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Significant oil and gas discoveries found at the southern hub of offshore deepwater Sabah proved the existence of a working petroleum system in the area. DWR, which is located on trend with the Gumusut/Kakap and Limbayong oil and gas discoveries, remains under-explored with a number of attractive prospects to be tested in the block. To further eval...
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... Trinity 2.5D model predicted that all prospects in the study area are likely to be oil-rich in the deeper Kinarut reservoirs, while gas-condensate are more likely to be present in shallower Pink and Kamunsu shallower levels. At present-day, the "Pseudo-Brown" source rock interval is predicted to be oil to gas matured (%VRo 0.7 for oil and %VRo 0.9 for gas), and thus capable of expelling hydrocarbons with predominantly gas-rich components ( Figure 6). While predominantly gas-rich, expelled fluids from the "Pseudo-Brown" source rock tend to retain the oil-rich component in larger relief structures in deeper reservoir intervals, while the gas-rich component is able to leak-up and enter the overburden by slow leakage over geological time (Figures 7 and 8). ...
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... These discoveries were made mainly in deepwater fold-thrust structures related to compressional tectonics; and document the existence of active petroleum systems in the region (Ingram et al., 2004;Algar, 2012). However, it remains today challenging to predict hydrocarbon accumulations and hydrocarbon types in the petroleum system offshore NW Borneo due to the complex structural architecture of the region, lacking source-rock information (Jong et al., 2014;Madon et al., 2015), and a general difficulty of mapping complex turbidite reservoirs (Grant, 2003, 2004204;Ingram et al., 2004;Rice-Oxley and Abu-Bakar, 2022). High fluid pressure generated during source rock maturation has been previously proposed to promote the development of decollements, which in turn affects deformation timing and deformation style of the overlying sedimentary succession (Morley, 1992;Parnell et al., 1998;Cobbold et al., 2004Cobbold et al., , 2009Deville and Sassi, 2006;Zanella et al., 2014;Rocha and Cristallini, 2015). ...
... In the deepwater NW Borneo fold-thrust belt source rocks have not yet been drilled, and therefore the source rock properties are poorly understood (Jong et al., 2014). However, mud volcanoes and pockmarks accompanied by hydrocarbons are broadly observed at the seafloor (Zielinski et al., 2007;Warren et al., 2010;Jong et al., 2014), implying that mature or overmature source rocks are widespread. ...
... In the deepwater NW Borneo fold-thrust belt source rocks have not yet been drilled, and therefore the source rock properties are poorly understood (Jong et al., 2014). However, mud volcanoes and pockmarks accompanied by hydrocarbons are broadly observed at the seafloor (Zielinski et al., 2007;Warren et al., 2010;Jong et al., 2014), implying that mature or overmature source rocks are widespread. ...
The generation of hydrocarbons in source rocks can lead to overpressure, which can support development of detachments and the deformation of sedimentary rocks. In turn, the deformation of rock units by e.g., large-scale overthrusting can lead to tectonic-driven burial. Tectonic-driven burial can generate overpressure, which in turn can influence the petroleum system. In fold-thrust belts, an integrated understanding of both, the tectonic and the petroleum systems is important for understanding the potentially complex interaction of faulting, folding and fluid geochemistry. This study combines structural restoration and basin-modeling techniques to provide a comprehensive view of the fold-thrust system offshore northwest Borneo.
