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A new Source-to-Sink approach to petroleum systems and prospectivity evaluation in
deep-water: case study from Sabah
Xavier Legrand (XTG), Max A. Meju (XAGP), Mei Lu Lee (XTG), Hafiz Nordin & Hamzah
Yunus (XM/Sabah).
Introduction
Since the 2000s, there has been an increasing study of the interplay between tectonics, erosionsedimentation and climate, and the implications for hydrocarbon prospectivity. Recent works,
supported by numerical and analogical modelling have shown the importance of the
deformation rate and the rainfall rate on the drainage network evolution and sedimentation
patterns (Viaplana et al., 2014). Unlike previously thought, it is emerging that in appropriate
physiographic contexts, the derived organic matter re-deposited close to and/or far from the
source (onshore and offshore) by gravity flows represents an effective source rock able to
generate large amounts of hydrocarbons. An example is the occurrence of the richest organic
facies associated with sandy interval (and not deep marine shale) in offshore NW Borneo. In
this petroleum system the maturity conditions (gas and oil window) are mainly driven by the
subsidence rate (sedimentation velocity vs burial) and the associated geothermic gradient.
Upstream, the sedimentation velocity depends on tectonic activity and erosion velocity in the
mountain. Downstream, the sedimentary accumulation is mainly controlled by isostacy, the
lithospheric elastic resistance and the eustatic variation of sea level in the basin (Fig. 1). Hence,
not only onshore foreland basins but also shelf and deep offshore areas are the seat of
siliciclastic sedimentary accumulations. Understanding the timing and controls on localization
of major fields in such environments is important for successful exploration offshore but
requires the correct evaluation of the contributions from sedimentation due to regional plate
tectonics and those from local gravity tectonics.
Figure 1: Schematic of interactive surface and deep processes from mountain belts up to
sedimentary basins (source-to-sink) as in NW Borneo. The relief, in response to crustal
thickening or exhumation, induces an increasing of the erosional activity that provides both
siliciclastic sediments and source rocks which contribute to the development of hydrocarbon
bearing reservoirs. In the most cases, the sedimentary infilling is a cyclic succession of marine
and continental supplies.
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Here, we combine insights from detailed 3D seismic reflection, experimental modelling,
structural restoration, CSEM imaging and magnetic mapping to improve our understanding of
the influence of basement heterogeneity on the partitioning of deposition and deformation in
our study area in offshore Sabah.
Beyond the direct applications in the NW Borneo margin, this new approach can be readily
applied to other continental margins.
Methodological Details and Improved Source-to-Sink Workflow
Elevated relief, in response to crustal thickening or exhumation, induces increased erosional
activity that provides both siliciclastic sediments and source rocks. Recent experimental work
show how tectonics, climate and intrinsic dynamics of drainage combine to modulate sediment
flux (Viaplana et al., 2014). These laboratory results have been independently confirmed in NW
Borneo by 3D seismic attribute computation (Fig. 2) and by a geomechanics-based solution of
2D seismic interpretation (Fig 3). The effect of downstream sedimentation on the upstream
erosion is to inhibit part of the erosion by decreasing the potential energy for building incised
valleys. Thus, a steady topography can be perturbed by a local aggradation of the sedimentation
around uplifted blocks (fault-related fold; Figs. 2 & 3).
Figure 2: Detailed map of the seafloor in offshore Sabah from 3D seismic-reflection survey.
Ponded turbidites, slope fans, debris flow and fault-related folds can also be imaged by CSEM.
Debris flow corresponds to a sudden erosion of the upstream part of the erosion/sedimentation
system upstream in relation to the regional tectonic and the geodynamic context.
In the conventional approach, correctly estimating such features requires a 3D geometrical
determination of the deposit taking into account outcrop, seismic and well data to estimate the
eroded rock volume in relation with the variations of the sedimentation velocity (e.g., Babault
et al., 2012; Giletycz et al., 2015). Also radiogenic absolute dating is required to date the
variations of erosion speed. If the sedimentation area and the erosion area are close, the transfer
time between source area and deposit area can be considered as negligible. Then a direct
correlation between the variations of erosion speed and sedimentation speed is reasonable, as
well as an evaluation of the burial rate. These require good-quality seismic data. In the presence
of poor-quality seismic data at depth, these determinations become highly uncertain. Absolute
timing of events also becomes uncertain. To overcome such problems, the experimental
approach and potential methods are necessary. We use an experimental approach (Viaplana et
al, 2014) and incorporate magnetic attributes and CSEM imaging results in our workflow which
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resulted in a more integrated approach. The magnetic data would enable us determine whether
the slope has significant inherited structures that may have influenced the sediment flux in the
feedback between erosion, transfer and sedimentation. CSEM imaging permits depth mapping
of marker horizons and turbidites especially in stratigraphic traps.
