(1) Scott Pickford Group Ltd. (A Core Laboratories Company), Croydon, England
ABSTRACT: Integrated Science to Predict Overpressure in a New Deep Water Frontier Area
The industry trend is to explore and develop deeper water regions - Gulf of Mexico, offshore Brazil, Northwest Europe, Southeast Asia, North and West Africa. All the pressure-related curves are close together because overlying heavy sediments are replaced by water and sediment loading can be fast. Frontier areas lack well data particularly in the top-hole section where shallow water flows occur.
This paper presents a methodology that uses changes in seismic velocities to infer the formation pore pressure. Velocities increase during compaction because porosity reduces and grain-to-grain contact increases. During rapid burial of low permeability sediments, fluid escape may not be in equilibrium. The pore fluid begins to support the overburden and becomes overpressured. In these zones the seismic velocities are lower because the porosities are higher.
Case studies illustrate how 3D assessment of the structural styles and depositional systems enhances interpretation. Using co-kriging, an initial velocity model is constructed from seismic stacking velocities and sonic logs. The optimum seismic image in depth is obtained by several iterations of pre-stack depth migration using tomography. By creating litho-facies associations a model is built up by analysis of sedimentary processes and use of sequence stratigraphy. Well petrophysical properties and global analogue rock properties data aid synthetic seismic modelling. These are used to generate and analyse seismic attributes. Pre- and post-stack acoustic impedance inversion, amplitude variation with offset and rock properties from seismic (lambda mu-rho) will help isolate lithology and pore fluid. Velocity is transformed to pore pressure as a function of effective stress.
Emphasis is given to a focused cross-disciplinary team to unlock the valuable information held within the available data. This allows a rapid and accurate assessment of abnormal pressures that can be incorporated into bidding, prospectivity, drilling and field development strategies, thus reducing risk.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado