvial fan environment. These gravel–sand zones are promising zones for groundwater abstraction which are concentrated in the central part of the study area.     

Quartzite transformation as a consequence of late tectonic events in the evolution of the Chuya and Gargan blocks

We consider the evolution of the tectonic setting and geochemical features of rocks in ultrapure quartz areas of the Baikal region. These are the Chuya uplift of the northern Baikal region, with the Tyya quartz deposit, as well as the Bural-Sar’dag and other deposits in the Gargan block of East Sayan. Similarities are observed in the sedimentation and tectonic evolution of the regions. The conditions of the deposition of quartzites (forearc basins within siliceous-carbonate rocks with slight tholeiitic volcanism and a small amount of terrigenous material) ensure their relative initial purity. Low-grade quartzite-bearing strata are bedded or thrust by collision over an older high-grade basement consisting of a tonalite–trondhjemite–granodiorite association (Gargan block) or active-continental-margin metavolcanics (Chuya uplift). Migmatization of the basement and the intrusion of granitoids therein cause its uplifting and fault displacements and erosion of the overlying rocks. If the thrusting is associated with retrograde metamorphism of the basement and progressive metamorphism of the cover, backward motion can cause only quartz recrystallization with its purification and the grinding of the other minerals. This leads to the formation of lenses of “superquartzites,” mylonites, cataclasites, and breccias in the autochthon–allochthon suture and within the allochthon.     

Comparison between 2D imaging and vertical electrical sounding in aquifer delineation: a case study of south and south west of Samawa City (IRAQ)

The vertical electrical sounding (VES) survey was carried out by using Schlumberger array in (12) points distributed along two profiles, except VES 6 was not located within any profile. The 2D imaging survey was carried out using Wenner array in four stations. The VES results revealed the presence of two groundwater aquifers. The first is Euphrates aquifer and the second is Dammam aquifer. While the results of 2D imaging survey distinguished two secondary aquifers in addition to previous aquifers located within Dammam formation. Also, 2D imaging gave a more accurate picture for the distribution of electrical horizons especially for the depths which ranges between 2.5 and 73 m. While the VES gives information for larger depths than that of 2D, which ranges between the earth’s surface and a depth 140 m. The geo-electrical sections of VES showed electrical horizons (layers) with sharp boundaries. But the 2D inverse models revealed the lateral and vertical variations of the resistivity within each horizon especially for shallow depths due to large volume of data available in 2D measurement. These variations were not shown in the geo-electrical sections of VES points. Comparison of VES and 2D imaging techniques revealed that the 2D imaging was the best for determined the shallow aquifers as observed from this study. It can be concluded that 2D resistivity imaging and VES can provide very useful guide for borehole drilling particularly where there is no existing well.     

Integrated approach for identification of potential groundwater zones in Seethanagaram Mandal of Vizianagaram District,Andhra Pradesh,India

Identifying a good site for groundwater exploration in hard rock terrain is a challenging task. In hard rocks, groundwater occurs in secondary porosity developed due to weathering, fracturing, faulting, etc., which is highly variable within short distance and contributing to near-surface inhomogeneity. In such situations topographic, hydrogeological and geomorphological features provide useful clues for the selection of suitable sites. Initially, based on satellite imagery, topographical, geomorphological and hydrogeological features, an area of about 149 km² was demarcated as a promising zone for groundwater exploration in the hard rock tract of Seethanagaram Mandal, Vizianagaram District, Andhra Pradesh, India. A total of 50 Vertical Electrical Soundings (VES) were carried out using Wenner electrode configuration. An interactive interpretation of the VES data sharpened the information inferred from geomorphological and hydrogeological reconnaissance. Ten sites were recommended for drilling. Drilling with Down-The-Hole Hammer (DTH) was carried out at the recommended sites down to 50 to 70 m depths. The interpreted VES results matched well with the drilled bore well lithologs. The yields of bore wells vary from 900 to 9000 liters per hour (lph).     

Integrated geophysical interpretation for the area located at the eastern part of Ismailia Canal, Greater Cairo, Egypt

The integrated geophysical interpretation for the different geophysical tools such as resistivity and gravity is usually used to define the structural elements, stratigraphic units, groundwater potentiality, and depth to the basement rocks. In the present work, gravity and resistivity data were utilized for detecting the groundwater aquifer and structural elements, as well as the upper and lower surfaces of the subsurface basaltic sheet in an area located at the eastern side of Ismailia Canal, northeastern Greater Cairo, Egypt. Two hundred and ten gravity stations were measured using an Autograv instrument through a grid pattern of 50?×?50 m. The different required corrections were carried out, such as drift, elevation, tide, and latitude corrections. The final corrected data represented by the Bouguer anomaly map were filtered using high- and low-pass filters into regional and residual gravity anomaly maps. The resulting residual gravity anomaly map was used for gravity modeling to calculate the depths to the upper and lower surfaces of the basaltic sheet. The resulting gravity models indicated that the depths to the upper surface of the basaltic sheet are ranged between 26 and 314 m, where the shallower depths were found around the southern and eastern parts. The depths to the lower surface of the basaltic sheet are varied from 86 to 338 m, and the thickness of the basaltic sheet is ranged from 24 to 127 m, where the biggest thicknesses were found around the southern and northern parts of the study area. Forty-two vertical electrical soundings (VES) were carried out using Schlumberger configuration with AB/2 spacings ranged from 1.5 to 500 m. 1D quantitative interpretation was carried out through manual and analytical interpretations. The VES data were also inverted assuming a 3D resistivity distribution. The results from the 3D resistivity inversion indicated that the subsurface section consists of sand, sandstone, and sandy–clays of Miocene deposits overlying the basalts. Such basaltic features (of Oligocene age) are underlain by Gabal Ahmar Formation of Oligocene deposits, which are composed of sand and sandstone. Therefore, two aquifers were deduced in the area. The first is the Miocene aquifer (shallower) and the other is the Oligocene aquifer (deeper).     

Vertical extrusion and horizontal channel flow of orogenic lower crust: key exhumation mechanisms in large hot orogens?

A large database of structural, geochronological and petrological data combined with a Bouguer anomaly map is used to develop a two‐stage exhumation model of deep‐seated rocks in the eastern sector of the Variscan belt. An early sub‐vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18?20 kbar to depths equivalent to 8?10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8–9 kbar. Following the folding and vertical extrusion event, sub‐horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record the early vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root in response to SE‐directed Saxothuringian continental subduction. Fabrics that record the later sub‐horizontal exhumation event relate to an eastern promontory of the Brunia continent indenting into the rheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogenic crust over the Brunia continent in a north‐directed sub‐horizontal channel. This sub‐horizontal flow operated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re‐equilibration. The erosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow of lower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin. Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SE‐oriented Saxothuringian continental subduction in the Early Visean, suggesting a major switch of plate configuration during the Middle to Late Visean.     

巴彦浩特盆地火成岩的综合地球物理解释

采用综合地球物理方法识别巴彦浩特盆地火成岩,首先利用高精度重磁资料和大地电磁测深资料,初步确定火成岩的平面位置和深度,然后利用钻遇火成岩的已知测井资料,建立人工合成地震记录,在地震剖面上标定出火成岩并分析其波组特征,采用BP神经网络进行火成岩的精细识别.研究表明,综合地球物理解释是提高反演解释精度的有效途径.     

Ground water potential in fractured aquifers of Ophiolite formations,Port Blair,South Andaman Islands using Electrical Resistivity Tomography (ERT) and Vertical Electrical Sounding (VES)

