The Role of Sedimentology in Accurately Characterising Newly Discovered Reservoirs

Early confirmation of depositional environment is essential if a new hydrocarbon play is to be properly understood and developed, as it is a major factor influencing the morphology, trend and continuity of a reservoir. Accurately predicting the reservoir morphology, trend and continuity limits the risk of drilling dry holes and leads to superior estimates of reservoir gross rock volume – probably the single most important factor when calculating hydrocarbon reserves. On the larger scale, regional seismic information is used to paint a broad picture of the reservoir, with more detail added as wireline log information comes to hand. The inferences drawn from these two methods of investigation should then be confirmed or refuted as quickly as possible by detailed sedimentological examination of the actual reservoir rock. This is best achieved by visual description of conventional core. The importance of sedimentological core description is highlighted in this paper by reviewing two case studies. The first study, completed in 2003, was conducted following the unexpected drilling of a dry hole. Sedimentological description of available core material led to a complete re-interpretation of the depositional environment, and consequent reassessment of reservoir trend and morphology. In the second study, completed in 2001, detailed core descriptions also caused a previously conceived, pessimistic geological model to be dismissed. The new model re-defined reservoir continuity and morphology, thereby adding substantially to estimated reserves.