SEISMIC INTERPRETATION AND STUCTURAL ANALYSIS OF CLOSURES IN THE GREATER UGHELLI CENTRAL AND COASTAL SWAMP DEPOBELTS OF NIGER DELTA BASIN

ABSTRACT
The Niger Delta is located on a typical passive margin with sediment thicknessesin excess of 12 km at the basin center. The combined occurrence of rift structures, basin subsidence, rapid progradation of deltaic systems, development of growth faults and fold thrust belts in the offshore Niger Delta produced a very unique petroleum habitat with different structural styles and play types. Most of the prospects and producing fields in the region are related to faulted structures. Structural elements such as fault patterns, kinematics, geometry, timing and size of structures therefore control the distribution of hydrocarbons in adjacent fault blocks in most petroleum habitats. The objectives of this study were to: (i) evaluate some failed exploration prospects within a hydrocarbon play; (ii) understand the field and prospect’s kinematics and growth history; (iii) evaluate the timing of fault movements and closure formation relative to hydrocarbon charge; and (iv) understand the fractal nature of faults, trap integrity, faults and structure geometry or size modification.
The methods adopted for this study include both quantitative and qualitative interpretation of well and seismic data.  Well logs, biostratigraphic and biofacies data were integrated to create a correlation panel to aid the understanding of reservoir continuity and gross depositional environment. A synthetic seismogram was generated from density log, sonic log and seismic wavelet to enable well-to-seismic tie for the identification of stratigraphic markers on the 3D seismic data.  A detailed seismic interpretation of faults and horizons was done followed by fault interpretation, quality checking, fault surface mapping, fault pattern, fault growth and kinematics analyses. Sequential restoration of the deformed horizons along the corresponding fault planes using both 2D and 3D approaches was done for the evaluation of episodes of fault motion and closure development through time. Fractal analysis of faults using geostatistical approach aided prediction of sub-seismic faults in three depobelts of the Niger Delta.
The correlation of stratigraphic markers in the three locations shows that they are laterally continuous and easily identifiable on the 3D seismic data with the aid of the synthetic seismogram. The objective intervals are shoreface sandstones between the 33.3 and 31.3 Ma maximum flooding surfaces (MFS) deposited in a middle neritic environment. The seismic interpretation and fault surface mapping yielded a robust sub-surface model which was the main input for the structural analysis. The result of the growth, kinematic analysis and sequential restoration showed that traps formed earlier at all the objective levels at field A, compared to closures B and C, because faulting and closure development started and ceased earlier (at about 18 Ma) around the vicinity of field A. Relative stability of the fault at field A enhanced hydrocarbon retention capacity of the trap at field A compared to closures B and C where multiple episodes of fault movements  modified and breached the paleo-closures (traps), and as a result, trapped hydrocarbons spilled, leaving a paleo-amplitude anomaly at closure B.The fractal analysis shows that the fractal dimension for size frequency distribution increases from Greater Ughelli to the Coastal Swamp Depobelt. The distribution of simulated sub-seismic faults in the fields studied shows a variation from clustering around the major faults where strain is high to anti-cluster distribution at areas with low strain concentration. Sub-seismic faults (also seen in cores) may account for some of the unexpected drop in pressure and production observed in some wells. The results of this study clearly demonstrate the impact of episodic fault movements on closure modification and hydrocarbon retention capacities of hydrocarbon traps in the Niger Delta.
 

CHAPTER ONE
INTRODUCTION
STATEMENT OF THE PROBLEM
The Niger Delta is located on a typical passive margin with sediment thicknessesin excess of 12km at the basin center. A combination of extensional structures generated by the rifting and subsequent basin fill by prograding deltaic sediments across original rift setting made the Niger Delta a prolific petroleum province. The combined occurrence of rift structures, basin subsidence, rapid progradation of deltaic systems, development of growth faults and fold thrust belts in the offshore Niger Delta produced a very unique petroleum habitat with different structural styles and play types. Most of the prospects in the Niger Delta are related to faulted structures. Therefore structural elements such as fault patterns, kinematics, geometry, timing and size of structures probably control the distribution of hydrocarbons in adjacent fault blocks. The success or otherwise of an exploration well depends on its location relative to the structural closure interpreted from the seismic data. Detailed structural analysis of fault patterns and fractal nature of prospective fields can provide a reliable kinematic and growth history upon which risks associated with fault movements, hydrocarbon trap integrity, sub-seismic faults and structure geometry/size modification could be evaluated before deciding on the drilling location. This study was aimed at evaluating some failed exploration prospects within a hydrocarbon play, with producing field, in order to understand the field and prospect’s kinematics and growth history, timing of fault movements and closure formation relative to hydrocarbon charge. The work also intends to examine the fractal nature of faults, hydrocarbon trap integrity, faults and structure geometry/size modification in the study area. The result of this study would help in de-risking prospects and reducing the chances of drilling dry wellsin the Niger Delta.
SPECIFIC OBJECTIVES OF THE STUDY
The main objectives of this study are to:
evaluate the influence of fault pattern and fault propagation in the distribution of hydrocarbons in the three fields situated along adjacent fault blocks in OML-38 and 41 and provide possible reasons for the lack of hydrocarbons in the prospective levels in closure B, while the correlative levels in adjacent fault blocks are hydrocarbon bearing;
understand their fractal nature (scaling properties) and simulate sub-seismic faults in the study area; and
estimate the number, size and distribution of small or sub-seismic faults and their influence on fluid flow and top seal leak probability in Greater Ughelli and Coastal Swamp depobelts.
AVAILABLE DATASET AND SOFTWARE
Dataset
The dataset used for this study include:
3D PRE-STACK SEISMIC DATA
Well Data
WELL LOGS(Gamma Ray, Caliper, Sonic, Density, Neutron, and Resistivity)
BIOSTRATIGRAHIC AND BIOFACIES
HEADER INFORMATION
DEVIATION SURVEYS FOR WELLS
RESERVIOR TOPS, CONTACTS AND SATURATION
CORES
Software
PETREL
123 DI
STRUCTURAL GEOLOGY TOOLKIT (SGT)
2D AND 3D MOVE
REGIONAL GEOLOGY OF THE NIGER DELTA
Location
The Niger Delta (Fig. 1) is situated in the Gulf of Guinea and covers an area of about 300,000 Km²(Kulke, 1995), has a sediment volume of 500,000km³(Hospers, 1965) and a sediment thickness that ranges from 9 to 12 km in the basin depocenter(Evamy et al., 1978; Kaplan et al, 1994). The delta is considered to be one of the most prolific hydrocarbon provinces in the world, and recent giant oil discoveries in the deep-water and hydrocarbon prospects below the penetrated intervals in the areas suggest that this region will remain a focus of exploration activities in years to come. From the Eocene to the present, the delta has prograded southwestward, forming depobelts that represent the most active portion of the delta at each stage of its development (Doust and Omatsola, 1990).
 
The Niger Delta basin evolved at the mouth of the rift-rift-rift (RRR) triple junction that initiated the separation of South America from Africa (200 – 110 Ma). The three arms of the junction opened at different times and rates. In this region rifting started in the Late Jurassic and continued into the Middle Cretaceous (Lehner and De Ruiter, 1977). In the Niger Delta Province, rifting ceased altogether in the Late Cretaceous. The present day evidence of the separation of the two continents is the marginal basins parallel to the coastline and the intra-continental rift structures such as the Benue Trough. 
The geology of the Niger Delta has been described in detail by many researchers (Ejedawe and other 1984; Doust and Omatsola, 1990 , Nwachukwu et al., 1995). The northern part of the Delta is delineated by the southern Benue trough. At the Northwestern rim of the delta is the Benin Flank which is a subsurface continuation of the West African shield. The Benin Flank terminates along the Benin hinge line -a Northeast-Southwest trending Cretaceous Basement fault system. At the southwestern rim of the Niger Delta is the Calabar Flank, which terminates along the Calabar Hinge Line – a Northwest-Southeast trending Cretaceous Basement fault system. In the offshore the Cameroon VoIcanic line to the east, the eastern boundary of the Dahomey basin to the West and the 4,000m bathymetric contours define the delta.
 
