The history of the geological development of the south of the Nepa-Botuoba anteclise during Nepa and Tira time

The application of the sequence-stratigraphic analysis of seismic profiles throughout most of the sedimentary cover of the Nepa-Botuoba anteclise (NBA) is confined by lateral extension of thick dolerite and salt series partly dampened and deformed the signal. This problem was solved by using the sequence-stratigraphic modeling method, based primarily on lithological-facial and facies-cyclic analyses of reference wells core and tracking the boundaries of sequences over the area. The basis for the research was a combination of geological and geophysical data, including the geophysical well logging data specifically radioactive, density and acoustic logs, as well as the results of layer-by-layer lithological and sedimentological core description. A regional sequencing-stratigraphic model along the northeast-southwest profile in the transition continent-basin zone has been constructed relied on the results analysis of all obtained data. Four third-order sequences have been distinguished in the interval of the Nepa and Tira horizons, as also the system tracts have been characterized. The fifth sequence is timed to the Danilovsky horizon. Sequences SQ1 and SQ2 are indicative of the area development at the Nepa stage. The main ablation of terrigenous material in Nepa time was in process in the southeast direction. On the southern limb of the NBA due to the gentle slope of the basin bottom, material evacuation was less active. The southern part of the profile shows the features of sedimentation in the transitional coastal-marine zone of the relatively small epicontinental basin, where the terrigenous material was accumulated from the NBA and the Angara-Lena step. Designing of the well profile in the interval SQ3 and SQ4 allowed to show the sedimentation basin development in the Tira time and the carbonate platform evolution. For the first time for this area, deposits of low-stand systems tract (LST) for the fifth sequence, being carbonate-clay fans from the Tira carbonate platform formed during the pre-Danilov stratigraphic hiatus, have been identified. The position was clarified and the previously established stratigraphic unconformities were confirmed. The results obtained are consistent with and supplement the previously obtained data on sedimentation stages in the Prilensk-Nepa structural-facies zone. The sequence-stratigraphic profile construction had made it possible to clarify the correlation and prognosticate the reservoir areal extent.

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