Sequence Stratigraphy and Structure of the Southern ...

IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG) e-ISSN: 2321–0990, p-ISSN: 2321–0982.Volume 5, Issue 6 Ver. I (Nov. – Dec. 2017), PP 32-47 www.iosrjournals.org

Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A new insight into the hydrocarbon systems and prospectivity at deeper stratigraphic intervals Chima, K. I.a,*, Ozumba, B. M. a, Adejinmi, K. T. a, Hoggmascall, N. M.a, Orajaka, I. P. b, Okoro, A. U. b, and Ahaneku, C. V b, a

Shell Petroleum Development Company Limited (SPDC), PMB, 263, Port Harcourt, Nigeria. Department of Geological Sciences, NnamdiAzikiwe University, PMB 5025, Awka, Nigeria 1

b

Abstract: The Niger Delta Basin of Nigeria is a mature petroleum province having been producing for over half a century. Structurally the delta is subdivided into several depobelts from the Northern delta to the Deep Offshore delta.The Coastal Swamp Depobelt (CSD), has been variously referred to as the ‘Golden Belt’ of the region as it plays host to the biggest trend of oil and gas producing fieldsin the Niger Delta. Until recently, most of the prospectivity has focused on the shallower depths of the basin up to a maximum of 11,500 feet (3,496 m). The latest effort is targeted at the deeper parts given the acquisition of new long cable3D seismic data in the area and the availability of production facilities. Preliminary seismic interpretation of the new data revealed the occurrence ofdifferent types of normal faults comprising synthetic, antithetic, major growth and K-type faults which complicate the structure of the southern CSD. Regional hanging /foot wall, simple roll over anticline and K-type faults constitute the potential structural traps, while stratigraphic pinch outs and erosional truncations below fluvial channel incisions constitute the subtle traps. The stratigraphic framework constitutes of two, third order genetic sequences, bounded by three maximum flooding surfaces and sequence boundaries.The three main systems tracts of lowstand (LST), highstand (HST), and transgressive (TST) were all identified in the area with varying thicknesses but showing overall decrease in net to gross basinward. While the sandier LST and HST units serve as potential reservoirs, the marine shales within the HST units act as source/seal for hydrocarbon. The application of the systems tracts to the deeper prospects has enabled a clearer definition of the hydrocarbon system elements especially the reservoir presence, quality, seal and source quality. Some potential deeper leads were identified. Keywords: Golden Belt, Sequence stratigraphy, K-type faults, Deeperprospectivity, Systems tracts. ----------------------------------------------------------------------------------------------------------------------------- ---------Date of Submission: 25-12-2017 Date of acceptance: 30-12-2017 ----------------------------------------------------------------------------------------------------------------------------- ----------

I.

Introduction

The Niger Delta Basin is located in the Gulf of Guinea, West Africa (Figure 1). This Cenozoic delta is one of the largest regressive deltas in the world with subaerial and submarine sediments covering ∼140,000 km2, and 12 km in thickness (Allen 1964, 1965; Evamyet al., 1978; Doust and Omatsola 1990). The Niger Delta petroleum province has been described as a natural laboratory, where the factors controlling the stratigraphic architecture of large deltas can be studied in analyzed (Jermannaudet al., 2010). This includes; the dynamic interaction between stretched lithosphere forming passive margin, large-scale gravity deformation overprinting delta clastic wedge, sea-level variation, global and regional tectonic evolution, as well as dynamic drainage system controlling sediment supply and distribution (Jermannaudet al., 2010). The sedimentary succession in the delta accumulated rapidly under high-frequency fluvio-deltaic-eustatic sea-level oscillations. The continuous mobilization of the thick, unstable mobile shales upon which the deltaic sediments were deposited led to the development of three structural domains in the delta, which include; the extensional zone-beneath the continental shelf, translational zone below the upper slope and compressional zone in the outer fold and thrust belt (Damuth, 1994; Morgan, 2004). Corredoret al (2005), further subdivided the delta into five (5) structural domains including; (1) Extensional province below the continental shelf characterized by seaward dipping (Roho-type) and counter-regional growth faults. (2) Mud diapir province below the upper slope characterized by passive, active and reactive mud diapirs Morley and Guerin (1996), shale cored ridges and massifs, shale overhangs, vertical mud diapirs that form mud volcanoes on the seafloor Graue (2000), and inter-mud

