Sequence Stratigraphic Interpretation of FX-1 and FX-2 wells, Onshore Western Niger Delta, Nigeria

The area of study is a portion of the Greater Ughelli Depobelt in Niger Delta Basin. The main aim of the paper is to interpret the sequence stratigraphy of FX-1 and FX-2 wells by employing data sets from biostratigraphic data and well logs. Standard laboratory techniques were used for data treatment while computer software such as Petrel and StrataBugs were used for data simulation, processing, integration and interpretation. Sedimentology, interpreted gamma ray and resistivity well logs integrated with biostratigraphic data were utilized to define the candidate maximum flooding surfaces and sequence boundaries. The wells have the following distributions of sequences: FX-1 well have five depositional sequences with eight candidate maximum flooding surfaces at depths 10011 ft., 9509 ft., 9437 ft., 6362 ft., 5752 ft., 5507 ft., 5161 ft. and 4816 ft. dated 34.0 Ma, 33.0 Ma, 31.3 Ma, 28.1 Ma, 26.2 Ma, 24.3 Ma, 23.2 Ma and 22.0 Ma and seven candidate sequence boundaries at 9616 ft., 6656 ft., 6116 ft., 5639 ft., 5424 ft., 4859 ft. and 4581 ft. dated 33.3 Ma, 29.3 Ma, 27.3 Ma, 24.9 Ma, 23.7 Ma, 22.2 Ma and 21.8 Ma, respectively. FX-2 well have four depositional sequences, five candidate MFSs were identified at 7764 ft., 7196 ft., 6721 ft., 5862 ft. and 5571 ft. dated 34.0 Ma, 33.0 Ma, 31.3 Ma, 28.1 Ma and 24.3 Ma and five candidate SBs at 6941 ft., 6029 ft., 5688 ft., 5653 ft. and 5542 ft. dated 32.4 Ma, 29.3 Ma, 27.3 Ma, 24.9 Ma and 23.7 Ma respectively. The correlation of the two wells and sequence stratigraphic interpretation is a supplementary understanding of the subsurface geology of the Onshore, western Niger Delta area of Nigeria. DOI: https://dx.doi.org/10.4314/jasem.v24i2.17 Copyright: Copyright © 2020 Amiewalan and Lucas. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 16 November 2019; Revised: 11 January 2020; Accepted: 22 February 2020

