Comparative Studies of Ekenkpon Shale Subsurface Distribution in the Calabar Flank

Introduction

Over the years, Cretaceous subsurface sedimentary rocks of the Calabar Flank have been a conceptual illustration from surface outcrop studies. Earlier studies by Nyong [1], Reijers and Petters [2] on the structural setting and subsurface distribution of sedimentary rocks in the Calabar Flank were based on field relations, stratigraphy and sedimentological descriptions of Cretaceous sedimentary rocks exposed at different outcrop locations. The Calabar Flank was first proposed by Murat [3] that part of Southern Nigeria sedimentary basin characterized by crustal block fault tending NW-SE. The Calabar Flank is bounded by the Oban Massif to the North and the Calabar Hinge Line separating the Niger Delta Basin in the South. It is also separated from the Ikpe Platform to the west by a NE-SW trending fault has been affected by two major tectonic phases, marine transgression and regressions which played an important role in the formation of the various sedimentary sequences. The study area is characterized by metamorphic gneisses, quartzite etc. The Cretaceous sediments comprise sandstones, shales and shaly limestone as well as impure limestone. According to Adeleye and Fayose [4] the study area affected by series of orogenic processes, basin tectonism as well as global eustatic changes in sea levels. Sedimentation began with the initial deposition of fluvio-deltaic cross-bedded sands of Awi Formation in the area. The associated transgressive and regressive cycles. Calabar Flank have varying composition of deposited sedimentary rocks [5]. Sedimentary succession is predominantly Cretaceous made up of Awi Formation, Odukpani Group, Mfamosing Limestone, Ekenkpon Shale, New Netim Marlstones, Nkporo Shales and Benin Formations respectively [6]. Sedimentation in the Calabar Flank began with the deposition of the Awi Sandstones which is uncomformably overlain the Mfamosing Limestone and continued with the deposition of the Ekenkpon Shale and New Netim Marlstone. Sedimentation of the Cretaceous sedimentary rocks ended with the deposition of the Nkporo Shale Late Campano-Maastrichtian [6, 7, 8, 9, 10] (Figure 1).

Click to enlarge

Earliest studies on the geology of the Calabar Flank carried out by Reyment [7] attempted to establish the biostratigraphy of the basin, Reyment [7] however identified it as part of the Benue Trough while Murat [3] suggested that the Calabar Flank underwent phases of tectonism and stratigraphic development in comparison to the adjacent Anambra Basin and lower Benue Trough hence classified it as a distinct basin. Nyong and Ramanathan [5] defined the location of the Calabar Flank as the easternmost part of the Gulf of Guinea which forms part of Nigerian continental margin between the Cameroon Volcanic Trend to the east, the Ikpe Platform to the west, the Oban Massif and Calabar Hinge Line to the North and South respectively (Figure 2).

Click to enlarge

The Calabar Flank is a diminutive Nigerian sedimentary basin; a fringe which forms on the edge of the Gulf of Guinea. The major difference between the Calabar Flank and the southern Benue Trough resulted from the initial rifting of southern Nigeria margin which led to the formation of two principal sets of faults, the NE-SW and the NW-SE trending sets of faults characterize the Benue Trough and the Calabar Flank respectively [3]. Murat [3] defined the tectonic components of the Calabar Flank to include the Ikang Trough and the Ituk High which were a mobile depression and stable mobile submarine ridge respectively. Nyong and Ramanathan [5] have shown that the Calabar Flank is underlain by horsts and graben structures (Figure 3).

The Ekenkpon Shale is derived from mainly felsic rocks having mainly clay minerals and formed in passive to tectonic settings. It was deposited in an oxic, continental to transitional marine environment [8]. According to Itam, et al. [9], the Ekenkpon Shale has been deposited under the influence of arid to warm climate, marginal marine setting using recovered palynormorph assemblages.

Click to enlarge

Materials and Methods

Materials used for this study are seismic, well logs, location map and mud log data. The well logs (A, B, C) that were drilled at different periods within the study area. The logs analyzed and used in this work include Gammar Ray (GR) log, Self-potential (SP) log, Resistivity log, Neutron and density logs. The logs signatures were interpreted and described of the study area. This study was carried out by comparing the surface Ekenkpon Shale evaluated by studies with its subsurface equivalent and this work. The procedure adopted in carrying out this study involved lithology identification, lithologic description and facies distribution using well logs, processed seismic and mud log data.

Summary and Conclusions

Previous works on Ekenkpon Shale were conceptual illustrations of the subsurface based on surface outcrop studies however, this work is data driven based on seismic, well log and core data which modifies the subsurface Ekenkpon Shale in the study area. Juxtaposing the structural and subsurface geological setting from the findings of this work against previous works it is evident that the lithostratigraphic sequences are not correlatable and differ especially in the distribution of facies such as Ekenkpon Shale, their lateral extents and their structural settings. The Ekenkpon Shale was only seen by well C in the as identified by Nyong and Ramanathan [5], Reijers and Petters [2] however this study reveals that the three wells penetrated the Ekenkpon Shale. This implies that the Ekenkpon Shale extended through wells A NE to well C SW of the study area and did not pinch-out as opined by previous works.