American Journal of Geosciences

Original Research Paper and Ichnology of Late Oligocene Delta Front Reservoir Sandstone Deposit, Greater Ughelli Depobelt, Niger Delta

Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti

Department of Geology, University of Port Harcourt, P.M.B. 5323, Port Harcourt, Nigeria

Article history Abstract: Sedimentological and ichnological study of cored reservoir Received: 28-02-2015 correlated with wireline logs between two wells, 0.8 km distance apart Revised: 05-06-2015 along dip direction, enabled vertical and cross sectional facies variability Accepted: 03-07-2015 assessment aimed at determining intra -body continuity in longitudinal

or down-dip direction; and sub-environments of deposition and factors that Corresponding Author: Raphael Oaikhena Oyanyan controlled depositional processes. Ten lithofacies described 62 meters cores Department of Geology, of the reservoir sands and seals. Sub-environments of deposition identified University of Port Harcourt, with lithofacies associations include proximal delta front-mouth bar, distal P.M.B. 5323, Port Harcourt, delta-front, prodelta-offshore, transgressive marine sandstone and tidal flat. Nigeria High mica content, poor sorting, very coarse quartz grains, high angle Email: [email protected] bedding contact, micro-slump folds and absent to sparse bioturbation at the base of an upward-coarsening sequence indicated mouth bar deposition and direct link to a distributary channel. The study of vertical and lateral intra- reservoir depositional trends indicated that structural and textural (grain sizes and biogenic features) heterogeneities in the deltaic deposit were controlled by variations in physical energy and mixed interactions of seal level changes, tide, wave, fluvial influx, storm, food supply and oxygen levels. Consequently, there is down-dip lithofacies heterogeneity, pinch out of lithofacies or gradation from coarse grains to finer grains and better sorting. Though ichnodiversity is fairly uniform between the two wells, ichno-abundance and burrow sizes decrease down-dip especially at the proximal delta front-mouth bar deposit. The results of this study improve our knowledge of the characteristics of a mouth bar deposit in a mixed-processes deltaic environment and it can be applied in the characterization of delta front deposit elsewhere with similar depositional processes and tectonic setting.

Keywords: Delta-Front, Sub-Environments of Deposition, Lithofacies Heterogeneity, Ichnofacies, Sandbody Continuity, Bioturbation

Introduction allocyclic processes. Autocyclic cycles result from natural redistribution of energy within a depositional Patterns of sedimentation and erosion in a fluvio- system such as channel meandering or switching and deltaic environment are controlled by many factors; delta avulsion, while allocyclic cycles results from among them include sea level changes, tectonic setting changes in sedimentary system as a result of external and nature of the source area, nature of basin, sediment causes such as eustatic sea level change, tectonic basin grain size and climate (Reading, 1986; Coleman and subsidence and climate change. Autocyclic cycles are Prior, 1980; Labourdette et al ., 2008). Reijers (2011) superimposed on allocyclic cycles. Niger Delta basin is updated the sedimentological model of the Niger Delta therefore said to be a mixed-processes delta with mixed sedimentary basin by Weber (1971) to a model that takes interaction of sea level changes, tide, wave, fluvial influx into consideration of the local and delta-wide effects of and storm. Dynamism in all these factors and processes sea-level cyclicity and delta tectonics. He indicated that in the deltaic sediment-transport and depositional system sediment deposition was affected by autocyclic and determines the continuity of flow units and flow barriers;

© 2015 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti. This open access article is distributed under a Creative Commons Attribution (CC-BY) 3.0 license. Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25 facies and biogenic textural heterogeneity that have Port Harcourt, Rivers state, Nigeria. It is about 164.16 impact on hydrocarbon recovery (Singh et al ., 2013; km 2 in size with oil, condensate and gas producing wells. Weber and Van Geuns, 1990; Tyler et al ., 1992). As also shown in the Fig. 1, it is bounded in the north by Effective production of hydrocarbon reservoirs a major growth that has three adjoining antithetic requires reliable prediction of facies related reservoir growth faults. Down-dip the major growth faults, are up properties and correlation at the inter-well scale. Optimal to eight syndepositional synthetic growth faults with exploitation of oil and gas assets is more likely when the their associated rollover anticline that form fault-dip geologic processes that dictated the characters of closure. The two wells, Gabi 55 and 56, used in this sedimentary reservoirs are well understood (Tonkin et al ., study are indicated in Fig. 1 with pink coloured ring. 2010). Some of the stratigraphic factors that affect Well Gabi 55 is located at the flank of a rollover production are reservoir continuity and connectivity anticline to a major synthetic growth fault, while well (Hovadik and Larue, 2007). Therefore, the aim of this Gabi 56 is at the crest. study include: (1) to evaluate the vertical and cross The studied sub-surface sedimentary rocks-D3 sectional or down-dip lithofacies variability and organism reservoir -were recovered from the Agbada responses to the dynamic interplay of rivers, sea level Formation- one of the three lithostratigraphic units in the changes, waves, storms and tides; (2) the assessment of Niger Delta basin (Short and Stauble, 1967). Past studies intra sand-body continuity/connectivity in longitudinal or in Niger Delta indicate that Agbada Formation has a down-dip direction; (3) the identification of sub- maximum thickness of 4000 m and characterized by environments of deposition in a delta-front environment. paralic to fluvial-marine sediments organized into Study Area and Geologic Setting coarsening-upward offlap cycles. While the underlying Akata Formation, has maximum thickness of 6500 m The study area is located in the Greater Ughelli depobelt of the Niger Delta, a major petroleum producing and mainly made up of over pressured marine shale with province with great importance to economy of Nigeria, thin silt and sandy interbeds. The topmost unit is the situated on the West Coast of Africa, between Latitude 3 Benin Formation, which has a maximum thickness of and 6° N and Longitude 5 and 8° E (Reijers et al ., 1997) 2000 m and consists of continental and fluvial sands, (Fig. 1). The study area is approximately 95 km from gravel and back swamp deposits.

