INTEGRATING SEISMIC AND PETROPHYSICAL LOG DATA IN RESERVOIR STUDIES IN “FAMITO” OIL FIELD ONSHORE NIGER DELTA NIGERIA

ABSTRACT

Reservoir characterization has taken prominence in hydrocarbon exploration due to its role in proper  understanding  of  the  reservoir  properties  such  as  lithofacies,  and  petrophysical parameters, to enhance success in the quantification of the hydrocarbon reserve in place thereby diminishing  exploration  failures  and  enhancing  successes.  This  research work  is  aimed  at integrating three-dimensional seismic data and petrophysical log data to characterize reservoirs of interest, map structures, delineate facies and interpret the various depositional environments in “FAMITO” field of Niger Delta. Mapping and interpretation of faults and horizons/reservoir tops were carried out across the seismic volume. Field-wide and intra-reservoir correlation (North- South direction), aided by gamma ray log, were carried out across the field of study. Three reservoir tops of interest (F-1000, G-1000, and H-1000 reservoirs), were delineated and correlated. Facies interpretations using the  integration of biostratigraphic and well log data showed that the reservoirs have shoreface and channel sands. The environments of deposition encountered, spans from fluvial through shallow marine. Time structural maps generated from these tops were depth converted using the velocity model. Studies reveal hydrocarbon closures and prospects bounded by faults which constitute the main trapping mechanism across the study area. Petrophysical analysis shows that the average porosity and water saturation values for F-

1000 reservoir is 0.2401 and 0.3866, G-1000 reservoir is 0.2527 and 0.4549, and that of H-1000 reservoir is 0.2436 and 0.2838 respectively. These values indicate that the identified prospective zones have good porosity and water saturation.  Seismic attribute maps (root mean square (rms), maximum amplitude and average energy) generated from the time structural maps show the presence of hydrocarbon accumulation. Hence, this work has shown that integrated studies are critical in characterization of reservoirs in the onshore Niger Delta.

CHAPTER ONE

INTRODUCTION

With maturing basins and even deeper and more complex frontiers, the importance of extending the life of and maximizing recovery from producing fields has never been greater. Hydrocarbon exploration and development has seen several step-change in technologies during recent decades and integration of seismic and petrophysical tools has now been seen as a robust technique whose value is now recognized worldwide in predicting hydrocarbon in place and lithologies for oil exploration thereby enhancing accurate reservoir characterization.

Reservoir characterization has evolved through several generations such as the originally petrophysics, geologic analogs, and more recently on multidisciplinary integration. The petrophysical approach began in the 1950s, where reservoir attributes were determined using logs, cores, and well tests. The second generation based principally on geologic analogs with the addition concept between wells. The analog technique improved inter-well prediction, but identifying the right analog often proved difficult, especially in structurally and stratigraphic complex settings. Multidisciplinary approach, attempts to integrate all available geologic, engineering, and geophysical data along with modern probabilistic and risk analysis techniques to produce a better reservoir model. This third generation reservoir made significant use of 3D seismic amplitude anomalies and other complex trace attributes to define field development programs. This multidisciplinary approach was used in this research work.

1.1      Background Study

The study area, “FAMITO” field, is situated in the Coastal Swamp Depobelt of the Niger Delta area (Fig 1.1) and belongs to Shell Petroleum Development Company (SPDC) concession. The seismic base map of the study area covers 4485 – 5585 inlines and 1274 – 1974 crosslines respectively. The Field was discovered in 1958 with well-001 first drilled at the peak of the structure with few hydrocarbon bearing reservoirs between 10150 and 10620 ftss. The field comes from the swampiest of the ecological zones, and consists of a massive swamp, dotted with islands of dry land covering about 10240 square kilometers.

Over forty eight wells have been successfully drilled in the field with lots of gas saturated reservoirs, and more are still ongoing to increment efficiency on gas production. Six wells, namely  well-005,  well-013,  well  025,  well-026,  well-029,  and  well-048  that  penetrated reservoirs of interest  were  used for  this  study and  was drilled  to  the  depth of 11299.52,

12890.03, 10978.02, 11661.52, 11868.02 and 11478.52 (ftss) respectively. (Fig. 2) shows the location of wells in the study area. Of the six wells, well-048 has complete composite well logs, which include Gamma Ray, Resistivity, Bulk Compensated Sonic (DT and DTS), Density (FDC), Neutron (CNL), Caliper and Checkshots. Well-013 and well-025 have biostratigraphic data, while well-013, well-026 and also well-48 have checkshots. The well log data are in LAS format while the seismic volume used is a 3-Dimensional pre-stack seismic volume that is in SEG-Y format,  with a  record length of 3200ms. The  field  was discovered in the  middle Miocene deposits and the hydrocarbon reservoirs are at the depths from 1.5 to 3.5 km in a terrigenous  Neogene  sequences.  Based  on  the  biostratigraphic  data  and  the  log  motif

interpretation, the environment of deposition is Channelized – Shoreface (fluvial and shallow- marine) for the sands which is consistent with the progradational nature of the Niger Delta.

