3D SEISMIC INTERPRETATION AND PETROPHYSICAL EVALUATION OF STROH- FIELD ONSHORE NIGER DELTA

ABSTRACT

Petrophysical  evaluation  and  3D  seismic  interpretation  were  carried  out  on  the hydrocarbon bearing reservoirs of Stroh field with the aim of maximizing the benefits of  petrophysical   evaluation  and  structural  interpretation  in  the   production  and development of hydrocarbon in the reservoir of the ‘Stroh field’ in Niger Delta. Eight hydrocarbon bearing reservoirs – sands ST1, ST3, ST6, ST7, ST8, ST11, ST13 and ST14 were identified from well logs and correlated across six Stroh wells. Sand ST11 was missing in Stroh-5 which crossed a  normal fault. Fluid types were determined from the well log signatures;  neutron  density crossplot was used to check for the presence of gas. All the  hydrocarbon bearing reservoirs were inferred to be oil with sand ST11 having a gas cap. Stroh-1 well saw 134.5 ft of oil in 7 sands and 26 ft of gas in 1 sand. Stroh-3st has 25.6ft of oil in one sand. Stroh-4 encountered 113ft of oil in 3 sands and 18ft of gas in one sand. Stroh-5 has 100ft of oil in 2 sands and 19ft of oil in  Stroh-6. Result from petrophysical  analysis shows that porosity ranges from

15.8%  to  28.3%,  volume  of  shale  from  2.5%  to  38.9%  and  permeability  which increases  with porosity ranges  from <10md  to  >10000md.  Bulk volume  of water shows possible variation of grain size. Top depth structure maps were produced from seismic  interpretation  for Sands ST6,  ST11  and ST13having  total  (47.23mmbo  &

32,319.05mmscf)  of oil and gas respectively.  One prospects – NE (7.9mmbo  low case, 10.81mmbo mid case and 12.03mmbo high case) was also identified for future drilling considerations. The result of seismic data interpretation shows that ten faults were identified F1 to F10 with two main regional bounding faults; F1 synthetic& F2 antithetic  faults  trending  E-W.  Trapping  mechanisms  are  fault  assisted  and  fault dependent. Time maps generated and depth converted to show structural variation and fluid  contacts.  The  results  of  petrophysical  analysis  of  the  reservoir  properties (NTG,Porosity,Sw)  and  seismic  data  interpretation  (GRV)  showed  a  perforation interval for ST13 as having the most profilic reservoir with a reserve of 25.28 mmbo, while ST6 has least prolific reservoir with reserve of 8.68 mmbo which serves as a guide for robust future  development strategy for the Stroh field.

CHAPTER ONE

1.0 INTRODUCTION

The  search  for  economic  accumulations  of  hydrocarbon  starts  with  the recognition of likely geological provinces, progressing to seismic  surveying, and the drilling of one or more wild-cat wells. If any well(s) encounter oil, and if that is the case, measurements made down the hole with wireline or LWD (Logging  While Drilling)  tools are used to assess  whether  sufficient  oil is present, and whether it can be produced. Clearly, the evaluation of sub-surface formations   requires  the  combined   efforts   of  geologists,   petrophysicists, drilling  engineers,   reservoir   engineers   and   geophysicists.   However,   the geologist and petrophysicist have the most influence (Glover, 2008).

Exploration   and  production  companies   utilize  a  variety  of  methods   to understand  their reservoirs and maximize the recovery of  hydrocarbons; the treatment  of seismic  and  well  log data  becomes  critically  important.  This evaluation involves integration of three  dimensional (3D) seismic data with well logs to delineate geologic structures and prospects and estimate volume of hydrocarbon in-place in  “Stroh Field” onshore Niger Delta. Examples of known  structural  traps  in  the Niger  Delta are rollover  anticlines,  flanks  of shale   domes   and   traps   related   to   faulting.   Identification   and   proper classification of these traps as prospects form a guide for further exploration, and in making economic decisions.

The objectives of subsurface petroleum geology are to find and develop oil and gas reserves. These objectives are best achieved by the use and integration of all the available data (well logs and seismic) and the correct application of these data. As a field is developed from its initial discovery, a large volume of well log, seismic and production data are obtained.

Petrophysical log interpretation is one of the most useful and important tools available to a Petroleum geologist. They are used in exploration to  correlate zones and assist with structure and isopach mapping, logs help define physical rock   characteristics   such   as   lithology,   porosity,   pore   geometry,   and permeability.  Logging data are used to identify productive zones, determine

depth  and  thickness  of  zones,  distinguish  between  oil,  gas  or  water  in  a reservoir, and estimate hydrocarbon reserves. The importances of petrophysics and well log analysis have become more evident as more  attention is being devoted to the ongoing management of reservoirs. The oil and gasindustry has realized the importance of detailed petrophysical analysis of the available data in monitoring, simulating and enhancing reservoir performance to maximize the return on investment.  On the  various type of logs, the ones used most frequently    in   hydrocarbon    exploration    are   called   Open   hole   logs. Measurements  made down  the hole with wireline or LWD (Logging While Drilling) tools are used to assess whether sufficient oil is present and whether it can be produced.  Rock properties  that  affect  logging  measurements  are porosity,  lithology, mineralogy,  permeability  and water  saturation  (Asquith and Krygowski, 2004).

