ABSTRACT

Prior to BP Exploration's drilling the well Antufash-1 in the Yemeni waters of the Southern Red Sea, reservoir quality was estimated to be poor; it was dry, plugged, and abandoned. The Miocene sandstones encountered were tight, with a mean porosity of 4% in the cored section and a permeability of only 0.07 md.

The prediction of low quality for the reservoir section of Antufash-1 was based on very few core analysis data. The diagenetic history of potential reservoir sands in the Antufash acreage was calculated from data on depth to prospect, burial and thermal history of the area, reservoir sand provenance, and depositional environment.

An initial assessment, using limited local well data, led to the conclusion that only at depths <0.5 km was it reasonable to expect high reservoir quality (>100 md). However, at depths >1.5 km, permeability was likely to be as low as 10 md. Throughout this depth range, the chances of halite cementation were also reasoned to be high. The rapid deterioration of reservoir quality with depth was attributed to the instability of the original volcaniclastic detritus. Such detritus was predicted to have converted to a mixture of zeolites and smectitic clay soon after deposition. The reactivity of the assemblage was also predicted to have been exaggerated by the high thermal gradients in the area.

The recommendation was to avoid large parts of the license area known to have received input of volcaniclastic sediment, and to develop prospects in the few areas thought to have had arkosic sand input. These sands, it was reasoned, would suffer less degradation of reservoir quality. The Antufash-1 well successfully proved the existence of such arkosic sands in the basin, and their diagenetic history was as predicted. Unfortunately, the sandstones were tight. Halite cement filled, as predicted, all remaining porosity.

1 Present address: Monument Oil and Gas plc, London, United Kingdom.

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