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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A .. diapirdepocenters. (3) Inner fold and thrust belt characterized by basinward verging imbricate thrust faults and associated folds with some detachment folds. (4) Transitional detachment fold zone below the lower slope, characterized by large areas of little or no deformation interbedded with large detachment folds above thickened section of Akata Formation (5). Outer fold and thrust belt characterized by both basin-ward and landward dipping faults. The bathymetry map of the Niger Delta showing the structural zones is shown (Figure 1).The sedimentary packages in the delta is subdivided into three lithostratigraphic units (Akata, Agbada and Benin Formation). The deltaic unit (Agbada Formation), was deposited under paralic condition, which facilitated the accumulation of thick successions of hydrocarbon-bearing sandstones within the highstand deposits, while marine shales within the transgressive deposits provide effective sealing for hydrocarbons (Kulke; 1995; Tuttle et al., 1999; Brownfield, 2016). The deepwater portion of the Niger Delta is characterized by the complex dynamic interaction between sediment gravity-flows and growth structures. Sediment gravity flows (triggered mostly by localized overstepping of the slope by vertically thrusted mud diapirs, growth faults and/or failures at the shelf-margin), often transport sediment downslope along channels and canyon complexes to the deep basin, where thick turbidite reservoir sequences are deposited (Posamentier and Kola, 2003; Adeogbaet al., 2005; Prather et al., 2012; Jobeet al., 2015, 2016; 2017; Joly et al., 2015; 2017; Hansen et al., 2017). The large aerial extent of the Niger Delta and the unique geological setting under which petroleum systems formed underscores its continued relevance to major oil companies, who have progressively explored the deeper part of the delta since 1960’s (e.g. Allen, 1965; Short and Stauble, 1967; Merki, 1972;Weber and Daukoru,1975; Lehner and De Ruiter, 1977; Evamyet al., 1978; Whiteman, 1982; Knox and Omastsola, 1988; Doust and Omatsola, 1989; Damuth, 1994; Morley and Guerin, 1996; Reijerset al., 1997; Wu and Bally, 2000; Hooper et al., 2002; Deptucket al., 2003; Steffens et al., 2003; Corredoret al., 2005; Bilotti and Shaw, 2005; Deptucket al., 2007). The proven oil and gas resources within the delta rank it as the twelfth largest petroleum province in the world, with 34.5 billion barrels of recoverable oil and 93.8 trillion cubic feet of recoverable gas (Tuttle et al., 1999).

Figure 1: Structural map ofthe Onshore Niger Delta showing the major structural segments (depobelts) and insert is the study area-southern Coastal Swamp Depobelt (CSD).Notethe largest onshore fields are located within the Coastal Swamp Depobelt (CSD). Although a lot of sequence stratigraphic studies have been published in the Niger Delta, the on-going effort at finding hydrocarbon leads at the deeper stratigraphic sections in the onshore blocks has spurred further studies at present. Previous sedimentological, biostratigraphic and sequence stratigraphic studies in the Niger Delta reveal that sea-level variations and basin tectonics controlled accommodation and depositional patterns in the delta (Ladipo, 1992; Stacher, 1995; Reijerset al., 1997; Owoyemi and Willis, 2006; Magbageola and Willis, 2007). Depositional sequences defined by Vail (1987), as comprising of strata bounded by unconformities and DOI: 10.9790/0837-0506013247


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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A .. their correlative conformities are only recognizable in some parts of the delta. However, regional genetic sequences defined by Galloway (1989), as comprising of strata bounded by maximum flooding surfaces within transgressive shales are easily identifiable in the delta (Reijers, 2011). We adopted the delta wide genetic sequence model in the analysis of the stratal packages in this study. The study area is located in the southern CSD of the Western Niger Delta (Figure 1). The CSD is an approximately 2,000 km2 sub-basin, generally referred to as the ‘Golden Belt’ of the Niger Delta because it is the location of many of the dense oil fields of the region (Figure 2). Although production figures are highly classified, most of the largest producing fields such as ForcadosYokri, Nembe Creek, Odema Creek, Bonny, Santa Barbara Otumara, Oloibiri, Ekulama-Belema, Tunu-Kanbo, Opukushi, Dodo etc., have been described to lie within the CSD (Tuttle et al., 1999; Reijers, 2011; Nwajide, 2011). Although many wells have been drilled in the southern CSD, most of them have been focused on prospects at relatively shallow depths, generally a maximum of 11,500 feet (3,496 m). Following the acquisition of new 3D seismic data by Shell Petroleum Development Company (SPDC) in 2013, coupled with the availability of oil and gas production facilities, there is now renewed effort at finding and producing more hydrocarbons in the area, including from deeper levels (>11, 500 feet). Hence, this study was aimed at identifying potential deeper prospectivity within the unpenetrated deeper stratigraphic sections in the southern CSD, which can in the future be developed into productive systems. In order to achieve this aim, we integrated high-resolution 3D seismic and well data to interpret the stratigraphic framework and field trapping structures within the southern CSD. We subdivided the studied sedimentary packages into genetic units and systems tracts to understand reservoir and seal distribution within the studied interval. These helped us to highlight potential deeper prospectivity in the unpenetrated stratigraphic sections.

