Transgressive-Regressive Cycles in the Malay Basin: New Insights

Abstract The Malay Basin is a prolific hydrocarbon producing basin located off the east coast of the Malay Peninsula (Fig. 1). It started as an extensional / rift basin during the Oligocene, followed by thermal subsidence in the Early Miocene. Compression started sometime in the Middle Miocene which resulted in localized inversion. Maximum basin inversion was recorded during the late Middle Miocene The stratigraphic scheme generally used for the Malay Basin is shown in Fig. 2. Understanding transgressive-regressive cycles, and their controlling factors, have important influence on Malay Basin sedimentation, and hence directly impacts exploration efforts in the basin. Seismic data and biostratigraphic analysis carried out since the 1980's showed evidence of periodic marine transgression as early as Late Oligocene (Group L). Although several marine transgression events were recorded (Fig. 3), the biostratigraphic data indicates that during much of Miocene the basin did not become fully marine until the Pliocene. The limited occurrence of a fully marine faunal assemblage indicates a restricted marine condition. In many instances the presence of common to abundant freshwater algae further indicates intermittent development of 'lacustrine' conditions within the Miocene (Fig. 4). In the Malay Basin the transgressive-regressive cycles are believed to be the result of interplay of tectonic and sea level fluctuations (Mazlan, 2011), particularly during the Miocene. Evidence of tectonic deformation includes the presence of basement highs in the southern end of the basin (Fig. 5), and these would have exerted a significant impact on the depositional environment, and consequently the sedimentation patterns. This basement high is thought to behave as a sill which prevents the Malay Basin from being fully connected to the sea. The long term uplift of the basement high in the south would have been superimposed with fluctuation of sea level (Mazlan, 2011; Fig. 6). There would be instances where sea level rises exceeded the rate of tectonic uplift. During such time the sill or 'gate' would have opened up and caused marine flooding into the basin. At other times, the basin would be disconnected from the sea and lacustrine conditions would be established.