N 0 0.5 1 1.5 2 2.5km

Gabi 58 Gabi 19 Gabi 54 Gabi 17 Gabi 16 Eke 6 Gabi 14 Gabi 53 Gabi 15

Gabi 18 Gabi 27

Study location Gabi 54 Gabi 55 Eke 10

Well position Gabi 21 Gabi 56

Growth fault Gabi 57

Fig. 1. Location map showing well locations and growth faults. Study oil wells indicated with pink coloured ring

13 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25

The aforementioned sediments of Niger Delta the description and nomenclature of sedimentary basin have been deposited since Palaeocene until structures identified. Trace fossils were recognized present day. The tectonic setting is connected to that using the recognition methods of (Chamberlain, 1978; of the southern Benue Trough, which is the mega Pemberton et al ., 2009) as well as trace fossils’ structure it’s coastal and oceanward part lies the Niger descriptions in the works of (MacEachern et al ., 2005; Delta basin. Benue Trough is a NE-SW folded 2007; Pemberton et al ., 2004; Rotnicka, 2005). The basin that runs diagonally across Nigeria. The tectonic degree of bioturbation in cores was classified with evolution of Benue Trough and Niger Delta are well bioturbation index of (Taylor and Goldring, 1993; documented in Niger Delta geologic literatures (e.g., Taylor et al ., 2003). Short and Stauble, 1967; Doust and Omatsola, 1990; Reijers, 2011). Results The structural patterns indicate that the delta comprises six depobelts that include the Greater Ughelli Sedimentological and Ichnological Analysis where the study area is located. The depobelts are growth fault bounded sedimentary units that succeed Lithofacies Analysis each other in a southward direction (Tuttle et al ., 1999). Ten lithofacies numbered 1 to 10 described D3 cored Biostratigraphic report of the studied field not interval in wells Gabi 55 and 56. The diagnostic features discussed here indicates the studied reservoir of each lithofacies are indicated in the core photos of sediments is Late Oligocene in age and validated the Fig. 3 and 4. The lithofacies, their descriptions and depobelt as Greater Ughelli which according to interpretations are follows. Reijers (2011) is dominated by wave, fluvial and tide, delta lobe switching and channelization. Lithofacies 1: Inter-bedded silty shale and fine- grained sandstone (Fig. 3a-c). A rock interval made up Dataset and Method of dark grey coloured shale with intervals of mm-cm thick siltstone and sharp based ripple laminated Wireline logs that include gamma ray, resistivity, sandstone that fines-upward. There are some sideritic bulk density and neutron were used to correlate D3 nodules in the shale interval. Mix current and wave reservoir sands between Gabi 55 and 56 oil wells, 0.8 ripples, flaser and lenticular beddings, abrupt km distance apart along dip direction as shown in Fig. deepening contact and rare truncations within the sand 2. A total of 43 and 19 m cores of D3 reservoir sands units. It is well-sorted and consolidated with some and seals from wells Gabi 55 and 56 respectively were fractures on the massive shale/silt shale intervals. It is examined for lithology, sediment texture (grain size characterised by very low ichnodiversity represented and shape), trace fossils, macro-body fossils, macro- by Planolites burrows and variable bioturbation (BI: diagenetic features and primary sedimentary structures for the identification of lithofacies and 0-3). Bioturbation intensity increases toward the interpretation of environments of deposition. The shaley sandstone interval and sharp boundary between scheme of Reineck and Singh (1986) was applied in sandstone and overlying shale.

Fig. 2. Log motifs of D3 cored intervals in Gabi 55 and 56 wells

14 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25

Fig. 3(a-i). Core photos showing diagnostic features in lithofacies that described D3 reservoir sands in well Gabi 55

Interpretation. Shale deposit is an indication of in deep subaqueous condition. Abrupt deepening, suspension settling during slack water condition, while truncations and an increase of bioturbation intensity the sand intervals represent periods of higher currents. toward sand-shale contact indicate retrogradational Sharp based clean sandstone intervals are tidal washover parasequence boundary (Van Wagoner et al ., 1990). sandstone or sub-tidal deposit in a lagoonal or tidal flat Lithofacies 2: Massive coarse-to fine-grained environment. Sideritic nodules indicate reducing condition sandstone (Fig. 3c). It is made up of coarse to fine

