Depositional environment and reservoir quality of Miocene sediments in the central part of the Nam Con Son Basin, southern Vietnam shelf

Abstract

Abstract The Nam Con Son Basin is a rift basin that formed offshore southern Vietnam at the time of opening of the East Vietnam Sea (South China Sea). The Cenozoic stratigraphy in the basin is subdivided into four megasequences based on the main tectonic stages of basin evolution: syn-rift 1, inter-rift, syn-rift 2 and post-rift. This study investigates the depositional environment and reservoir quality of the Miocene sediments in the central part of the basin, and thus discusses the later part of the inter-rift, the second syn-rift and the post-rift megasequences. We focus on seismic interpretation, sediment core analysis, biostratigraphy and well-log analysis. The Lower Miocene depositional succession is dominated by stacked, coarsening-upward/progradational packages, possibly related to the interplay between high sediment supply and fluctuating sea-levels. The depositional environment is fluvio-deltaic to paralic. The Middle Miocene depositional succession is characterized by an upward deepening trend which seems related to northeast-southwest rift propagation. During this time, several intra-rift packages demonstrate increasing rift-forming activities that resulted in deposition of footwall-derived turbidite deposits in the central grabens/half grabens and non-deposition/erosion on structural highs. Shallow marine facies dominated in the west while the deep marine settings occurred in most of the eastern part of study area. Carbonate platforms were initiated and widely developed on the structural highs in the southeast and north as a result of overall marine transgression and backstepping of the clastic shoreline toward the west. The upper bounding surface of this succession is a regional unconformity marking the boundary between the syn-rift 2 and the post-rift megasequences. During the Late Miocene, deep marine facies extended across the entire study area with extensive submarine fan deposits and submarine channels downcutting W-E and S-NE, which extended across the shelf and slope. The Miocene sandstones comprise mostly fine to very fine-grained, feldspathic litharenite to lithic arkose. Deep marine turbidite sandstones of the Upper Miocene succession exhibit the best reservoir qualities (both porosity and permeability) of all the sedimentary facies examined. Paralic sandstones of the Lower Miocene succession and footwall-derived turbidite sandstones of the Middle Miocene succession have similar reservoir qualities. Relatively good reservoir quality sandstones of the Middle and Upper Miocene successions were deposited as thick massive turbidites. However, parallel-laminated turbidites and slumps/debris flows often show poor to moderate reservoir qualities. Precipitation of carbonate cement during burial diagenesis has destroyed the reservoir quality of the Miocene sandstones.