In the deepwater region offshore northwest Borneo a major fold-thrust belt is present. The thrust belt is characterized by a mix of gravity-driven folding and faulting in a southwestern domain, and deep-seated crustal deformation in a northeastern domain. Oil and gas preferentially accumulated in thrust-top anticlines. The NW Borneo fold-thrust belt has a low taper angle; likely related to fluid overpressure along the basal detachment. The basin models presented in this study are based on an integration of regional 3D seismic-reflection interpretation, borehole analysis and 2D kinematic restoration. 2D petroleum systems modeling shows oil and gas generation and expulsion from Middle Miocene coaly source rocks since the Late Miocene, migration of the hydrocarbons through carrier beds and faults, and the accumulation of oil and gas in thrust-hangingwall anticlines. Vertical gas leaks modeled are comparable with gas clouds observed on 3D seismic-reflection data. The risk of biodegradation was estimated for reservoirs shallower than 1000–1500 m, and their burial is influenced by uplift due to thrusting. Combined tectonic and thermal modeling of the fold-thrust belt indicates the initiation of shortening at the time when the maturation of the source rock at the basal detachment was within the oil window. The timing of the maximum shortening rate, however, exhibits regional variations across the fold-thrust belt. In the gravity-driven fold thrust system in the southwest, the primary control on the peak of shortening is the maturation of the source rock at basal detachment level (gas window). In contrast, in the northeast the timing of deep-seated crustal compressive tectonics is interpreted to have exerted the dominant control on the petroleum system.
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https://doi.org/10.1016/j.geoen.2023.212060
... This study aims to define and understand the various reservoir compartments from a deepwater Sabah case study, applying both structural and stratigraphic approaches over the Kinarut to Pink stratigraphic intervals (Figure 1). Jong et al., 2014). ...
One of the key challenges in deepwater exploration is the prediction of reservoir geometries. Basin floor fans, deepwater channel systems and crevasse splays, ponded turbidites and MTDs are the main depositional geometries that can result in reservoir compartmentalization and baffling in a shale-dominated environment (Shoup et al., 2020). This study aims to define and understand the various reservoir compartments from a deepwater Sabah case study, applying both structural and stratigraphic approaches over the Kinarut to Pink stratigraphic intervals.
... The thickness of the seal MTC in the field ranges between 40 and 75 m (~130-250 ft) (Algar et al., 2011). Fluid escape features are common in the area (Van Rensbergen et al., 1999;Jong et al., 2014;Madon et al., 2015) and a prominent gas chimney/cloud sits above the trap of the Kikeh field (Fig. 19A). The gas cloud is the product of seals leaking by capillary failure, whereby gas is lost preferentially over oil because of the molecular size difference, resulting in the reservoir being enriched in oil (Algar, 2012;Jong et al., 2014). ...
... Fluid escape features are common in the area (Van Rensbergen et al., 1999;Jong et al., 2014;Madon et al., 2015) and a prominent gas chimney/cloud sits above the trap of the Kikeh field (Fig. 19A). The gas cloud is the product of seals leaking by capillary failure, whereby gas is lost preferentially over oil because of the molecular size difference, resulting in the reservoir being enriched in oil (Algar, 2012;Jong et al., 2014). Although no rafted megablocks are observed in the published seismic lines, based on core data, Algar et al. (2011) reported rafted sandstone megablocks up to 5 m thick encased in a silty claystone matrix ( Fig. 19D and E). ...
Mass transport complexes and their associated mass transport deposits—both referred to here as submarine failure deposits—are virtually ubiquitous in the modern and ancient sedimentary record of many deepwater basins. As exploration expands to new frontiers, submarine failure deposits are being identified as components of the petroleum system in numerous prospects. Although such deposits were historically considered facies to avoid from a viewpoint of petroleum exploration, it is now recognized that they can act as source, reservoir, or seal elements. In this paper, we set out to investigate the role of submarine failure deposits as effective seals in the petroleum system using published data and propose a methodology to risk some first-order factors at a macro-, meso-, and micro-scale that influence the seal quality of these submarine failure deposits. We accomplish this by discussing the properties intrinsic to submarine failure deposits that affect the seal quality at different scales. Based on published literature, at least six offshore fields from the Gulf of Mexico and NW Borneo are reported to have a submarine failure deposit as an effective seal. These fields combined account for ∼0.9 billion barrels of oil equivalent of cumulative discovered reserves globally and prove the potential of submarine failure deposits as effective seals and a future undervalued play concept. We use three case studies to illustrate our methodology. Our methodology can be further customized using datasets from industry and public records. The seal risk matrix presented here is based on more than a decade of research and has been already used by several exploration companies with encouraging feedback. We acknowledge the limitations of the methodology, but future interdisciplinary research and integration of new datasets and results will improve the de-risking of submarine failure deposits in exploration. Additionally, this methodology can potentially be applied to assessing seal potential of submarine failure deposits for carbon capture sequestration and storage projects.