15 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25 quartz grains and massive with faint laminations sharp basal contact to mud bed indicate deposition towards the top. Light grey in colour. It is from terminal distributary channel as mouth bars are consolidated, poorly to moderately sorted, micaceous initiated by bed load deposition and are formed from and carbonaceous. It is 0.6 m thick, fines upward and the coarsest deposits carried by the river (Olariu and underlain by an erosive base with basal lags. Interval Bhattacharya, 2006). is unburrowed (BI = 0) and characterised by Lithofacie 5: Ophiomorpha burrowed fine-to coarse- increasing gamma ray log values (Fig. 2). grained sandstone (Fig. 3d). It is fine-to coarse quartz- Interpretation. Structureless and massive deposits grained sandstone deposit that is micaceous and indicate quick deposition. The small thickness, poor to occasionally granular. Light greyish brown in colour. It moderate sorting and erosive base with basal lags are is fairly massive with sporadic faint cross-stratification. typical of transgression in shallow marine environment Moderately to commonly bioturbated by horizontal and (Weber, 1971). Carbonaceous contents indicate vertical/oblique Ophiomorpha burrows (BI = 3/4). It was terrestrial influence, while mica flakes indicate constant only identified in the up-dip well (Gabi 55). and high rate of sediment supply to shelf from river Interpretation. Abundant Ophiomorpha burrows, (Dias et al ., 1984; Selley, 1995). suspension feeder structures and elements of Skolithus Lithofacies 3: Gravelly sandstone/sandy ichnofacies, reflect sediment deposition in oxygenated, conglomerate (Fig. 3d). The rock unit is made up of high energy and shallow water depositional setting. very coarse quartz grains to granules and pebbles, Typical depositional environment is upper shoreface or with lots of mica flakes and some carbonaceous proximal delta front (Pemberton et al ., 2009). patches. Poorly sorted and well consolidated. Pebbles Lithofacies 6: Cross-stratified fine-to medium- are sub-rounded to well-rounded. It is greyish brown grained sandstone. This facies was identified in both up- in colour. It occurs within massive fine-to coarse- dip and down-dip wells. It is upward-cleaning cross- grained sandstone and also capping the upward stratified fine-to medium-grained sandstone with lamina- coarsening sequence with funnel log motifs or upward set thickness ranging from 1.0 to 3.5 cm. It is moderately decreasing gamma ray log values. It is massive with to well sorted, moderately consolidated and micaceous. no visible primary sedimentary structures but slightly In the up-dip well (Gabi 55), the lithofacies is bioturbated with Ophiomorpha burrows (BI = 1). It characterised by rare hummocky lamination, burrows was only identified in the up-dip well, Gabi 55. and light brown in colour (Fig. 3e). There are Interpretation. Ophiomorpha burrows are elements Ophiomorpha burrows on top of the rock unit, while rare of Skolithus ichnofacies associated with high energy Diplocraterion burrows occur at the lower part of the environment and also suggest shallow marine unit, with rare Palaeophycus only at bed boundaries. depositional setting (Pemberton et al ., 2009; Bioturbation is generally sporadic (BI = 0 to 2). In the MacEachern et al ., 2005). Gravelly sandstone with no down-dip well (Gabi 56), the lithofacies is brownish primary sedimentary indicate quick gravity flow grey in colour and grades upward from carbonaceous deposition, while sandy conglomerate capping a mud draped and sparse to uncommon Ophiomorpha coarsening upward sequence indicate high energy and burrowed planar cross-bedded interval to unburrowed wave reworking processes that remove the finer matrix. trough cross-bedded interval (BI = 0-3) (Fig. 4a and b). Typical environment is proximal delta front-mouth bar. Interpretation. Planar/trough cross-stratification Lithofacies 4: Coarse-to very coarse-grained indicates migration of 2/3dimension subaqueous dunes, sandstone (Fig. 4a). It is occasionally pebbly with while the mud drapes in the down-dip lithofacies sharped basal contact and brownish grey in colour. indicate periods of decrease in flow velocity in which Poorly to moderately sorted and friable to moderately mud is deposited on lee slope. The change from brown consolidated. It is massive and no visible traces of colour in up-dip well to brownish grey in the down-dip bioturbation (BI = 0). It occurs at the top of an upward well indicates paleo-seaward increase in water depth. coarsening succession in the down-dip well, Gabi 56. It Rare hummocky lamination reflects occasional wave is suspected that the above described gravelly influence. The dominance of Ophiomorpha burrows sandstone/sandy conglomerate in the up-dip well (Gabi especially on top of the rock unit is indicative of a stressed environment associated with high energy and 55) grades down-dip to this lithofacies. Interpretation. Massive and structureless deposit high rate of sediment supply. Palaeophycus and indicates rapid emplacement, with no space of time for Ophiomorpha burrows are elements of Skolithus bioturbation by benthic organisms (MacEachern et al ., ichnofacies, while diplocraterion is a common element 2005). The greyish brown colour indicates subaqueous in the distal end of the Skolithos ichnofacies deposition in an oxygenated shallow water depth such (Pemberton et al ., 2009; Seilacher, 1967). Typical as in proximal delta-front. The very coarse grains and environment of deposition is proximal delta front.

