Abstract

Investigation of the deeper hydrocarbon potential offshore Sabah, northwestern 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 Stage 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, elevated seismic amplitudes over 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 both reservoir presence and reservoir 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 Horz 5 surface at the base of the basin fill exhibits an edge from positive to negative amplitudes towards the perimeter of the basin (Fig. 4). The trough amplitude has an overall 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 Horz 70 surface shows a similar edge from positive to negative amplitudes toward the basin margin, but is not as sharp as the lower section (Fig. 5). The trough 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 Horz 70 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, 2000, 2003). A hydrocarbon accumulation in the Trona basin will have to rely on a stratigraphic trap 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.

The contemporaneous Titanite basin is another accommodation space on the slope but has a different seismic fill pattern from the Trona basin. From the acoustic amplitude of Horz 10 in the lower part of the basin stratigraphy, it is difficult to discern any depositional patterns with the amplitudes predominately reflecting seismic illumination and residual seismic energy patterns rather than geological features of the basin fill (Fig. 7). The isochron of the lowest interval in the basin however does show a thick in the northern part of the basin that diminishes toward the south suggesting a possible sediment entry location into the basin from the north. It should be noted that the amplitude pattern of Horz 10 does not show any relationship to the isochron again suggesting no correspondence of amplitudes to the basin fill. The acoustic amplitude distribution of Horz 55 in the upper part of the basin stratigraphy again is difficult to establish any depositional patterns. Lineations in the amplitude have a straight, regularly spaced pattern and most likely reflect residual seismic energy patterns from overlying geology. The isochron of this upper interval is more informative showing a thickness pattern that appears to be related to a secondary sediment entry point from the east. There is a weak relationship between the amplitudes and isochron of this interval but no depositional patterns can be defined. At the top of the Titanite stratigraphy an erosional surface has been mapped across the entire basin that most likely is linked to movement of underlying thrust faults (Fig. 9). The erosional surface can be mapped cutting across the complete basin stratigraphy and even into underlying layers giving rise to a huge containment risk for the basin.

Reservoir presence is a greater probability in the Trona basin than in the Titanite basin from the seismic evidence which leads to the Trona basin also having a great probability of containing reservoir quality sediments. Very early in the fill history of the two contemporaneous basins there does appear to be a linkage between them (Fig. 10). As observed in the isochron from the lowest interval in the ponded stratigraphy in the Trona basin, an arm of the basin towards the west indicates a secondary exit point from the basin. This exit point leads directly to a sediment entry point seen in an isochron in the equivalent lower portion of the Titanite basin, indicating spill over from the Trona basin. With the coarsest part of any turbidity flow remaining in the Trona basin, most likely the Titanite basin only received the finer-grained portion (silt and mud) that was higher up in the flows and able to spill over the sill separating the two basins. The lack of any depositional amplitude patterns in the Titanite basin indicating low density contrasts of the deposits from this interval, but still showing a thickness biased toward the sediment entry point, also indicates the probability that predominately only silt and mud were brought into the basin. This connection between the basins was very short lived as all the sediment transport indicators in the Trona basin after this indicate an exit point to the north away from the Titanite basin and therefore ceasing any sediment contribution to it (Fig. 11). The isochron of the Titanite basin from the upper section shows a thickness pattern that suggests a sediment entry point from the west that may be shared with the Trona basin. However, the lack of discernable depositional patterns in the amplitudes of the same interval in the Titanite basin does not lead to the conclusion that it was a robust sediment supply system with coarser-grained clasts, but would have delivered lower reservoir quality sediments. The Trona feeder system appears to be the dominant sediment supply to the area and would have received the coarser-grained, higher reservoir quality sediments.

Sitting in the slope that prograded over the Trona and Titanite basins, the elevated amplitudes of the Realgar prospect sit in a structurally low position, but do appear to be fault bounded (Fig. 3). Strike and dip seismic panels through Realgar show the overlying prograding shelf edge in the dip direction with undulating seismic reflectors indicating numerous channels in the strike direction placing Realgar in a slope environment (Fig. 12). Reflector terminations in the Realgar stratigraphy all point in the dip direction with a downlap configuration (Fig. 13). The terminations show the distal portion of the prograding slope clinoforms and define the advance of the base of slope across the area placing the lower portion of the stratigraphy in Realgar beyond the base of slope with the upper part on the slope. Volume visualization techniques were used to illuminate depositional patterns from the seismic data to define reservoir presence and potential reservoir quality. Opacity was applied to an ~20 ms slab of seismic data that was flattened on interpreted horizons through the Realgar stratigraphy. The volume around Horz10 in the lower part of the Realgar stratigraphy holds amplitudes that show a cone shaped pattern being narrower updip, paleo-transport direction is from southeast to northwest, and spreading out downdip. The amplitude pattern is interpreted to be the result of sediment gravity flows slowing down and spreading out as they go across a gradient change at the base of slope (Fig. 14). The volume around the trough above Horz 40 shows amplitude lineations across the complete flattened area indicating individual channels on the slope (Fig. 15). The base of slope and slope environments at Realgar contains channel and overbank settings with reservoir probably both inside the channel and the overbank setting. Typically, channels will hold higher net to gross, coarser grained, less muddy sands with higher average porosity and permeability while overbank sands will have less net to gross as well as being finer grained with lower permeability and porosity. However, lineations in the amplitudes are defined by both troughs and peaks indicating there are probably both sand and mud filled channels. In the volume opacity images, the elevated amplitudes of the Realgar prospect are clearly seen with strong, straight edges indicating probable fault offset. Following a single amplitude lineation that defines a channel from the southeast of the fault into the Realgar prospect shows in increase amplitude from one side of the fault to the other (Fig. 15). Assuming that a single channel will have similar rock properties along this length, the increase in amplitude seen in the Realgar prospect could be caused by hydrocarbons being trapped by faults and giving a potential direct hydrocarbon indicator (DHI) (Figs. 14 and 15).

For the Trona basin, there is strong evidence for a robust delivery system depositing reservoir quality sediments into a restricted slope basin setting. The Titanite basin, however does not appear to be on the same sediment delivery pathway as Trona with the seismic evidence indicating limited clastic input that is postulated to have been predominately silt and mud. The depositional environment changes from a setting beyond the base of slope at the deepest amplitude level of Realgar to more slope channel settings upwards through the stratigraphy with reservoir probable both inside the channel and in the overbank setting. Hydrocarbon containment in the Trona basin is more probable with stratigraphic pinchout in the ponded section but holds a greater risk of connectivity to feeder channel systems in the healing sections. However, there is no seismic amplitude support for hydrocarbons at Trona. For the Titanite basin, there is a large containment risk due to erosion across the entire basin. Amplitude variation in similar reservoir from outside to inside potential DHI’s at Realgar indicates a viable fault trap. However, the faults are difficult to define in vertical seismic sections, perhaps indicating thin sands in order to allow for juxtaposition of mud across low throw faults.