Sequence stratigraphy and depositional sequence ...

Petroleum Research 3 (2018) 25e32

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Sequence stratigraphy and depositional sequence interpretation: A case study of “George” Field, offshore Niger Delta, Nigeria O. Abiola*, M.T. Olowokere, J.S. Ojo Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 July 2017 Received in revised form 24 December 2017 Accepted 28 December 2017

The seismic sequence stratigraphic analysis revealed four depositional sequences (DS-1, DS-2, DS-3 and DS-4). The accompanying systems tracts were interpreted and mapped in the study area based on the log motifs of the reference well and the spatial distribution of the recognized constrained surfaces: maximum flooding surfaces (MFSs), sequence boundaries (SBs) and transgressive surfaces (TSs) on the seismic data. Depositional systems in the study area comprise lowstand systems tracts (LSTs), transgressive systems tracts (TSTs) and highstand systems tracts (HSTs). The LSTs are represented by coeval facies dominated by deposition basinward of the shelf-edge during maximum regression and are characterized by shallow-water deposition from gravity flows and/or traction processes within the shelf-edge or canyon-head delta. The sediments associated with lowstand systems tracts recognized in the study area are the fluvial channel sands and slope fans (SF). The transgressive sand units were interpreted as shoreface sands deposited in the shelf region during rising sea levels. Highstand systems tracts are characterized by intervals of coarsening and shallowing upwards, with both fluvial and deltaic sands prograding laterally into neritic shales. In the study area, the units are very thick. The highstand and lowstand system tracts exhibit blocky log patterns and are associated with the reservoirs while the transgressive system tracts serve as seals to the reservoirs. The environment of sediments deposition in this area is delta plain, shelf, slope to toe of slope. © 2018 Chinese Petroleum Society. Publishing Services by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Sequence stratigraphic analysis Depositional systems Log motifs Systems tracts Environment of sediments deposition

1. Introduction The derivation of stratigraphic information from seismic data has its origin in the early 1970s with the advent of improved quality two-dimensional seismic data (Posamentier et al., 2007). Sequence stratigraphic studies can play a significant role in reservoir studies and access its connectivity (Mansouri-Daneshvar et al., 2015). Continued success in the search for oil and gas reserves therefore depends upon thorough understanding of the subsurface geology of exploration fields. This involves the ability to accurately predict and delineate the spatial and depth distribution of subsurface geologic facies (source rock, reservoir rock and seal) and the ability to discriminate the fluids saturating the reservoirs (oil, gas or brine) and possibly quantifying such (Aminu and Olorunniwo, 2012). For better understanding of the offshore sedimentary basin, the application of sequence stratigraphic method will enhance the geographic control and correlation of worldwide pattern of sea

* Corresponding author. E-mail address: [email protected] (O. Abiola).

level changes (Payton, 1977). The method facilitates the identification of major progradational sedimentary sequences which offer the main potential for hydrocarbon generation and accumulation. Depositional processes and pattern of some sequences can be observed and traced in correlation with global changes in the sea level (Vail et al., 1977). Seismic reflection is the major tool that could be used in the stratigraphy interpretation, coupled with well logs, it can be used to interpret depositional processes and environment. The deep offshore of Niger Delta basin is an oil-rich belt or shelf that consists of alternating stacked pattern of lowstand and highstand slope fan, which delineate the reservoir pattern and competency.

2. Geological background of the study area The study area is situated on the Niger Delta, Nigeria (latitudes 3 and 6 N and longitudes 5 and 8 E), (Fig. 1a and b). The George Field falls within the parasequence of set of the Agbada Formation. The structure is characterized by simple rollover anticline that is bounded to the north by a major growth fault. The crest of the

https://doi.org/10.1016/j.ptlrs.2017.12.001 2096-2495/© 2018 Chinese Petroleum Society. Publishing Services by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

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O. Abiola et al. / Petroleum Research 3 (2018) 25e32

Fig. 1. (a) Map of the Niger Delta showing the base map of the study area, (b) Base map of the study area covered by the seismic data and well locations.

