Drilling into a Charge Shadow Zone of a Prolific Niger Delta Field: Learnings from Well-X

Abstract

Three decades on, a brownfield still springs surprises in the Deepwater Niger Delta. An Exploration well, Well-X was recently drilled targeting an amplitude-supported opportunity within a prolific geological interval in a Deepwater field. The well discovered excellent reservoir of 78 feet thick sand with 100% net-to-gross and 32% porosity, but turned out wet, with no evidence of hydrocarbon. The post well evaluation indicate that the excellent reservoir properties contributed to the false-positive amplitude anomaly that is typically attributed to a direct-hydrocarbon indicator in prospective reservoirs. A post-mortem was conducted to understand which element of the petroleum system failed resulting in the wet well outcome. Access to hydrocarbon charge and trap integrity were re-assessed as two possible geological failure mechanisms. Results of the investigation and preponderance of available evidence indicated that lack of charge access was the most likely reason for failure as there was no trace of thermogenic hydrocarbons in the entire stratigraphic interval drilled. The post-mortem also revealed that deep-seated faults extending into the charge kitchen act as major hydrocarbon migration conduits into target reservoirs within the field. Unfortunately, Well-X target reservoir was not connected to any of such faults and is located in what is now considered a ‘Charge-shadow’ zone of the field. A Fluid Inclusion Study (FIS) required to validate the charge theory is currently being embarked upon. A key learning from this well outcome is that access to charge is not ubiquitous, even in prolific fields or basins. A good understanding and clear definition of plausible migration pathways are critical to exploration success.