De-risking Deep Cluster of Gas Prospects in a Complex Horst Structure: A Near Field – Near Infrastructure Exploration Case Study, Onshore Niger Delta

Abstract Nigeria's "Decade of Gas" initiative is well underway. The Renaissance Joint Venture has sought to maintain a rich exploration portfolio of attractive drill-worthy prospects with material gas volumes sufficient to sustain supply for Nigeria Liquified Natural Gas (NLNG) export and growing domestic gas demands. The Niger Delta remains one of the few prolific hydrocarbon basins in the world with large prospective gas resources. Most of these gas resources are in deep prospects, which are often located beneath existing producing fields. De-risking these prospects will aid the nation's natural gas reserves growth and enhance full utilization of gas as an energy transition fuel. This contribution describes a Near Field Exploration project initiated with the goal of addressing gas supply shortfall for NLNG Trains 1-6 and sustaining the existing gas plant infrastructure near the producing field, ensuring a minimum supply requirement of 200MMscf/day starting from 2027. A focused maturation effort was undertaken to estimate robust volume ranges and decipher the inherent risks and uncertainties of three high-pressure deep prospects, located in the Coastal Swamp depobelt. The prospects are fault-dependent footwall closures within a tilted horst (back-to-back) structure, comprising Serravallian sequences of channel and shoreface reservoirs with shale top seals deposited in the Outer Shelf to Shelf Margin Gross Depositional Environment (GDE). While the prospects are non-amplitude supported, some of the target reservoirs have been tested by analogue wells in a different fault block nearby and found to be gas bearing, confirming the presence of a working petroleum system at depth. Our current subsurface evaluation, using observed field and offset well data, predicts the base case hydrocarbon phase as gas. The key subsurface risks are reservoir presence and quality, while key uncertainties include high formation pressures and hydrocarbon column height. This paper seeks to demonstrate the integrated approach adopted by the project team, to adequately describe the complex subsurface structural styles and de-risk presence of lowstand deltas associated with the cluster of drill-worthy prospects leveraging seismic attributes and calibrated relative acoustic impedance data respectively. We also share the results of our assessment of potential reservoir bypass risk within the back-to-back structure to investigate the likelihood of shelfal reservoirs pre-Serravallian 2.