Мурманская МИЛЯ. 2018 г. №4.

EXPLORATIONS Sea varies from 7,000 to 500 m from east to west. The Admiralteyskaya-1 well located in the arch of the Admiralteysky megaswell in the Prednovozemelskaya structural area penetrated 60 meters of late Carboniferous carbonates overlapped against the erosional truncation by Permian terrigenous sediments. The Upper Devonian mid-Permian GS is sub-divided into two seismic sub-sequences namely the upper Devonian to lower Carboniferous sub-sequence and the mid- Carboniferous to mid-Permian sub-sequence which constitute the lower parts of the huge fans of the Sedov and Western Fobos troughs (Fig. 4). The maximum thickness of sedimentary lenses assigned to the Fobos trough fan makes 6,000 m. The Sedov trough fan is featured by the lower thickness value which can reach 5,000 m. The clinoform structure of the upper Devonian to mid-Permian GS allows high evaluating its’ oil and gas potential for the large stratigraphic traps of different types from the lithologically limited traps to the fault ones should be found there (Fig. 4) /3,4,5/. The overlying mid to upper Permian GS limited by the unconformity surface with the toplap elements at the top is divided into two large geoseismic sub-sequences. The sequence thickness varies from 250 to 4,500 m, in addition the lower sub-sequence at the western margin of the Alexandrovskaya high zone has been thinned out. On the whole the mid to Upper Permian GS demonstrates more steady sedimentation in contrast to the ‘avalanche’ sedimentation of the upper Devonian to mid-Permian GS. However the areas of stratigraphic traps development can be still observed there. Near to the upper Permian and Triassic sediments boundary presenting the unconformity of the toplap type, for instance, a huge bright spot anomaly which might be associated with the large hydrocarbon accumulation has been recorded (Fig. 5). The Triassic GS is limited at the top by a strongly pronounced erosional truncation surface which is most prominent in the Prednovozemelskaya structural area. A stratigraphic gap has been observed on well data between the Triassic and Jurassic sediments. Thus surface B features another pause in the sedimentation within the northern part of the East Barents megatrough which resulted into the emergence and denudating of kilometers long Triassic sediments in its’ marginal parts. The mid-Triassic formations at the sedimentary basin eastern margin have got prominent clinoform structure which is not typical for its’ other parts. The Jurassic GS is limited at the top by reflecting horizon B (J-K) associated with the upper Jurassic black clay and which is the regional marker in the Mesozoic part of the stack. The sequence thickness in the North Barents syneclise reaches 2,000 m. The Jurassic deposits within the Alexandrovskaya high zone and the Prednovozemelskaya structural area are being thinned out against the erosional truncation extending beneath the seabed. The largest gas and condensate fields in the Barents Sea such as the Shtokmanovskoye, Ludlovskoye and the Ledovoye fields are associated with the Jurassic deposits exactly. The Cretaceous GS. The Cretaceous deposits at the margins of the North Barents syneclise are eroded and extend beneath the seabed. A long pause in sedimentation of the Cretaceous sequence has been recorded within the post-Neocomian age. The early Cretaceous erosion depth (reflecting horizon Tn(K1nc)) which affected the Jurassic and the upper Triassic deposits in the Prednovozemelskaya structural area, is estimated by the first kilometers. The Cretaceous sequence thickness in the North Barents syneclise is over 1,000 m. The apparent dip of the Neocomian clinoforms to the centre of the North Barents syneclise is observed in the south-east direction as well as in the south direction from Franz Josef Land side. Thus, the highest volume of sediments in the northern part of the East Barents megatrough had accumulated during the upper Devonian and the Triassic ages. In addition surface A, separating those sequences shows no indication of any substantial tectonic processes such as rifting for instance. Intensive sedimentation however in substantially lower volumes took place in the Mesozoic age as well especially during the Cretaceous period. Most local anticline highs are associated with the Mesozoic part of the sedimentary cover exactly. It has been mentioned earlier that in the Paleozoic and Mesozoic sedimentary cover three sedimentation gaps were recorded, namely the mid-Devonian (unconformity III2), the lower Jurassic (unconformity B) and the post-Neocomian gap (unconformity Гп) which appear most distinctively in the marginal parts of the sedimentary basin, namely within the Alexandrovskaya high zone in the west and the Prednovozemelskaya structural area in the east. It is characteristic that in periods of tectonic stabilization the sedimentary cover denudation at the structures adjacent to the MURMANSK MILE • 4-2018