anticline is flat/elongated and runs parallel to the bounding fault. The stratigraphic sequence (Fig. 2) in the field consists of marine shales of the Akata Formation which is about 6100 m thick, the Agbada Formation which is 4500 m thick, and the Benin Formation

which is about 1820 m thick (Short and Stauble, 1967). Deep offshore Niger Delta is situated over oceanic crust emplaced during Cretaceous- Paleogene, during the initial stage of the spreading of the South Atlantic. Initial sedimentation began within Upper


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Fig. 2. Chrono-stratigraphic diagram showing the three Formations of the Niger Delta. (After Shannon and Naylor, 1989; Doust and Omatsola, 1990).

Cretaceous e Lower Oligocene with the deposition of hemipelagic mudstones of the Akata Formation. From the Late Oligocene through recent, the deposition was dominated by progradation of the Niger Delta onto a slope-rise environment, which was conducive to turbidite deposition of the more coarse-grained siliclastics of the Agbada Formation (Short and Stauble, 1967; Avbovbo, 1978). The latter contain the Lower and Middle Miocene reservoir-seal couplets responsible for the major deep-water hydrocarbon accumulations discovered to date. The underlying Akata Formation is believed to contain the main source rock intervals. Tertiary extension of the Niger Delta shelf was the driving process for the gravity driven structures of the deep-water (Short and Stauble, 1967; Avbovbo, 1978). According to Kulke (1995), Shannon and Naylor (1989), Doust and Omatsola (1990), the Tertiary sequence consists of alternations of clastic lithofacies that occur in stacked sections of (regressive) offlap cycles. These lithofacies comprise sandstones, silts and shales of much similarity, regardless of their ages or

positioms in the sequence. Thus, in a vertical sense, the sequence can be subdivided into three lithofacies in ascending order of the Akata, Agbada and Benin Formations (Fig. 2). The overall regressive clastic sequence reaches a maximum thickness of 30,000e40,000 ft (9000e12,000 m) at the approximate depocenter in the central part of the delta (Avbovbo, 1978). 3. Materials and methods The data sets used for this study comprises geophysical well logs (GR, Resistivity, Sonic) from four wells, checkshot data and 3-D seismic sections (496 In-lines and 780 Cross-lines) covering a total area of about 85.8 km2. The procedures for seismic stratigraphic interpretation have been well presented in Mitchum and Vail (1977), Mitchum et al. (1977), Sangree and Widmier (1977). These are: (i) seismic sequence analysis, which includes recognition and correlation of seismic sequences; and (ii) seismic facies analysis,

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which includes recognition, mapping and interpretation of seismic facies. A modified form of these procedures was used in this study. 3.1. Recognition of systems tracts on wireline logs Vertical stacking patterns and grading trends in the well logs (e.g. GR) were used to delineate the parasequences. Identified parasequences are progradational, aggradational and retrogradational (Fig. 3a and b). Stacking patterns of parasequences refer to the architecture of a vertical succession of parasequences. In a progradational stacking pattern, the facies at the top of each parasequence become progressively more proximal higher in the succession and displays upward coarsening pattern. In retrogradational stack, the

facies become more distal upwards and displays fining upward sediment grain sizes. Aggradational stack is characterised by similar facies at the top of each parasequence and displays uniform pattern (Milton and Emery, 1996). Other descriptive terms for these parasequences are funnel shaped (cleaning up motif), bell shaped (messy up motif) and blocky shaped for progradational, retrogradational and aggradational, respectively. Lowstand systems tract was recognized by its diagnostic upward coarsening progradational stacking patterns, and the occurrence of sequence boundary below and maximum regressive surface at the top. The upward coarsening pattern of parasequences persists vertically until the maximum regressive surface is reached. In instances where the sediments associated with the LST deposits

Fig. 3. (a) GR or SP log motifs for the deposits associated with lowstand systems tract (After Vail and Wornardt, 1991); (b) GR or SP log motifs for the deposits associated with transgressive and highstand systems tracts (After Vail and Wornardt, 1991).


