Low Energy Shallow Marine Depositional Systems of SE Asia: Implications for Reservoir Characterisation
Joseph J. Lambiase
Lambiase Geoscience Pte. Ltd
Many of the reservoir sandstones of SE Asia were deposited in the low energy shallow marine environments that develop in semi-enclosed seas with small tidal ranges. Generally, these environments are described as “microtidal” and are expected to follow standard models for microtidal coastlines with respect to sand body geometry, facies distribution and sedimentary structures. However, ongoing research indicates that the Miocene outcrops and modern low energy systems of northern Borneo exhibit a significant departure from the established sedimentologic and stratigraphic models with respect to facies distribution, sand body geometry, stratigraphic succession, sedimentary structures and permeability distribution on both deltaic and non-deltaic coastlines. Furthermore, the modern systems demonstrate that subtle variations in the relative influence of the wave and tidal regimes (plus the fluvial regime on deltaic coasts) generate strikingly different sets of coastline morphologies and facies distributions. These various morphologies and facies distributions are capable of depositing the range of stratigraphic successions seen in outcrop, although none of them duplicate the standard microtidal coastline model.
The variety and complexity of the modern systems and low energy outcrop successions, many of which are direct analogues for producing reservoirs, indicates that the standard sedimentological and stratigraphic models are far too simplistic to accurately predict subsurface reservoir properties in most cases. (For example, in some very low energy environments it is even difficult to distinguish wave-dominated facies from tide-dominated facies based on a standard interpretation of sedimentary structures.) Indeed, preliminary analysis of a producing field with connectivity problems suggests that most of the problems result from inaccurate predictions of sand body geometry because the predictions were based on a standard facies model.
The need for new models is readily apparent; consequently, the outcrop and modern examples are being studied to derive a set of sedimentologic and stratigraphic models designed to distinguish among the various types of low energy depositional system based on the facies succession, sedimentary structures and faunal assemblages that can be interpreted from standard borehole data. The anticipated result is a significant improvement in the prediction of sand body geometry, connectivity and heterogeneity.
Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery