Summary The discovery of Nertseta field, the largest oil find in Russia in 2016, has revitalized interest in petroleum potential of the frontal part of the Polar Urals thrust belt. This field includes oil pools bypassed in the past, which substantiate the reinterpretation of the legacy data with use of new geological concepts. This work showed that the critical elements in understanding of the petroleum systems are: (1) spatio-temporal relationship between trapping formation and hydrocarbon generation and expulsion; and (2) identification of the porous reservoir facies in the highly heterogeneous carbonate sequences. It is interpreted that in the northeastern part of the Polar Urals thrust belt multiple traps formed in course of the Late Permian Uralian orogeny had been destroyed by the younger structural overprint related to the Triassic Pay Khoy folding. Much better conditions for oil and gas accumulation are interpreted to be present in the southeastern part of the Polar Urals thrust belt. They include thrust related trapping configurations and salt-cored anticlines. Of prime interest are the Upper Devonian and Lower Permian reefs, their drapes, zones of karstification.
Трансформация энергетической отрасли актуализирует пересмотр приоритетов нефтегазовой геологоразведки. В условиях дефицита инвестиций наибольший интерес представляют зоны нефтегазонакопления, которые обладают потенциалом высокой плотности запасов и располагаются по соседству с существующей инфраструктурой. Этим критериям в России и Казахстане с наибольшей вероятностью соответствуют складчато-надвиговые зоны восточного и южного обрамления супербассейнов Восточно-Европейской платформы. До коллизионной складчатости они принадлежали значительно более крупной Уральской континентальной окраине. Она являлась долгоживущими очагом нефтегазообразования, вклад которого в развитие нефтегазовых систем этих бассейнов недооценен. Использование современных технологий геологоразведки с применением новых знаний, с большой вероятностью, позволит существенно нарастить запасы нефти и газа в этих районах. В статье рассматриваются геологические предпосылки открытия в них новых месторождений. Приводятся примеры, которые показывают перспективные направления геологоразведочных работ.
We have developed a new interpretation of the Povarnitsa High, located in the southern part of the transition zone between the Chernyshov swell and the Kosyu-Rogov trough in the Polar Urals foreland. Petroleum exploration in this area took place in the 1980s and resulted only in an uneconomical oil deposit. Reprocessing and reinterpretation of vintage data and a newly acquired 3D seismic survey along with nonseismic geophysical data provided new insight into the geologic setting of the area. It found a subthrust extension of the Povarnitsa High, overlain by a backthrust sheet floored by upper Ordovician evaporites. This backthrust sheet interpretably resulted from the squeezing of the salt diapir during the late Permian contraction. Stratal patterns of the Silurian-Carboniferous section adjacent to the salt structures suggest early precontraction initiation of the halokinetic deformation, which influenced the distribution of reservoirs such as reefs and oolites. We consider the salt-cored subthrust structural culmination a principal hydrocarbon prospect. It includes multiple reservoir-seal pairs in the Silurian-Permian section at depths ranging from 1 to 4 km. The updated structural model of the Povarnitsa High area suggests that the vintage wells penetrated prospective intervals at the periphery of the closure approximately 1 km downdip from the subthrust crest of the prospect.
ABSTRACT Exploration boreholes and seismic reflection data in the foothills zone of the northeastern Caucasus obtained during the last decade reveal considerable differences between the surface and subsurface structures of the area. The new data suggest that this zone may be viewed as a buried thrust belt. The allochthonous assemblage of the belt is formed mainly by stacked north-verging thrust sheets made up mostly of Mesozoic carbonates and sandstones bounded at the top and bottom by conjugate detachment surfaces. The thrust sheets are interpreted to be inserted into the clastic section of the Terek-Caspian foredeep along the base of Oligocene - Early Miocene mudstones. The blind subsurface thrusts have been active since the Late Miocene. Strata above and below the allochthonous unit are characterized by independent styles of deformation. The mildly deformed foredeep clastics create a hinterland-facing monocline that is passively uplifted by underthrusting. These rocks mask the subsurface structures. Tectonic wedging in the Dagestan thrust belt was facilitated by the mechanical weakness of the overpressured mudstones of the Maykop Formation (Oligocene - Lower Miocene) which prevented transmission of the compressional stress across it. The interpreted geometry of the thrust belt front implies a shortening ranging from 20 to 50 km. This interpretation of the regional structure suggests a petroleum exploration play consisting of structural traps within the buried antiformal stacks. Regionally, oil- and gas-bearing Upper Cretaceous and Upper Jurassic carbonate rocks involved in thrust sheets and sealed by Maykop mudstones are considered primary prospecting targets. RESUME Des forages d'exploration et des donnees de reflection sismique provenant de la zone des contreforts des monts Caucase du nord-est obtenus durant la derniere decennie, revelent des differences considerables entre les structures de surface et de subsurface de la region. Ces nouvelles donnees suggerent que cette zone peut etre envisagee comme une ceinture de chevauchement ensevelie. L'assemblage allochtone de la ceinture est constitue surtout de nappes de charriage empilees, situees a la limite nord et composees surtout de roches carbonatees et de gres mesozoiques limites au haut et au bas par des surfaces de decollement de meme direction. Les nappes de charriage sont interpretees comme etant inserees dans la coupe clastique de l'avant-fosse Terek-caspienne le long de la base des pelites de l'Oligocene - Miocene inferieur. Les chevauchements de la subsurface sans affleurements ont ete actifs depuis le Miocene superieur. Les strates au-dessus et en-dessous de l'unite allochtone sont caracterisees par des styles de deformation independants. Les sediments clastiques legerement deformes de l'avant-fosse creent un monoclinal faisant face a l'arriere-pays et passivement souleve par sous-charriage. Ces roches cachent les structures de la subsurface. L'insertion tectonique dans la ceinture de chevauchement Dagestan fut facilitee par la faiblesse mecanique des pelites en surpression de la formation Maykop (Oligocene - Miocene inferieur) qui empecha la propagation de l'effort de compression a travers cette derniere. L'interpretation de la geometrie du front de la ceinture de chevauchement laisse supposer un raccourcissement allant de 20 a 50 Km. Cette interpretation de la structure regionale suggere une region d'exploration petroliere qui consiste en pieges structuraux au sein des empilements de structures anticlinales ensevelies. A l'echelle regionale, les roches carbonatees du Cretace superieur et du Jurassique superieur renfermant du petrole et du gaz, impliquees dans les nappes de charriage et scellees par les pelites de la formation Maykop, sont considerees comme des cibles d'exploration primaires. Traduit par Marc Charest.
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