REGIONAL STRATIGRAPHIC AND STRUCTURAL FRAMEWORK OF PARTS  OF THE  EASTERN  COASTAL  SWAMP DEPO-BEL T OF THE NIGER  DELTA.

ABSTRACT

Facies geometry, stratigraphic configuration, structural style, hydrocarbon  type and distribution within the paralic Agbada Formation of Middle to Late Miocene age across several fields in the Eastern part of the Coastal Swamp depo-belt were studied using regional 3D seismic,  wire-line well  logs  and  biostratigraphic   data.  The  study  involved  analyses  of sequence  stratigraphic framework  across nine fields using information  obtained  from twenty-  four wells. Ten major stratigraphic bounding surfaces (five each of sequence boundaries  (SB) with ages ranging from 13.1 Ma through  8.5 Ma.  and maximum  flooding surfaces (MFS) with ages between  12.8  Ma. and 7.4 Ma respectively) were identified, correlated and mapped across several wells and seismic sections.  Four  depositional  sequences  were  delineated  and  stratigraphic  flattening  at various MFS(s) indicates that there is a shift of the depositional center from north to south.  Three major stacking patterns (progradational, retrogradational and aggreadational) were delineated and interpreted as Lowstand Systems Tract (LST), Highstand Systems Tract (HST) and Trangressive Systems Tract (TST) using their bounding surfaces.  The alternation of the reservoir sands of the LST and HST and the shale units of the TST offers good stratigraphic traps for hydrocarbon. The Gross Depositional  Environment  spans through  incised Canyons,  Channels,  Inner Mid  Shelf, Shelf Margin  and Slope Margin. Paleobathymetric  maps show generally, that sediments  were deposited  within  Neritic  through  Bathyal  environments  at different  times,  aligning  with  the progradational pattern of deposition of the Niger Delta.  Structural analyses reveal the occurrence of Back  to  Back  Horst  Block  (Trapezoid  Zone),  Collapse  Crest  Structures,  Simple/Faulted Rollovers,  Regional  Foot  Walls/Hanging   Walls  and  Sub-detachment   structures  dominating within the mainly extensional zone and these constitute the major hydrocarbon traps in the area. Revalidated and newly identified leads at intermediate and deeper horizons indicate that detailed mapping of stratigraphy and structures are critical in hydrocarbon evaluation of deeper prospects in the Niger Delta.

CHAPTER ONE

General Introduction

1.1  INTRODUCTION

The Late Cenozoic  strata of the Niger  Delta  Basin  are among  the most  challenging  targets  for both  stratigraphic  and  structural  interpretation in petroleum  exploration  because  of the  several factors involved in their deposition. They are thick, complex  sedimentary units deposited rapidly during high-frequency, fluvio-deltaic-eustatic sea level oscillations. The surface upon which they were deposited  is underlain  by thick, unstable  mobile  clay; this loading has produced  a complex series of gliding surfaces and sub-basins.  In these sub-basins,  deposition  commonly  is controlled by  large  contemporaneous   glide-plane  extensional  faults  and  folds  related  to  diapirism,  shale sills, and underlying  structures. Many  of the sediments  were deposited  within  neritic  to bathyal water depths and are highly  variable  in their patterns  of deposition.  Because  of this complexity, the Niger Delta Basin remains highly attractive, but truly challenging  in today’s expensive  deep• water drilling environments.

The understanding  of facies geometry, stratigraphic  configuration,  structural  trend, hydrocarbon type and distribution  within the paralic  sequence  of Middle  to Late Miocene  age across  several fields  in  the  Tertiary  Niger  Delta  basin  fills,  is  expected   to  improve   immensely   with  the application   of    the   concept   of  sequence   stratigraphy.   Recent   developments    in   sequence stratigraphy  (Posamentier  and Allen,  1999;  Posamentier,  2000;  Catuneanu,  2002;  Catuneanu  et al., 2005) offer  a more definitive  approach  to stratigraphic  interpretation  of these strata.  Greater emphasis  on interpretation  of well-log  and biostratigraphic  information, closely  integrated with seismic data,  increases the resolution  for prediction  of reservoir,  seal and source rocks.

1

This work presents the results of a regional geological analysis of the Niger Delta basin.  Aspects related  to the structural  framework, stratigraphy  of the siliciclastic  sequences, and the tectonic• sedimentary   evolutions   are   discussed.   In   this   study,   a   generalized   description,   regional correlation,  synchronization,  and  interpretation  of genetic  relationship  of the  most  significant structural features are given.  The following main features constitute the structural  framework  of the  Niger  Delta  basin:  antithetic  tilted  step-fault  blocks,  synthetic  untilted  step-fault  blocks, structural  inversion  axes,  hinges  with  compensation   grabens,  homoclinal   structures,  growth faults  with  rollovers,  shale  diapirs,  and  structural   features  related  to  igneous  activity.  The hypothesized  contemporaneous  development  of the two faulted block  systems mentioned  above constitutes  a new viewpoint  regarding  to the evolution  of the structural framework  of the Niger Delta basin.

