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2 2023

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Facies Analysis and Depositional Environmental Interpretation of The Upper Oligocene, Block 09- 2/10, Cuu Long Basin

Nguyen Muoi Duy, Le Anh Ngoc, Nguyen Hoa Minh, Bui Ngan Thi

Inżynieria Mineralna

2023

no. 2, vol. 1

155--162

The article presents the facies and environment of the Upper Oligocene sediments in the area of block 09-2/10 based on the analysis of seismic facies and well data of the neighboring areas. The results of the interpretation of seismic data indicate that the upper Oligocene sediments are limited by the top C, top D seismic reflectors. The environment is formed from lagoons, lakes to deep lakes. Seismological facies analysis identified 03 facies including seismic facies with medium to poor reflection amplitude, medium continuous, low frequency reflecting the lacustrine sediments (80%) in most of the lake center. Strong reflective seismic facies, sigmoidal clinoforms reflect the lakeside sediments (15%) distributed in the lakeside shelf. The seismic facies with average and continuous amplitude poorly reflect alluvial sediments (5%) in the Northwest region. The direction of sediment transport is mainly from the Northwest and Southwest in the area. The sandstone sequences are distributed on the slopes of the lake and the lake bottom, which is potential reservoir.

Morphometric analysis of pebbles in verification of transport processes and interpretation of palaeoenvironment: A case study from the Ogwashi Formation (Oligocene), Niger Delta Basin, Nigeria

Ogbe Ovie B., Osokpor Jerry, Emelue Chukwunomso V., Ocheli Azuka

Geologos

2023

Vol. 29, No. 1

21--31

The present paper discusses a palaeoenvironmental interpretation of the Oligocene Ogwashi Formation (Niger Delta Basin, Nigeria) through morphometric analysis of pebbles, a research method essentially depending on the quantitative evaluation of pebble size and shape which change during transport processes in a range of depositional environments. The relationship of bivariate and ternary-diagram plots of independent functions was determined from freshly exposed sandstone sections in quarries at Ibusa, near Asaba, Nigeria. The grain size of pebbles ranges from fine (11.00 mm) to very coarse (41.33 mm), with a mean size of 21.05 mm (coarse pebbles). A bivariate plot of the flatness index vs maximum projection sphericity index shows 37% of the pebbles to fall within a beach (marine) environment, and 33% within a fluvial environment; the remaining 30% are uncertain. The maximum projection sphericity index vs oblate-prolate index bivariate plot indicates that 47% of pebbles fall within a marine environment and 22% within a fluvial environment, while the remaining 31% are uncertain. The sphericity-form ternary diagram plot shows that the pebbles primarily comprise bladed, platy, very bladed and very platy of near-equal proportions with minor elongate and very elongate stones, which suggests that the pebbles are diverse in origin. The sphericity-form bivariate plot indicates that the pebbles are largely of disc and blade shapes with a minor number of spherical and rod shapes. The near-equal proportion of pebbles in different segments of the bivariate and ternary diagram plots, and the wide distribution of the pebble grain sizes and shapes suggest the Ogwashi Formation is composed of redeposited sediments that likely were transported and deposited in a mix of marine and fluvial settings with possible transitional environments. In other words, the pebbles can be interpreted as marine-influenced fluvial sediments in marginal-marine settings.