Seismic  data  allow  us  to  extend  interpretation  beyond  the  limits  of  well control. These help in the construction of a continuous subsurface picture of the horizon being mapped and provide additional more complete information about the characteristics of the faults that intersect these horizons. Subsurface geological  maps  are  important  vehicle  used  to  explore  for  undiscovered hydrocarbons and to develop hydrocarbon resources. Subsurface maps should accurately represent geology of the study area and both well logs and seismic interpretation represent physical measurement of the subsurface.

The  wellbore  provides  the  only  path  from  the  surface  to  the  reservoir. Therefore, one of the purposes of a well completion will be to  connect the reservoir to the surface so that fluids can be produced. To a large extent, the successful production and depletion of a reservoir depends upon the successful completion and workover operations applied to a well.

1.1 Statement of Research Problem

It  has  been  observed  that  some  thick  sands  do  not  necessarily  contain economic accumulation of hydrocarbons compared with some thin sand with economic viability. This can be attributed to the trapping mechanism that is responsible for the accumulation. Therefore, there is need to properly identify

and delineate structural traps and prospects in a field for exploratory purposes. Furthermore,  proper  characterization  of the  petrophysical  properties  of the reservoir is also the key to accurately quantify reservoir  fluid. To determine the  optimum  hydrocarbon  recovery,  appropriate   evaluation  method  was adopted in the Stroh field.

1.2 Location of Study Area

Stroh Field is located  in the Greater  Ughelidepobelt  of the Niger Delta  oil province (see Fig 1). The Niger Delta is situated on the gulf of Guinea.

Fig.1: Location Map of the Study Area (Adopted: Doust and Omatsola,

1990).

The studied area is located in greater ughelidepobelt as indicated with “The cross-hatched box”.

1.3 Climate and Vegetation

The climate of the Niger Delta Region varies from the hot equatorial  forest type in the southern lowlands to the humid tropical in the northern highlands and the cool mountain type in the Obudu plateau area.

The wet season is relatively long, lasting between seven and eight months of the year, from the months of March to October. In the northern and  north- western parts of the Niger Delta Region, the rains may be delayed by as much as four weeks, thereby extending the dry season which, in recent times, tends to last for four to five months. There is usually a short break around August, otherwise termed the “August break”. The dry season begins in late November and  extends  to  February  or  early March,  a  period  of  approximately  three months.  During the dry season,  the  northeast  trade wind blowing  over the Sahara  Desert  extends  its  dehydrating  influence  progressively  towards  the equator,  reaching  the  southern  coast  of Nigeria  in late December  or early January. The period is known as the “Harmattan”, which is more noticeable in some years than others. Mean annual rainfall ranges from over 4,000mm in the coastal towns of Bonny and Brass in Rivers and Bayelsa States respectively, and  decreases inland to 3,000mm in the mid-delta around Ahoada, Yenagoa and Warri in Rivers, Bayelsa and Delta States, respectively; and slightly less than 2,400mm in the northern parts of the region such as Imo and Abia States The warmest months are February, March and early April in most parts of the Niger Delta Region. The coolest months are June through to September during the peak of the wet season.

The Niger delta is home to the world’s third largest mangrove forest, the most extensive freshwater swamp forests in West and Central Africa, and the site of Nigeria’s   remaining   primary   forest,   including   a  high   concentration   of biodiversity and several centers of endemism.  Cloud  cover is high, relative humidity  always  above  80%.  Soils  are  hydromorphic  and  poorly  drained. There are few remaining areas of  pristine  vegetation and the contemporary biogeography is largely comprised of a mosaic of arable farmlands (cassava, maize, etc.), tree crops (oil palm, rubber, plantain etc.) and patches of natural

vegetation.  The  remaining  natural  vegetation  includes  lowland  rainforest, freshwater swamps, tidal mangroves, saltmarsh and tidal mudflats, and coastal forest on the barrier sand ridges.

1.4 Drainage

The area around this coastline is interrupted by series of estuaries that form the Niger Delta swamp at the middle where the lower Niger River system drains the waters of Rivers, Niger and Benue into the Atlantic Ocean. This delicate

mangrove swamp of the Niger Delta covers a coastline of 560 km2, about two-

thirds  of the  entire  coastline  of Nigeria  and  the wetland  in this region  is traversed  and  criss-crossed  by a  large  number  of rivers,  rivulets,  streams, canals and creeks.

The Niger Delta is a rich mangrove swamp in the southernmost part of Nigeria covering over 20,000km² within wetlands of 70,000km² formed primarily by sediment deposition.

It is the largest mangrove swamp and wetland in Africa, maintaining the third largest drainage basin in the continent, and is also the third largest wetland in the world after Holland and Mississippi.  The major drainage systems of the delta consist  of seven discrete  river systems which lie  squarely in the wet equatorial climatic belt.

1.5 Scope of Study

This research study covers formation evaluation of “Stroh Field”, through seismic (structural) interpretation and petrophysical analysis, volume estimation of hydrocarbon in place, identification of prospects of processed

3D seismic data in SEG-Y format.

1.6 Aim and Objectives of the Study

Aim: The aim of this work is to demonstrate the importance and maximize the benefits  of  petrophysical   evaluation  and  structural  interpretation   in   the production and development of hydrocarbon in the reservoir of  the “Stroh- field” onshore Niger Delta

Objectives:

The following objectives were set out:

1.        Identify hydrocarbon bearing sands, delineate fluid types and contacts

2.        Generate time and depth structure maps for hydrocarbon bearing zones

3.        Determine petrophysical parameters for the Stroh field

4.        Estimate volume of hydrocarbon originally in-place.

5.        Identify new prospects

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