1.2  LOCATION OF THE BASIN AND THE STUDY AREA

The Niger Delta Basin,  situated at the apex of the Gulf of Guinea on the west coast of Africa, is one  of the  most  prolific   deltaic  hydrocarbon   provinces   in  the  world  (Figure   1.1 ).   The sedimentary  basin occupies  a total area of about 75,000 km and is at least  11  km deep in its deepest parts.  Current daily oil production  is 2.1  million bbl, and daily condensate production is

85,000 bbl (Haack et al., 2000). The study area spans through five Blocks (which for proprietary reasons are named OMLs I, II, III, IV and V) on the onshore part of the Eastern Coastal Swamp of the Niger Delta Basin.  The field lies between Latitudes 4° 201   0011   N and 4° 501   0011  N and Longitudes 6° 301  0011  E and 7° 101   0011  E and covers an area of 3610.656 km (Figure 1.2).

2

Figure 1.1: Niger Delta: Location Map of Study area showing Topographic and Oil and Gas Fields (Courtesy:  Shell)

3

•

—-

                 [:___ —–. T’7,.,·

…

–—––                    –                                ‘I

.-                                         – 4                        l

——–                                                   = –   I

e                                [

I

+                                   I

I

»”                                «                                              I

I

I I

I I

I I

I I

I I

I

I I

I I

I I

I I

I I

I I

I I

I I

I

Figure 1.2: Map of the Study area showing the various Fields and Blocks (OMLs). (Fields and Blocks: Renamed for proprietary reasons reseons)

4

1.3   LITERA TURE RE VIEW

Detailed discussion on the history,  evolution,  and structural features of the Niger Delta can be found in the works of Allen (1964), Hospers (1971), Burke et al., (1971) and Whiteman (1982). Stoneley (1966) and Burke et al. (1972) analyzed and discussed the mega tectonic setting of the Niger Delta.  The syn-sedimentary  tectonics  of the Tertiary delta was extensively  described by Evamy et al. (1978).

Previous studies reveal that the tectonic framework of the continental margin in the Niger delta is controlled by Cretaceous  fracture zones expressed as trenches and ridges in the deep Atlantic. The fracture zone ridges subdivide the margin into individual basins, and, in Nigeria, form the boundary faults of the Cretaceous Benue-Abakaliki  trough, which cuts far into the West African shield.  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).

Detailed  studies  on  tectonics,  stratigraphy,  depositional  environment,  petrophysics, sedimentology  and hydrocarbon  potential  are well  documented  in the  literature  (Weber  and Daukoru,  1975;  Doust and Omatsola,  1990;  Reijers  and Nwajide,  1996,  Nton and Adebambo,

2009; Nton and Adesina, 2009) among others. The Niger Delta, on the passive western margin of Africa,  has long been recognized as a classic example of continental-margin  structural collapse under sediment loading (Daily,  1976; Khalivov and Kerimov,  1983; Morley,  1992; Morley et al.,

1998; Rensbergen et al., 1999; Edwards, 2000; Rensbergen and Morley, 2000).

5

The modem Niger Delta has distinctive basinward  variations in structural style that define (1) an inner extensional  zone of listric growth faults beneath the outer shelf; (2) a translational  zone of diapirs  and  shale  ridges  beneath  the  upper  slope;  and  (3)  an  outer  compressional   zone  of imbricate  toe-thrust  structures  beneath  the  lower  slope  (Hooper  et  al.,  2002).  These  areas  of contrasting  structural  style are linked on a regional  scale by slow gravity  collapse  of this thick deltaic prism (Damuth,  1994).

Although broad regional relationships between patterns of deposition  and deformation  caused by structural  collapse  within  the  inner  extensional  zone  of the  Niger  Delta  have  been  proposed (Knox and Omatsola,  1989),  details of high-frequency  sequence development  within this setting are less well documented.  Most recent stratigraphic  studies of the Niger Delta deposits based on modem   three-dimensional    (3-D)   seismic   records   have   focused   on   relationships   between depositional  patterns  within  the  compressional  toe  of this  elastic  wedge  along  the base  of the continental slope (Morgan,  2004; Adeogba et al., 2005; Corredor et al., 2005).

Short and Stauble (1967) defined three formations  within the  13,000 ft thick Niger Delta elastic wedge  based  on  sand/shale  ratios  estimated  from  subsurface  well  logs:  (1)  basal,  offshore• marine, and pro-delta  shale of the Akata  Formation; (2) interbedded  sandstone  and shale of the dominantly deltaic Agbada Formation;  and (3) the capping sandy fluvial Benin Formation.