16 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25

Fig. 4(a-h). Core photos showing diagnostic features of lithofacies that described D3 reservoir sands in well Gabi 56

Lithofacies 7 : Mud draped high angle cross- Planolites , rare Pischichnus , Fugichnia , Chondrites laminated sandstone. This lithofacies was identified and rare Phycosiphon and rare Synaeresis crack (Fig. both in the updip and down-dip wells. The lithofacies 3f and h). The thickness of the lithofacies increased in is very fine-grained, well-sorted, consolidated, down-dip direction. Distinguishing features of the micaceous and light brown in colour. Sedimentary lithofacies in the down-dip setting are occasional structures include current ripple cross-laminations, micro-slumped folds, rare Synaeresis crack and hummocky/wave ripples laminations, climbing diminutive ichnofossils such as Cylindrichnus, ripples, high angle cross-laminations, micro-slumped Lockeia and Fugichnia (Fig. 4c-e). folds and scoured surfaces. The high angle cross- Interpretation. Mud draped cross-laminations laminations are draped by single to double mud flasers reflection migration of sinuous crested ripple with mud to occasional 2-4 cm thick shaly mud. Bioturbation is deposition on the lee slope during periodic drops in variable (BI = 0-3) and ichnofossils include depositional current. The mud inter-beds represent fluid

17 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25 mud deposit on clinoform surface during waning flow. 2005; Pemberton et al ., 2009). Typical environment is Rare hummocky cross-lamination indicates occasional tide and wave influenced distal delta-front. influence of storm waves. Periodic waning flow or low Lithofacies 9 : Inclined inter-laminated mudstone and energy allows rapid infauna colonisation that result in siltstone with some lamina or thin layers commonly sporadic bioturbation. Fugichnia , an escape burrow, convoluted. Convolute laminae are similar to (Bouma, indicates rapid deposition. Lockeia is an indication of 1962) Tc sequence. It has basal contact that is sharp and frequent episodic depositional events, while rare concave-upward especially at the up-dip well. In the Synaeresi s crack is an evident of salinity fluctuation due down-dip part, some sandstone intervals are characterised to high sediment influx. Cylindrichnus indicates the by mud draped wave ripple laminations and soft-sediment proximal end of the Cruziana ichnofacies while Chondrites folds. It is well consolidated and bioturbation is absent to indicates low oxygen zone (MacEachern et al ., 2005; sparse (BI = 0-1) (Fig. 3i and 4h). 2007; Pemberton et al ., 2009). Typical environment is Interpretation. High angle inclination and convolute middle/less distal delta-front. lamination indicate rapid deposition from storm Lithofacies 8: Inclined fine-grained sandstone and generated hyperpycnal flow on an inclined deposition shale heteroliths. It is well-sorted, well consolidated and surface such as seaward margin of a deltaic setting (Coleman and Prior, 1980). The sharp contact represents micaceous. It underlain a coarsening upward sequence the asymptotic base of a clinoform or delta front or storm and characterised by upward decreasing gamma ray wave base. Sparse bioturbation is characteristic of values (Fig. 2). The sand units are characterized by small oxygen-restricted environment. Typical environment is scale hummocky and swaley cross-laminations, wave more distal delta front/prodelta. ripple laminations or oscillation ripples and rare load Lithofacies 10: Grey coloured massive shale, grading structures. Sand and shale thickness are variable (1 to 5 upward to lenticular-wavy but rarely contorted siltstone cm) but sand shale ratio increases upwards (Fig. 3f and and shale couplets. The lithofacies is rarely fractured and g). It grades down-dip to two sub-lithofacies in well formed the basal part of an upward coarsening sequence. Gabi 56-sand dominated heteroliths (8a) and mud No bioturbation (Fig. 3i). dominated heteroliths (8b) (Fig. 4e, f and g). The sand Interpretation. Massiveness and silt/clay content dominated heteroliths is dominantly made of sand with indicate rapid deposition of suspended load in a low shale intercalations that decreases upward in thickness energy environment. The lack of burrows indicates and volume. The mud dominated heterolith that grade deposition in a deep and anoxic environment. Fractures are upward to sand dominated heterolith consist of 1-10 mm an evidence of an overpressure condition (Ingram et al ., thick massive dark grey shale and 1-2 mm laminated 1997). Typical environment is offshore or outer shelf. silty shale with intercalated thin very fine-grained sandstone as starved current ripples or lenticular beds. Sub-Envronments of Deposition Ichnofossils include Planolites, Chondrites , rare The characteristics and the associations of the Piscichnus (fish resting burrow), rare Synaeresis crack, lithofacies described above led to the identification of Palaeophycus , rare Phycosiphon and Cylindrichnus on five depositional facies or sub-environments of the sand-shale boundary. Bioturbation intensity is deposition. The vertical stacking and distribution of variable (BI = 0-4,) (Fig. 3f and g; Fig. 4e-g). lithofacies in the up-dip and down-dip wells are shown Interpretation. High angle bedding implies in Fig. 5 and 6 respectively. The sub-environments of deposition on an inclined surface. Sand and shale inter- deposition are as follows: beds represent deposit of tidal current of fluctuating Transgressive Sandstone and Tidal Flat Deposit strength. The upward decreasing gamma ray values indicate mouth bar or barrier bar deposition (Serra, 1989). A fining upward succession made up of lithofacies 2 Load structure is soft-sediment deformation structure, an (massive coarse- to fine-grained sandstone, underlain by evident of rapid deposition. Wave ripple laminations basal lagged erosive surface) grading to lithofacies 1 (inter-bedded silty shale and fine-grained sandstone) is indicate fair weather wave reworking. Hummocky and interpreted as transgressive sandstone and tidal flat swaley cross-laminations reflect the influence of storm deposit that records deposition during the transgressive waves and indicate deposition between storm and fair- phase of deltaic sedimentation. The carbonaceous and weather wave base. Rare Phycosiphon and Palaeophycus micaceous contents are reflection of deposition in an are bioturbation indices of storm beds (Rotnicka, 2005). environment less winnowed by waves and longshore Rare Cylindrichnus reflect facies-crossing elements of current and also close to distributary channel and mouth the proximal end of Cruziana ichnofacies, while bar deposition (Selley, 1995). Chondrites , though also facies-crossing elements, is an Lithofaces 1 interpreted as transgressive tidal flat indicative of low oxygen zones (MacEachern et al ., heterolith, characterized by abrupt deepening surfaces,

