(1) ConocoPhillips, Houston, TX
ABSTRACT: 3D Seismic Pore Pressure Analysis in Deep Water Fold and Thrust Belts: A Map and Volume Based Flow Balance Approach
Traditionally, pre-drill seismic pore pressure analysis has been a one-dimensional exercise. Even most 3D analyses are actually dense arrays of 1D calculations. To calibrate velocity to pore pressure, vertical profiles are commonly calculated to match pressures measured in offset wells. In shale-dominated basins, seismic velocity primarily indicates shale properties while pressures are measured in permeable sands. This duality of data types presents no problem when pressures in sands match those in adjacent or enveloping shales. However, in basins with high structural relief, differences between fluid pressures in sands and fluid pressures in shales due to flow focusing (hyper-pressure, centroid, and lateral transfer) can lead to errors in calibration and calculated pore pressures.
This paper proposes a revised map and volume based approach to: (1) calibrate 3D seismic pore pressures to measurements in high relief structures; and (2) calculate pore pressures in sandstones. The method is based both on Darcy flow and on balancing fluid exchange between sands and shales. An example from South East Asia illustrates how the flow balance method can provide solutions where traditional seismic methods fail to match observed data. However, results are sensitive to parameters that may be poorly constrained such as sand extent and shale permeability. Parameter evaluation will result in a range of possible pore pressure calculations to be weighed in light of available data and a geologic model. Despite inherent uncertainties, the flow balance method is a step towards properly accounting for flow focusing in deepwater fold and thrust belts.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.