Ancient Mangroves: Swamped with Hydrocarbon Potential
Fossil mangrove deposits have received relatively little attention in the published literature, despite their obvious potential as hydrocarbon-prone source rocks. Recent fieldwork examining both ancient and modern deposits suggests that mangroves may also host significant oil deposits. Studies on Miocene coastal clastic deposits in the Sandakan region of eastern Borneo have identified several well preserved mangrove systems. Detailed sedimentological analysis has allowed the development of new criteria enabling the recognition of these facies in other locations. These criteria include the sedimentological characteristics of both channelised and sheet-like sandstones, as well as macro- and micro-palaeontological observations.
Several morphologically distinct sandbody classes within these successions have confidently been assigned to channel and inter-mangrove broads, based on analogues from both the Everglades of Florida and modern Bornean mangroves. The channelised sandstones have extremely steep banks and appear to accrete vertically, thought to be due to the constraining mangrove root systems. The sheet-like sandstones are massive with rare rooted intervals.
Unpublished data from Brunei suggests that mangrove mudstones may provide a significant source of oil, sourced mainly from the waxy cuticles of mangal species. Using the analogue data to give an indication of the lateral extent of the fossil mangrove sandbodies, plays can be developed invoking stratigraphic traps sourced from the associated mudstone deposits. Additional plays have been erected for carbonate mangrove systems which, while sharing some of the features of clastic mangrove systems, have additional mangal indicators such as preserved rooted systems, fenestra and an absence of large channels. These features relate to their development in arid climates.