A methodology for remote pore pressure prediction using seismic attributes is presented, based on rock physics responses to various overpressuring mechanisms. A series of laboratory acoustic tests were performed on reservoir sandstones and shales, simulating normal compaction, disequilibrium compaction, fluid expansion and tectonic mechanisms of overpressuring. Sandstones showed lower Vp/Vs ratios during fluid expansion overpressuring as compared to both normal and disequilibrium compaction. For shales on a tectonic stress path, velocities and elastic constants all increase with increasing mean effective stress. P- and S-wave anisotropy are initially high and diverge with increasing mean effective stress. Seismic attributes of these laboratory waveforms were derived to verify which attributes were most sensitive to changing pore pressure conditions. Positive correlations between effective stress and several instantaneous seismic attributes were established allowing direct mapping of seismic attribute changes into definite values of effective stress. This methodology has been tested on a 3D seismic dataset from the Northwest shelf of Australia and shows good agreement with both the distribution of normally pressured and overpressured wells as well as the magnitude of the overpressures present.
1. INTRODUCTION
Pre-drill prediction of overpressure is usually achieved through manipulation of seismic data, founded on the empirical relationship between effective stress and seismic velocity [1,2,3]. Velocity-based methods usually require a normal compaction trend and deviations from this normal compaction trend are taken to be indicative of the presence of overpressure. However, velocity-based pore pressure prediction methods are not reliable under all geological conditions. The relationship between stress and seismic P-wave velocity is, in general, non-unique because P-wave velocity is affected by other factors such as lithology and stress history. The reliability of estimated P-wave seismic velocities from surface measurements also decreases with target depth, signal to noise ratio and structural complexity for example. Consequently reliable remote prediction of abnormal pore pressure requires research into alternative approaches. The application of seismic attributes to predict overpressure based on VSP analysis was proposed by [4]. They conducted an analysis on field data and concluded that instantaneous seismic attributes are sensitive to both variations in lithology and pore pressure. Consequently they suggested that sequence attributes may be more relevant for direct detection of overpressured areas, although the methodology they proposed was qualitative in nature. However, our understanding of factors controlling seismic attribute response at the field scale is limited, but this can be enhanced through the derivation of seismic attributes on core samples at ultrasonic frequencies under controlled stress and pore pressure conditions. Previous work has shown that stress path is an important control on ultrasonic response in sandstones [5]. These authors subjected reservoir rocks to pressure conditions simulating normal compaction, disequilibrium compaction and fluid expansion and showed that velocity response and Vp/Vs ratios were stress path dependent. The full elastic tensor and anisotropy parameters for a shale were derived by [6] under low stress conditions and this work is expanded here to higher stress levels and anisotropic stress states.