Application of High-Resolution Biofacies for Better Depositional Environment Interpretation in the Malay Basin Tertiary Sedimentary Successions

Abstract

Interpreting detailed environment of deposition (EOD) as accurate as possible for the delineation of reservoir geometry and continuity is crucial in any field development plan, especially in reservoir geological modeling. Conventionally, a conceptual depositional model is developed from the sedimentological study of cored sections and constrained by wireline logs and seismic signatures. If available, any results from routine biostratigraphic analysis will be incorporated to provide additional constraints for the interpretation of EOD. In the Malay Basin, although biostratigraphic analysis is routinely undertaken on most wells, biofacies and sedimentological interpretations are often in conflict, and thus give contradictory results regarding to environment of deposition and salinities. For example, a peculiar anomaly observed in several cored sections shows that in numerous instances, upsection change to shallower water as suggested from sedimentological criteria coincides with increased in salinities based on microfauna/flora. In another observation, coal layers that were interpreted as brackish water coals from sedimentological study can be shown to be autochthonous freshwater coals when analysed using high-resolution biofacies. Our approach to high-resolution reservoir-scale biofacies analysis will be discussed in detail. For reservoir-scale interpretation using bed-to-bed biofacies analysis, biofacies signals are obtained through a carefully designed high-resolution systematic sampling program. Samples are selected from each lithofacies encountered in cored sections and analysed in a manner sufficient to display the temporal succession of biosignals. For each lithofacies, a minimum of three samples were taken to represent lower, middle and upper units, while lithofacies showing a continuous succession were sampled at regular interval. With respect to coals, samples were taken at a much closer interval as to ensure sufficient representative of biosignals are captured. As a result, the overall sample density may become greatly increased, sometimes with samples analysed at 1cm intervals, but more typically a sample every 50 – 100 cm. Since the results answer fundamental sedimentological questions, the work is easily justified. Using this approach, temporal biofacies successions are better understood, and interpretation of EOD is greatly improved to a higher degree of confidence than is possible using sedimentology alone. Also, shales associated with allocyclic and autocyclic sedimentary controls can be readily differentiated, allowing the sedimentary succession to be placed in a realistic sequence stratigraphic perspective. Examples of selected cored intervals from different stratigraphic units in the Malay Basin are discussed.