Introduction

This chapter focuses on the specific role of amplitude interpretation for the purpose of reservoir evaluation. A number of techniques based on seismic amplitude have been described in previous chapters, including AVO analysis and various inversion techniques, and the interpreter's choice will depend to a large extent on the quality and type of data available as well as the problem at hand. In general, relative techniques, such as AVO analysis for defining fluid related anomalies, are appropriate in the exploration phase whereas in development projects more sophisticated techniques requiring significant well control, such as deterministic and stochastic inversion, are warranted. At each stage the interpreter can attempt to use seismic amplitude to describe the critical aspects of the reservoir as well as defining limits of uncertainty. It should be noted that to be consistent with the aim of the book in terms of the physical interpretation of seismic amplitude, purely statistical or ‘data mining’ type techniques have not been included in the discussion.

In field development, seismic interpretation is an integral part of the field geological model; key reservoir parameters that can be characterised include geological facies, reservoir properties (including porosity, net-to-gross and water saturation) and reservoir geometry and connectivity. In addition to the static element of the reservoir, time-lapse seismic offers the chance to evaluate dynamic changes. Seismic amplitudes also have a central role in the risk evaluation of prospects characterised by DHIs. In addition, modern amplitude technologies can produce results with reasonable certainty such that they can be incorporated into the process of reserves determination.

Introduction

This book is about the physical interpretation of seismic amplitude principally for the purpose of finding and exploiting hydrocarbons. In appropriate geological scenarios, interpretations of seismic amplitude can have a significant impact on the ‘bottom line’. At all stages in the upstream oil and gas business techniques based on the analysis of seismic amplitude are a fundamental component of technical evaluation and decision making. For example, an understanding of seismic amplitude signatures can be critical to the recognition of direct hydrocarbon indicators (DHIs) in the exploration phase as well as the evaluation of reservoir connectivity or flood front monitoring in the field development phase. Given the importance of seismic amplitude information in prospect evaluation and risking, all technical disciplines and exploration/asset managers need to have a familiarity with the subject.

Philosophy, definitions and scope

The central philosophy is that the seismic interpreter working in exploration and appraisal needs to make physical models to aid the perception of what to look for and what to expect from seismic amplitude responses in specific geological settings. This usually involves the creation of synthetic seismic models for various rock and fluid scenarios based on available well log data. In rank exploration areas the uncertainties are generally such that only broad concepts, assumptions and analogies can be used. By contrast, in field development settings where data are readily available, physical modelling can lead to a quantification of reservoir properties from seismic (with associated error bars!).

Introduction

Interpreting seismic amplitudes requires an understanding of seismic acquisition and processing as well as modelling for describing and evaluating acoustic behaviour. Separate books have been written about each of these subjects and there is certainly more to say on these issues than can be presented here. The aim of this chapter is to provide a framework of basic information which the interpreter requires in order to start the process of seismic amplitude interpretation.

Seismic geometry

Seismic data are acquired with acoustic sources and receivers. There are numerous types of seismic geometry depending on the requirements of the survey and the environment of operation. Whether it is on land or at sea the data needed for seismic amplitude analysis typically require a number of traces for each subsurface point, effectively providing measurements across a range of angles of incidence. The marine environment provides an ideal setting for acquiring such data and a typical towed gun and streamer arrangement is illustrated in Fig. 2.1a. Each shot sends a wave of sound energy into the subsurface, and each receiver on the cable records energy that has been reflected from contrasts in acoustic hardness (or impedance) associated with geological interfaces. It is convenient to describe the path of the sound energy by rays drawn perpendicular to the seismic wavefront; this in turn clarifies the notion of the angle of incidence (θ in Fig. 2.1a). Usually, the reflections recorded on the near receivers have lower angles of incidence than those recorded on the far receivers.

Introduction

A fundamental aspect of any seismic interpretation in which amplitudes are used to map reservoirs is the shape of the wavelet. This chapter presents introductory material relating to the nature of seismic wavelets; how they are defined, described and manipulated to improve interpretability. Seismic resolution, in terms of recognising the top and base of a rock layer, is controlled by wavelet properties. However, owing to the high spatial sampling of modern 3D seismic, resolution in the broadest sense also includes the detection of geological patterns and lineaments on amplitude and other attribute maps.

Seismic data: bandwidth and phase

The seismic trace is composed of energy that has a range of frequencies. Mathematical methods of Fourier analysis (e.g. Sheriff and Geldart, 1995) allow the decomposition of a signal into component sinusoidal waves, which in general have amplitude and phase that vary with the frequency of the component. An example is the seismic wavelet of Fig. 3.1, which can be formed by adding together an infinite set of sine waves with the correct relative amplitude and phase, of which a few representative examples are shown in the figure. The amplitude spectrum shows how the amplitude of the constituent sine waves varies with frequency. In Fig. 3.1 there is a smooth amplitude variation with a broad and fairly flat central peak. This is often the case as the acquisition and processing have been designed to achieve just such a spectrum. The bandwidth of the wavelet is usually described as the range of frequencies above a given amplitude threshold. With amplitudes that have been normalised, such as those shown in Fig. 3.1, a common threshold for describing bandwidth is half the maximum amplitude. In terms of the logarithmic decibel scale commonly used to present amplitude data this equates to −6 dB (i.e. 20 log10 0.5).