The purpose of this study is to apply seismic and petrophysical techniques, with the integration of seismic  attributes  in reservoir characterization for  better  delineation of reservoir quality especially at deeper horizons.

1(A) Insert map of Africa showing the location of the study area, (B) Insert map of Niger

Delta depobelts in Map of Nigeria, (C) Concession map of the Niger Delta.

Fig. 2 Seismic Survey Base Map of the studied field showing the location of the wells.

1.2     Statement of the Problem

The onshore Niger Delta basin has assumed the status of a mature petroleum province in terms of oil and gas exploration and production. Exploration and production activities have been active in the basin for over five decades now. Thus, due to seismic ambiguities caused by propagation of seismic energy during acquisition, difficulties have been encountered in lithologic description and interpretation of structural, stratigraphic and petrophysical properties of the reservoir over these years, which constitute great challenges during drilling, resulting in reduced volume of crude oil from several fields. With the renewed call by the Federal government on Nigeria for increased reserves, there is need to boost reserves of old producing oil fields in the basin using a multidisciplinary approach such as the  integration of seismic and petrophysical log data to enhance the characterization of prospective reservoirs.

1.3      Aim and Objectives of Study

The purpose of this work is to demonstrate how to obtain superior reservoir architecture and characteristics  such  as  structure,  facies  and  depositional  environments  by  integrating  both seismic and petrophysical log data.

The objectives of this study will be to use the conventional well logs, biostratigraphic data and

3D-Prestack seismic data

  To discriminate between lithologies thereby enhancing reservoir delineation from non- reservoirs.

  To determine the system tract and environment of deposition of the field from where the quality of the reservoir sand will be determined.

  To interpret the structural, stratigraphic and petrophysical properties of the reservoir in order to enhance the understanding of the reservoir for better characterization.

1.4      Significance of Study

The expected outcomes of this research study include:

  Precise interpretation of the structural, stratigraphic and petrophysical properties of the reservoir, lithofacies classification and as well the environment of deposition associated with the identified reservoirs.

  Reduction of uncertainties associated with reservoir characterization when planning in- field development systems.

1.5      Review of Previous Literature

The last 40 million years have been important in shaping the geology of the Niger Delta. The petroleum potentials associated with this area have attracted so many geological and geophysical studies which range from petroleum geology, sedimentology and stratigraphy, biostratigraphic and paleontology and to AVO/AVA. However, few researchers have done their work using petrophysical techniques to quantify the amount of hydrocarbon in place, poisson impedance and other elastic impedances to discriminate and predict lithofacies and fluids in mature fields of Niger Delta and around the world. These include the works of Ameloko and Oseghe (2013), Ozumba (1999), Russell et al., (2006), Singh et al., (2007), Ujuanbi et al., (2008), Amigun and Odole (2013), and Zhou and Hilterman. (2010).

According to Short and Stauble (1967), the Niger Delta comprises of a regressive sequence of deltaic and marine clastics, defined by three major lithofacies. From the base, predominantly

marine shale, made up of Akata Formation, followed by paralic sequence of Agbada Formation and topmost non-marine alluvial (continental) sands of the Benin Formation.

Short and Stauble (1967) and Weber and Daukoru (1975) outlined the three major depositional cycles in the coastal sedimentary basins of Nigeria. They first began with an Albian marine incursion and terminated during the Santonian time; the proto-Niger Delta started during the second cycle, the growth of the Niger Delta continued from Eocene to Recent time. At several stages during the late Quaternary, sedimentation was interrupted by uplift and erosion, whereby several cycles of channels were cut and filled which resulted to submarine canyons (Evamy et.al., 1978).

Weber  (1971)  reported the  cyclic  nature  of sedimentation of the  Tertiary paralic  deposits. According to him, a complete cycle consists of thin, fossiliferous transgressive marine sands followed by an onlap sequence which commences with marine sediments and another transgression may terminate the cycle.

Doust and Omatsola (1990) recognized six depobelts in the Niger Delta, which are distinguished primarily by their age. They are: Northern delta (Late Eocene – Early Miocene), Greater Ughelli (Oligocene – Early Miocene), Central swamp 1 (Early – Middle Miocene) Central swamp 11 (Middle Miocene), Coastal swamp 1 and 11 (Middle Miocene) and Offshore mega structures (Late Miocene).