Figure 3: Conceptual illustration of tectonics-sedimentation interaction using a 2D
geomechanics-based solution of seismic structural interpretation for a regional transect in
offshore Sabah.
To use CSEM as a complimentary tool in our source-to-sink workflow, it must provide
diagnostic geological information when the seismic data are limited. Specifically, it should
provide reliable correlation across the study area. It should also be capable of distinguishing
between regional tectonic and gravity deformation signatures. As shown in Figure 4a, there is
a dominant approximately N-S basement structural trend in the study area according to vintage
1965 Shell SB aeromagnetic contour map of the region (Grant, 2004). This trend is the regional
tectonic trend. The strike of the overlying sediments imaged by CSEM and ascribed to gravity
deformation in the structural interpretation of Figure 4b is different. A CSEM depth-section (03500 mbsl) along this gravity deformation strike is shown at the same horizontal scale for
comparison (Meju and Chuan, 2013). Two resistive marker sequences (A and B in Fig 4b) are
identified. The resistive sequences occur at 900mbs and 2700mbs in Wakid locality. Note the
remarkable along-strike continuity of these key resistive (sandy?) sequences deduced by CSEM
imaging, necessary for correlating 2D geomechanics-based solution of seismic structural
interpretation of dip lines in this area. The top resistive marker B is related to gravity tectonics
with NW-directed transport (typified by the NE-SW trending Kinarut ridge), while the deep
resistive marker A is related to an earlier SW-directed transport and deformation activity
(typified by the NW-SE trending Kebabangan-Kamunsu fault-propagation fold).The continuity
of sequence A is disrupted at locations of major fault-related fold by a mechanism of
segmentation (Fig. 2)
Figure 4: (a) Basement structural grain from 1965 Shell SB aeromagnetic contour map, scale
1:500,000 (Grant, 2004); red rectangle shows an area where 3D CSEM data are available. (b)
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Structural trend of basement-cover rocks. Base map is a structure map of SB4. NW-SE section
is a 2D geomechanics-based reconstruction of structural interpretation in depth. Strike-line
CSEM depth-section (0-3500 mbsl) for the cover rocks is embedded in (a) and (b) and shows
along-strike continuity of key formations needed for correlating 2D structural interpretation of
dip lines.
Also, the resistive marker units clearly suggest the presence of a structural high along a
dominant E-W axis linking Medang-Malikai and Wakid-Kinarut where the marker sequence-2
is more resistive (Fig. 4b). Importantly, the CSEM result can be used to map the lateral limits
of significant reservoir prone zones in our source-to-sink approach. In combination with
magnetic data, deep-seated structural features related to regional tectonics and shallow
structures related to gravity driven deformation were distinguished clearly.
Conclusion
The interaction between deformation and erosion in mountain belts on one hand, and transport
and sedimentation in basin areas on the other hand, can be used as prognosis tools in petroleum
exploration. 3D seismic provides detailed images of the seafloor permitting the interpretation
of sediment fluxes, based on experimental modelling of drainage networks. Geomechanicsbased restoration was used to decipher the structural features at depth interpreted from seismic
lines and reduced the uncertainty on the deformation timing. Where seismic data is of poor
quality at depth (e.g. due to widespread presence of shallow gas) and where a high rate of fault
segmentation occurs, along-strike correlation is difficult, necessitating the incorporation of
analogue models, magnetics and CSEM in our workflow. We have successfully demonstrated
that we can distinguish regional tectonic deformations and gravity-driven deformations using
this integrated approach. Also the petroleum system which is a component of the drainage
network, responds as a mirror effect of the coupling between Earth’s surface processes and
deep crustal deformation. This new approach illustrated in NW offshore Borneo can be applied
in hydrocarbon exploration in other continental margins.
Acknowledgements
We thank PETRONAS for permission to present these results.
References
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APGCE 2015
Kuala Lumpur, Malaysia, 12 - 13 October 2015
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