... In the Dangerous Grounds, Reed Bank, and adjacent areas during the initial rifting phase, major structures formed are grabens associated with high angle normal faults; large half-grabens formed controlled by listric faults and detachments; and presence of rotated fault blocks in the hyper-extended continental crust (Peng et al., 2019). The Sabah Basin, including the Dangerous Grounds, witnessed the deposition of thick clastic sedimentary succession as a result of uplift during Miocene and rapid erosion of the Sabah land massif under favourable climatic conditions, which resulted in the northwest progradation of deltas under regressive condition (Jong et al., 2014). ...
The onshore and shallow marine conventional hydrocarbon resources around the world are mostly at the maturation phase and hence, exploration activity globally, showing increasing interest into progressively deeper water hydrocarbon prospects such as South China Sea basins. However, a comprehensive understanding of basin tectonics, depositional history, and petroleum systems of the basin is required to deduce the exploration prospectivity of these deeper reservoirs. The Sabah Basin is a prolific hydrocarbon province in southeast Asia and has a significant economic impact on this part. Recent discoveries in the deep-water hydrocarbon prospects in the NW Sabah Trough prompted a comprehensive investigation of the surrounding area, e.g., Dangerous Grounds for hydrocarbon prospectivity. Previous studies in the Dangerous Grounds mainly focused on the sparse 2D seismic and gravity-magnetic data analysis. A systematic review of stratigraphic evolution has not been presented thus far in the published literature. In this paper, recently acquired high-resolution 3D seismic data with the core and cuttings and conventional well log data from the adjacent areas have been combined to characterize the stratigraphic evolution of the Dangerous Grounds in the NW Sabah platform. Seven horizons interpreted from the 7570 sq. km 3D seismic data, calibrated with the global sea level and geological time scale, to establish the stratigraphic evolution in the study area. Based on distinct structural and seismic characteristics, and calibrated with the adjacent regions well data, two mega-sequences, namely, lower syn-rift and upper post-rift mega-sequences, have been identified and evaluated. The Paleocene to Early Oligocene syn-rift sequence consists of clastic sedimentary fill in grabens and half-grabens is overlain by Late Oligocene to recent post-rift sedimentary successions represented by siliciclastic and carbonates deposits. Major depositional units within these two mega-sequences, including carbonate, channel, mass transport deposits, and turbidites, have been identified and evaluated. During Late Oligocene to Middle Miocene, the area has witnessed the deposition of carbonate platforms and reefs on top of pre-existing structural highs. Late Oligocene to Middle Miocene channel-levee complexes with convex upward sand filling features is prominent in the seismic data. Mass Transport Deposits and turbidites have also been observed above the Middle Miocene Unconformity with distinct seismic characteristics. The pelagic and hemipelagic nature of the sedimentary succession deposited from Pliocene to recent in a passive margin condition have also been interpreted from their unique seismic features. Paleocene-Early Oligocene syn-rift siliciclastic hydrocarbon play, Late Oligocene-Middle Miocene carbonate play, and Late Miocene turbidite play within the study area have been interpreted.
... The offshore NW Sabah Basin is a foreland basin associated with an active fold and thrust belt (Figure 6), where, from the latest Miocene to Holocene epochs, a consistent palaeo-shelf edge position has been established with a restricted narrow shelf area due to constant hinterland uplift and very rapid subsidence rates of the basinal area (Ingram et al., 2004;Behain, 2005;Grant, 2005;Lambiase & Cullen, 2013;Jong et al., 2014Jong et al., , 2016Kessler & Jong, 2015;Khamis et al., 2018a, b). ...