Figure 2: Map of the Niger Delta showing the structural and stratigraphic framework and the dominant structural gravity slide models. DOI: 10.9790/0837-0506013247


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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A .. II. Geologic Setting The Niger Delta sits on the southern culmination of the failed arm of a rift triple junction (Benue Trough), which developed following the separation of Equatorial Africa and South America, in which the rifting ceased in the Late Cretaceous (Burke, 1972; Lehner and De Ruiter, 1977; Whiteman, 1982). The delta proper was initiated in the Paleocene (Reijers, 2011), but rapid progradation commenced in the Eocene (Doust and Omatsola, 1990). Sedimentary dispersal in the delta was regionally controlled by marine transgressive/regressive cycles related to eustatic sea-level changes with varying duration, while differential subsidence locally influenced sediment accumulation (Reijers, 2011). The sedimentary fill of the basin has been subdivided into (from oldest to the youngest) Akata, Agbada, and Benin Formations (Short and Stauble, 1967; Knox and Omatsola, 1989; Lawrence, 2002), (Figure 3). Major punctuation in deposition in the Late EoceneEarly Pliocene gave rise to the development of extensive submarine canyons traditionally referred to as ’channels’ (Burke, 1972; Petters, 1984; Knox &Omatsola, 1989; Reijerset al., 1997). These channels (e.g. Afam, Buguma, Soku, Opuama etc., Figure 3), have erosional bases and their fills are characterized by a number of internal erosional unconformities that have also been reported from the offshore (e.g. Owoyemi, 2006). In contrast, such erosional features are scarce in the central part of the Niger Delta, where transgressive shales clearly mark maximum flooding shales (Reijers, 2011). Extensional seaward dipping growth faults oriented east-west in map view, stretch laterally along depositional strike across nearly the entire delta, defining ‘‘mega-structures’’ and associated depositional belts/sub-basins (Evamyet. al., 1978; Hooper et. al, 2002). Depobelts are major fault-bounded stratigraphic packages resulting from sequential, basinward outbuilding of the delta through time (Evamyet. al., 1978; Knox and Omatsola, 1989). Rapid progradation of the thick delta caused gravitational collapse of the underlying, overpressuredAkata Formation (shale-dominated), thereby creating structural traps in both up-dip extensional and down-dip contractional domains (Knox and Omatsola, 1989; Doust and Omatsola, 1990). In the depobelt model developed of Knox and Omatsola (1989), loading by prograding delta wedge caused the underlying unstable marine shales to move upwards and basinward. This mobile shale mobilization in turn, caused collapsing along major growth faults, creating accommodation for further sediment loading. As shale withdrawal within a depobelt neared completion, subsidence rate along faults decreased, causing sedimentation to shift to new depocenterbasinward. Each depobelt is associated with different structural traps. This pattern of deposition continues in the present-today, with extensional development of growth faults on the modern shelf and slope, and compressional uplift occurring near the toe of the slope (Armentroutet. al., 2000; Hooper et. al., 2002; Bellingham et. al., 2014). The evolution stages of the depobelts is shown (Figure 4). The Akata-Agbada system has previously been described as the only petroleum system in the Niger Delta (Ekweozor and Daukoru, 1994; Kulke, 1995). The marine shales of the Akata and the lower Agbada Formations were identified as the potential source rocks, while the deltaic sandstones within the Agbada and turbidites deposits in the deep Akata Formation are the dominant reservoir rocks. Clay-smeared faults, shale gouges, interbedding shales and sand/shale juxtaposition are the dominant sealing mechanisms. Hydrocarbon traps are dominantly structural (faults and anticlinal structures) with some stratigraphic traps localized at the flanks of the basin (Kulke, 1995). Schematic diagram illustrating the different hydrocarbon trapping styles in the delta is shown (Figure 5). However, in the latest assessment of the undiscovered petroleum resources in the Niger Delta Brownfield (2016), two separate reservoir systems namely Akata and Agbada Reservoir Units were identified contrary to the Akata-Agbada petroleum system. In the delta accrding to (Brownfield, 2016), estimated mean volume of undiscovered, technically recoverable conventional oil and gas resources within the Agbada Reservoir Unit is 1,616 million barrels of oil, 9,454 billion cubic feet of gas and 494 million barrels of natural gas liquids(What is your source- please reframe from quoting reserves numbers unless already published). Similarly, in the Akata Reservoir Unit, the estimated mean volume of oil is 13,918 million barrels, 48,767 billion cubic feet of gas, and 5,832 million barrels of natural gas liquids. The estimated mean size of the largest oil and gas field expected to be discovered within the Agbada Reservoir Unit are respectively 274 million barrels and 981 billion cubic feet. In the Akata Reservoir Unit, they constitute 4,119 million barrels of oil and 13,355 billion cubic feet of gas, (Brownfield, 2016).

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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A ..

Figure 3: Schematic cross section of the Niger Delta showing the lithostratigraphic units (Akata, Agbada and Benin Formation. Also shown are some late Miocene canyons that incised Benin-Agabada Formation. (Redrawn from Shannon and Naylor (1989); Doust andOmatsola (1990); Brownfield (2016).

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Figure 4: Evolutionary stages of the Cenozoic Niger Delta Sub-basin. Rapid sediment loading led to the withdrawal of unstable marine shales, which in turn caused structural collapse along major growth faults. Consequently, delta wedge prograded further seaward into the newly created sub-basin (Modified after Knox and Omatsola, 1989; Magbabeolaet al., 2007).

Figure 5. Schematic diagram illustrating the different hydrocarbons trapping styles in the Niger Delta (Modified after Doust and Omatsola, 1990).

III.

Data set and Methods

The well and 3D seismic data used in this study were provided by SPDC. The seismic survey is a zerophase, Society of Exploration Geophysicists (SEG) normal polarity data with dominant frequency of 60 Hz. It covers 1,086 km2 but part of the data was masked due to confidential reason, leaving approximately 445 km 2 survey, which was interpreted. Generally, the seismic data quality decreased below 3.5 s two-way travel time (TWTT), thereby affecting the confidence in horizon interpretation below this interval. The well data used in this study consists of wire-line log suites, deviation, biostratigraphic and checkshot data sets (Table 1). The logs were mostly acquired within the hydrocarbon bearing intervals in the Agbada Formation. The biostratigraphic data consists of planktonic foraminifera population and diversity (PPOP & PDIV) and benthonic foraminifera DOI: 10.9790/0837-0506013247


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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A .. population and diversity (FPOP & FDIV), as well as palynological (P), foraminiferal (F) biozones, interpreted from ditch cuttings and side-wall cores. These data sets were integrated with well log signatures to interpret the major stratal bounding surfaces (maximum flooding surfaces-MFSs and sequence boundaries-SBs. Paleobathymetry interpreted from biostratigraphic data provided useful information about paleo-water depths, which also facilitated the interpretation of maximum flooding surfaces and sequence boundaries. A regionally continuous stratigraphic marker (9.5 Ma MFS) was interpreted in the control well (AzaraNorth-1), using a combination of gamma-ray, resistivity and neutron-density log signatures. This MFS was interpreted on high gamma-ray log motif, corresponding to low resistivity, high neutron-density cross plot, and high foraminiferal population and diversity. Consequently, this MFS was interpreted across the remaining Ituku1, Sagamu River-1 and Azufu-1 wells located more distally. Stratigraphy was flattened on this datum (assumed to have been deposited nearly horizontally at the time of maximum transgression of the shelf) to aid the interpretation of the major depositional cycles bounding surfaces. The intervening SBs was interpreted at the bases of thick, blocky sands corresponding to low gamma-ray log motifs, relatively high resistivity, low neutron-density cross plot, minimum planktonic foraminifera population and diversity. A total of three MFSs and SBs each were interpreted in the proximal Azara-North-1 well). Relative ages (9.5 Ma MFS-12.8 Ma MFSs and 8.5 Ma SB-12.1 Ma SB), were assigned to these stratigragraphicsurfaces using the Niger Delta Chronostratigraphic Chart (Stacher, 1994), (Figure 6). These surfaces were matched with their corresponding seismic horizons and correlated regionally across the four wells from proximal to distal part of the study area. Genetic sequences defined within MFSs according to Galloway (1989), were subdivided into lowstand, transgressive and highstand systems tracts using the parasequence stacking methodology of Van Wagoner et al (1990). The stratal stacking methodology used in this study is shown (Figure 7). Table 1: Summary of the well-data used in this study. Data type Gamma-ray Resistivity Neutron Density Sonic Checkshot Biostrat Biozones

