Evaluating the Stratigraphy, Reservoir Quality and Seal Potential of a Paleo-Stepped Slope Setting - Offshore NW Borneo
Authors: Scott Erik , Abraham Wayne 
Published: May 2016

Extended Abstract
Investigation of the deeper hydrocarbon prospectivity offshore Sabah, northwest Borneo (Fig. 1) has revealed two basins along a stepped slope profile followed by a prograding slope that has the potential to hold reservoir quality deposits in the middle Miocene aged Stage 4B and lower 4C stratigraphy (Fig. 2). The Trona and Titanite basins are separate accommodation spaces created by a complex structural history involving the evolution of a fold thrust belt (Fig. 3). Seismic characteristics of full stack time data from both basins show continuous reflector terminations in an onlap configuration with substantial thinning towards the basin margins predominately in the lower stratigraphy that transitions to include semi-continuous seismic reflectors with limited thinning in the upper stratigraphy. The Realgar prospect, which consists of elevated seismic amplitudes over the background bounded by apparent faults, sits above the Trona basin in an interpreted slope setting (Fig. 3). Detailed analysis of the fill of the basins, the sedimentological relationship between the basins and characterization of the subsequent overriding slope allowed for the reconstruction of the depositional history to assess reservoir presence, reservoir quality and seal quality. Mapping of almost every trough-peak pair in the basins and through the slope, followed by horizon-based seismic attribute extractions and volume visualization provide the needed understanding to reduce the uncertainty of the reservoir risk as well as the seal potential of any potential hydrocarbon columns.
Seismic amplitude extractions on surfaces in the Trona basin provide evidence for sediment transport and deposition throughout the fill of the basin. The amplitude of the Horz5 surface at the base of the basin fill exhibits an edge of positive amplitudes towards the perimeter of the basin (Fig. 4). The trough amplitude has an overall map pattern of being narrower in the southeast and expanding wider into two arms toward the northwest portion of the basin. Lineations in the amplitudes from southeast to northwest can also be observed indicating potential axes of flow in a distributary complex and/or weakly confined channel system. These geometries, coupled with the shelf and shoreline lying towards the southeast, indicates that sediment entry into the basin was from the southeast and exited to the north or west. The isochron of the lower part of the basin fill that exhibits seismic reflector onlap shows a basin centered thick indicating ponding of the sediment gravity flows entering the basin (Fig. 4). The basin thick is elongated in the north-south direction mirroring the dominant basin configuration and flow path from basin entry to basin exit during this ponding stage of the basin fill. A minor arm of the basin extending to the west, which can be seen in the Horz 5 surface amplitude and the lower stratigraphy isochron (Fig. 4), was occupied quickly at the beginning of the basin fill. Most likely, only the finest-grained, upper portions of any turbidity flow moving through the basin was able to exit to the north, with the exception of possibly early on in the history of the basin fill some flows exiting through the minor arm of the basin towards the west, while the bulk of the sediments in the flows remained in the basin. In the upper portion of the basin fill, the amplitude of the Horz70 surface shows a similar edge of positive amplitudes toward the basin margin, but is not as sharp as the lower section (Fig. 5). The amplitude pattern is overall wider than the lower section with distinct branches in the updip direction, a strong elongation to the north, as well as internal lineations. The isochron of the upper section shows a thickness shift updip toward the basin entry in the south. This shift in thickness indicates that the basin has transitioned from ponded deposition filling the available accommodation space to deposition that is healing the slope to move it to the regional gradient. The upper section isochron also shows distinct thickness patterns in the deposition center that match the branches seen in the Horz70 surface amplitude. These branches are interpreted to be channels that were established at the end of the basin fill linking up the sediment entry point of the basin to the exit point in the north that allowed for sediment gravity flows to bypass the basin and transport sediment further down the slope showing that the slope has returned to equilibrium. Shallow seismic analogs for the filling in of accommodation space on a stepped-slope profile with the sequence of ponded deposits transitioning to healing deposits have been documented in offshore west Africa and the Gulf of Mexico (Prather, 2003; Prather, 2000). A hydrocarbon accumulation in the Trona basin will have to rely on a stratigraphic trap of sands pinching out into a mudstone in the updip sediment entry area as this is the structurally highest point today. The potential for a stratigraphic trap is different between the ponded and healing stratigraphy (Fig. 6). The lower ponded section has significant updip thinning with virtually all seismic reflectors onlapping onto the basin margin towards the sediment entry point indicating deposit pinchouts and the high chance for a stratigraphic trapping configuration. The upper healing stratigraphy has much less updip thinning with almost no seismic reflectors onlaping onto the ponded sediments or the basin margin towards the sediment entry point. The upper stratigraphy also holds isolated extra seismic reflectors close to the sediment entry point that corresponds to the interpreted bypass channels seen in the amplitudes and isochron that may provide a connection to the updip sediment feeder system and limit any potential stratigraphic trap. Expected elevated amplitudes if hydrocarbons are present at this depth below mudline are not observed at this location, adding an additional risk to the viability of the trap.