INTRODUCTION
1.1 Background of Study
This
work discuses amplitude variation with offset (AVO) modeling and processing
applied to an offshore dataset from Niger Delta. Seismic modeling forms the
basis for understanding the seismic signature. It helps in the prediction of
reservoir characteristics away from well control points. Reliable estimation of
petro physical parameters is needed as input for such studies. These petro
physical estimates are an integral part of more advanced reservoir
characterization and modeling. First, the AVO principles are described and
various prestack attributes are presented. Subsequently, the elastic approach
is discussed and finally the benefits of seismic modeling with advantages of
multi-disciplinary reservoir studies are demonstrated.
Amplitude variation with offset (AVO) is a
prominent seismic attribute which is widely employed in hydrocarbon detection,
lithology identification and reservoir fluid identification, as a result of the
fact that seismic amplitudes at the boundaries are affected by variations of
the rock’s physical properties just above and below the boundaries . A
compressional seismic energy incident obliquely at an interface generates P-
and S-waves which are both reflected and transmitted at the interface,
utilizing the concepts of conservation of stress and displacement across an
interface, the amplitudes of the reflected and transmitted waves at the layer
boundary can be derived.
Recently,
there has been a lot of interest in the extraction of information about the
fluid content of the reservoir using Amplitude Variations with Offset Analysis,
or AVO. Goodway et al (1997) proposed the lambda-mu-rho technique, which has
met with much success. Hedlin (2000) proposed the pore-modulus method, which
was based on work by Murphy et al (1993). Most recently, Hilterman (2001)
1.2 Statement of Problem
The variation in the amplitude of a seismic reflection with
source-receiver offset is known to contain information about the rock type and
of fluid present in the pore spaces of the rocks. In particular, analysis of amplitude
variation with offset has been used with some success for detection of light
hydrocarbon (gas). Unfortunately, AVO analysis is very challenging, especially
for thin, high-impedance reservoirs. More specifically, the success rate is
dependent on whether the method of analysis and algorithms used are tailored
toward the needs of the survey.
In this study, AVO
modeling and analysis were carried out using data acquired in the Niger Delta
offshore to provide the basis for a detailed litho-fluid study of a given
reservoir in the area.
1.3 Objective
of Study
The objective of this study is to determine the
presence of gas in a given reservoir using Fluid Replacement Modeling, and
mapping the amplitude variation with offset response at the top of the reservoir
from synthetic seismic data created at the well location, in order to
underscore the AVO Class at the reservoir top. This will assist in subsequent
interpretation of the reservoir and its litho-fluid characterization.
1.4 Significance of Study
Amplitude Variation
with Offset (AVO) results provides elastic rock properties which can be used to
determine lithology and fluid content of reservoirs.The ultimate goal of this
study was to obtain reliable quantitative estimate of relevant reservoir rock
and fluid parameters in the reservoirs.The AVO effect represents a potentially
powerful tool to discriminate between water- and hydrocarbon-saturated
reservoirs.
1.5Scope of Study
The
study focuses on the Amplitude Variation with offset (AVO) modeling and
analysis, using data obtained from offshore Niger Delta. The modeling and
analysis was carried out at a known reservoir identified on the basis of the
well logs provided. The study involved gassman’s fluid substitution which
resulted in the generation of new well log data that were subsequently used in
the generation of synthetic seismogram. This was followed by the modeling of
the seismic signatures at the reservoir top in order to underscore the AVO
response. The modeled seismic response provided a guide during AVO analysis of
the actual pre-stack data provided for the study.
1.6 Study Area
The data used for this study are obtained from a
deep water block, south-eastern part of Niger Delta. Fig 1 below shows the
study area. The Niger Delta is a prolific hydrocarbon province
with a regressive succession of clastic sediments which reaches a maximum
thickness of 10-12 km. The province contains only one identified petroleum
system, known as the Tertiary Niger Delta. The delta is divided into an upper
series of massive sands and gravels (Benin Formation), deposited under
continental conditions. This grades downward into interbeddedshallow marine and
fluvial sands, silts and clays, which form the paralic sequence of the Agbada
Formation. The Agbada Formation grades into the massive and monotonous marine shale.