Azara North-1 ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

Well name Ituku-1 ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

Sagamu River-1 ✓ ✓ X X ✓ X ✓ ✓

Azufu-1 ✓ ✓ X ✓ ✓ X ✓ ✓

Figure 6. A section of the Niger Delta Chronostratigraphic Chart with the interval studied between Upper Miocene (Modified after Stacher, 1995). DOI: 10.9790/0837-0506013247


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Figure 7: Well log sequence stratigraphic interpretation using Azara North-1 located within fault block 3 shows how the different types of well data provided in this study were integrated to interpret the key sequence stratigraphic surfaces.

IV.

Results and interpretation

4.1 Structural Framework Within the limit of the seismic survey used in this study, two major normal faults labeled (A & B), as well as numerous synthetic and few antithetic faults have been identified in the southern CSD. The position of the landward dipping counter regional fault labeled (C) was inferred at the southernmost limit of the seismic survey based on the seismic shadow effect commonly associated with fault activities in the area. The faults generally trend NE-SW in map view, with lateral extent of approximately 9 km and 7.5 km respectively for A and B. These faults (A and B), located in the north-east part extend throughout the entire stratigraphic interval with large vertical displacement of strata, while numerous synthetic and antithetic faults located in the southwest portion have less vertical extent and are associated with smaller stratal displacement. The down to basin faults A and B are somewhat listric, and subdivide the study area into three major fault blocks labeled (block 1, 2 and 3), (Figure 8). We do not have any well within the fault block 1. Azara North-1 is located within the fault block 2, associated with few synthetic faults, while Ituku-1, Sagamu River-1 and Azufu-1 wells lie within the fault block 3, which is complicated by numerous synthetic and few antithetic faults. Time structure map generated from the regionally interpreted horizons (especially the deeper 11.5 and 12.8 Ma MFSs), show changes in the isochrone values across the major faults A and B (Figure 9). This observation suggests that these faults are syn-depositional growth faults and probably controlled increase in accommodation in the southern DOI: 10.9790/0837-0506013247


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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A .. CSD during the deposition of the stratigraphic section within the 11.5 and 12.8 Ma MFSs. The counter regional fault C could not be interpreted across the entire seismic volume to ascertain its impact on stratigraphy. 4.1.1 Field trapping styles The structural and stratigraphic features identified in the southern CSD include regional hanging wall rollover/footwall (RHWR/FW), simple hanging wall rollover, K-type faults, and subtle traps associated with the interpreted lowstand delta as well as erosional truncations (Figure 10A, B).

Figure 8. Attribute map showing the major growth faults (A & B), inferred counter regional fault (C), numerous synthetic and few antithetic faults. The three fault blocks are also show. The regional hanging wall rollover/footwall closures exhibit convex upward geometry with decreasing net to gross (N/G) from the inner shelf through the mid to outer shelf as evidenced from gamma ray log signatures in the penetrated shallower sections. A simple hanging wall structure is associated with fault B at the upper slope setting. The interpreted K-type fault is characterized by multiple synthetic faults, which terminate against antithetic fault and located within the alluvial plain to the inner shelf with high net to gross. Stratigraphic pinch outs occur within the depositional feature interpreted as lowstand delta. The feature was interpreted as lowstand delta because it marks the onset of reflection inclination at the outer shelf/shelf margin transition. It also lies above the 12.1 Ma SB, which was interpreted in the Azara North-1 well in the proximal section. Some stratigraphic traps also occur in the erosional truncations and channel fills associated with the shallow 8.5 Ma SB.

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Figure 9. Time structure maps of 9.5, 11.5 and 12.8 Ma MFSs. Changes in isochron values across major faults A and B and across these syn-depositional growths indicate that they were active during the deposition of the stratigraphic packages bonded by 11.5 and 12.8 Ma MFSs.

