Lithostratigraphy and Sequence Stratigraphy of Ultra-Deep E-Field, Eastern Niger Delta: Reservoir, Geological and Biostratigraphical Evidence

Matthew O Akpan ,Paul A Udofia ,Thomas A Harry

doi:10.7176/jees/10-7-06

Abstract

The ultra-deep offshore, eastern Niger Delta is marked by rapid, cyclic deposition of thick units of siliciclastic sediments ranging from deep marine to non-marine environments, deposited into rapidly subsiding sub-basins occurring along the slope of the continental margin. This rapid deposition resulted in thick third-order sequences and systems tracts. Patterns of deposition were analysed from seismic reflection configuration and well-log patterns. Lithofacies patterns critical for systems tract recognition were interpreted from well logs and tied to seismic sections where possible. Sediment accumulation plots were constructed and employed to interpret the location of stratigraphic condensation, key surfaces, diffuse boundaries between systems tracts and evaluate the significance of condensed sections. The origin of these condensed sections is caused by major allocyclic changes associated with transgression and shifting of the deltaic depocenter that fed the area. The regional change in condensation through time was interpreted as reflecting avulsion of the shallow marine sediment source. The compilation of sediment accumulation plots also showed a major increase in sedimentation approximately 2.4 Ma; caused by the influx of the prograding shallow marine sediments. Wells located in distal regions in this field are more condensed [steeper slope] than proximal locations. The resulting analyses of this study showed that the basin-floor fan has the highest rate of deposition and could be identified as a gentle slope in the line of sediment accumulation. In the distal regions of the field, TST’s are characterized by sediment starvation because most of the sediments are trapped in the proximal areas. The maximum flooding surfaces [MFS’s] were recorded in deep water as condensed sections. Secondary condensed sections were delineated and interpreted to have deposited above the top basin-floor fan surface [tbfs] and top slope fan surface [tsfs]. In addition to traditional first downhole occurrence biostratigraphy, the database also contains information on nannofossil abundance. The sands encountered in the reservoirs are correlatable indicating a relatively longer period of depositional cycle. Keywords : Lithostratigraphy, Sequence stratigraphy, Eustacy, Reservoir geology, Biostratigraphy, Ultra-deep Offshore, Eastern Niger Delta. DOI : 10.7176/JEES/10-7-06 Publication date :July 31st 2020

Highlights

1.0 Introduction There is a wide spectrum of bottom currents, for example, the bottom currents sensu stricto that operate in deep water such as in the E-Field of the Niger Delta
These currents are those that are part of either the thermohaline- or wind-driven major circulation patterns. Such currents generally have an overall alongslope flow, but in detail their velocity and direction are extremely variable in both time and space. They are affected by seafloor morphology, Coriolis force, circular motions unrelated to contours, and eddy kinetic energy changes
The paleo-geographic reconstruction using isopach map of the payzone thicknesses penetrated in some wells in this field has provided a greater insight for understanding of the paleo-river dynamics and its morphology

Summary

Introduction

There is a wide spectrum of bottom currents, for example, the bottom currents sensu stricto (contour currents) that operate in deep water such as in the E-Field of the Niger Delta. These currents are those that are part of either the thermohaline- or wind-driven major circulation patterns. Such currents generally have an overall alongslope flow, but in detail their velocity and direction are extremely variable in both time and space. Additional types of deep (bottom) currents not driven by sediment suspension have been observed to flow mostly perpendicular to the slope. These include currents related to internal waves and tides (formed between subsurface water layers of varying density), canyon currents (frequently reversing flow of clear or very low-density turbid waters), down- and up- welling flows (currents on the continental slopes that may be generated by density, wind, storms, or obstructions)

Methods

Conclusion