Abstract

This paper presents an integrated approach using the 3D seismic and well data to enhance our understanding of the lateral and/or vertical distribution of the Tar Mat.

The study was carried out utilizing a recent stat-of-the-art, high resolution and high quality 3D ocean-bottom seismic dataset (OBC) acquired offshore Abu Dhabi and several wells with an excellent suite of logs, thousands of feets of core data and geochemical studies.

A Model Based Acoustic Impedance Inversion was conducted following the 3D seismic reservoir mapping. A comprehensive porosity prediction analysis and validation were conducted for each well. The observation of the abrupt destruction of porosity in the well data associated with Tar Mat presence in the core led to the idea of computing the porosity derivative cube from the seismically predicted porosity cube. This significant and dramatic change in porosity associated with the Tar presence suggested that this porosity destruction might be visible in the seismically predicted porosity cube.

The derivative of the porosity volume after post-stack Impedance inversion was generated to visualize the rate of changes in porosities. The high negative porosity derivative in a highly porous section may represent the top of a Tar mat. The high positive porosity derivative values also can be used to indicate Tar free developed porosity. Good match was found between the generated porosity derivative volume and the top tar from wells.

Cross-plots between the seismic acoustic impedance and porosity for all wells (including Tar wells) suggest difficulty to distinguish between Tar and lithology change for porosities less than 12.5%.

The lateral Tar distribution was found to be predictable utilizing this approach, through blind test well validation. The seismic Tar mat prediction on the porosity volume has provided new and important interpretation of the top of the Tar in the inter-well region and for the static model. Different Tar prediction schemes from seismic have been evaluated for further refinement. Differentiating tight rocks from the porosity plugged with tar remains ambiguous in the lower reservoir tight rocks. Therefore, a detailed sampling and geochemical analysis of the tar is being performed on the core to determine its base.

Study area

The subject structure is undeveloped and located about 120 km offshore North-West of Abu Dhabi (Figure 1). The undeveloped structure is a North-South elongated anticline, approximately 15 Km by 7 Km in size. The structure relief with 1200ft is considered one of the largest in the area. The structure developed as a result of periodic deep-seated salt plug associated with basement faulting. The field was first recognized in 1955 following the interpretation of the earliest seismic survey acquired in ADMA-OPCO concession in 1954. The first exploratory well-1 was drilled 1969 followed by 8 additional wells, between 1970 and 2007, with the objective to appraise the structure and evaluated the reserves.

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