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Figure 10 A, B. Uninterpreted and interpreted seismic sections showing the structural and stratigraphic features in the southern CSD. 4.2 Sequence stratigraphic framework 4.2.1 Stacking patterns and key stratigraphic bounding surfaces Progradational and retrogradational stacking patterns were recognized and interpreted in this study. The progradational stacking patterns are characterized by funnel-shaped (coarsening upward) gamma ray log patterns, which generally exhibit progressive upward increase in sand with corresponding decrease in shale across the wells. Stratal packages associated with this type of stacking patterns have been interpreted as sediments deposited within crevasse splays, river mouth bars, delta front and shoreface environments (Emery and Meyer, 1996; Kendall, 2004). The retrogradational stacking patterns are characterized by bell-shaped (fining upward) gamma ray log patterns, which generally exhibit progressive upward increase in shale content with corresponding decrease in sand. Sedimentary deposits exhibiting this type of stacking patterns have been interpreted as fluvial point bars, tidal bars, deep-tidal channel and transgressive shelf environments (Emery and Meyer, 1996; Kendall, 2004). These stacking patterns are bounded by marine flooding surfaces and their correlative surfaces. The stacking patterns methodology used in the identification and correlation of the key sequence bounding surfaces is shown (see Figure 7). Using this stratal stacking patterns, we identified three MFSs and SBs dated from oldest to the youngest (12.8 Ma, 11.5 Ma and 9.5 Ma), and (12.1 Ma, 10.6 Ma and 8.5 Ma) respectively. Maximum flooding surfaces cap transgressive systems tracts deposited during the episodes of maximum flooding of the shelf and clastic sediment starvation seaward. Sequences boundaries, which represent substantial erosional unconformities and seaward progradation of systems tracts were interpreted at the bases of abrupt upward increase in sand packages between two adjacent MFSs (Figure 10). The 8.5 Ma SB, which marks the base of Benin Formation shows evidence of erosional unconformity throughout the entire seismic volume, while the 10.6 and 12.1 Ma SB were occasionally associated with erosive surfaces. The three MFS and SBs were all identified in the proximal Azara North-1 well but, the more distal Ituku-1, Sagamu River-1 and Azufu-1 wells did not penetrate the older 12.8 Ma MFS and 12.1 Ma SB as shown below (Figure 10).

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Figure 10. Well log correlation showing the interpreted genetic sequences, stacking patterns, systems tracts and their bounding surfaces. 4.2.2 Systems tracts and genetic sequences A systems tract is defined as the linkage of contemporaneous depositional systems, which subdivide a sequence into smaller genetic units (Brown and Fisher, 1977). Lowstand, transgressive and highstand systems tracts were interpreted based on their stratal stacking patterns, the nature of their bounding surfaces and positions within a sequence. Correlation of the systems tracts across the wells from the northeast (landward) to the southwest (seaward), reveal variation in their thicknesses and percentage distribution, such that the LST attained 58%, 31% for the HST and 11% for the TST (Figure 11). In seismic section, the LSTs are generally characterized by discontinuous to moderately continuous, low amplitude to transparent seismic facies labeled (F1) in this study. They are interpreted as stacked fluvial channel deposits. The TSTs and HSTs are generally characterized by parallel, continuous, high amplitude seismic facies labeled (F2). They are interpreted to record shallow marine succession deposits under subaqueous conditions. These two seismic facies alternate throughout the seismic section in a similar way as demonstrated elsewhere in the CSD (e.g. Magbagbeola and Willis, 2007). Two genetic sequences were recognized in this study (Figure 10). The younger (Sequence 2) is capped at the top and base by the 9.5 Ma MFS and 11.5 Ma MFS respectively. This is sequence lie within the Agbada Formation. It was correlated across Azara North-1, Ituku-1, Sagamu River-1 and Azufu-1 wells. It attained a thickness of 3400 feet in Azara North-1, 3000 feet in Ituku-1, 4200 feet in Sagamu River-1 and 3600 feet in Azufu-1 well. The HST unit generally thinned in the seaward direction, while the LST increased in thickness in the seaward direction. The TST also increased in thickness in the seaward direction. The older (sequence 1) is bounded at the top and base by the 11.5 Ma MFS and 12.8 Ma MFS respectively. This sequence lie within Agbada and probably Akata Formation, which is yet to be penetrated in the CSD Onshore Niger Delta. It was fully penetrated by Azara North-1 well where it attained a thickness of 5000 feet. Ituku-1, Sagamu River-1 and Azufu-1 wells only penetrated the upper part of this sequence. The DOI: 10.9790/0837-0506013247


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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A .. thickness of this sequence is interpreted to increase in the seaward based on horizon interpretation in seismic section (see Figure 9A). These sequences were interpreted as third order genetic sequence because their duration of formation range from 0.5-1 Ma (Doust and Omatsola, 1990; Reijers, 2011).Potential reservoirs in these sequences generally occur within the LST and HST units, which are generally sandier. The marine shales within the HST units also act as source/seal for hydrocarbon, while the TST shale units act as regional top seal.

