Enhanced Analysis of 3-D Seismic Survey Over A Carbonate Province
Authors: Thapar M.R. 
Published: December 2007

Offshore South East Asia 82 Conference, 9x003D12 February, Singapore.

Seismic response from carbonate rocks varies from weak reflections (dim spots) associated with reef-like features to strong reflections from carbonate rocks having a much higher velocity than the surrounding rocks. Amplitude, frequency, and velocity of seismic reflections can be diagnostic of hydrocarbon presence.

A three-dimensional (3-D) seismic survey was carried out for Philippines cities Service, Inc. by Geophysical Service, Inc. in February, 1978. One of the objectives of this 3-D survey is to define the shape and size of the productive Nido features. It is also important to determine the relative productive Nido features. It is also important to determine the relative locations of these features. Finally, the 3-D seismic survey data are to be employed for locating new exploratory wells to increase the oil reserves within the Nido area. A three-dimensional seismic survey is well suited for resolving reflections anomalies from reef-like (dome-shaped features by employing some of the state-of-the-art processing techniques.

We are able to display seismic data in various colors by using the CIT-CHROME seismic color processing system. These color plots can be attributed to seismic parameters like amplitude, frequency, absorption, energy, and energy frequency. Our experience with frequency anomalies over known hydrocarbon-saturated zones indicates that low frequency anomalies can be an indicator for the presence of hydrocarbons.

All of 88 seismic lines are processed in color to show "Energy Frequency" of the seismic reflections. These "Energy Frequency" color plots are laminated on plexiglass sheets to form a 3-D color display. A close examination of this 3-D display reveals some interesting geophysical observations regarding the location of some of the noncommercial wells.

Introduction

The seismic response from carbonates varies from weak reflections (dim spots) associated with reef-like features to strong reflections due to a large acoustic impedance contrast between the carbonates and the surrounding formations.

A three-dimensional (3-D) seismic survey was carried out for Philippines Cities Service, Inc. by Geophysical Service, Inc. in Philippines Cities Service, Inc. by Geophysical Service, Inc. in February, 1978. One of the main objectives of this survey is to define the shape and size of the productive Nido features. It is equally important to determine the relative locations of these features. The 3-D seismic data are useful in locating exploratory wells.

This 3-D seismic survey covered an area of 17.2 km × 8.7 km., with 88 seismic lines. These seismic lines were recorded at 100 meter intervals.

We have processed these seismic lines in color to create a 3-D color display. Horizontal constant time slices were also processed in color to make a movie film of the seismic data with depth.

The seismic attributes included in the CIT-CHROME seismic color process are listed in Figure 1. Parameters like relative true amplitude, structure true amplitude, frequency, energy and attenuation/absorption can be displayed as a function of color. The measurement of wave propagation quantities is a useful tool for interpretation and processing of seismic data.

Cit-Chrome color process can help the interpreter in detecting hydrocarbon-indicator anomalies, structural features like highs and fault, lithology and stratigraphic traps. Minor character changes in seismic reflections are accentuated by the use of multiple colors. The changes in geophysical parameters displayed in color are measured with increased resolution as compared to the black-and-white seismic display.

It appears that color processing can aid the interpreters to explore for oil and gas-saturated zones. Cit-Chrome color processing is helping us to do a better job of processing and analyzing the seismic data.

Cit-Chrome Color Processing System

Cit-chrome color processing is a three-step process. Seismic data is first processed for color separation and then displayed on black-and-white films. The final step is the making of a color display by the use of DuPonts Cromalin process, or on an Application color plotter. Several different seismic parameters, like true amplitude, structure true amplitude, frequencies, and energy can be displayed in color by Cit-chrome color processing system.

Relative True Amplitude In Color

In a black-and-white display of a relative true amplitude seismic section, a geophysicist sees the amplitudes in a qualitative manner; e.g., amplitudes appear either large, medium, or small. A relative true amplitude in a black-and-white section for an offshore line is shown in Figure 2.

Two levels of amplitudes (high and low) can be observed on the black-and-white display (Figure 2). It is quite difficult to detect any variations within the large amplitudes observed on this display. Also, an opposite-polarity display is required to detect anomalies in the negative oscillations (troughs) of this section. A color display of the same true amplitude seismic data is shown in Figure 3. In this color display not only many more levels of variation can be noticed, but also both polarities as compared to the black-and-white plot can be observed. Only one of the six wells marked on the plot can be observed. Only one of the six wells marked on the section is dry. The remaining five wells to the left are all producers. The producing sands are marked by the bright reflection amplitudes shown in red color.