A Case Study of Pre-stack Depth Migration Application over a Salt Dome Area
Authors: Zhang Hongjun , Ping Junbiao , Yang Feng , He Qi , Chen Hongtao 
Published: May 2016

There is a variety of salt domes with complex shapes in our work area. As high-velocity geologic units, subsurface salt domes bring about a series of problems for seismic data processing in time domain and interpretation in the seismic exploration. For example, false images of subsalt structure, abnormal stack velocity field, and difficult time-depth conversion of structural mapping, are all caused by complex wave-fields from irregular salt domes.

Pre-stack depth migration is designed for imaging complex structures with strong velocity variations, especially horizontal variation. Complex salt domes are high velocity anomalous zones, which introduce strong velocity variation in horizontal and vertical directions. Consequently, depth migration should be effective in identifying subsalt structural traps. Our applications indicate that pre-stack depth migration produces reasonable seismic images for salt dome flanks surrounding sediments and underlying formations. As an illustration, we present a case study in this paper.

This paper first analyzes the geological settings in our work area; then designs target-oriented multi-step modeling principle to establish the model above salt dome, the model of salt dome body, the model of the flanks and the bottom of salt dome, and the model beneath salt dome, which are constrained by the information of geology, logs and drilling information; next, different pre-stack depth migration methods are applied in this work area data, including Krichhoff pre-stack depth migration, one-way equation pre-stack depth migration and RTM; after that, demonstrates the successful imaging of subsalt carbonate rocks and the true shape of the underlying structures through the 3D pre-stack depth migration; finally, shows some results of pre-stack reservoir prediction. The results have proven that 3D pre-stack depth migration is an effective tool for subsalt oil and gas reservoir identification and evaluation; firstly, high-precision imaging of complex salt bodies, salt dome flanks and subsalt strata has been achieved with pre-stack depth migration; secondly, the structural map from pre-stack depth migration shows reliable geological structures in detail; thirdly, subsalt carbonate reservoir prediction and characterization have been successfully carried out with RAP pre-stack depth migration data; finally, well log constrained pre-stack multi-parameter inversion, such as neutron gamma, natural gamma and flouid factor, is a good approach to predict reservoir.

Introduction

A lot of salt domes with complex shapes are located in the central block of eastern margin of the Pre-Caspian basin. As high-velocity geologic anomalies, subsurface salt domes bring about a series of problems for conventional processing and interpretation in the seismic exploration. For example, pseudo faults and false images of subsalt structure, abnormal stack velocity field, and difficult structural mapping seismic data from time domain to depth domain, are all caused by complex wave-fields from irregular salt domes.

Complex salt domes introduce strong velocity variation in horizontal and vertical directions because they are high velocity anomalous zones. Pre-stack depth migration is designed for imaging complex structures with strong horizontal velocity variation. Consequently, depth migration should be effective in identifying subsalt structural traps. Our applications indicate that pre-stack depth migration produces reasonable seismic images for both salt domes and surrounding sediments and is also big helpful to do subsalt carbonate reservoir prediction and characterization.