Abstract

This paper is a case study of the S structure and is drawn from an integrated study involving interpretation of a 3D survey acquired in 1992. The S structure, located offshore Abu Dhabi, is a relatively low relief feature discovered in 1970. To date nine wells have been drilled on the structure. The major oil reservoirs are in the Thamama (Lower Cretaceous) and Arab (Upper Jurassic) litho-stratigraphic units. The prime reason for acquisition of the 3D seismic was to better quantify the structural uncertainties. The results of the study have been used to plan future drilling location and estimate their likely impact on uncertainty reduction.

The low relief of this structure makes depth conversion an important issue. Layer cake and single layer depth conversions were evaluated. The velocity models for the overburden were determined using a combination of well data and stacking velocities. The uncertainty in velocity determination has been estimated in order to allow the generation of minimum and maximum structural cases.

Paleo-structure maps were generated in order to study the evolution of the S structure. The paleo-reconstructions were derived from a combination of the 3D seismic, 2D seismic and the available well control in the sub-regional area. A detailed geological layering scheme derived from core analysis, thin sections and log correlation was integrated with the structural interpretation via geocellular modeling software.

The integrated model was used for sensitivity analysis, volumetrics and visualizing the reservoir description.

Introduction

The principle objective of this study was to construct reliable maps of the structure for the major reservoir intervals and integration of the structural mapping with the detailed reservoir layering model.

Prior to the 3D survey the structural mapping was based largely on the well data and a sparse grid of 2D lines. The well data did not indicate any faulting however the 2D seismic data was open to interpretation with respect to faulting in some of the undrilled area. Some small throw faults were postulated but their orientation and magnitude was unclear.

The low relief of the "S" structure means that the volumetrics are sensitive to the depth conversion. A relatively small change in the velocity field can lead to large changes in the area at any given structure contour. It was important to quantify this uncertainty and investigate how lateral and vertical variations in the facies impacted the velocity field.

The "S" structure is separated from the nearby "F" structure by a saddle area. The absence of well control in the saddle area leads to some ambiguity regarding the maximum closing contour and possible oil water contacts.

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