Ground water occurs in weathered formations of unsaturated zone and fractured rocks of saturated zone. The ground water occurring in the unsaturated zone is not sustainable while the ground water occurring in the fractured rocks are sustainable if properly exploited. But, targeting the productive fractured rocks needs careful evaluation and systematic approach of geophysical survey owing to the heterogeneity, magmatic and metamorphic activities of multiple episodes of rocks. Hence, judicious planning in ground water exploration is warranted because of the huge money involved in drilling, manpower and time factor. In this context, an attempt has been made to locate the fractured rocks of ground water potential in the Ophiolite formations of Port Blair, South Andaman Islands using Electrical Resistivity Tomography (ERT) and Vertical Electrical Soundings (VES) since the ground water database of Andaman and Nicobar islands is poor as not much work has been carried out so far and the ground water is not properly utilised. The ERT have been carried out along different azimuth of fractures to ascertain the resistivities in vertical and horizontal direction and the conductivity and/or the resistivity of the varied fractures was also evaluated by spot VES. The 2-D Electrical Resistivity Images in conjunction with the geoelectrical parameters brought out by VES revealed that E-W fractures are expected to be productive fractures showing more conductivity as it is compared with the NE-SW and NW-SE fractures. The potentiality of the E-W fractures was also validated with the borehole data.     

Connection of the Late Paleolithic archaeological sites of the Chuya depression with geological evidence of existence of the Late Pleistocene ice-dammed lakes

The complexity of the age dating of the Pleistocene ice-dammed paleolakes in the Altai Mountains is a reason why geologists consider the Early Paleolithic archaeological sites as an independent age marker for dating geological objects. However, in order to use these sites for paleogeographic reconstructions, their locations, the character of stratification, and the age of stone artifacts need to be comprehensively studied. We investigate 20 Late Paleolithic archaeological sites discovered in the Chuya depression of the Russian Altai (Altai Mountains) with the aim of their possible use for reconstructions of the period of development of the Kurai–Chuya glacio-limnosystem in the Late Neopleistocene. The results of our investigation show that it is improper to use the Paleolithic archaeological sites for the dating of the existence period and the draining time of ice-dammed lakes of the Chuya Depression in the modern period of their study owing to a lack of quantitative age estimates, a wide age range of possible existence of these sites, possible redeposition of the majority of artifacts, and their surface occurrence. It is established that all stratified sites where cultural layers are expected to be dated in the future lie above the uppermost and well-expressed paleolake level (2100 m a.s.l.). Accordingly, there are no grounds to determine the existence time of shallower paleolakes. Since the whole stone material collected below the level of 2100 m a.s.l. is represented by surface finds, it is problematic to use these artifacts for absolute geochronology. The Late Paleolithic Bigdon and Chechketerek sites are of great interest for paleogeographic reconstructions of ice-dammed lakes. The use of iceberg rafting products as cores is evidence that these sites appeared after the draining of a paleolake (2000 m a.s.l.). At this time, the location of these archaeological sites on the slope of the Chuya Depression allows one to assume the existence of a large lake as deep as 250 m synchronously with the above paleolake or later. The location of the lowermost archaeological sites is evidence that a paleolake could have existed at an altitude below 1770 m a.s.l. in the Late Neopleistocene–Early Holocene. The absolute geochronology of the archaeological sites (cultural layers in multilayered sites, split surfaces on dropstones, etc.) can be useful for further reconstructions of the existence time, depths, and a number of ice-dammed lakes in the Kurai–Chuya system of depressions.     

New data on chemical composition of Jurassic–Lower Cretaceous sedimentary rocks of the Bureya basin (Far East of Russia)

The first data on the whole-rock chemical composition of Jurassic–Lower Cretaceous sedimentary rocks cropping out in the Soloni–Umal’ta river interfluve (Bureya sedimentary basin) are used for revealing the distribution of their rock-forming elements. It is shown that the clastic material originated mostly from acid igneous rocks, while their intermediate varieties, as well as quartz-rich sedimentary and metamorphic rocks, played a subordinate role. It is assumed that the bulk of the clastic material was transported from the west and southwest (Bureya massif) and a smaller share from the east. The most significant differences between the Lower–Middle Jurassic and Upper Jurassic–Lower Cretaceous rocks mark a break in sedimentation.     

