Use of AVO Processing Techniques for Shallow Gas Assessment
Authors: des Vallieres T.O. , Nicoletis S.A. , O&apos;Sullivan M.P. 
Published: October 2008

The paper describes the interest of using AVO (amplitude versus offset) processing techniques in offshore drilling site surveys for shallow gas assessment. Presently, shallow gas evaluation is mostly based on the aspects of the amplitude anomalies of seismic reflectors. This practice is undoubtedly conservative as amplitude anomalies may not correspond to gas charged horizons; hence, the need to qualify seismic amplitude anomalies, in particular in environments where such anomalies are numerous. With this objective, AVO processing techniques enable discrimination of events which are gas charged or not. The paper presents results of HR surveys with map and seismic sections for which AVO techniques were used. Examples from THAILAND, TONKING GULF and INDONESIA are presented, showing practical consequences for drilling site selection and drilling programs.

INTRODUCTION

In offshore drilling site surveys, the requirement for a more accurate assessment of shallow gas is presently increasing. Up to now the standard practice for determining shallow gas is based on the amplitude anomalies or "bright spot" effect on the High Resolution (HR) seismic sections. As is well known, characterisation of bright spots is worked out from equalized and relative amplitude sections (variation of the reflection amplitude, variation of the signal frequency, polarity reversal, decrease of velocity and pull down effect, attenuation of the energy under gas, diffraction at the extremities of the reflection...). However, the method is somewhat inaccurate as the above criteria may not show clearly and as anomalous amplitudes do not only correspond to gas charged layers. Since the objective is security, interpreters are conservative and, as a consequence, many places may erroneously be considered potentially hazardous for drilling. The problem is aggravated when the environment is actually intensively gas charged (like a delta for instance) : the anomalies found are so numerous that almost no "safe" drilling location can be recommended. Hence the need of better qualification of gas charged horizons.

The AVO TOOL FOR HR SEISMIC

The AVO (Amplitude Versus Offset) processing techniques, in theory, enable the identification of the presence of disseminated gas within sediments. These techniques are based on the variation of the amplitudes of seismic reflections with the offset source- receiver. They are today, commonly used in deep seismic for oil and gas evaluation but not in shallow HR seismic. Following R.T. SHUEY, 1985, it can be shown that the reflection coefficient R can be expressed as a function of the incidence angle ? : (mathematical equation)(available in full paper) In case of a gas charged layer, both impedance and Poisson ratio decrease across the interface. The product Ro*G is therefore doubly affected by the presence of gas and provides an effective tool for its identification. With the primary objective of qualifying gas charged and none gas charged layers within the range mudline to 800 m deep, a number of systematic trials were performed during the last 18 months on various fields where shallow gas occurs. The data acquisition parameters are the classical ones for HR seismic : 48 traces, 12.5 m between traces, shot point interval 6.25 m, I ms sampling rate, record length 1.5 s. In order to ensure the range of investigation required (0-800 m) a 600 m long streamer is necessary for velocity analysis control as well as for a comprehensive AVO processing. In addition the recording geometry (positioning - trigger) must be really accurate, that is constantly controlled.