 
 
 
 


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Figure 1. Location map of the Niger Delta region showing the delta outline, some sedimentary
basinsand tectonic features. (a) Regional map of Nigeria showing the sedimentary basin and the basement complexes (Modified from Onuoha and Ekene, 1999). (b) High-resolution bathymetric image of the Niger Delta obtained from a dense grid of 2D seismic reflection profiles and the global bathymetric database (Smith and Sandwell, 1997)
 
Tectonic Framework
The tectonic framework of the continental margin along the West Coast of equatorial Africa is controlled by Cretaceous fracture zones expressed as trenches and ridges in the deep Atlantic and basement faults in the continents (Fig. 1). These fracture zones subdivide the margin into individual basins and formed the boundary faults of the Cretaceous Benue-Abakaliki trough in Nigeria. The trough represents a failed arm of a rift triple junction associated with the opening of the South Atlantic. In this region, rifting started in the Late Jurassic and persisted into the Middle Cretaceous (Lehner and De Ruiter, 1977). In the Niger Delta region, rifting ceased altogether in the Late Cretaceous after which gravity tectonism became the primary deformational process. Shale mobility induced internal deformation and occurred in response to two processes (Kulke, 1995). First, shale diapirs formed from loading of poorly compacted, over-pressured, prodelta and delta-slope clays (Akata Formation) by the higher density delta-front sands (Agbada Formation). Second, slope instability occurred due to a lack of lateral and basinward support for the under-compacted delta-slope clays (Mobile Akata Formation;Fig. 2). For any given depobelt, gravity tectonics were completed before deposition of the Benin Formation and are expressed in complex structures, including shale diapirs, roll-over anticlines, collapsed growth fault crests, back-to-back features, and steeply dipping, closely spaced flank faults (Evamy et al.,1978; Xiao and Suppe; 1992, Haack et al., 1997). These faults mostly offset different parts of the Agbada Formation and flatten into detachment planes near the top of the Akata Formation.
 

Figure 2.  Schematic of seismic sections from the Niger Delta showing the mobile shale above the basement and associated deformation patterns. Observe the up-dip extensions and the down-dip compression (After Haack et al., 1997).

