Analysis of Depositional Environments of Uk Field Reservoir Sands in Niger Delta Basin, Nigeria

Abstract

The environments of deposition play a vital role during hydrocarbon formation, migration, trapping and storage. Since the reservoir rocks are a function of their depositional environments, the successful tapping of hydrocarbon from its host rock when wells are drilled depends largely on the petrophysical characteristics of the reservoir rocks which in turn originated from and are influenced by their depositional environment. The reservoir facies in Niger Delta shows a broad range of characteristic sedimentological complexities that gave rise to subsurface geological, drilling and production problems occurring in UK Field. The problems include inaccurate determination of environments of deposition, imperfect stratigraphic correlations and reservoir top uncertainty across the UK Field which are targeted by this research in other to help resolve these challenges facing oil and gas industries in the Niger Delta basin of Nigeria. The research findings will assist in the evaluation of depositional environments and well-to-well lithologic correlation within the UK Field and Niger Delta Basin at large. It will also help to unravel major causes of reservoir top uncertainty in UK Field. Also, it will help in future planning and drilling of new wells within UK Field. The determination of the depositional environments of UK Field reservoir sands were carried out to determine the depositional environment of reservoir sand bodies based on data from seven (7) wells. The determination of depositional environments of sand facies penetrated by wells UK1, UK2, UK3, UK4, UK5, UK6 and UK8 was achieved through a side-by-side comparison of their log suites to standard log motifs. Results of facies analysis showed that the reservoir sands belong to mostly (i) fluvial channel, (ii) barrier bar, (iii) lower-middle shoreface, (iv) distributary mouth bar, (v) distributary channel, (vi) point bar and (vii) tidal channel environments that belonged to parts of a deltaic system. Lithologic correlation result reveals the existence of good correlation among all wells in UK Field due to good geological similarities except well UK8 that failed to correlate perfectly with others thereby establishing the existence of reservoir top uncertainty within UK Field. Therefore, reservoir top uncertainty within UK Field is geologically controlled.

Exploring Reservoir Potential of the "X Field" Offshore, Niger Delta Basin: An Emphasis on Lithofacies, Depositional Environments, and Petrophysical Characteristics

“…The porosity of the field appears to be largely unaffected by compaction and diagenetic processes, as opposed to the depositional processes and contexts of deposition. The lateral variation in porosity might have been influenced by changes in the depositional environment and the gradual increase in depth due to the movement of the coastline and the shift in deposition towards the south and the sea, similar to the results presented by Olaviwola [26], Friday [27], and Archie [20]. This variation is noticeable in the gamma-ray log patterns, where sands deposited in low-energy environments exhibit a decrease in porosity due to little or no reworking (attributed to the presence of shales, silts, and clays in this environment).…”

“…The predicted habitat for shale ranges from fluviodeltaic to deltaic front to prodeltaic to shelf margin to marine. The gamma ray logs of various reservoir sands show different log forms, indicating deposition in different environments, such as fluvial or tidal flood plains, deltaic distributaries, deltaic fronts, shorefaces, tidal flats, tidal channels, and proximate offshore areas [26,27]. Differences in well log patterns may indicate lateral variations in rock character, which can be detected in connection with the section and interpreted with knowledge of the depositional environment from the cored section.…”

“…The features of sandstone, particle size, spontaneous potential, and gamma ray log patterns can be used to predict the depositional environment [26][27][28][29][30]. Based on these patterns, potential deposits near the delta complex's outer reaches include deep-sea channel sands, low-stand sand bodies, and proximal turbidites [10,29].…”

“…Based on gamma ray log patterns and core plug descriptions, the depositional environment of the reservoir sands has been identified as ranging from the fluvio-deltaic plain to the shelf margin or slope. Fluvial and fluviomarine processes are responsible for producing higher-quality reservoirs than marine processes [26,27,29]. Sandstone sorting influences the amount of shale in each unit, the depositional environment and processes, and the diversity in porosity and permeability of reservoir sand units.…”