The area of study is a portion of the Greater Ughelli Depobelt in Niger Delta Basin. Two wells (FX-1 and FX-2) used for this research are one of the several developmental boreholes penetrated in the oil-rich Niger Delta, situated Onshore, Niger Delta Basin, Nigeria. FX-1 well is located at geographic coordinates of Longitude E5 0 33'55''.58 and Latitude N6 0 18'32''.64 while FX-2 well is located at geographic coordinates of Longitude E5 0 33' 36' '.86 and Latitude N6 0 18' 25''.43. Fig.1a. Distance between the two wells is 2,022.30 ft. / 617 m and the direction for FX-1 well is NE while the direction for FX-2 well is SW as shown in figure 1b. The two wells are fairly deep and they penetrated both the Benin and Agbada Formations of the Niger Delta. Sequence stratigraphy is the investigation of rocks that are hereditarily correlated in the framework of chronostratigraphically confined surface. Van Wagoner et al., (1990). The three principal schools of thought on sequences are: The Exxon (1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988) school is centered on unconformity. The depositional sequence is based on highstand and lowstand systems tracts, confined by subaerial unconformity, followed by a regressive surface of marine erosion, and then a correlative conformity (regularly hard to identify). According to Hart, (2005), the model is widely accepted and applied in the hydrocarbon industry. The Galloway, (1989) school is established on maximum flooding surfaces. The depositional sequences is constrained by maximum flooding surfaces regularly referred to as regressive-transgressive sequences (R-T). The benefit of this genetic stratigraphic model is that only one type of surface (maximum flooding surface) is defined and is easy to identify. Conversely, an interpreter should take caution since there is the likelihood of unconformities lying inside the sequences. Hart, (2005). The Emery, (1993) school is known as the Emery and Johannessen model of transgressiveregressive (T-R) sequences is founded on subaerial unconformity and transgression surfaces. The depositional sequences are bounded by subaerial unconformities and maximum regressive surfaces with two systems tracts. The transgressive systems tract lies between the sequence boundary at the base and the maximum flooding surface at the top. The regressive systems tract lies between the maximum flooding surface at the base and the sequence boundary at the top. It is the newest model, simple to apply, but not Sequence Stratigraphic Interpretation of FX-1 and FX-2 Wells….. 304 AMIEWALAN, FO;LUCAS, FA widely used. Hart, (2005). The fundamental unit of sequence stratigraphy is the depositional sequence. A sequence is defined as a relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities. van Wagoner et al., (1990). It emphasizes phases of deposition and non-deposition (associated with periods of rising and falling of sea level). Several workers such as Evamy et al., (1978) ;Tuttle, et al., (1999); Weber and Daukoru, (1975) ;Obaje, (2013); Ojo and Gbadamosi, (2013); Okengwu and Amajor, (2015); Alege, (2017); Ozumba and Amojor (1999); Ajayi and Okosun (2014); Ifeoluwadun and Saka (2018) have worked on the biostratigraphy, sequence stratigraphy, structural geology, sedimentology and petroleum geochemistry of the Niger Delta. Nonetheless, in most of the published and unpublished works, incorporated method in sequence stratigraphic interpretation is absent. The use of sequence stratigraphy methods to sedimentary basin investigation has brought about a new technique of dividing, analyzing and mapping sedimentary rock. Kelly et al., (2000). Unrecognized stratigraphic traps can be revealed and identified which could significantly intensify the finding of hydrocarbon potential in the study area. Furthermore, an improved understanding of the depositional environment, lithological facies and numerous depositional sequences of the area of study which could be extended laterally to a regional extent in the Niger Delta. The aim of this study is to demarcate the major stratigraphic units and systems tracts in the study area of the Niger Delta, by integrating stacking pattern, sedimentologic and biostratigraphic data to delineate candidate chronostratigraphic surfaces.
Geological Setting: The Niger Delta is situated at the apex of the Gulf of Guinea on the west coast of Central Africa. As posited by Whiteman (1982), the Niger Delta clastic section was formed along the aulacogen that was initially established in the course of the disintegration of the South American and African plates in the Late Jurassic. According to Evamy et al., (1978); Doust and Omatsola, (1990), the regressive Niger Delta consist of a wedge of clastic sediments of up to 12 km thick formed by a succession of offlap cycles. Short and Stauble, (1967); Knox and Omatsola, (1989); Tuttle et al., (1999, etc.) recognized three main Formations in the Niger Delta sedimentary basin, namely: the Akata Formation, Agbada Formation and Benin Formation. The Akata Formation is composed of marine prodelta shales and turbidite sands that have been deposited since the Paleocene and is believed to be the main source rock within the basin. The Agbada Formation is made up of paralic siliciclastics and are considered to be the main deltaic sequence that establish the petroleum reservoirs of the basin capped by sandy fluvial Benin Formation which consists largely of non-marine sands with a few shaly intercalations. Esan, (2002). Deposited since the Oligocene.

MATERIALS AND METHODS
Three hundred and eighty two (382)

Determination of sandstone and shale Lithology:
Given data in ASCII format were used to generate sets of wireline logs (gamma ray, resistivity, neutron and density log) using the Petrel software which were presented at steady processes to enhance log developments and as well to support the trend of facies stacking patterns. The interpretation of well log responses (gamma-ray and resistivity logs) was established with ditch cutting features of sediments evaluated using both visual inspection and reflected binocular microscopy. The Gamma-ray log was utilized to define the lithologies at the depth intervals. The higher the value of the log signature the more shalier is the Formation. Sandstones that is clean usually reveals a low level of natural radioactivity, while clay minerals and fluid particles in shales show higher levels of radioactivity owing to adsorption of the heavy radioactive elements. Sandstones were defined with low API log signatures fluctuating between 0 and 20 API units. Sandy-shales varies from 20 to 100 API units whereas shales have API unit as from 100 and above. This illustrates reduction in the rate of sedimentation and general reduction in energy, recognized as fluvial environments and transgress sequences. The outlining method enabled us to estimate and establish the lithological sequence of the Formations of the study area.