18 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25 sideritic concretions, flaser and lenticular beddings, reworking of the shoreface or mouth bar deposit as it truncations, tidal washover sands and bioturbation, was drown during transgression. Its silty content and indicates open marine tidal flat or lagoon deposit in a stratigraphic position also buttress its proximity to retrogradational depositional system (Reineck and Singh, shoreline and active mouth bar deposition. Though this 1986; van Wagoner et al ., 1990; Davis Jr and Dalrymple, lithofacies association was not cored in the down-dip 2012). While lithofacies 2 interpreted as transgressive well (Gabi 56), it was correlated to it with wire line logs. sandstone is similar to the Niger Delta transgressive It overlain a coarsening upward sequence and marine sand (onlap sands) described by Weber (1971). characterised generally by upward increasing gamma ray Therefore, the facies association records transgressive log values (Fig. 2 and 5).

Fig. 5. lithofacies log of D3 reservoir sands in well Gabi 55, showing environments of deposition

19 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25

burrows. It is 17 m thick and exhibit coarsening upward trend. Very coarse grain and poor sorted texture and sedimentary structures (massive, cross-bedding and rare hummocky laminations) indicate deposit coeval to a high energy distributary channel. Abundant Ophiomorpha (suspension feeder structures and element of Skolithus ichnofacies), rare Palaeophycus and Diplocraterion burrows reflect deposited sediment in oxygenated, high energy and shallow water depositional setting (MacEachern et al ., 2007; Pemberton et al ., 2009). The inclination of strata implies delta slope progradation, while, high mica content, very coarse grains and poor sorting indicate a direct link to a distributary channel (Dias et al ., 1984; Olariu and Bhattacharya, 2006). These are some of the features that differentiate delta front-mouth bar deposit from that of shoreface. Delta front-mouth bar deposit replaces shoreface where lithofacies characteristics indicate direct link to a distributary channel. Whereas shoreface represents delta front sediments that have been reworked or highly winnowed and re-deposited by wave, tide and longshore current. Distal Delta Front A coarsening-upward successions that grades upward from concave sharp based inclined inter- laminated sand and shale with convolute structures (lithofacies 9) through inclined heterolithic fine- grained sandstone and silty shale (lithofacies 8) and capped by mud draped high angle cross-laminated sandstone (lithofacies 7) is interpreted as distal delta front or distal-mouth bar facies association (Fig. 5 and 6). It underlain upward-coarsening successions and characterised by upward decreasing gamma ray log values. The top to the mid part of the succession is characterised by high angle cross-laminations, small scale hummocky and swaley cross-laminations, wave ripple laminations or oscillation ripples, rare load structures, flaser bedding and 2-4 cm mud bed

intercalations and sporadic or sparse bioturbation (BI Fig. 6. Lithofacies log of D3 reservoir sands in well Gabi 56, showing environments of deposition. For legend of = 1 to 2) by rare Fugichnia , rare Synaeresis crack, sedimentary structures (Fig. 5) localized sand filled Chondrites , rare Planolites and Palaeophycus burrows and stunted Phycosiphon,

Cylindichnus and Lockeia burrows. The base is Proximal Delta Front-Mouth Bar characterised by convoluted or contorted bedding with This lithofacies association is a coarsening upward sparse to no traces of bioturbation. successions that start with cross stratified fine- to Sporadic or sparse bioturbation and stuntedness of medium-grained sandstone (lithofacies 6) and grades some burrows indicate suppressed biogenic activities upward through lithofacies 5 and 4 and then to sandy attributed to the stress in environment caused by conglomerate (lithofacies 3) (Fig. 5 and 6). It is fluctuating salinities or temperatures combined with a generally moderately to poorly sorted and has high large suspended-sediment load and rapid deposition mica flakes content. It is characterised by abundant (MacEachern et al ., 2007). The downward decrease in Ophiomorpha , rare Palaeophycus and Diplocraterion bioturbation intensity implies downward increase in