Most of the hydrocarbons are in the sandstones of the Agbada Formation, where
they are trapped in rollover anticlines fronting growth faults in channels and
barrier sandstone bodies. Offshore deep-water Nigeria is entering its third
decade of exploitation. The sediments in the study area are deposited in the
deltaic and prodeltaic environments, with the reservoirs mainly dominated by
interplay of lower and upper shore facefacies, distributary channel facies and
tidal deposits.
Fig1: Map of the Niger Delta showing the study area.
Major structural features of the delta are labeled shown over a bathymetric map
from.
GEOLOGY
OF STUDY AREA
The Niger Delta in Southern Nigeria has been
pro-grading outward to the Atlantic Ocean since late Cretaceous times and is
in-filled with Tertiary and Quaternary sediments which decrease in age
progressively southwards. The deposit comprise from north-east to south-west,
the Imo shale. A unit of Paleocene to Eocene (lower Tertiary) is the blue gray
shale with thin sandstones and limestone. The Eocene to Oligocene is the Ameki
Formation, comprising clays, sandstones and limestone. Oligocene to Miocene
clays comprises sands and grits with occasional lignite (carbonaceous deposits)
of the Ogwashi-Asaba Formation. The Miocene to Pilocene, Benin Formation is
composed of coasted-plain sands and pebbly sands with clay lenses and lignite.
These sediments were deposited in a variety of environments from marine,
through deltaic, estuarine and coastal swamp to lagoon and fluvial. In all, the
sediments pile reaches a thickness of around 12,000m (Osueni 2009). The study
area is geologically characterized by deposits, laid during the Tertiary and
Cretaceous periods on the South Western extension of the Niger Delta Basin,
(Reyment, 1965).
The various formations in the geology of Edo State
are the Benin, BendeAmeki, Ogwashi- Asaba, Imo, Nsukka Formation and the
various Quaternary Deposits. In this study the entire investigated area is
underlain by sedimentary rocks of the Niger Delta Basin of southern Nigeria,
(Precambrian basement complex of southern Nigeria) with about 90% of sandstone
and shale intercalation. It has coarse grained to locally fine grained in some
area, poorly sorted, sub angular to well rounded, which bears lignite streaks
and fragments (Kogbe, 1976). Its origin and evolution have been discussed by
several workers including Hospers (1965), Burke et al. (1972) and Nwachukwu
(1972). The origin is believed to be linked to a series of tectonic activities
that occurred in the south Atlantic region during the late Cretaceous times
(Murat, 1972). The Sediments penetrated by the Gbakebo “B” well located at
Okitipupa Ridge on the western flank of Niger Delta form part of the late
Cretaceous and Tertiary sequences of the southern Nigerian Basin (Kogbe, 1976).
Deposition of sediments in the Niger Delta Basin began in the Tertiary and
continued into post Tertiary times. The Niger Delta sediments include Benin,
Agbada and Akata Formations and they range in age from Eocene to Recent (Short
and Stauble, 1967; Asseez, 1976).
The
Agbada Formation is a down-dip continuation of Eocene-Miocene Ameki and
Ogwashi-Asaba Formations, while the Akata Formation is a down-dip continuation
of Paleocene- Imo Formation (FranklandCordy, 1967). The geology of the study
area is characterized by deposits laid during the Tertiary and Cretaceous
periods. The area is underlain by sedimentary rocks constituting part of the
formation which is made up of over 90% massive, porous, coarse sand with
clay/shale inter-beds having high ground water retention capacity. Soil
particles vary from coarse grained to fine grained is some areas, poorly
sorted, sub-angular to well-rounded particles with lignite fragments.