Abstract

An integrated study of seismic reef signatures and geological development of early Miocene carbonate buildups in the North Sumatra Basin enables a better understanding of seismic reef facies. Analysis of variable seismic signatures on 2D and 3D conventional seismic sections leads to the identification of off-reef, transition zone and on-reef facies which is useful in reef exploration and reservoir delineation. Significant Early Miocene reef seismic signatures include divergent reflections or "doublets", and "wild-trace zone". These phenomena, combined with other conventional seismic reef indicators such as time-structure relief, lateral seismic character and polarity changes, and scattered reflections within the reef body, are useful but not necessarily diagnostic of reefs. These observations have been successfully applied in the South Lho Sukon-D1 gas discovery, where seismic interpretation is difficult.

In this paper a summary is given of an integrated geophysical / geological analysis of some Early Miocene reef examples in the North Sumatra basin. Seismic models are used to explain examples of typical seismic reef expression recognized on several seismic lines which have been tied to some discovery wells. Two types of major seismic reef expression (the Arun reef complex and the South Lho Sukon isolated reef) are categorized. Also, the pitfalls and failure of the integrated seismic interpretation technique, which can occur because of wrong interpretations or in-appropriate application of typical seismic reef expression to exploration are discussed. Nevertheless, identification of the seismic expressions presented here allow us to reduce the risk of interpreting such features as bald basement or thin limestones as reefal bodies.