20 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25 anoxic condition. Loading features resulted from longitudinal or down-dip direction. It shows lateral sediment instabilities and density contrasts between changes in thickness, geometry and lithology-which are rapidly deposited clay, silt and sand. The occurrences components of reservoir heterogeneity. As shown in Fig. of diminutive burrows on sand and mudstone contact 7, the sub-environments of deposition in which the D3 and the preponderances of unbioturbated mud beds can reservoir sands and seals were deposited and their best be explained by rapid deposition in distal delta- wireline-log shapes are quite correlatable between the front. The combination of mudrapes, current ripple two wells and imply lack of lateral mega or field scale laminations and hummocky and swaley cross- geological heterogeneity, typical of layer cake reservoir laminations is an indication of mixed processes-tide, architecture (Weber and van Geuns, 1990). However, the wave and fluvial-environment and deposition between foregoing lithofacies analysis of D3 reservoir reveals the storm and fair-weather wave base (Walker and Plint, following macro/mesoscopic reservoir scale 1992). This is corroborated with rare presence of heterogeneities in the depositional dip direction. Phycosiphon and Palaeophycus burrows that are Ichnodiversity and Abundance bioturbation indices of storm beds (Rotnicka, 2005). The contorted bedding similar to Bouma (1962) Tc Ichnodiversity is fairly uniform between the two sequence is interpreted as the deposit of hyperpycnal wells, but ichno-abundance and burrow sizes decreases underflows agitated by storm or initiated during high- from the updip well (Gabi 55) to the down-dip well discharge events (i.e., river floods) at delta-front (Gabi 56) especially at the proximal delta front- mouth (Bhattacharya and MacEachern, 2009). According to bar deposit as indicated by Ophiomorpha burrows. The Coleman and Prior (1980), in delta front environments, down-dip decrease in ichno-abundance is attributed to mass-movement processes such as small localized down-dip increase in hydraulic forces associated with slumps often result in distorted laminations. Therefore, wave energy that keep sediment in suspension, thereby they are related to slope instability induced by high increasing the water turbidity that gradually decrease sedimentation rates. The sparse bioturbation (BI =1-2) suspension feeding behaviour and also by increase in to no bioturbation (BI = 0) at the base of this lithofacies water depth and traction current due to increase in association clearly differentiates it from the lower hydraulic gradient before the basin-ward tectonic uplift shoreface deposit characterised by common to by shale diapir as well as down-dip increase in distance abundant bioturbation (Van Wagoner et al ., 1990). from food supply source which is the distributary Offshore-Prodelta channel (MacEachern et al ., 2005). The distal delta-front is variably bioturbated in both Dark grey coloured massive shale, grading upward wells and most burrows typically occurred at sandstone to lenticular-wavy but rarely contorted silty sandstone and shale interfaces. The variations in the degree of and shale couplets (lithofacies 10) reflecting gradual bioturbation in distal delta-front points to fluctuations in coarsening-upward succession and underlying the delta salinity and oxygen levels, sedimentation rates and front lithofacies successions described above is varying amounts of suspended material in the water interpreted to represent offshore-prodelta transitional column (MacEachern et al ., 2005; 2007). setting. It is characterised by homogeneous gamma ray Finally, the ichnoassemblage in the two wells indicates log value (Fig. 2, 5 and 6). It is sparsely bioturbated to a vertical trend underlain by storm defaunated interval unbioturbated (BI = 0-1) and the underlying shale is (prodelta), followed by Zoophycus ichnofacies, sporadically fractured and locally sideritic. The locally through Cruziana and mixed Cruziana-Skolithu s sideritic massive shale with sparse or no bioturbation ichnofacies and then to Skolithus ichnofacies, structures record deposition in an anoxic environment, reflecting vertical increase in physical energy, food and below storm wave base, in offshore setting oxygen levels (Seilacher, 1967; Pemberton et al ., 2009; (MacEachern et al ., 2005; 2007). The fractures are pore Gingras et al ., 2007; McIlroy, 2008; MacEachern et al ., fluid escape structures resulting from high pressure 2005; 2007; 2012). caused by rapid sediment deposition in deltaic The Zoophycus ichnofacies is made up of Chondrites environment (Nwozor and Onuorah, 2014). and rare Phycosiphon which are indicative of oxygen limited environment with dysaerobic substrate Discussion condition and low deposition (MacEachern et al ., Down-Dip Correlation of Lithofacies and Ichno- 2012). The Cruziana ichnofacies is typified by Fossils’ Characteristics and Stacking Pattern Cylindrichnus, Palaeophycus, Teichichnus and Fugichnia , while Skolithus ichnofacies is made up of Development Ophiomorpha and Diplocraterion burrows Cross sectional facies variability assessment is a (MacEachern et al ., 2007; 2012; Pemberton et al ., 2009; method of studying intra sand-body connectivity in McIlroy, 2008; Pemberton, 1998).

21 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25

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Lateral and Vertical Changes in Lithofacies is down-dip interfingering of upper facies with the The proximal delta-front mouth bar deposit is lower one. For example, the baser part of distal delta characterised by down-dip pinch out of some front facies interfinger or inter-tongue with that of lithofacies or down-dip gradation from coarse texture prodelta, while that of proximal delta front also to finer one. For example, as shown in Fig. 7, gravelly interfinger with that of distal delta front. Therefore, as sandstone/sandy conglomerate and ophiomorpha the proximal delta front reservoir thickness decreases burrowed coarse-grained sandstone in well Gabi 55 down-dip, that of distal delta front increases (Fig. 7). grades to massive coarse-grained sandstone in well The distal delta-front, split down-dip into multiple, Gabi 56. The lithofacies changes reflect gradation vertically stacked, upward-coarsening bedsets from bar crest (closed to river mouth) to bar flanks or separated offshore or prodelta mudstones due to front deposition where the influence of wave is paleoseaward deepening of paleobathymetry and stronger (Olariu and Bhattacharya, 2006). Also, there increase in tidal energy.

