Bouguern A.

University of Boumerdes, Algeria, Bouguern.Ahbouguern @yahoo.fr

ANALYSIS OF THE AMPLITUDE VERSUS OFFSET (AVO). APPLICATION ON A SEISMIC

PROFILE OF THE BASSIN OF AHNET, ALGERIAN SAHARA

Abstract

The purpose of this work consists in estimating the efficiency of the method of the direct search for

hydrocarbons. To realize it, a classic treatment and one in protected amplitudes were applied to the seismic

profile 04 TGE 10, passing by an already productive well in gas. It seems that the treatment in protected

amplitudes put in evidence an abnormality in amplitude (bright spot). The analysis of the attributes AVO

confirmed this abnormality.

Key words: protected amplitudes ,attributes, analysis AVO.

Introduction The objective of this work consists in showing what can bring the technique AVO in the field of the

direct detection of hydrocarbons, and by way of consequences to apply it to other studies when anomalies of

amplitude are detected which can be probably connected to the presence of hydrocarbons.

This work highlights the results of a classical treatment (fig.1) , followed by a treatment in protected

amplitudes (fig.2) achieved on the 04TEG 10 seismic profile, in the bassin of Ahnet which is situated in a

favorable oil environment. The main objectives in the region are the ones of lower Dйvonien and

Ordovicien.

1. Geologic context The prospect Hassi Hassine Est, belongs geologically to the bassin of Ahnet, situated between two

structures of the North-East and South-West directions: that of Hassi Hassine to the West and the

continuation of the Gour Mahmoud structure to the East by means of a structural nose. The superficial

grounds begin with the formation of continental intercalary and the formations of Palйozoпque begin with

Vicien clays. Les structures on this block are from the North-South to the North West- South East. They are

from the principally Hercynian age.

The main petroleum objectives in this region are those from the inferior Devonian and from the

Ordovicien.

2. Processing of seismic data The identification of the geologic horizons was achieved thanks to the data of the seismic core

drillings and those of the seismic vertical profile ( VSP).The processing of the seismic profile is made under

UNIX environment with the computer program Gйovecteur.

The AVO technique is applied to the 04 TGE 10 profile crossing a productive gas well. The section

in protected amplitude presents an AVO anomaly at the Emsein reservoir level (1350ms-1450ms), (CDP630

to CDP690).

The AVO technique, allowed us to obtain sections with constant offsets and sections of the

attributes. For this purpose three sections of constant offset were achieved: near offset, middle offset and far

offset. From which we notice the increase of the amplitude with increase of the offset. The sections of the

AVO attributes have shown that:

− On the intercept section (R0), the negative intercept is due to a big contrast of acoustic impedance

between the sandstone of Emsein and the clays of Silurian.

− The section of the gradient shows the scope of the variation of the amplitude with the offset (fig.2),

the increase or decrease is connected to the contrast sign of the Poisson’s coefficient.

− The section fluid factor: the interpretation of this section is based on the variation of the waves; the

results obtained do not depend on the class of the sandstone. The studied anomaly under study is not

very visible in this attribute, it correlates to the characteristics of the reservoir which is of 14m of

thickness (little thick).

Conclusion The AVO analysis made on the 04 TGE 10 profile confirmed the anomaly, (bright spot) observed on

the seismic sections at the Emsein level (objective).The sections of the AVO attributes gave positive results.

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Fig. 1: Seismic section with classic processing.

Fig. 2. Seismic section in protected amplitudes .