do not display the characteristic upward coarsening log patterns, other criteria such as multistory upward fining pattern and blocky pattern within pelagic sediments are considered. Transgressive systems tract is delineated using retrogradational parasequences, which exhibit upward fining patterns. Their thicknesses may reduce or may be higher than the underlying one. This finingupward pattern of parasequences persists vertically until the maximum flooding surface is reached. Highstand systems tract is delineated by progradational regressive well log stacking patterns. They are bounded below by distal downlap into maximum flooding surface of the underlying transgressive systems tract and the upper boundary is the overlying sequence boundary produced by relative fall of sea level (Fig. 3). 3.2. Well to seismic tie and integration The key sequence stratigraphic element was transferred to the interpreted seismic sections. This was done through the use of check-shot data. The check-shot data showed the variation of depth with time. The seismic profiles represent the closest seismic lines to the wells, and the interpretations were transferred to other seismic lines. After identification of depositional cycles, sequence stratigraphic surfaces (maximum flooding surfaces, transgressive surfaces, sequence boundaries) and systems tracts were identified on the well logs. 4. Results and discussion Figs. 4 and 5 are the interpreted well-to-seismic section with the

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three systems tracts, highstand systems tracts (HST), transgressive systems tracts (TST) and lowstand systems tracts (LST), showing the distribution of lithofacies. Four depositional sequences (DS-1, DS-2, DS-3 and DS-4) and the accompanying systems tracts (Figs. 6 and 7) were interpreted and mapped in the study area. Depositional systems in the study area comprise lowstand systems tracts (LSTs), transgressive systems tracts (TSTs) and highstand systems tracts (HSTs). The LSTs are represented by coeval facies dominated by deposition basinward of the shelf-edge during maximum regression and are characterized by shallow-water deposition from gravity flows and/or traction processes within the shelf-edge or canyon-head delta. The sediments associated with LSTs recognized in the study area are the fluvial channel sands and slope fans (SF). The transgressive sand units could be interpreted as shoreface sands deposited in the shelf region during rising sea levels. Depositional sequences DS-1 and DS-4 formed the deepest (oldest) and topmost (youngest) sequences respectively. DS-1 is enveloped on top by the DS-2, which was revealed only in wells A and C that probed deeper stratigraphic sections of the well field. Highstand systems tract (HST) of the DS-1 sequence, estimated to be about 600 ft was deposited in the middle neritic setting depicting mainly progradational-aggradational stacking patterns. The transgressive systems tract (TST) sand units have been interpreted as shoreface sand deposited in the shelf region during rising sea levels. The DS-2 is approximately 495 ft thick. The lowstand systems tract (LST) of this sequence formed thick sand deposits interpreted as basin floor fans (BFF), deposited in the outer neritic to bathyal depositional settings. The TST of DS-4 was observed to be unconformably overlying

Fig. 4. Well-to-seismic interpretation of the study area showing interpreted various systems tracts.

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Fig. 5. Well-to-seismic interpretation of the study area showing interpreted various systems tracts.

the DS-3 and underlies by a TST of DS-2 which is about 800 ft thick. DS-2 overlies the DS-1 and is capped by DS-3. The sequence was identified at the depth of 9200 ft in the down dip wells (A and C) and from a depth range of 6000 and 7000 ft in the up dip wells B and D. The sequence displayed predominantly fluvial and tidal

processes (progradational stacking pattern) as shown in the parasequence stacking pattern of the wells. The LST of this sequence contains reworked channel sand deposits which were more pronounced in the down dip wells. DS-4 is the topmost (youngest) sequence in the study area and rested unconformably on the DS-3.

Fig. 6. Seismic interpretation of the study area showing interpreted various systems tracts.


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Fig. 7. Seismic interpretation of the study area showing interpreted various systems tracts.

