RESERVOIR POTENTIAL OF AXIAL FLUVIAL DELTA VS ALLUVIAL FAN DELTA IN SYN-RIFT LACUSTRINE: A MODERN STUDY IN LAKE SINGKARAK, SUMATRA

摘要

Paleogene syn-rift lacustrine rocks in western Indonesian basins are viable and prolific petroleum plays. However, due to active tectonics and confined environment, reservoir distribution and geometry of these Paleogene rocks are highly complex. A field study of analogous modern alluvial-fan and axial-fluvial deltas in Lake Singkarak has been performed by investigating data from multiple components of the depositional system to better understand and identify stratigraphic relationships and facies geometries. The results of this study illustrate how an integration of grain texture, faunal analysis, depositional facies, and stratigraphic stacking patterns in the modern environment can characterize the complexity of potential reservoir geometry and quality distribution, both laterally and vertically. Seven depositional facies have been recognized in the Sumpur axial-fluvial delta: fluvial, upper and lower distributary channel, subaqueous distributary channel, mouth bar, shoreline, and abandoned delta. From a sand quality and facies geometry perspective, lower distributary channel, subaqueous distributary channel, and mouth bar facies are associated with the most favourable reservoir potential. The Malalo alluvial-fan delta can be categorized into four depositional facies: upper, middle, lower, and subaqueous fan facies. The highest reservoir quality exists in the lower and subaqueous fan facies. These two deltaic systems exhibit the highest quality reservoir character in the more distal setting, with the axial-fluvial delta being more regionally extensive than the alluvial fan delta. An improved understanding of the heterogeneities within the Sumpur axial-fluvial and Malalo alluvial-fan delta depositional systems contributes new insights into the Singkarak lacustrine system. We propose a new model of the Singkarak axial-fluvial and alluvial-fan delta as a result of this study. This model provides facies architecture, the probability of delta evolution, and an improved reservoir analogue, all of which are valuable as reservoir prediction tools to reduce subsurface uncertainty for exploration and development activities in similar lacustrine petroleum-rich systems.