... The East Baram Delta province where the study area of "Block X" is located (Siti Aishah Abdullah et al., 2018), is a margin originating in the Eocene when the proto-South China Sea oceanic crust started subducting below the NW Borneo continental margin (Figure 7). The basin consists of a thick, clastic sedimentary succession deposited as a consequence of Miocene to Holocene uplift and rapid erosion of the Sabah land massif, which resulted in the NW progradation of regressive clastic deltas (Jong et al., 2014;Khamis et al., 2018a, b). The sediment input greatly exceeded accommodation space along the narrow shelf, triggering shelf-margin instability and resulting in episodic, massive slope failures with slumps and prolific turbidite deposition from the Middle Figure 6: NW Sabah fold and thrust belt, the index map shows the line location with study area of "Block X" as indicated. ...
... NW Borneo regional map, where the NW Borneo margin is transected by the West Baram Line (WBL) and Tinjar Line (TL). The study area of "Block X" is shown located in the East Baram Delta basin (fromJong et al., 2014 and modified from Cullen, 2010). ...
Natural gas hydrates (NGHs), sometimes referred to as "flammable ice", are crystalline solids, consisting of hydrocarbon gases with low molecular weight, such as methane, ethane and propane, bound with water molecules within cage-like lattices. The water molecules and low molecular weight NGH lattices are stable within a specific range of temperatures and pressures, and the source of the gases can be biogenic or thermogenic in origin. NGHs are common in the upper hundreds of metres of sub-seafloor sediments on the continental margins at water depths greater than about 500 m. Seismic reflection profiles and wireline well logs are common indicators used to identify the presence of NGHs, which are often encountered during offshore deepwater exploration drilling. They may cause geohazards such as slope instability, expulsion of the seafloor, shallow water flows and shallow gas if the stability of penetrated NGHs is disturbed and starts to dissociate. Methane gas hydrates represent a significant potential energy resource, as illustrated in this case study from offshore NW Sabah and may represent one of the world's largest reservoirs of carbon-based fuel, with some estimates suggest that the hydrocarbons bound in the form of NGHs may rival the total energy resources contained in other conventional hydrocarbon sources. Methane can be extracted from NGHs through three methods: depressurization, inhibitor injection and thermal stimulation. However, risk associated with NGHs extraction can contribute to environmental concerns such as global warming and a decrease in microbial communities associated with methane hydrate ecosystem. Presently, in many countries, national programs exist for the research and production of natural gas from NGH deposits. As a result, hundreds of deposits have been discovered, with a few hundred wells drilled and kilometres of NGH cores studied. Hence, in the future (pending improved gas price and extraction technology), methane gas hydrates could be a vast source of natural gas supply.
... Also in many deltas and deepwater foldbelts, it is clear that both oil and gas have been charged, but some traps are found containing oil only without even a gas cap. In those basins, it has been concluded that seals may sometimes be leaking for gas but not (strongly) for oil (Algar, 2012;Jong et al., 2014;Okawa and Jong, 2014). Seals that are leaky for gas may be shale seals that have not yet been buried very deeply and are still relatively porous and permeable. ...
The petroleum system concepts links accumulations of hydrocarbons to a source rock, thus placing emphasis on the origin of the hydrocarbons. The elements of a petroleum system include a source rock, reservoir, trap and seal. Geological processes involved are trap formation and generation-migration-accumulation. The formalization of the petroleum system concept by Magoon and Dow (1994) has greatly assisted in making explorers aware of the fundamental logic of where and how hydrocarbons accumulations occur in the subsurface and thus in making hydrocarbon exploration more effective.The presence of the four essential ingredients (trap, reservoir, seal and charge), the way these ingredients occur in a basin and the heat flow history all depend strongly on the basin type: failed rift basin, passive margin, foreland basin, deep-water foldbelt, intracratonic basin, Cenozoic delta, etc. Each of these basins have distinct structural styles and depositional fills that relate directly to the trap styles, source rocks, reservoirs and sealing lithologies that may be expected in each of these basins (Fig. 31). The Play-Based Exploration workflow is developed in recognition of the importance of the basin development and provides a geological context for the evaluation and assessment of individual prospects. The PBE workflow should provide opportunities for explorers to exploit their geological curiosity in a geologically sound way; to come up with play and prospect concepts that are both imaginative and, at the same time, consistent with all data, observations and understanding of geological concepts at all geological scales. As such, the PBE workflow is a natural modern evolution of the petroleum systems concept that was developed several decades ago. The PBE workflow recognises three ‘tranches’, that are normally displayed in a so-called PBE pyramid:1) Basin evaluation: To provide a solid geological foundation for all further analysis. Basin evaluation should result in an understanding of the basin type and development, its main tectonic phases and basin cycles, its depositional fill, its heat flow history, and of the (potential) petroleum systems in the basin.2) Play evaluation: At this stage the focus is still regional and basin wide, but aims at defining and understanding play levels with their associated risks and opportunities. The end product should be an inventory of leads at the different play levels.3) Prospect evaluation: Individual prospects are fully evaluated, in the context of a sound understanding of the basin development and of the play the prospect belongs to. The end product of this final tranche of the PBE workflow is a portfolio of drill-ready prospects with an assigned Probability of Success (POS) and an assessment of the range of hydrocarbons volumes that may be found if drilling the prospect results in a discovery.
... Tectonic history: It is known that many Sarawak onshore traps with shallow marine reservoirs and seals originally contained hydrocarbons but leaked later. This was caused by a Pliocene phase of compression combined with significant uplift (of some 600 m since the onset of Pliocene, and 20 m since the beginning of the Holocene) 2014b;Jong et al., 2016a). It is believed that reactivation of older faults and the formation of joints may have broken both top and fault seals. ...
... Top seal leakage due to later stage faulting can be another geological risk, but numerous major fields in the area do have extensive faulting over the structure (and extensive shallow gas clouds), while nevertheless trapping hydrocarbons. It has been noted that differential top seal leakage might have allowed gas to escape from accumulations in the area resulting in the loss of light-end hydrocarbons from the reservoir, thus enriching them to give more oil-rich deposits (Algar, 2012;Jong et al., 2014b;Jones et al., 2016). Some crestal faults are seen to extend up from the reservoir formations to the seafloor, providing potential vertical leakage pathways, which may open and reseal as a function of pressure. ...
... We observe a full spectrum of oils (low API to condensate), and very dry to wet gas. Regional work by various researchers is leading to the rationale that all oil and gasses found trapped in reservoirs around Borneo stem from a mixed plantderived and coaly source rocks, with a high wax content derived from mangrove detritus (e.g., Algar, 2012;Jong et al., 2014bJong et al., & 2017a. Consequently, the variety of observed hydrocarbons is explained by osmotic processes, in which hydrocarbons are separated (or retained) according to capillary pore size in reservoir and thief zones (Figures 8 and 9). ...
Several siliciclastic fault-trap settings of the NW Borneo margin, from the West Baram Delta covering offshore Sarawak and Brunei to offshore NW Sabah, are compared with each other to assess the intricate relationship between hydrocarbon retention, seal capacity and reservoir parameters. Hydrocarbon column length is found to be a derivative of several parameters potentially affecting the integrity of a hydrocarbon trap. The presence of an effective and laterally continuous top seal is perhaps the most impor tant success parameter; though relatively thin top seal can be surprisingly efficient. Seal capacity is featured by parameters such as mineralogy, grain size, contiguity, diagenesis and lateral continuity. Contiguity of hydrocarbon reservoirs is also important as discontinuous reservoir bodies commonly lead to very short and variable columns. The overall sand-to-shale ratio governs to some extent clay gouging capacity, with hydrocarbon columns tending to be longer in overall clay-prone environments, such as those found in outer shelf and deepwater turbidite environments. In these depositional settings, P50 columns are in the order of 250 m because sand-to-clay juxtaposition is more likely in fault-controlled traps. A better fault seal is often realized due to good shale gouge. However, hydrocarbon columns tend to be short (P50 of around 30 m) in sand rich shallow marine to deltaic settings given the discontinuity of reservoirs, leaky top seal, abundant yet discontinuous reservoir sand bodies and poor fault sealing capability. It is also observed that there are patterns of parameters such as sealing, reservoir, pressure and drive that have been identified, and combinations that appear viable (probabilistic success patterns) and non-viable for hydrocarbon retention (probabilistic failure patterns). The authors thus suggest to develop plausible patterns/scenarios and apply probabilistic simulations to each of the various combinations to assess the likely outcomes for column length predictions.
