A study has been undertaken to use the edited and conditioned log data from 48 exploration wells throughout the Malay Basin. The depth trends of elastic properties (density, ρ, compressional velocity, Vp and shear velocity, Vs) together with the porosity (ø) values of “pure” sand and shale lithologies of Malay Basin have been carried out using this database (Singh and Mohamud, 2008). Their study concluded that, the high values of Vp, Vs and densities along with low porosities exist at the deeper part of the basin. This is the normal trend worldwide. However, other external elements such as tectonic events, which may have some influence on the depth trends of elastic properties, were not considered in their study. The present paper will shed some lights on what the effects of such events may have on the variation of these properties along a stratigraphic horizon that crosses different tectonic zones. Malay Basin is known to have undergone a series of structural events. Major tectonic events which took place early to mid Miocene demarcated the SE portion from the NW portion, causing it (the SE part) to undergo a thermal/tectonic subsidence phase, accompanied by basin inversion (Figure 1). Special attention has been given to the prolific I-horizon which was penetrated by 23 wells out of 48 in the database. This horizon is well represented throughout the basin and has experienced periods of both relatively quiet and intense tectonic stresses. Ultrasonic measurements of core data from the I-Horizon are being collected to compare<br>with the log data to check for consistency. The authors believe that the tectonic evolution and stress level imposed on all the pre-inversion stratigraphic horizons and at the reservoirs level within the I-Horizon may have some influence on the reservoir’s elastic properties. Any marked differences in depth trends from two different tectonic environments along this horizon would cause some variations in the seismic responses. This may also include variations in the AVO trend, which is one of the main predictive tools for Direct Hydrocarbon Indicator (DHI) quantitative interpretation. Acoustic impedance (AI) of sand and shale has been chosen as the fundamental parameter used as the basis in analyzing the impact of tectonic activity on rock elastic properties in the Malay Basin. The true vertical depth (Depth) vs. AI trend was plotted for all the wells by using filtered end member data and the AI behavior was observed. The AI reflects the stiffness of the lithology and normally increases with depth. The comparative plot between these two lithologies for each well is shown in Figure 2. The distribution leads to a rather distinct demarcation between the southeast region marked as Zone A to the northwest and central region, which marked as Zone B respectively (Figure 3). Comparing the combined AI trends plots for all the wells for both zones had showed a contrasting regression line between sand and shale at depth greater than 1500m (Figure 4).


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