Stacher (1994), produced a delta wide framework of Cretaceous Chronostratigraphic surfaces, and a sequence stratigraphic chart for the Niger Delta, using digitally stored biostratigraphic data, obtained from over 850 wells.

Ozumba (1999), developed a sequence stratigraphic framework of the western Niger Delta, using foraminifera and wireline 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.

Posamentier and Kolla (2003) analyzed 3-D seismic data in predominantly basin-floor settings offshore Niger Delta. These revealed the extensive presence of gravity-flow depositional elements. Five key elements were observed. And they are; turbidity-flow levee channel, channel- over  bank  sediment  waves  and  levees,  frontal  splays  and  distributary channel  complexes, crevasse-splay complexes, debris-flow channels, lobes and sheets. The reservoir architecture of each of these depositional elements is a function of the interaction between sedimentary process, seafloor morphology, and sediment grain-size distribution.

Adeogba et.al (2005) have interpreted a near surface, 3-D seismic dataset from Niger Delta continental slope, offshore Nigeria and revealed important stratigraphy and architectural features. Architectural features and sediment deposits interpreted from seismic character and seismic stratigraphy in the absence of borehole data  include  mass-transport complexes, distributary channels, submarine fans and hemi-pelagic drape complex.

Rusell et al., (2006) in their approach used a method which combines seismic attribute, rock physics modeling and neural network to predict the behavior of reservoir and to reduce uncertainty. The method uses different well logs like gamma ray, density, density-porosity, neural network and multi-attribute regression to obtain the optimum ordering of the attributes and to increase the resolution of the final result. Their results were able to show that the use of data crossplots enhanced the ability to map the extent of clean sand facies.

Singh et al .(2007) described crossplots as a powerful tool for direct hydrocarbon detection and a simple way to determine AVO attributes that are suitable for the discrimination of lithofacies in a particular  reservoir.  They  used  attribute  sets  derived  from  inversion  of  seismic  data  for

identification of hydrocarbon (HC) sands. Their result confirmed that the power of Poisson impedance attribute has been demonstrated by comparing well log attributes and attributes based on simultaneous inversion to infer areas of flat-spot features.

Ujuanbi et al., (2008) used elastic impedances rather than reflection coefficient as a model parameter to discriminate oil sand from shale in the Niger delta region of Nigeria. He also showed that basic rock physics, Amplitude Variation with Offset (AVO), and seismic amplitude inversion can be used to show the effectiveness of this technique in an oil sand reservoir. His results confirmed the effectiveness of this technique for litho-fluid discrimination irrespective of the geological setting.

Zhou and Hilterman (2010) in their study on Poisson impedance used fluid factors and lambda- rho to predict fluid and lithology by considering the pore fluid sensitivity of these attributes with both well log and seismic data in Tertiary unconsolidated sediments. In their study, they showed that method of target correlation coefficient analysis (TCCA) can be used to calculate Poisson impedance using C-values to improve the reliability of reservoir predictions.

Amigun and Odole (2013), in their petrophysical properties evaluation for reservoir characterization of Niger Delta using a field they called SEYI oil field said that reservoir with porosity ranging from 0.22 to 0.31 indicates a suitable reservoir quality, permeability values from 881.58md to 14425.01md attributed to the well sorted nature of the sands and hydrocarbon saturation range from 20.29% to 91.97% implying high hydrocarbon production.

Aigbedion, I and Aigbedion, H. O, (2011) in Hydrocarbon Volumetric Analysis Using Seismic and Borehole Data over Umoru Field, Niger Delta-Nigeria concluded that the integration of well and seismic data provides insight to reservoir hydrocarbon volume which may be utilized in exploration evaluations and in well bore planning.

Ameloko and Oseghe (2013) in petrophysical characteristics and reservoir quality of the Inda Field, Niger Delta stated that the quality of the reservoirs in the part of the “Inda field” under study is poor owing to the analysis of the petrophysical parameters determined from the two wells. The average porosity values are moderate and approximately the same, but have very low permeability due to the presence of high volume of shale in the reservoirs.

Imasuen and Osaghae (2013), in their formation evaluation of Wells X, Y and Z in G-field Onshore Niger Delta stated that porosity range from 12% to 30% implies good to very good reservoir and that resistivity ranging from 3Ωm to 200Ωm indicates the presence of hydrocarbon in Niger Delta.

Omolaiye and Sanuade (2013) did Petrophysical analysis of the B-Reservoir in Eyram Field, Onshore Niger Delta and concluded that for effective reservoir management, field development strategies that various petrophysical parameters and contact types should be taken into consideration and be implemented.

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