Abstract

The Arun Reef is a large gas/condensate bearing shelf reef complex developed over a north-south trending faulted pre-Tertiary basement high. Other known reefs found in the area suggest the Arun is only one part of a greater reef tract that may be categorized as a fringing reef trend.

The Arun complex does not appear to be a single reef type that can be divided in the simplistic manner of reef core, fore-reef and back-reef facies. From cores, however, six main types of carbonates have been recognized which compare favorably with the number of facies established for some recent and fossil reefs reported in the literature.

The entire reef complex has been completely recrystallized and diagenetically altered to the extent that little if any of the original rock texture can be recognized. One of the most important effects of diagenesis was the removal of faunal debris with the resultant development and localization of porosity with moldic and vugular porosity being the dominant types. Dolomitization at the very base of the reef and microfracturing on an extensive scale have also occurred.

The restricted nature of the faunal assemblage and absence of planktonic foraminifera make precise age determination difficult. The mutual ranges of Lepidocyclina c.f. canelli and Miogypsina thecidaeforma, however, indicate an age of Early to Middle Miocene (Upper Te to Lower Tf).

Seismic modelling was carried out to resolve uncertainties regarding the nature of the eastern flank of the reef and to determine the lateral extent of reef limestone. The results of the models pointed out several important characteristics of the seismic events associated with the reef edge, but also emphasized some of the pitfalls inherent in the interpretation of a reef from seismic data alone.