Figure 11: Pie chart illustrating the distribution of potential reservoir, seal and source rocks within systems tracts across the studied wells.

V.

Discussion

5.1 Sequence stratigraphy and relative sea-level The sequence stratigraphic succession interpreted in this study was correlated with the eustatic curve in the Niger Delta Chronostratigraphic Chart (Figure 6) to understand how relative sea-level changes in the Middle Miocene influenced sedimentary evolution in the studied part of the southern CSD. It was observed from the chart that relative sea-level was athighstand during the Langhian Stage (associated with the 15.0 Ma MFS), which was not penetrated by the wells studied. However, relative sea level is interpreted to have begun falling during the SerravallianStage (bounded by the 12.8 Ma MFS and 11.5 MFS), but rose gradually from the Tortonian Stage (defined by the 9.5 Ma MFS) to the Messinian Stage. This trend follows other interpreted eustatic sea-level trends (e.g. Haqet al., 1987). The more than 1000 feet thick, dominantly shally sediments recoded in the Azara North-1 well with deeper penetration is interpreted to represent the uppermost part of the LanghianStage deposits. The thick sedimentary packages with overall upward increase in net to gross in the sequences 1 and 2, notably in Azara North-1, Ituku-1 Sagamu River-1 and Azufu-1 is interpreted to represent sediments deposited during the SerravallianStage fall in relative sea-level. The thin marine shales associated with high foraminifera abundance and diversity, which is defined by the 9.5 Ma MFS (see Figure 7), is interpreted to represent sediments deposited during the TortornianStage relative sea-level rise. The interpreted shelf edge trajectory appear to have risen throughout most of the Langhian stage, but prograded gradually during the Serravallian as evidenced in seismic section by major seaward inclination of seismic reflections (Figure 10A, B). This observation suggests that the shelf edge aggraded during the LanghianStage relative sea-level highstand, but prograded gradually during the SerravallianStage relative sealevel fall. Although, the inferred Counter Regional Fault C appears to have caused some degree of tilting in stratigraphy, the consistency of stratal terminations on the hanging wall of Fault B, which separates the proximal shelf from distal shelf support our interpreted shelf margin..

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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A .. 5.2 Sand distribution within the genetic sequences Systems tracts analysis across the studied wells, show that highest sand distribution occurs within the LST and HST units compared to the TST units, which are mostly shales, (Figures 7, 10-11). In Sequence 2, sand distribution is highest in Azara North-1 with an estimated value of 40%, compared to shales with 60%. Overall sand content is interpreted to decrease towards Azufu-1 well in the seaward direction where it attains a minimum of 35%. In Sequence 1, the overall sand distribution in Azara North-1 is 30%, and decreases also towards Azufu-1 well as in sequence 2, where it could attain a minimum of 20-25%. However, the lack of complete log information in Ituku-1, Sagamu River-1 and Azufu-1 wells made it impossible to determine the actual sand distribution within this sequence. 5.3 Deep prospectivity Four potential deeper leads were identified in this study. The first and second are located in Fault Block 2, while the third and fourth leads are located in Fault Block 3 (Figure 12). Proven hydrocarbon bearing intervals (high resistivity zones) in the penetrated shallower stratigraphic sections are associated with high to moderate amplitude seismic facies (Figure 10). Similar seismic facies, which were observed within the deeper stratigraphic sections guided our interpretation of the probable hydrocarbons in the potential deeper prospects. Deeper Leads 1 and 2 are associated with regional hanging wall block of Fault A/foot wall block of Fault B. These leads are located between outer shelf to upper slope settings with generally low net to gross. Potential reservoirs are most likely to be associated with the highstand deposits of the unpenetrated Serravallian Stage stratigraphic sequence (13.1 Ma SB-15.0 Ma MFS). The interpreted 12.8 Ma MFS horizon shows occurrence of two structural dip closures associated with the penetrated regional hanging wall blocks/foot walls capped by this horizon (red polygons in Figure 9A). Similar features are expected to trap hydrocarbon in the deeper Leads 1 and 2, which lie directly beneath these trapping structures with similar geometry. Also, the low net to gross (relatively high shale content) associated with the outer shelf-upper slope setting, where these prospects occur offers favourable top sealing conditions for potential reservoirs. The marine shales associated with the 12.8 Ma MFS may also contribute to the regional sealing of these reservoirs and may exhibit a possible source potential given depth of burial , geothermal gradient and the organic richness of condensed sections. Lead 3 is associated with the interpreted lowstand delta, which is located just below the interpreted shelf margin. Potential reservoirs in this prospect include the lowstand deposits associated with Sequence 1 as well as some stratigraphic pinch outs. Marine shales associated with the 11.5 Ma MFS is interpreted to act as regional top seal to these reservoirs. Lead 4 constitute regional hanging wall trap located in the upper slope. Potential reservoirs are most likely highstand deposits, while the marine shales associated with the 12.8 Ma MFS will act as regional top seal. The interpreted time structure map show no visible closure associated with Leads 3 and 4. Based on this observation, we recommend further maturation and deriskingof Leads 1 and 2, which have less risk compared to Leads 3 and 4. However, the poor seismic resolution associated with the hanging wall of Fault B could have negatively impacted horizon interpretation further away from the fault, hence, affecting the quality and integrity of the time structure map.

