Depositional Environment of Nampol Formation and its Implications as Potential Source Rocks in the Sumbermanjing Area, South Malang

Abstract

The decline in oil and gas production inevitably forces explorationists to be more creative in finding new potential reserves. Nampol Formation is expected to have the potential to become hydrocarbon source rocks due to their organic matter content. However, the depositional environment of Nampol Formation has not been thoroughly researched. The focus of this research is to reveal the depositional environment of Nampol Formation that may lead to a suitable condition for the deposition of potential source rocks layers. Detailed geological mapping in several selected traverses and micropaleontological analysis was carried out to determine lithological characteristics and depositional environment of Nampol Formation. The microfacies approach was also used to determine the carbonate facies zone of this unit. Source rock samples taken from outcrops on the surface then were analysed to evaluate organic matter content, type of kerogen thermal maturity, and its potential to produce hydrocarbons. The Nampol clastic limestone unit in the study area is composed of clastic limestone with intercalation of black shale, claystone, siltstone, sandstone and calcareous claystone. The microfacies approach shows that the wackestone and mudstone belong to the SMF 15-M (Ooid wackestone) and SMF 23 (Non-fossiliferous mudstone) which can be interpreted belong to Facies Zone (FZ) 8 in the Flügel carbonate facies model (2004). This zone is located in the Restricted interior platform or platform environment with closed water circulation which is situated behind the barrier reef (back reef). Based on the results of geochemical analysis, source rock samples from Nampol Formation can be considered as a gas prone source rock, with type III kerogen, total organic content ranging from 3.48 – 26.18 wt%, po-sess the potential to produce