... Red box represents the main stratigraphy column for exploration targets. Modified afterJong et al. (2014a). ...
This study aims to provide supporting evidence for a semi-qualitative understanding of the various deformational episodes experienced in the studied structures of deepwater Sabah toe-thrust trends, covering from the deeper Kinarut reservoir interval to seabed in a temporal and spatial framework. A quantitative analysis of the deformation profiles was conducted to observe their effect on sedimentary patterns, integrated with amplitude analysis and spectral decomposition volume interpretation. The basic principal of line balance restoration was employed to unravel the deformation history of three investigated structural trends: Trend 1 (“M-La-S”), Trend 2 (“L-B-P”) and Trend 3 (“Pg-Lt-U”), with the fundamental assumption that a geological section will restore at any particular moment in time to an unstructured section according to the law of superposition and horizontality. These toe-thrust anticlines were examined by calculating the rate of shortening at different geological times over the section utilising a number of line sections. All three trends have experienced a dominant NW-SE oriented compression from Early Miocene until Late Miocene, during the closing of the proto-South China Sea and the subsequent opening of South China Sea. These regional tectonic events impacted directly on the structuration style in the study area. The quantitative analysis has shown that the structuration and deformation history of each of the studied structures varied along the structural trends. Late stage deformation episodes, which possess a higher stress regime play a significant role on the rate of shortening on each anticline and its fold geometry. These variations influence sediment fairway distributions and impact on the selection of exploration targets and reservoir objectives. The study demonstrated a fundamental genetic link between structural growth and sedimentary fairway loci. It is concluded that the key to reducing reservoir risk is to first understand the kinematic growth profiles and deformation history of the studied structures. Well-developed structural kinematic models and the establishment of the deformation history of the studied toe-thrust anticlines in deepwater Sabah will have a paramount impact for more reliable predictions of sedimentary fairway distribution patterns, where effort to locate sandy reservoir remains a critical challenge for deepwater exploration.
Key words: deformation, Miocene, NW Sabah, structural kinematics, toe-thrust.
... Even though the definitive source rock intervals have not been fully penetrated in the Sabah Basin, numerous commercial hydrocarbon discoveries have essentially proven a working petroleum system within the basin. The primary source rocks in the deepwater Sabah Basin are comprised of coaly organic matter originating from land plants, which was transported into the deepwater region along with the reservoir sands in turbidity flows (Algar, 2012;Jong et al., 2014b). Core material from the major oil and gas discoveries in the region commonly have examples of high concentrations of coaly organic material within the reservoir sand units. ...
