Depositional environments of the osa horizon in the central and eastern parts of the Nepa arch

Category: 15-1
I.P. Malkov


UDC 550.8 (571.1) 


 I.P. Malkov 

Lomonosov Moscow State University, Moscow, Russia


Abstract. Results of executed work allowed to build cross-well correlations and lithofacies maps of the Lower Cambrian deposits of the Osa horizon in central and eastern parts of the Nepa arch and to study reservoir quality in this region. A reconstruction of depositional environments of the Osa horizon was made based on comprehensive analysis of core data, thin-sections and well logs.

            The research concluded that in the central part of the Nepa arch Osa horizon was deposited in partially isolated back reef and lagoon environments with active tide influence. However in the eastern part of the Nepa arch Osa horizon is formed by barrier carbonate buildups.

            Deposits of the Osa horizon are represented by six cyclites which progressively get shallower upwards and are separated by transgressive surfaces. Transgressive surfaces of the first two cyclites coincide with erosional surfaces. Sequence and thickness analysis of the cyclite’s section showed progradation of the first, second and third cyclites as well as aggradation of the fourth, fifth and sixth cyclites. Increase of thickness and change of facies of the Osa horizon from west to east suggest that deposits in the studied area were formed in the back reef environment of the Osa reef barrier.

            Deposits of the Osa horizon were affected by intensive diagenesis. However, sedimentary heterogeneity of carbonates controls modern distribution of reservoir quality. According to the results of petrophysical analysis, rocks with the best reservoir qualities are represented by two facies. The first one is crystalline dolomites which were formed from framestones. Such dolomites have intercrystalline and vuggy porosity inherited from framestones. The second type is oolitic dolomites with interparticle porosity. 

Keywords: Osa horizon, Nepa arch, logs interpretation, partially isolated back reef and lagoon zone.



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