Subsalt and Base-of-Salt Reflector Relationships and Hydrocarbon Trap Implications from a Large 3D, Depth Migrated, Seismic Volume
Authors: Jamieson George A. 
Published: October 2008

This paper discuss improvements in seismic imaging, Particulars in the sub salt region, which were identified from A large 3D post stack depth migrated seismic survey in the Vermilion region of offshore Louisiana. These imaging Improvements have increased the interpretability of the Sub salt seismic data.

A background velocity model was developed using Dix Converted smoothed stacking velocities which were Calibrated to wells. Rapid velocity variations caused by salt Were accounted for by a number of migration iterations. Compared to conventional time data, this procedure more Clearly delineated and correctly positioned both the top and Base salt reflectors and, in many areas, improved sub salt Reflector imaging. This imaging improvement has thus Raised confidence levels in both sub salt mapping and play Development in this region. Several sub salt plays are Outlined which could not be invoked with sufficient Confidence from conventional time migrated data.

INTRODUCTION

An extensive salt basin was deposited in the Gulf of Mexico during mid-Jurassic time. Large amounts of this salt have since been mobilized and have moved out of their original depositional setting and now reside in younger section1. The South Addtiions region of offshore Louisiana is one area where salt occurs largely as vast allochthonous (detached) sheets. An outline of salt distribution in the South Additions region of offshore Louisiana is shown in Figure 1. These complex salt bodies pose major problems when trying to image beneath them due to the much higher velocity of salt compared to the surrounding sediments. The need for better salt and subsalt seismic imaging has been spur Tedon by the relatively new subsalt play, recent successes of which have attracted a great deal of attention in the oil industry.

Remarkable advances in both seismic and computingtechnology have aflowed the use of sophisticated processing algorithms on extremely large data volumes, providing powerful tools with which to tackle salt-related seismic imaging problems. One of the resulting tools is 3D seismic post stack depth migration. A 3D volume, extending over more than 60 OCS Blocksin the Vermilionregion, has had a post stack depth migration applied and forms the data base for this paper.

SALT SHEET FORMATION

Halokinesisin the Gulf of Mexico is an important control on sedimentation, facies distribution and, ultimately, hydrocarbon accumulation2,3. Allochthonous salt is wide spread across the South Additions region of the offshore Louisiana shelf (Figure 1). The formation of these sheets was triggered by high sedimentation rates, particularly during Miocene time. This first cause Significant salt diapirism which was then followed by detachment of large salt bodies from the autochthonous Salt. These salt masses then moved laterally down slope, generally residing in successively younger geologic section.

The complex multiphase salt movement which has occurred in the Gulf of Mexico is summarized in Figure 2. This salt Movement can be tracked and, to some extent, explained in terms of salt evacuation surfaces or welds. On seismic Data these are often manifested as thin, high amplitude events between salt masses, unconformities, fault planes or Combinations of these. Welds can be subdivided into Primary, secondary or tertiary types3. Primary welds aessociated with evacuation surfaces related to the Movement of salt away from its original depositional Position.