Previous  sedimentological,  biostratigraphical   and  sequence-stratigraphic   studies  (Ladipo  et al.,

1992;  Stacher,  1995;  Reijers  et al.,  1997)  revealed  the combined  influence  of eustatic  cyelicity and  local  tectonics.  Recent  studies  on  the  offshore  Niger  Delta  (Owajemi  and  Willis,  2006;

6

Magbagbeoloa  and Willis, 2007) demonstrate that these concepts are still valid but perhaps could benefit  from the stratigraphic  information and the new approaches presented  here. Depositional sequences  as defined  by  Vail  (1987)  and  consisting  of strata  bounded  by  unconformities  and their  lateral  equivalents  are only recognised  in specific  sectors  of the  delta.  In contrast,  delta•

wide  genetic  sequences  as defined  by Galloway  (1989)  and consisting  of strata bounded  by

maximum flooding surfaces within transgressive  shales are more readily identifiable in the Niger Delta.  Individual  sea-level  cycles  are  reflected  in  the  Niger  Delta  in  various  sedimentary sequences.  Interferences  of cycles  with  different  periods  result  in  megasequences   that  are chronostratigraphically confined and sedimentologically  characterised.

Sequence  stratigraphic  concepts  are increasingly finding  new  and unique  applications  in the regressive siliciclastic deposits of the Niger Delta.  Haq et al. (1988),  found that the most useful criteria for the recognition  of sequence boundaries  in the acreage  in the Niger  Delta  include truncation of underlying reflections,  drape,  dip discordance,  or onlap of younger reflection over topography on sequence boundary,  contrasts in seismic attributes across the sequence boundary and the sequence termination of faults at the sequence boundary.

Pacht and Hall (1993) applied the sequence stratigraphic concept to exploration in the offshore of the Niger Delta.  Stacher (1994),  revised the earlier SPDC Bio and Time-Stratigraphic  Scheme and put the scheme in a sequence stratigraphic framework allowing correlation with Haq et al., (1988) sea level curve using the Harland et al., (1992) global time scheme. Bowen et al. (1994), established an integrated geologic framework of the Niger Delta slope, by applying established

7

sequence  stratigraphic  concepts,  on  the  newly  acquired  seismic  data  sets  of the  Niger  Delta, coupled with biostartigraphic  data,  from twenty-six  (26) key wells.

Over the years,  delta wide framework of Cretaceous chronostratigraphic  surfaces,  and a sequence stratigraphic  chart for the Niger Delta has been produced,  using biostratigraphic  data, obtained from several wells.  Ozumba (1999) developed a sequence stratigraphic framework of the western Niger  Delta, using  foraminifera  and wire  line  log data  obtained  from  four  wells  drilled  in the coastal and central swamp depobelts.  He concluded that the late Miocene sequences were thicker than the middle Miocene sequences.

Asseez (1976) reviewed the stratigraphy,  sedimentation  and structures of the Niger Delta.  Merki (1972),  described  the  structural  geology  of the  Tertiary  Niger  Delta,  which  is on the  overlap sequence  that  is  deformed  by  syn-sedimentary   faulting  and  folding.  Ekweozor  and  Daukoru (1984 and  1994), presented  a detailed report  on the petroleum  geology  and stratigraphy  of the Niger  Delta  showing the relationship  between  depositional  patterns, structures  and stratigraphy and their  influence  on the oil generation  in the Niger  Delta basin.  Knox  and Omatsola  (1989) used escalator regression model and impact on hydrocarbon  distribution  and its development.

This current work focuses on understanding the facies geometry, stratigraphic configuration, structural trend,  hydrocarbon  type and distribution  within the paralic sequence of Middle to Late Miocene age across several fields in the Eastern Coastal Swamp of the Tertiary Niger Delta.

8

1.4  AIMS/SCOPE OF STUDY

This research aims at:

1.        Building  a high-resolution  structural  and stratigraphic  framework  for parts  of the eastern Niger Delta by using biostratigraphy and suites of geophysical well logs, along with seismic and sequence stratigraphy. This will offer an integrated sequence/seismic stratigraphic interpretation for predicting reservoir, seal and source rocks of petroleum.

11.          Determining the influence of structural evolution and stratigraphy on the hydrocarbon system in the fields in the study area. This structural and stratigraphic framework will provide better understanding  of stratigraphic  and  structural  evolution  on  retention  or  non-retention  of hydrocarbons;  hydrocarbon  trends;  Hydrocarbon  types  (gas  and/or  oil)  and  distribution, useful in influencing exploration and exploitation decisions.

111.          Identifying new and/or revalidate  leads (especially deeper  leads) in the  study area, using cutting-edge computer tools such as Petrel, nDI-Geosign and ArcGIS ..

₦5,000.00 – DOWNLOAD FULL PROJECT DOWNLOAD FULL PROJECT Added to cart

---

---

---

---

Related Project Topics :