Geoelectrical method in the investigation of groundwater resource and related issues in Ophiolite and Flysch formations of Port Blair, Andaman Island, India

Geoelectrical resistivity investigation using Vertical Electrical Sounding (VES) technique was conducted at Port Blair, South Andaman Island, to locate the fractures in different formations and to decipher its groundwater potential. A total of 40 VES were carried out covering the entire study area using Schlumberger electrode configuration out of which 34 VES fall in Andaman Flysch formations and the remaining VES in Ophiolite formations. The interpreted resistivity results were integrated with nine borehole lithologs for the subsurface analysis. The combination of VES with borehole litholog data has provided a close correspondence on subsurface hydrogeological conditions. The interpreted VES data of various formations showed drastic variations in the resistivity ranging from higher in Ophiolite, moderate in Andaman Flysch and very low in valleys of Andaman Flysch formations. The study further revealed that the weathered and fractured volcanics of Ophiolite groups of rocks and sandstone that occur in the Andaman Flysch formations constitute the productive water bearing zones categorized as good groundwater potential zone. Based on the geoelectrical parameters, viz., thickness of layers and the layer resistivity values, a groundwater potential map was prepared, in which good, moderate, and poor groundwater zones were demarcated. Further, numerical, spatial and litho-geoelectric models of resistivity were analyzed in terms of groundwater potential and these models have thus enabled to prepare a comprehensive groundwater development and management plans proving its efficacy in this art of exploratory investigations.     

塔里木盆地中央隆起区现今地温场分布特征及其与油气的关系

沉积盆地热状态研究不仅对于理解盆地成因演化具有重要意义, 而且还与油气生成和保存息息相关.根据塔里木盆地中央隆起区近120口钻井的试油温度资料和296块岩石热导率的测试结果, 获得了该区现今地温梯度、深部温度(1 000~5 000 m埋深及烃源岩顶界面)及大地热流的分布特征.结果表明, 该区热状态整体偏低(平均地温梯度为23.3 ℃/km, 平均大地热流为47.3 mW/m2), 二叠纪的岩浆活动对现今地温场已无影响.不同埋深的地层温度表现出与地温梯度及大地热流相似的分布模式, 即隆起区高、凹陷区低, 这一展布特征受基底起伏和形态控制.烃源岩顶部温度表明, 巴楚组、卡拉沙依组和良立塔格组等烃源岩层目前仍处于油气的有利保存状态; 巴楚隆起、卡塔克隆起西北部和古城墟隆起东部的中下寒武统烃源岩则处于良好的油气保存状态, 其他地区处于不利的油气保存状态.特别是, 该区已探明的油气田往往位于相对高温区, 并提出深部热流体的向上运移和聚集过程可能是造成异常高温的因素.这一发现表明盆地现今地温场特征与油气田分布具有良好的对应关系, 可为今后油气勘探提供地热学依据.      

Geoelectric section of the lithosphere of the Amur-Zeya sedimentary basin deduced from magnetotelluric soundings

The results of magnetotelluric sounding are analyzed along the Korfovo-Astashikha-Novosergeevka profile 200 km long in the south of the Amur-Zeya sedimentary basin. The Korfovo-Astashikha and Korfovo-Novosergeevka profiles were sounded in the AMT and AMT + MTS regimes with a step between the observation points of 1 and 5 km, respectively. The shape of the MTS curves, their variations along the profiles, the shape of the polar plots of the main and additional impedance, and the parameters of the heterogeneity (N) and asymmetry (skew) are characterized. The dimensions of the geological medium is estimated and methods of the interpretation of the magnetotelluric data are chosen. The geoelectric sections are constructed for the depths of 3 and 150 km. The structure and electric properties of the sedimentary cover, the Earth’s crust, and the upper mantle are characterized. The thickness of the sedimentary cover in the grabens of the basin attains 1.5–1.7 km. Blocks with various resistivities were identified in the basement. Based on the contrasting changing of the electric resistances, the thickness of the Earth’s crust was determined as 38–40 km, which agrees with that established by the seismic data. The geoelectric structure of the upper mantle of the basin is relatively simple. A layer of elevated resistivity from the first hundreds up to a thousand Ohm · m was identified in the background of the low electric resistivity (20–30 Ohm · m) of the mantle in the depth range of 50–80 km. This layer is discrete and divided on the blocks by the zones of the decreased resistivity penetrating to the middle part of the Earth’s crust and coinciding with faults of various origins. The petroleum prospectives are estimated for the individual grabens of the basin.