Residual Hydrocarbons – A Trap for the Unwary

Abstract Residual hydrocarbons occur in a range of habitats and can take various forms: they may be oil or gas, they may be present within high-quality or poor reservoirs; and may appear as a single palaeo-column or as thin columns interspersed with beds of producible hydrocarbons. Residual hydrocarbons are sometimes easy to identify but when not recognised they may lead to optimistic reserves estimates. This paper illustrates the range of habitats of residual hydrocarbons and provides techniques to enable their identification, such as novel log cross-plots and log overlays. In most cases the non-productive nature of the residual hydrocarbon can be confirmed using flow tests, with the possible exception of very low permeability reservoirs containing residual gas. The results of not recognising residual hydrocarbons include (a) overbooking of reserves which may have serious commercial consequences, (b) the application of incorrect saturation-height functions, (c) pessimism in calculated formation-water resistivities estimated from water legs containing residual hydrocarbons, (d) lower effective permeabilities in water legs which may affect waterflood design, and (e) excessive expenditure on wireline formation testing and drill stem testing. From an exploration perspective, residual hydrocarbons provide direct evidence of migrated hydrocarbons and/or breached oil fields and hydrocarbon loss due to regional post-migration tilt. Thus they provide important clues to the location of additional traps along the migration pathways. We present examples of residual hydrocarbons from the Indian subcontinent and elsewhere. The subcontinent is particularly prone to the presence of residual hydrocarbons as many basins have undergone polyphase deformation and multiple periods of secondary migration and fill. By raising the awareness of residual hydrocarbons, we hope that this paper leads to greater accuracy in reserves estimates, and perhaps even the discovery of additional hydrocarbons.