AMPLITUDE VARIATION WITH OFFSET MODELING FOR RESERVOIR FLIUD DISCRIMIANTION

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CHAPTER ONE

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.           


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