The Arang Petroleum Play in The West Natuna Basin: A New Perspective From Geochemical Study and Petroleum System Modelling

Abstract

One of the main producing gas reservoirs in the Medco E&P Natuna Ltd.’s (Medco E&P) operated South Natuna Sea Block B PSC (Block B) is the shallow Miocene Arang Formation (Arang reservoir or Arang). The main characteristics of the Arang gas is the dominance of the dry component. The previous understanding was that the gas was generated from the Arang intra-formational shale source rock. An integrated geochemical and regional 3D petroleum system model has been conducted over the Block B area. This effort suggests that the main contributing source rock to the Arang reservoir is in fact the syn-rift Belut Formation. The study also aimed at locating the potential areas for primary microbial gas accumulation in the Arang reservoir, especially where traps are not connected to thermogenic kitchens. The results of composition analysis from producing Arang gas show variable compositions from dry gas only to gas with small amount of oil. Gas wetness appears to increase with proximity to thermogenic kitchen. The carbon isotope values of the gas fraction in the Arang reservoir suggests mainly a thermogenic gas origin with additional primary microbial gas characterized by much lighter Carbon isotope ratios. This study concludes that the thermogenic Arang gas originate from multiple source rocks with various maturity levels ranging from early to late maturity stage, mixed with some paleo biogenic gas. The Belut source rock has reached an adequate maturity level to generate gas in some sub-basins such as the Bawal, Belanak and Belut Grabens. Despite Arang has reached early to peak maturity oil window in Block B, the low expulsion value disregards the Arang as a key contributor for hydrocarbon generation and expulsion in Block B. The majority of the discoveries and producing gas fields from the Arang reservoir are located above the mature Belut Kitchen. The fact that there are some Arang discoveries away from mature kitchen suggest that lateral migration also occurred within the Arang.. Complex vertical and lateral migration-remigration process within the Arang is a result of interplay between continuous structural growth and possible seal integrity change during the syn-inversion period.