Regional scale rock physics modeling for quantitative analysis of seismic amplitudes in the Malay basin, Malaysia
Authors: Mohamud Nurhakimah Binti , Singh Yeshpal 
Published: May 2016

The objective of this study is to build a rock properties database and to understand vertical and lateral trends which can explain seismic amplitude response(s) in terms of lithology and fluid fill in the Malay Basin. The elastic rock properties, defining the seismic response were extracted from the logs of 60 wells distributed throughout the Malay Basin. To ensure high quality of elastic logs, each well underwent iterative petrophysical and rock physics analysis to produce high quality elastic logs (Vp, Vs, and density) and consistent petrophysical logs (mineralogy, saturation, porosity, etc.).

The rock physics analysis was carried out to relate well log derived rock properties and core information with seismic amplitudes. A few models have been analysed to investigate the best effective medium models that best represent the sandstones and respective shale in the Malay Basin. Homogeneous mixing model was used to integrate the net properties along the borehole. The core information has been incorporated to validate the rock physics models describing rocks in the study area. The optimum rock physics model, relating elastic properties to porosity, lithology, thickness, fluid properties and diagenetic texture variation, was established for each well through correct parameterization. A regional rock physics model map has been generated for the Malay Basin. It has been observed that the Malay Basin can be represented by "Soft Sand", ‘Constant’, and "Stiff Sand" models.

Basin-wide rock physics trend analysis is important in predicting rock properties at varying depths and in between well locations. Fluid substitution modeling using Biot-Gassman relations was performed to obtain the 100% brine (wet) case. Vertical trend analysis for 100% shale and 100% sand trends for density, porosity, Vp and Vs was carried out for the wet case in the conditioned logs. Both vertical as well as spatial trends have been calculated based on well log correlation of conditioned logs. The rock physics models have been used to understand the sand-shale property contrasts in the Malay Basin. It has been observed that density and porosity contrasts are higher, and hence more important, in comparison to velocity contrasts to improve understanding of seismic amplitudes in the Malay basin. The basin-wide regional rock physics database helps to condition more wells, which in turn enrich the database, and provide a better handle on seismic amplitudes in the basin.

Introduction

The Malay Basin is situated in the southern part of Gulf of Thailand, between Vietnam and Peninsular Malaysia. It is an elongated Northwest-Southeast trending basin (Figure 1). Petroleum exploration in the Malay Basin started in the early 1970s, since then seismic has played a vital role in delineating structures and defining hydrocarbon traps.60 wells out of 2300 wells have been selected as the starting point to build a regional understanding of rock physics modeling in the study area. Deepest logged intervals, deep penetration, reasonable quality, and shallowest log readings were among the main criteria for wells selection.