Publications

Reconstruction of paleogeography is commonly performed based on log motive recognition and biostratigraphic analysis. Although this conventional method may give a satisfactory result in formation scale, its capability to decipher paleogeography in sequence stratigraphic levels is very limited. As evidenced by the biostratigraphic data in South Natuna Sea Block B PSC, West Natuna Basin, the entire Muda Formation had been interpreted as shallow marine, while independent evidence from the 3D seismic unveiled several cycles of base level fall. Moreover, log pattern recognition cannot deliver an unequivocal indication of depositional environment without a proper constraint. This paper provides a case study in the Base of Muda Formation, X-Field, as a showcase on how an integrated approach can effectively solve the problems. The methodology emphasized four fundamental aspects: (1) regional depositional trend, (2) direct environment indicator, (3) log pattern, which corresponds to fluctuation of depositional energy and (4) paleo-morphology. The initial step entailed identification of incised valleys to predict the regional trend of deposition. Furthermore, the positions of direct environment indicators, e.g., coal and limestone, were incorporated to control the interpretation of terrestrial-marine boundary. These two fundamental elements were ultimately established in order to provide a proper constraint in the log pattern recognition. Finally, the methodology also considered the influence of geological structures to the paleo-morphology during deposition. This study found that the Base of Muda Formation was predominantly fluvial with tidal influence in the southeastern region of the AOI, as indicated by channels, coal, and marine fossils. The Northern region of the AOI was mostly shallow marine, in which deltaic system might develop. Furthermore, the underlying reverse faults were responsible for local exposures in the central region. These exposed highs formed a barrier-like topography that could support an environment with restricted circulation and strong tidal current.