One of the key uncertainties associated with a subsurface study is structural uncertainty, especially as it relates to resource volume estimation and reservoir connectivity across a structure of interest. Connectivity and volumetrics therefore play a key role in determining the viability of projects as they have impact on hydrocarbon recovery, well count and project economics. Hence, a robust and realistic understanding of the structural configuration of an area of interest in a subsurface study cannot be over emphasised. Seismic interpretation is done to define the structural configuration of an area of interest and a key stage in the seismic interpretation process is the loop picking stage. Loop picking can be particularly challenging when there is minimal well control, poor seismic data or a plague of bifurcating loops (doublets) around the area of interest. The Zed_X1 reservoir (the subject of this paper) is a gas reservoir located in a Partially-Appraised Field (PAF) in the Niger-Delta region of Nigeria. As is typical of PAFs, the Zed_X1 field has limited subsurface data (only two well penetrations in the hydrocarbon accumulation) hence high structural uncertainty away from well control. One of the critical aspects of structural uncertainties in this reservoir is the depth of the saddle which occurs in the middle of the reservoir. On seismic, the area around the saddle occurs as doublets. Any of these doublets will translate into different reservoir volumetrics and thus development scenarios as the structure is generally flat. That is, picking the shallower bifurcating loop will generate a single culmination that can be drained by one drainage point, while picking the deeper one will translate into a deeper saddle that splits the culmination into two unconnected accumulations, hence requiring two drainage points to maximise development. The shallower arm of the doublet and deeper arm of the doublet loop interpretations were carried out and validated by wells that penetrated the structure, particularly the two wells that logged the same Gas Water Contact (GWC) on either side of the saddle. This paper documents the potential impact that Loop Picking uncertainty can have on reservoir development decisions. The full range of resource volumes and connectivity is presented by generating different scenarios in the saddle area. The impact of this on the development of the field and on project economics is also highlighted.
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