22 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25

The cored intervals in the two wells display vertical end of a small-scale clinoform deposit (low angle oblique changes in lithofacies based on variations in grain-sizes, form), truncating older shelf deposit (Porębski and Steel, sedimentary structures, bioturbation intensity and 2003). The presence of contorted layers, concave- ichnofossils. The upward changes in lithofacies reflect upward sharp basal contact, variable bioturbation and the increasing sediment supply and stronger fluvial and tide absence of an overlying coastal delta plain facies such as ebb-oriented currents and wave reworking processes a distributary channels and coal bed substantiate higher on the delta-front. High sediment supply and deposition close to shelf-margin as consequent of late fluvial influence is indicated by high mica content, lowstand relative sea level rise, after the sea level fall planar/trough cross-bedding, micro-slumped folds, load that took the shoreline to shelf-margin before the Oligocene regression of Niger Delta basin (Reijers, structures and rare synaeresis cracks. Wave and storm 2011; van Heijst et al ., 2002; Porębski and Steel, 2003; influence is indicated by the presence of wave ripples, Mellere et al ., 2002). hummocky lamination and contorted bedding and tidal influence is indicated by mud drapes and heterolithic bedding (i.e., centimeter-scale interbedded shale and Conclusion sandstone). The soft sediment deposition recorded as The deposition of D3 reservoir sands was contorted beds at the lower segment indicates delta controlled by variations in physical energy and mixed progradation to shelf edge and or across the shelf break interaction of seal level changes, tide, wave, fluvial (i.e., the clinoform rollover or top slope). influx and storm, food supply and oxygen levels. The effects of these factors change along deltaic sediment- Down-Dip Changes in Shalines and Degree of transport and depositional route-in the case here, Sorting along-dip direction. Consequently, the reservoir sands The degree of sorting and shaliness generally is characterised by the followings: increased down-dip. Two mud beds separating upward- • coarsening sandstone bedsets in the proximal/mid delta Variability of lithofacies. Ten lithofacies described front, interpreted to represent deposits of intermittent the cored samples of the D3 reservoir sands and the marine flooding, are laterally correlative and relatively associations of the lithofacies enabled the continuous; and they have potentials to form flow identification of sub-environments of deposition that include proximal delta-front mouth bar, distal delta- barriers (Fig. 7). Larue and Legarre (2004) interpreted front, transgressive marine sandstone and open such laterally continuous shale intervals as minor marine marine tidal flat and offshore-prodelta. flooding-surface mudstones that vertically • The degree of sorting and shaliness generally compartmentalized reservoirs in an oil field of Western increased down-dip Niger delta. The distal delta-front is heterolithic or made • Down-dip pinch out of some lithofacies or gradation up of interstratified sandstone and shale with shale to lithofacies of finer grains and better sorting intervals that increases in thickness in down-dip • Though ichnodiversity is fairly uniform from direction and hence, of greater reservoir heterogeneity proximal to distal depositional setting, the ichno- and stratigraphic complexities. abundance and burrow sizes decrease down-dip Stratigraphic Surfaces and Stacking Pattern especially at the proximal delta front-mouth bar Development • Sandstone bedsets are separated by mm to cm thick mudstone and hence of high potential for vertical The recognition of Transgressive Surface of Erosion subsurface fluid (oil and gas) compartmentalization (TSE) and transgressive marine sandstone deposit • Down-dip correlations of lithofacies between two suggest the presence of a Maximum Flooding Surface wells indicate high intra sand-body (MFS); and the identification of a flooding surface (an continuity/connectivity abrupt deepening surface) at 4083.4 m of well Gabi 55 cored interval (Fig. 3c) suggest that the studied reservoir Acknowledgement sand body is a progradational parasequence overlain by a retrogradational parasequence set and the characteristics This paper is part of the PhD thesis submitted to of delta-front deposit described in this study are also the Department of Geology, University of Port similar to the strata characteristics of a deltaic Harcourt, Nigeria by the first author. The study was parasequence of Van Wagoner et al . (1990), except with ably supervised by the second author. Gratitude is the occurrence of contorted lamina at distal-delta front extended to the lecturers and Graduate studies and concave-upward sharp basal contact that grades committee of the department for their support. Special down-dip to a gradational contact. The concave-upward thanks to the Department of Petroleum Resources sharp basal contact is suspected to be asymptotic lower (DPR), Ministry of Petroleum of the Federal Republic