Stratigrahy of the Niger Delta
The Tertiary section of the Niger Delta is divided into three lithostratigraphicunits. These units are currently classified as formation however,Reijers 2011proposed a re-classification of each of these units as Group.The delta-top Benin formation(red in Fig. 4) overlies the delta-front AgbadaFormation (yellow) and the pro-delta Akata Formation(green). The composition of the subsurface BeninFormation reflects the present-day Quaternaryland and swamp outcrops (Fig. 3); the AgbadaFormation reflects the beach ridges and the Akataformation the offshore sands, silts and clays. Previous sedimentological, biostratigraphicaland sequence-stratigraphic studies (Ladipo,1992; Stacher, 1995; Reijers et al., 1997) revealedthe combined influence of eustatic cyclicity andlocal tectonics (Reijers 2011). Recent new studies in the offshore(Owejemi and Willis, 2006; Magbagbeoloaand Willis, 2007) demonstrate that these conceptsare still valid as seen in offshore wells data.
Regional stratigraphic studies of the Niger Delta show that depositional sequence as defined by Vail, 1987 (strata bounded by unconformities andtheir lateral equivalents) are only recognised inspecific sectors of the delta. Conversely, delta-wide genetic sequences as defined by Galloway, 1989 (consisting of strata bounded by maximum flooding surfaces within transgressive shales) are more readily identifiable in the Niger Delta. These sequence boundaries (SB) and the maximum flooding surfaces (MFS) are well represented in Figs. 4 and 5.
The type sections of these formations are described in Short and Stauble (1967) and summarized in a variety of papers (e.g. Avbobvo, 1978; Doust and Omatsola, 1990; Kulke, 1995; Haack et al., 1997). The Akata Formation at the base of the delta is of marine origin and is composed of thick shale sequences (main source rock), turbidite sand and minor amounts of clay and silt.From the Paleocene to the Recent, the Akata Formation formed during lowstands when terrestrial organic matter and clays were transported to deep water areas characterized by low energy conditions and oxygen deficiency (Statcher, 1995). AkataFormation is estimated to be up to 7,000 meters thick (Doust and Omatsola, 1990). The formation underlies the entire delta, and is typically over pressured.
Deposition of the overlying Agbada Formation, the major petroleum-bearing unit, began in the Eocene and continues into the Recent (Fig. 4). The formation consists of paralic siliciclastics sediments and is over 3700 meters thick and represents the actual deltaic portion of the sequence. The clastics accumulated in delta-front, delta-topset, and fluvio-deltaic environments. In the lower Agbada Formation, shale and sandstone beds were deposited in equal proportions, however, the upper portion is mostly sand with only minor shale inter-beds.
The Benin Formation overlies the Agbada Formation and is Eocene to Recent in age. The formation consistsof deposits of alluvial and upper Coastal Plain Sands that are up to 2000 m thick (Avbovbo, 1978).
 
 
 

Figure 3. Geological map of the Niger Delta and surroundings (Reijers, 2011)
 
 
 
 
Depobelts
Deposition of the three lithostratigraphic formations occurred in each of the five offlapping clastic sedimentation centers that comprise the Niger Delta. These depositional centers (depobelts) are 30-60 kilometers wide, prograded southwestward 250 kilometers over oceanic crust into the Gulf of Guinea (Statcher, 1995). The depobelts are defined by synsedimentary faulting that occurred in response to variable rates of subsidence and sediment supply (Doust and Omatsola, 1990). The interplay of subsidence and rates of supply resulted in formation of new depobelts when further crustal subsidence of the basin could no longer provide accommodation, the focus of sediment deposition shifted seaward, forming a new depobelt (Doust and Omatsola, 1990). Each depobelt is a separate unit that corresponds to a break in regional dip of the delta and is bounded landward by growth faults and seaward by large counter-regional faults or the growth fault of the next seaward belt (Evamy et al., 1978; Doust and Omatsola, 1990). Five major depobelts are generally recognized (Fig. 6), each with its own sedimentation, deformation, and petroleum history  or play (Ejedawe, 2012). Doust and Omatsola (1990) described three depobelt provinces based on structure. The northern delta province, which overlies relatively shallow basement, has the oldest growth faults that are generally rotational, evenly spaced, and increases their steepness seaward. The central delta province has depobelts with well-defined structures such as rollover structures with crests that shift seaward for any given growth fault. The distal delta province is the most structurally complex due to internal gravity tectonics on the modern continental slope. Some common oilfield structures in the Niger Delta are shown in Fig. 7.
 
 

Figure 4. Stratigraphic data sheet of the Western Niger Delta, Reijers 2011
 

Figure 5. Stratigraphic data sheet of the Eastern Niger Delta,Reijers 2011
 
 

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Figure 6. Niger Delta generalised mega-structural and stratigraphic scheme of the Niger Delta. (a) & (b) show section through the depobelts and number of wells that penetrated different plays. (c) shows the gross depositional environment (GDE) and ageof the depobelts (Ejedawe, 2012)

Figure 7. Examples of Niger Delta oil field structures and associated trap types. Doust and Omatsola (1990) and Statcher (1995).

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