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GEOLOGY, ECOLOGY, AND LANDSCAPES 2020, VOL. 4, NO. 3, 216–221 INWASCON https://doi.org/10.1080/24749508.2019.1616154 RESEARCH ARTICLE Structural to stratigraphic framework of “Banghan” ﬁeld, onshore Niger Delta, southern Nigeria Osuwake Omini Etimita and Francis T. Beka Department of Geology, University of Port Harcourt, Port Harcourt, Nigeria ABSTRACT ARTICLE HISTORY Received 11 July 2018 Banghan ﬁeld shows growth fault structures with dips ranging from 32° to 71° at the top and Accepted 4 May 2019 it decreases towards the base of the fault. The syn-depositional growth fault throw is 10 m to 91 m. The synthetic growth faults trend NW-SE while antithetic and other subsidiary faults KEYWORDS trend NE-SW. Fault assisted closures correspond to the crest of rollover structures that are Structural map; biofacies; possible trapping mechanism for hydrocarbon. The lithologic units are Burdigalian to horizon; fault; sequence; Tortonian in age. The lithofacies include bioturbated heteroliths, shale’s, and laminated heteroliths; stratigraphy; sandstones. The ﬁeld had undergone at least six cycles of deposition. The sequence of well logs; lithofacies; system tract deposition and various chrono surfaces identiﬁed using well logs, biofacies, seismic and core data shows that the sediments deposited obey Walther’s law. Introduction identiﬁed within the Niger Delta Petroleum Province are found below the Benin Formation (Over The Niger Delta Petroleum Basin is ranked among 2000 m thick) and they include Shale diapirs, roll-over the world’s proliﬁc petroleum producing Tertiary anticlines, growth fault, back to back and steep dipping delta. The Tertiary Niger Delta is divided into three closely spaced ﬂanks faults (Evamy et al., 1978;Xiao & diachronous formations, namely, Benin Formation Suppe, 1992).The growth faults present had been (Continental sands), Agbada Formation (Paralic sili- described by Shell-BP and Gulf Nigeria geologist, ciclastics) and Akata Formation (Marine shales) Short and Stauble (1967), Avuru, Adeleke, and according to their order of sedimentation. These for- Gbadamosi (2011), Merki (1972), Weber and Daukoru mations are diﬀerentiated on the basis of Sand to (1975)and Evamy et al. (1978). The characteristics of Shale ratio. the reservoirs in the Agbada Formation are controlled The Akata Formation (Miocene to Recent) is esti- by depositional environment and the depth of burial. mated to be 7000 m thick (Doust & Omatsola, 1990; The stratigraphic evolution of the Tertiary Niger Delta Oladotun, Olugbenga, Chukwudike, & Olatunji, 2016). was described by Evamy et al. (1978), and Doust and They Akata, Agbada, and Benin Formations are inter- Omatsola (1990) further described its stratigraphic col- ﬁngering lithofacies equivalent which represent prodelta, umn while Avbovbo (1978) produced the structural map deltaic front and delta top environment, respectively. of the Niger Delta formations. Petters (1991) described Doust and Omatsola (1990) suggested that the distribu- the sequence stratigraphic building blocks of the Niger tion of petroleum is likely related to heterogeneity of Delta Basin succession to have 11 third-order sequences source rock type. Petroleum in the Niger Delta is pro- which are mainly Transgressive System tracts (TST) and duced from sandstone and unconsolidated sands predo- Highstand System tracts (HST). The planktonic forami- minantly in the Agbada Formation. The Tertiary Niger nifera that were recovered from some regional marker Delta development was a function of subsidence and shale suggest that the third-order transgressive pulses sedimentation rate which alternates based on sea level occurred during Late Paleocene, Early Eocene, Late rise and fall that results in progradation, retrogradation Eocene, Late Oligocene to Early Miocene, Late to Early and aggradation deposition. Miocene, Middle Miocene, Late Miocene, and Pliocene. The Agbada Formation (Eocene to Recent) is the The Transgressive System Tract (TST) was followed by major petroleum-bearing unit that is estimated to be ﬂuviomarine laminated silt, sand and clays which graded 3700 m thick and it is the deltaic portion of the Niger into lower to upper shoreface sand and coastal plain Delta stratigraphic sequence. These siliciclastic sedi- deposits (Oresajo, Adekeye, & Haruna, 2015; Petters, ments are deposited in deltaic front, delta top-set, and 1991; Ukpong & Anyanwu, 2018). ﬂuvio-deltaic environment. Generally, the sand to shale The study area is located in onshore Niger Delta ratio decreases with increase in depth. The structures Petroleum Basin (Figure 1). The Niger Delta is CONTACT Osuwake Omini Etimita firstname.lastname@example.org Department of Geology, University of Port Harcourt, East-West Road, Choba, Port Harcourt P.M.B 5323, Nigeria © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. GEOLOGY, ECOLOGY, AND LANDSCAPES 217 Figure 1. Location map of Niger Delta showing study area. and progradational stacking patterns of gamma-ray situated in the Gulf of Guinea in equatorial West logs (Bertram et al., 1996). Africa between Latitudes 3°N and 6°N and Longitudes 5°E and 8°E. The sedimentary basin of the Niger Delta occupies a larger region than the Results and discussion geographical extent of the modern delta constructed by the River Niger to River Benue drainage systems. The ﬁeld is intensely faulted and its fault assisted The aim of this study is to make use of wireline closures are revealed dominantly in the Northern logs, seismic, sidewall core description and biofacies area of the ﬁeld. These closures correspond to the data to determine the structure and stratigraphy of crest of the rollover structures which serve as possible Banghan ﬁeld. The