The Use of Seismic Attributes for Depth Conversion and Property Distributions, E-M Field, Offshore South Africa

Abstract

Abstract The E-M field is a gas reservoir that has been under production for nearly a decade. This paper presents the effort of the team to revise and improve the sub-surface model to delineate new drilling targets. Closure of the field to the east was uncertain, but critical to the field development plan. Inversion of the seismic data created an absolute acoustic impedance cube and a derived effective porosity cube. Attributes were extracted from each of the various seismic data, using direct and interval extractions following the interpreted surfaces. Careful inspection of the seismic amplitude in cross section revealed a flat spot, indicating a potential fluid contact. This feature was confirmed in several of the extracted attributes which were then used to constrain an iterative depth conversion. Fault interpretations in time were then adjusted to match the new depth horizons creating enechelon faults in a fashion analogous to surface outcrops observed in the Cape Town region. Uncertainty also exists with respect to the vertical isolation of the reservoirs. Log responses record a thin shale, below seismic vertical resolution, in all of the drilled wells. However, the areal extent of the shale is unknown, and vertical communication is a possibility. Stochastic representations of the potential shale extents were introduced into the dynamic fluid-flow simulation, and fed through an experimental design to test the impact on vertical connectivity. Using a proxy model from these results a Monte-Carlo simulation provided a probability distribution for the production response. The methods presented here are applicable to fields wherever sharp acoustic impedance contrasts exist between fluid types, particularly in gas reservoirs or reservoirs with an existing gas cap. Seismic attributes have refined the depth conversion for the E-M field, and the resulting geomodel provides new drilling targets for the operator.