THE GOL’CHIKHA FORMATION (upper Bathonian-lower Boreal Berriasian) OF THE YENISEI-KHATANGA DEPRESSION (west of the North Siberian lowland)
The uppermost Bathonian-lowermost Boreal Berriasian clay horizons (Gol’chikha Formation) of the Yenisei-Khatanga regional depression are regarded as probable oil source strata. Considerable core recovery in the Middle Jurassic to Lower Cretaceous sections from the boreholes drilled in the Paiyakhskaya well site and the presence of oils in the overlying strata of the Shuratovo Formation permit us to carry out integrated stratigraphic (bio-, litho-, chemo-, and seismostratigraphic) and geochemical (organic matter and oils) studies of the entire section of the Gol’chikha Formation and boundary beds, to reveal oil-producing horizons, and to compare the genotype and maturation level of their oils with those of the potentially oil source organic matter (OM) of the rocks. A detailed biostratigraphic zonation of the sections of the Gol’chikha Formation based on microfossils has been carried out. Comparison of δ<sup>13</sup>C<sub>org</sub> variations in the Volgian and in the lower beds of the Boreal Berriasian with those in the Barents Sea shelf and in the northeast of East Siberia provided the basis for more accurate definition of the boundaries of stages and substages in the intervals free of fossils in the Paiyakhskaya area. The studied section of the Gol’chikha Formation has been divided into eight lithologic members calibrated with bio- and seismostratigraphic units. The distinctive features allowing the definition of the upper boundary of the Gol’chikha Formation are proposed using GIS data. Analysis of bio- and chemostratigraphic data allowed the correlation of the seismic reflecting horizons defined in the Gol’chikha Formation and its boundaries with the geologic section and relevant litho- and biostratigraphic units. It has been established that the strata with the highest content of organic matter consist of the upper part of the Gol’chikha Formation (the Upper Volgian and basal Boreal Berriasian). According to the vitrinite reflectance data, the OM catagenesis in the Upper Volgian interval corresponds to the oil window, which is confirmed by pyrolysis data. Thus, these strata can be considered oil-producing. The low δ<sup>13</sup>C<sub>org</sub> values confirm the predominantly marine OM composition. Analysis of oils from the Cretaceous productive strata of the Paiyakhskaya area shows that they formed from the marine OM of the upper part of the Gol’chikha Formation at the same accumulation stage. The comprehensive studies of the Gol’chikha Formation in the Paiyakhskaya well site and complete stratigraphic coverage of the sections confirm that they can be considered a hypostratotype.