The sequence was deposited within the neritic paleo-depositional environment. 4.1. Reservoir potential of the study area The potential reservoirs in the study area were mainly the channel sands and shoreface sands of lowstand systems tracts (LSTs) and highstand system tracts (HSTs) respectively. These displayed low gamma ray and high resistivity values. The LSTs are represented by coeval facies dominated by deposition basinward of the shelf-edge during maximum regression and are characterized by shallow-water deposition from gravity flows and/or traction processes within the shelf-edge or canyon-head delta. The sediments associated with LSTs recognized in the study area are the fluvial channel sands and slope fans (SF). 4.2. Source rock potential Several thick shale units of the transgressive systems tracts (TST) identified in the study area were considered potential source rocks for hydrocarbon found in the reservoirs. The shales of the TST in which the MFS were delineated could form seals to the reservoir units. 4.3. Trapping mechanism The faults constitute the major traps for hydrocarbon accumulation within the study area. Also, the shale unit of transgressive systems tract (TST) and the shale units within the highstand systems tracts (HST) could form top and bottom seals for hydrocarbon in the reservoir sand. The reservoir rocks of the lowstand systems tracts (LST) and the highstand systems tracts (HST) and the seals from prodelta shales of the transgressive systems tract (TST) combine to constitute stratigraphic traps for hydrocarbon

accumulation in the field. In each of the sequences, the lower parts of the sections were marked by deposits arising from relatively low sea level, forming channels and slope complexes. The middle sections were deposited during a generally high relative sea level, while the uppermost sections were deposited during gradual drops in relative sea level lowering (Highstand). These inferred variations in the relative sea level defined the third order depositional systems that comprised the lowstand systems tract (LST) at the base of the section, transgressive systems tract (TST) in the middle of the section and the highstand systems tract (HST) at the top of the sections. In terms of hydrocarbon exploration, the sand units of the lowstand systems tracts (LST) and the highstand systems tracts (HST) formed the basin floor fans, channel and shoreface sands of the delta. The shales of the TST in which the MFS were delineated could form seals to the reservoir units. A combination of the reservoir sands of the lowstand systems Tract (LST) and the highstand systems tract (HST) and the shale units of the transgressive systems tract (TST) can form good stratigraphic traps for hydrocarbon and therefore, should be targeted during hydrocarbon exploration. 4.4. Economic significance These events have economic significance because these changes in the sea level cause large lateral shifts in the depositional patterns of seafloor sediments. These lateral shifts in deposition create alternating layers of good reservoir quality rock (porous and permeable sands) and poorer-quality mudstones which is capable of providing a reservoir ‘’seal’’ to prevent the leakage of any accumulated hydrocarbons that may have migrated into the sandstones. 5. Conclusion The seismic sequence stratigraphic analysis revealed four

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depositional sequences (DS-1, DS-2, DS-3 and DS-4) and the accompanying systems tracts were interpreted and mapped in the study area based on the log motifs of the reference well and the spatial distribution of the recognized constrained surfaces, maximum flooding surfaces (MFSs), sequence boundaries (SBs) and transgressive surfaces (TSs) on seismic data. Depositional systems in the study area comprise lowstand systems tracts (LSTs), transgressive systems tracts (TSTs) and highstand systems tracts (HSTs). The LSTs are represented by coeval facies dominated by deposition basinward of the shelf-edge during maximum regression and are characterized by shallow-water deposition from gravity flows and/ or traction processes within the shelf-edge or canyon-head delta. The sediments associated with lowstand systems tracts (LSTs) recognized in the study area are the fluvial channel sands and slope fans (SF). The transgressive sand units could be interpreted as shoreface sands deposited in the shelf region during rising sea levels and highstand systems tracts (HSTs) are characterized by intervals of coarsening and shallowing upwards, with both fluvial and deltaic sands prograding laterally into neritic shales. In the study area, the units are very thick. This may be attributed to very high rates of subsidence, high sediment input and instability. The direction of sediments supply within the study area is from the west to east as interpreted by the variations in the seismic facies (Abiola and Olowokere, 2016). The depositional environments within the study area might be controlled by these factors, the amount and type of sediment input, fluctuations in relative sea level, tectonic activity and regional growth faulting. Both high and low density turbidity current deposits were suggested as the dominant depositional processes and the environment of sediment deposition in this area is delta plain, shelf, slope to toe of slope (Abiola and Olowokere, 2016). References Abiola, O., Olowokere, M.T., 2016. Seismic facies analysis and depositional process:

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