Basin margin sediment wedge build out of the Eastern Niger Delta: application of shelf-edge trajectory pattern studies

Journal of Petroleum Exploration and Production https://doi.org/10.1007/s13202-021-01100-w ORIGINAL PAPER-EXPLORATION GEOLOGY Basin margin sediment wedge build out of the Eastern Niger Delta: application of shelf‑edge trajectory pattern studies Okwudiri A. Anyiam1 · Nicholas Hoggmascall2 · Daniel K. Amogu2 Received: 15 July 2020 / Accepted: 25 January 2021 © The Author(s) 2021 Abstract The understanding of how basin margin sediment wedge builds out causes shelf-edge migration with time is approached based on shelf-edge trajectory pattern analysis using a high-resolution mega-merge seismic data from the eastern Niger Delta, Nigeria. The study focuses on a seismic dip transect traversing the Greater Ughelli, Central Swamp, Coastal Swamp and the Shallow Offshore Depobelts of the Niger Delta. On the regional dip transects, shelf-edge sediments occur as clinoformbearing wedges at and immediately updip of the shelf-slope break. The shelf edge is deeply buried (> 2–4 s, twt), around the Greater Ughelli and Central Swamps. But with changing structural style, sudden change of ascending shelf edge around the Central Swamp was observed. The huge listric growth fault in the Coastal Swamp; around Bonny area, once again cut the shelf edge into half, rotated it along the listric fault and buried it distally. Several depositional packages show low to moderate ascending shelf-edge trajectory with progradational to aggradational clinoform growth that is characterized by thin sand sheets across most of the shelf and upper slope, though few are also characterized by progradational clinoform growth with thick sand on the shelf, upper-tolower slope and basin floor. The deposition is usually on the Outer Shelf Terrace (OST) which is regressive in a flat and rising trajectory style. This study has demonstrated that accommodation and sediment flux are the dominant controls on how the study basin’s sediment wedge built out, whereby limited accommodation promotes sediments with significant shelf-edge advance and descending trajectories, while increasing accommodation promotes ascending trajectories and increased deposition on the outer shelf. The greater sediments on the Outer Shelf Terrace and the shelf margin than on the slope gives more hydrocarbon prospectivity search around the outer shelf and shelf margin. Keywords Shelf margin · Sediment supply · Shelf edge trajectory Introduction Apart from the deepwater oil wells of the offshore Niger Delta, most oil and gas wells are concentrated within the shallow depths of the shelf. The sediment package is relatively small in the Northern Delta depobelt, but gradually increased basinward as prograding sediments build on the hanging wall of the listric faults. These hanging wall sediments build out get deeply buried as younger faults continue to cut into previous walls until the sediments get to the edge of the shelf. The shelf-margin deep packages form potential * Okwudiri A. Anyiam 1 Department of Geology, University of Nigeria, Nsukka Enugu, Nigeria 2 Shell Petroleum Development Company, PortHarcourt City, Nigeria hydrocarbon plays in the Niger Delta Basin, especially around the Central and Coastal Swamp depobelts. Recently, much discussion has centered on the deeply buried ‘shelf margin’ plays in the delta. In these settings, calibration by well penetrations is rare and much of our understanding of prospectivity relies on seismic imaging below 3 s (twt). The shelf-margin sediments are formed when there is relative sea level fall of the lowstand with corresponding high sediment supply (Mellere et al. 2002). The sediments are mainly terrigenous clastics derived from the eroded exposed shelf/ coastal plain and delivered to the regressive shelf (Burgess et al. 2008). Depending on the sediment flux and gradient of the shelf, these coastal plain facies can move beyond the shelf or near the shelf break (Coleman et al. 1983; Sutter and Berryhill 1985; Coleman and Roberts 1988), from where they can be deposited on the shelf slope and deep marine. It is important to understand the evolution of shelf-edge sediments so as to predict the delivery of sediments beyond the 13 Vol.:(0123456789) Journal of Petroleum Exploration and Production mouth of the delta. On a scarp shelf slope, such sediments bypass the terrestrial topset area and are delivered to the top of the continental slope, from where it can be routed through several conduits down to deep-marine environment (Porebski and Steel 2001). The sediments are deposited as turbidites in the deep marine depositional setting, which are the major target for hydrocarbon exploration (Mayall et al. 1992; Hart et al. 1997). The route through which these sediments migrate from the shelf, through the slope to the deep marine, defines its trajectory (Steel and Olsen 2002; Sydow et al. 2003). It varies in its inclination or slope (HellandHanson and Martinsen 1996), as a result of varying relative sea level change and sediment flux (Steel and Olsen 2002; Hampson and Storms 2003). The present study focuses on a seismic/geological dip transect of the eastern Niger Delta, traversing from the Greater Ughelli, through the Central Swamp and Coastal Swamp to the Shallow Offshore. In such a dip section, the delta progradation can be traced from the Oligocene south to the Pliocene and shows some distinctive features. It is aimed at using shelf-edge trajectory pattern analysis to understand how basin margin sediment wedge build out cause shelfedge migration with inferences on creation of accommodation and sedimentation rate. The study also seeks to describe the trap geometry of such basin margin settings along the line of section, provide evidence of play testing and to comment on the opportunities. Benin Formation was deposited, which is still being deposited today (Fig. 1) The sediment fill in the Niger Delta Basin is characterized by three diachronous lithostratigraphic units that range from the Eocene to Recent (Fig. 2). These three litho units indicate that the basin experienced an overall regression through time as the sediments go from deep sea mudstones and shales to fluvial denser sand sized grains. The Akata Formation is composed of thick shales, turbidite sands with small amounts of silt and clay. The clay content is ductile as it was squeezed into shale diapirs in the basin. The Akata shales are under-compacted, over-pressured and underlay the Agbada Formation and was formed an anoxic conditions. The thickness of the formation is estimated to be up to 7000 m (Doust and Omatsola 1989; Tuttle et al. 1999). The Agbada Formation is of marine paralic facies consisting of alternating sands, silts and shales. It is characterized by coarsening upward-ward grain size and bed thickness successions. This formation is the major hydrocarbon-bearing facies in the basin and it is estimated to be 3700 m thick (Turtle et al. 1999). The majority of the structural traps of the Niger Delta were developed during syn-sedi (...truncated)