23 Raphael Oaikhena Oyanyan and Michael Ndubuisi Oti / American Journal of Geosciences 2015, 5 (1): 12.25 DOI: 10.3844/ajgsp.2015.12.25 of Nigeria and Total Exploration and Production Ingram, G.M., J.L. Urai and M.A. Naylor, 1997. Sealing Company Limited, Port Harcourt, Nigeria for the Processes and Top Seal Assessment. In: provision of the data used for this study. Hydrocarbon Seals: Importance for Exploration and Production, Moller-Pedersen, P. and A.G. Koestler Author’s Contributions (Eds.), Elsevier, Amsterdam, ISBN-10: 0080534287, Raphael Oaikhena Oyanyan: Carried out the PhD pp: 165-174. the research from which the paper was written. He wrote Labourdette, R., J. Casas and P. Imbert, 2008. 3D the manuscript. Sedimentary modelling of a Miocene deltaic Michael Ndubuisi Oti: Supervised and coordinated reservoir unit, Sincor field, Venezuela: A new the research and read the manuscript. approach. J. Petroleum Geol., 31: 135-152. DOI: 10.1111/j.1747-5457.2008.00412.x Ethics Larue, D.K. and H. Legarre, 2004. Flow units, connectivity and reservoir characterization in a This article is original and contains unpublished wave-dominated deltaic reservoir: Meren reservoir, material. The corresponding author confirms that all of Nigeria. AAPG Bull., 88: 303-324. the other authors have read and approved the manuscript and no ethical issues involved. DOI: 10.1306/10100303043 MacEachern, J.A., K.L. Bann, J.P. Bhattacharya and References C.D. Jr, Howell, 2005. Ichnology of Deltas: Organism Responses to the Dynamic Interplay of Bhattacharya, J.P. and J.A. MacEachern, 2009. Rivers, Waves, Storms and Tides. SEPM, pp: 37. Hyperpycnal rivers and prodeltaic shelves in the MacEachern, J.A., K.L. Bann, G. Pemberton and M.K. cretaceous seaway of north America. J. Sed. Res., Gingras, 2007. The Ichnofacies Paradigm: High- 79: 184-209. DOI: 10.2110/jsr.2009.026 Resolution Pale environmental Interpretation of Bouma, A.H., 1962. Sedimentology of some Flysch the Rock Record. In: Applied Ichnology, Deposits: A Graphic Approach to Facies Interpretation. MacEachern, J.A., K.L. Bann, M.K. Gingras and 1st Edn., Elsevier, New York, pp: 168. S.G. Pemberton (Eds.), SEPM, Tulsa, Chamberlain, C.K., 1978. Recognition of Trace Fossils in Cores. In: Trace Fossils Concepts, Basan, P.B. ISBN-10: 1565761332, pp: 27-64. (Ed.), SEPM Short Course, pp: 119-166. MacEachern, J.A., K.L. Bann, M.K. Gingras, J.P. Coleman, J.M. and D.B. Prior, 1980. Deltaic Sand Zonneveld and S.E. Dashtgard et al ., 2012. The Bodies: A 1980 Short Course. 1st Edn., Books on Ichnofacies Paradigm. In: Developments in Demand, ISBN-10: 0608087297, pp: 175. Sedimentology, Elsevier B.V. (Ed.), pp: 103-138. Davis Jr, R.A. and R.W. Dalrymple, 2012. Principles of McIlroy, D., 2008. Ichnological analysis: The common Tidal Sedimentology. 1st Edn., Springer ground between ichnofacies workers and Netherlands, ISBN-10: 978-94-007-0123-6, pp: 621. ichnofabric analysts. Palaeogeography, Dias, J.M.A., O.H. Pilkey and V.M. Heilweil, 1984. Palaeoclimatol. Palaeoecol., 270: 332-338. Detrital mica: Environmental significance III north DOI: 10.1016/j.palaeo.2008.07.016 Portugal sediments. Comun. Serv. Mellere, D., P. Plink-Björklund and R.J. Steel, 2002. Geol. Portugal, t. 70: 93-101. Anatomy of shelf deltas at the edge of a prograding Doust, H. and E.M. Omatsola, 1990. Niger Delta. In: Eocene shelf margin, Spitsbergen. Sedimentology, Divergent-Passive Margin Basins. Edwards, J.D. 49: 1181-1206. and P.A. Santogrossi (Eds.), American Association DOI: 10.1046/j.1365-3091.2002.00484.x of Petroleum , Tulsa, Nwozor, K.K. and L.O. Onuorah, 2014. Geopressure ISBN-10: 0891813268, pp: 201-238. Analysis and Reservoir Fluid Discrimination in a Gingras, M.K., L. Bann, J.A. MacEachern and G. Central Swamp Field, Niger Delta, Nigeria. Pemberton, 2007. A Conceptual Frame Work for the Petroleum Coal, 56: 124-140. Application of Trace Fossils. In: Applied Ichnology, Olariu, C. and J.P. Bhattacharya, 2006. Terminal MacEachern, J.A., K.L. Bann, M.K. Gingras and S.G. Pemberton (Eds.), SEPM, Tulsa, Distributary channels and delta front architecture of ISBN-10: 1565761332, pp: 1-26. river-dominated delta systems. J. Sedimentary Res., Hovadik, J.M. and D.K Larue, 2007. Static 76: 212-233. DOI: 10.2110/jsr.2006.026 characterizations of reservoirs: Refining the concepts Pemberton, S.G., 1998. Ichnological dynamics of the of connectivity and continuity. Petroleum Geosci., 13, Triassic Sag River Fm., Alaska. Report for Arco 195-211. DOI: 10.1144/1354-079305-697 Alaska, pp: 15.

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