understanding of the structural trapping mechanism of the reservoir. and stratigraphic framework by this study provides The lateral transmissibility of the growth faults may insights to the structural and stratigraphic evolution be controlled by the extent and nature of clay smear, on retention or non-retention of hydrocarbons which cataclastic shear zone, hydrodynamics and the relative is vital in hydrocarbon exploration and exploitation position of fault to hydrocarbon kitchen. The growth decision-making for this study area. fault (Figure 3) dip ranges from 32° to 71° and it decreases towards the base of the fault. The growth fault presence decreases towards the surface. The Materials and methods growth faults throw ranges from 10 to 91 m with domi- The data set analyzed in this research includes seis- nant synthetic growth fault trending NW-SE while the mic, well logs, core description, and biofacies data. subsidiary and antithetic growth faults trends NE-SW. Seismic data (Figure 2) were analyzed to produce These faults will result in the compartmentalization of structural map and it was tied to well data, core hydrocarbon reservoirs within this study area. interpretations, and biofacies data to identify deposi- The biofacies analysis of well data shows that the tional sequences. lithologic units found within Banghan ﬁeld are Horizons with good lateral reﬂection continuity Miocene in age (Figure 4). The ages range from and faults were picked and mapped manually on Burdigalian to Tortonian and the study area had each line and transverse proﬁle of the entire 2D undergone at least six cycles of deposition within seismic reﬂection survey sections of the study area. two super cycles (Figure 4). The maximum ﬂooding The horizons picked correspond to depositional surfaces are deﬁned based on marker shales (Table 1) sequence boundaries and seismic sequences. Seismic identiﬁed and when tied to the Niger Delta Chrono checkshot data were analyzed to provide time-depth Stratigraphic chart, their ages were inferred. match (time in milliseconds and depths in feet) which The lithofacies found within this ﬁeld are bioturbated aided production of 3D isopach maps. The biofacies heteroliths, shales and laminated sandstones. The hetero- data were used in dating chrono-surfaces within the liths and laminated sandstones are the dominant hydro- study area and lithologic units were delineated using carbon reservoir within the study area. The chrono units gamma ray logs (Schlumberger, 1972). Chrono units (Figure 5) are inferred from gamma-ray logs based on were inferred base on retrogradational, aggradational retrogradational, aggradational and progradational 218 O. O. ETIMITA AND F. T. BEKA Figure 2. (2a) Seismic Base Map of Banghan Field (2b) Seismic section along with in-line 10920. Figure 3. Isopach map on time (ms) and depth (ft) domain. GEOLOGY, ECOLOGY, AND LANDSCAPES 219 Figure 4. Biofacies Analysis, Chronosurface Dating, and Correlation of Banghan 1 and Banghan 2 Well Data (EP = Epoch; SC = Super Cycles; ST = Stages; SA = Surface Age). characterized by drainage area, diﬀerential geospatial Table 1. The ages of marker shale identiﬁed within Banghan Field. integrity and capacity. The sandstones in the study area Age (Ma) Marker Shale are mud supported, laminated with well sorted, ﬁne to 9.5 Uvigerina-5 medium grey or milky colored grains. The shales were 12.8 Cassidulina-7 dark grey, silty, slightly calcareous and frangible based on 15 Bolivina-25 15.9 Chiloguembelina-3 core descriptions. Subsequently, by integrating the results from well logs, biofacies, core description and seismic analysis, stacking pattern. The transgressive sequences show a cross-section of the study area showing lithology increasing shales which are very good hydrocarbon seal and stratigraphic sequences at a depth less than 19442 because of their permeability and lateral extent. These ft (5925.2 m) is generated (Figure 6). The identiﬁed seals are fundamental and their characteristics control depositional sequences are results of sea level rise and the presence, distribution, absence, and movement of fall and these sediments were deposited in accordance hydrocarbons. Generally, hydrocarbon continuously with Walther’s law of facies succession. migrates until it encounters a seal which may be 220 O. O. ETIMITA AND F. T. BEKA Figure 5. Banghan 1, Banghan 2 And Banghan 3 Log Correlation Based on Identiﬁcation of Chrono Units (HST – Highstand System Tract; TST – Transgressive System Tract). Figure 6. Cross Section of Banghan Field Obtained from Sequence Stratigraphic Analysis by Integration of Data Available. Conclusion Disclosure statement The Banghan ﬁeld is attributed with syn-deposi- No potential conﬂict of interest was reported by the authors. tional growth faults that are common within the Niger Delta Petroleum Province. The growth faults throw ranges from 10 to 90m. The synthetic growth ORCID faults trend NW-SE while antithetic and other sub- sidiary faults trend NE-SW. The ﬁeld structures Osuwake Omini Etimita http://orcid.org/0000-0002- show a high tendency to trap hydrocarbon along 0022-5774 fault assisted closures which corresponds to roll- over crest. The lithologic units in the study area are Miocene in age and the lithofacies found are References bioturbated heteroliths, shales, and laminated sand- Avbovbo, A. A. (1978). Tertiary lithostratigraphy of the stones. The various maximum ﬂooding surfaces Niger Delta. 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Geology Ecology and Landscapes – Taylor & Francis
Published: Jul 2, 2020
Keywords: Structural map; biofacies; horizon; fault; sequence; heteroliths; stratigraphy; well logs; lithofacies; system tract
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