This study presents a play-based evaluation of the southern part of the deepwater NW Sabah fold-thrust belt, a key exploration area in Malaysia. The key objective was adding value to the existing database through an integrated approach. This goal was achieved by analysing four critical geological risk elements: reservoir presence, structural evolution, top seal integrity, and timing of hydrocarbon charge and migration, to identify prospective areas for future exploration by integrating all available geological, geophysical and geochemical information into a consistent petroleum system framework. Using the basin-play-prospect maturation workflow, data spanning the geophysical domain (with inputs such as seismic evaluation, structural mapping and attribute analysis) to the geological realm (such as well correlations, fairway mapping, sedimentological studies, biostratigraphic investigations and source rock maturation modelling), are combined with structural kinematic evolution to generate detailed play-based element maps. The application of the tried and tested play-based evaluation methodology from basin evaluation through to prospect maturation has been carried out. This has led to a comprehensive play element analysis yielding a composite risk segment map within a consistent petroleum system framework. In addition, the study has provided sensible explanations for dry hole analysis, an important reality check, but most importantly it has generated a fresh insight into the overall prospectivity of the study area. This enhanced multi-discipline analysis is beneficial for reducing exploration risk for future expenditure in a time of depressed oil prices that calls for a more innovative approach for deepwater exploration. In summary, integration of available data and the application of new in-house ideas and solid geoscientific knowledge has added value through the generation of increased prospectivity, however for further ground-truthing the real litmus test has to come from future drilling.
Key words: deepwater Sabah, play-based exploration, petroleum system, risk segment
... Oil and gas, generated by type III source rocks, actively charge these anticlines, which represent the main hydrocarbon traps of the region (Ingram et al., 2004;Warren et al., 2010;Jong et al., 2014). The stratigraphic column is divided into several units, composed of sand-prone turbidite fans, which constitute the main reservoirs for hydrocarbons, plus associated mass-transport deposits (MTDs) and hemipelagites, representing regional and local seals (Algar et al., 2011). ...
... The stratigraphic column is divided into several units, composed of sand-prone turbidite fans, which constitute the main reservoirs for hydrocarbons, plus associated mass-transport deposits (MTDs) and hemipelagites, representing regional and local seals (Algar et al., 2011). In the region, these Miocene-Early Pliocene units are named the Kebabangan, Kinarut, Kamunsu, Pink, Yellow and Lingan (Grant, 2004;Ingram et al., 2004;Jong et al., 2014). In this context, free and dissolved gases migrate through leaky seals across the more recent stratigraphy (Algar, 2012). ...
Fluid-escape pipes represent seismic evidence for the focused cross-stratal migration of fluids. In natural gas hydrate systems, these features serve both as conduits for methane-rich fluids and as preferred locations for the formation of gas hydrates. In this study, 3D seismic, well-log and core data from offshore Sabah (NW Borneo) are used to investigate the controls on the occurrence of fluid-escape pipes and their impact on hydrate distribution in a system dominated by the vertical leakage of thermogenic hydrocarbons.
The pipes are observed within a gas hydrate stability zone (GHSZ) that extends 100 m below a bottom simulating reflector (BSR), located at 155 m below the seafloor (mbsf). Pipes are restricted to an area with evidence of free gas-bearing sediments, suggesting a causative link where the free gas promotes the build-up of critical fluid pressures. The stacking of the upper terminus of fluid-escape pipes at discrete stratigraphic intervals suggests that fluid flow to the seabed has been episodically enhanced. Possible triggers for cyclical increases of pore fluid pressures are sea-level and temperature fluctuations, tectonic activity and gas leakage from deep reservoirs.
This fluid flow system further impacts the gas hydrate distribution. The fluid-escape pipes can be locations where hydrates occur at high concentrations up to the seafloor if the pipe is presently active. Therefore, the observed up-bending of the stratigraphic reflections along the pipes are interpreted as a combination of a net volume increase of the host sediment owing to hydrate formation and seismic velocity pull-up effects. Away from the pipes, hydrates do not occur until 65–152 mbsf and are present only at low to moderate concentrations. At this site of focused fluid flow, fluid-escape pipes constitute, by volume, only 7–11% of the gas hydrate occurrence zone. Nevertheless, we predict that they could host between 20 and 50% of the whole hydrate volume. It is therefore likely that, in similar systems, a volumetrically significant portion of the total hydrate reservoir is hosted within fluid-escape pipes. The distribution of these features should thus be considered as a critical parameter for hydrate volume estimates.
