Facies modeling of Cenozoic successions in the Gunsan Basin using statistical methods

The Gunsan Basin is located between the eastern Chinese coast and the Korean Peninsula. The basin originated during the Cretaceous due to tectonic activity. After several rifting events, the final rift occurred during the Oligocene. Since the Miocene, the postrift phase has been ongoing to the present day. This study focuses on the interval of the final rifting event of the Cenozoic. The Gunsan Basin remains a frontier basin for hydrocarbon exploration, with only five wells drilled between 1975 and 1991. The study area is located in the eastern sag, known as the East Subbasin, approximately 26 km from the nearest well. Typically, more than 10 wells are needed near a reservoir to predict sand bodies effectively. In this study, we employed geostatistics to generate facies models. The Sequential Indicator Simulation (SIS), one of the stochastic methods, is particularly effective for modeling facies in areas with sparse well data. The pixel-based SIS approach is using trend maps, especially when lateral information is unavailable. These trend maps, derived from the RMS (Root Mean Square) attribute, are based on amplitude and help delineate facies. During the Miocene, the paleoenvironments in the depocenter and margin were lacustrine and littoral, respectively, and the lithology was interpreted as mudstone and sandstone. The transition from littoral to lacustrine environments is attributed to thermal subsidence. Through attribute analysis, we indirectly infer the rift system and the associated facies changes.