SEISMIC EXPRESSION OF ABNORMAL GEO-PRESSURE IN THE BARROW SUB-BASIN

Abstract

Drilling uncertainties related to abnormal geopressure are common in the Barrow and Dampier Sub-basins of the North West Shelf. These uncertainties contribute to increased drilling risk and costs. There have been a number of published studies in this area which have been directed towards understanding the mechanisms and modelling of the expected pressures. These studies, however, have been in general isolated and have concentrated on non-seismic related methods. This paper provides an empirical analysis of the seismic response in an area with known variation of overpressure, and critically is integrated with a comprehensive research effort looking at aspects of overpressure from a laboratory, empirical and theoretical perspective.

The study was conducted using data taken from permit WA-25-P (P25) in the Barrow Sub-basin. These data included 3D surface seismic and VSP, well logs, mud weight and pressure data from the wells. The results of a mineralogical analysis conducted on core samples and basin-wide geological modelling studies were also incorporated into the study. The Muderong Shale, which comprises the principal seal in the area and is believed to be overpressured was selected as the prime target for the analysis. Initially we assess the potential of existing methods, such as velocitybased methods, for remote prediction of excessive pore pressure in the area P25. This is extended to amplitude related effects involving an analysis of reflectivity in the presence of a velocity transition zone over the overpressured interval. Finally the relationship of well data, VSP and surface seismic derived attributes is described.

The available data in the P25 area was sparse and consequently we could not rely on statistical based associations. Current industry methods that rely on a limited number of calibration points suggest that the application of either velocity or AVO based methods may produce unreliable predictions of pore pressure. Ambiguities in inferring overpressure introduced by a variable mineral composition of shales and the presence of a strong velocity gradient, which distorts the wave shape, reduces the reliability of these methods.

A detailed analysis using VSP data acquired in a highly overpressured well was found to be crucial for understanding the response of various seismic attributes to changes in effective stress. This enabled us to propose a new qualitative, but efficient approach for remote prediction of overpressure, particularly suited for underexplored areas such as P25. The applicability of the method, which uses single and combined seismic sequence and trace multi-attributes to predict overpressured zones, is demonstrated with the Venture-Carey 3D data recorded in the Barrow Sub-basin.