Figure 12. Seismic line cartoon showing the interpreted deeper prospects in the study area. DOI: 10.9790/0837-0506013247


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Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A .. VI.

Conclusions

Well and 3D seismic data were integrated to interpret the structural and stratigraphic framework of the Middle-Late Miocene sediments in the Southern Coastal Swamp Depobelt(CSD) of the Niger Delta. The structural framework is characterized by two major growth faults, one counter regional growth fault, numerous synthetic and few antithetic faults, which generally trend northeast-southwest. The major growth faults subdivide the study area into three fault blocks. Time structure maps reveal that syn-depositional growth Faults A and B where active and controlled accommodation during the deposition of sediments within the deeperstratigraphic sections. Seismic interpretation reveals dominantly structural hydrocarbon trapping styles, with some stratigraphic traps. The structural traps include regional hanging wall rollover/footwall (RHWR/FW), simple hanging wall rollover and K-type faults, while the stratigraphic traps include stratigraphic pinch outs associated with lowstand delta as well as erosional truncations below fluvial channel incision. The stratigraphic framework constitutes two third order genetic sequences, bounded by three maximum flooding surfaces and sequences boundaries, whose ages range from 12.8, 11.5 and 9.5 Ma (MFS), and 12.1, 10.6 and 8.5 Ma (SB). The deepest stratigraphic surfaces (12.8 and 12.1 Ma), were only penetrated by the deepest Azara North-1 well. Systems tracts thicknesses vary across the wells, such that their average distribution is composed of 58% for the LST, 31% for the HST and 11% for the TST, with overall decrease in sand content basinward. Potential reservoirs are postulated to occur within the LST and HST units, which are generally sandier, while the marine shales within the HST units also act as source/seal for hydrocarbons. The marine shales within the TSTs act as regional top sealinaddition. Interpreted shelf edge trajectory appears to have risen during the LanghianStage relative sea-level highstand, leading to sediment aggradation, but prograded gradually during the SerravallianStage relative sea-level fall thus giving rise to the progradational sediment stacking recorded in Sequence 1 and part of Sequence 2. The thin marine shales associated with abundance and diversity peaks of foraminifera, and defined by the 9.5 Ma MFS, record the onset of relative sea-level rise during the Tortornian Stage. Four potential deeper leads were identified. Leads 1 and 2 are located in the Fault Block 2, and are associated with regional hanging wall roll over/foot wall traps. Leads 3 and 4 are located in the Fault Block 3, and are associated with lowstand delta and simple roll over anticline respectively.Leads 1 and 2 are likely to be associated with structural dip closures as evidenced from time structure map of the shallower stratigraphic section with similar trapping geometry. Hence, are ranked higher than the Leads 3 and 4, which do, not showing structural closures.

Acknowledgments The authors wish to express their gratitude to Shell Petroleum Development Company (SPDC) for granting access to data and work station used in this study. We are grateful to Schlumberger for providing the Petrel(TM) software used in interpreting the data. Professor Cornelius S. Nwajide provided very insightful reviews that greatly improved the paper. This paper equally benefited from incisive reviews and discussions with Bilal U. Haq. Finally, many thanks to OtukaUmahi, AfolabiFatunbi, SegunAkinlorabu, ObobiOnwuka, Chidozie Dim and David Anomneze for providing software support.

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Chima, K. I "Sequence Stratigraphy and Structure of the Southern Coastal Swamp of the Niger Delta, Nigeria: A new insight into the hydrocarbon systems and prospectivity at deeper stratigraphic intervals." IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG) 5.6 (2017): 32-47.

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