Biostratigraphic analyses:
The standard micropaleontological preparation technique for foraminiferal and palynological samples were used in the Laboratory. The palynological analysis involved the use of standard palynological preparation for recovering acid insoluble organic-walled microfossils from sediments with the technique of Wood et al., (1996), which is the use of HCl, HF, HNO3 involving heavy-liquid separation with ZnBr2 (2.2 s.g.) and separating of the remains with a 20 μm sieve. Foraminiferal and palynology slides were examined using reflected and transmitted light binocular microscopes respectively. The distribution chart (statistical data) of the specimens were documented and StrataBugs software was used for data processing and integration with the well logs data. The absolute ages of the foraminiferal and palynomorphs biozones, maximum flooding surfaces and the third order sequence boundaries were based on correlation zones and cycles of numerous authors such as Blow, (1969Blow, ( , 1979; Bolli and Saunders, (1985); Germeraad et al., (1968); Evamy et al., (1978) ;Legoux, (1978).
Sequence Stratigraphy: Wireline logs, sedimentology and biostratigraphic data (foraminiferal and palynology) were integrated to construct sequence stratigraphic framework based on the approach of Armentrout et al., (1990). Variations in stacking patterns as observed on the produced gamma ray log aided the recognition of systems tracts. The microfaunal and micro floral zonation scheme chart of Haq et al., (1988) was utilized to delineate numerical stage boundaries of MFSs and SBs identified. The concepts of Emery, (1993) were employed to delineate the depositional sequences. The well log patterns were used to define the sequences to diverse categories of systems tracts.
Well correlation: FX-1 well with intervals 15 ft. -10,185 ft. was selected as the composite standard section (CS) for the study area due to its depth, most closely sampled, having abundant fossil and quite widespread with no indication of main facies breaks. The depths of the Oligocene/Miocene boundary were taken for both wells. Using the petrel software, sedimentological, palynological, foraminiferal and sequence stratigraphy interpreted for both wells formed the basis of correlation.

RESULTS AND DISCUSSION
Sequence Stratigraphic Interpretation of both wells are provided as follows. The highest value of a retrogradational stacking pattern of the gamma ray log, is inferred as candidate maximum flooding surface while low value of a progradational stacking pattern is deduced as sequence boundary and starting of a subsequent transgression.
The recognized sequence boundaries were considered by significant sand unit and low gamma ray value. It is the interval separating the overlying shallow microfacies from underlying deep water facies with minimal biofacies abundance and diversity. Also, recognized as the zone of most basin ward shift in marine biofacies which correspond with coarsening upward sequence.
The MFSs are categorized by dense and widespread mudstone components identified by high gamma-ray value and lowest resistivity readings. (Vail and Wornardt, 1990). They most often indicate sedimentation rates. Armentrout and Clement, (1990), with maximal biofacies abundance and diversity. Depositional sequences interpretation in FX-1 well: Five depositional sequences (SQ1-SQ5) were demarcated with eight maximum flooding surfaces and seven sequence boundaries. The depositional sequences are discussed below from the oldest interval to the youngest intervals. Table 1

Chronostratigraphic Correlation:
Chronostratigraphic correlation is related with the idea of comparing rock sediments in two geographic areas having the same marker fossil specimen which indicates that the sediments were deposited probably at the same time frame. In line with Jaramillo et al., (2009), the correlation analysis for the studied wells started by indicating FX-1 well with intervals 15 ft. -10,185 ft. as the composite standard section (CS) due to its depth of penetration, sampled at close depth, abundant fossil with varieties reasonably wide-ranged.  Fig. 7. With the aid of sedimentological / wire line data and obtainable biostratigraphic information, regional geologic correlation has been carried out across the area of study from Well-1 through Well 2.