Features of oil densities in the Nepa-Botuoba anteclise

The origin of oil within the Vendian terrigenous deposits overlying the crystalline basement of the Nepa-Botuoba anteclise remains a subject of ongoing debate. This study examines the lateral and vertical distribution patterns of oil density across the anteclise to elucidate constraints on hydrocarbon source, migration pathways, and generation history. A review of the scientific literature indicates a general consensus that reservoir oil density tends to decrease with increasing depth. For the present analysis, oil density data were obtained from the State Balance of Mineral Reserves of the Republic of Sakha (Yakutia) for fields distributed across the Nepa-Botuoba anteclise and the adjoining Predpatomsky regional trough. Comparison of 124 oil density determinations from 24 fields revealed a distinct trend of decreasing density upsection. The distribution of oil density data according to the lithological and tectonic elements of the Nepa-Botuoba anteclise established a relatively strong correlation between oil density and depth within the Nepa-Peleduy arch and a weak correlation within the Mirny ledge. A significant overall weakening of the correlation between oil density and depth was observed in carbonate deposits relative to terrigenous ones. The results indicate that the consistent decrease in oil density with shallower depth in the Nepa-Peleduy arch is caused by a filtration effect under conditions of lithological control of the deposits and the influence of the permafrost zone during production. The weak correlation in the Mirny ledge is hypothesized to result from repeated reformation of the deposits during tectonic processes, which does not exclude the presence of oils from different genetic sources. Further research is required to clarify the genesis of the oils in the Mirny ledge.