Baram Delta Research Articles

Exploration history and petroleum systems of the onshore Baram Delta, northern Sarawak, Malaysia

Exploration history and petroleum systems of the onshore Baram Delta, northern Sarawak, Malaysia

Sedimentologic and sequence-stratigraphic characteristics of wave-dominated deltas

Wave-dominated deltaic strata form prolific hydrocarbon plays in many mature basins across the world. Examples include the Jurassic Brent Group play in the North Sea, offshore United Kingdom and Norway (e.g., Husmo et al., 2003); Eocene Jackson Group and Oligocene Frio Formation plays, Texas, onshore United States (e.g., Fisher et al., 1970; Galloway and Morton, 1989); and Tertiary plays in the Niger Delta province, offshore Nigeria (e.g., Evamy et al., 1978), the Baram Delta province, offshore Brunei (e.g., Rijks, 1981), and the Columbus Basin, offshore Trinidad and Tobago (e.g., Sydow et al., 2003). Wave-dominated deltas also form key exploration targets in frontier basins (e.g., Triassic Snadd Formation, Barents Sea, offshore Norway; Klausen et al., 2014, 2016). The overall architecture of many wave-dominated deltaic reservoirs is determined by their sequence-stratigraphic framework together with their structural configuration. Sequence-stratigraphic frameworks contain elements such as shoreface–shelf parasequences bounded by flooding surfaces and fluvio-estuarine complexes that fill incised coastal valleys developed at sequence boundaries. The internal facies distributions and stacking patterns of such elements are ordered, which aids prediction of reservoir character and behavior. Outcrop analogs of wave-dominated deltaic reservoirs display aspects of ordered stratigraphic architecture across a range of spatial scales that are equivalent to those observed in core, well-log, and seismic data and can therefore be used to support interpretation of their subsurface counterparts. The Cretaceous Blackhawk Formation contains wave-dominated deltaic strata that are superbly exposed in the Book Cliffs of eastcentral Utah (Figure 1A). The extensive, continuous exposures of the Book Cliffs outcrop belt have allowed the wave-dominated deltas and related deposits to be mapped for tens of kilometers (e.g., Young, 1955; Balsley, 1980), and, consequently, these strata were the “birthplace” of high-resolution sequence stratigraphy using outcrop data (Van Wagoner et al., 1990). The Blackhawk Formation and related …

Geochemistry of Miocene sedimentary rocks from offshore West Baram Delta, Sarawak Basin, Malaysia, South China Sea: implications for weathering, provenance, tectonic setting, paleoclimate and paleoenvironment of deposition

Geochemistry, paleoweathering, provenance, tectonic setting, paleoclimate and paleoenvironment of deposition of offshore Miocene sedimentary rocks in the West Baram Delta have been examined through multi-element geochemistry and mineralogy. Materials and methods used in the study include ninety four core samples from four subsurface wells, XRF, thin sections and FESEM. Four main groups of samples are identified in the wells: porous sandstones, cemented sandstones, siltstones and mudstones. The sandstones are classified as sublitharenites, litharenites and Fe-rich sandstones based on chemical composition and mineralogy. Variations in compositional trends suggest strong lithological and diagenetic controls on the geochemical composition of the samples. Petrography of the samples indicates that they are composed mainly of quartz, K-feldspar, siderite and clay minerals. The samples are characterized by moderate to high degree of chemical weathering with CIA values between 52–76 and PIA values from 65–100. Provenance analysis of the samples shows predominantly felsic and intermediate igneous provenance with minor mafic contribution. The samples are inferred to have originated from a passive margin tectonic setting that followed the continental collision and rifting stages of the foreland basin development phase of the Sarawak Basin. The paleoclimate of deposition of the samples is interpreted to be warm and humid enhancing the chemical weathering. The paleoenvironment of deposition is predominantly suboxic to anoxic.

Integrated Asset Modeling: An Approach to Long-Term Production Planning

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 176322, “Samarang Integrated Operations: Integrated Asset Modeling— An Innovative Approach for Long-Term Production Planning Focused on Enhanced Oil Recovery,” by W. Sifuentes and J. Moreno, Schlumberger; P. Kumaran and R. Hamdan, Petronas; M. Muhamed Salim, M. Mohd. Som, H.W. Lee, A. Ahmad, N. Pangereyeva, S.S. Biniwale, N.J. Rodriguez, J.T. Nitura, R. Alia, O.A. Talabi, and V. Halabe, Schlumberger; and S. Pedraza, SPE, prepared for the 2015 SPE Asia Pacific Oil and Gas Conference and Exhibition, Bali, Indonesia, 20–22 October. The paper has not been peer reviewed. With declining trends in production and dwindling reserves for a 35-year-old offshore field, the Samarang Redevelopment Project was initiated with a vision toward implementing integrated operations as an asset-management decision-support tool. To realize expected reserves and optimize the production, an integrated-asset-model (IAM) concept is introduced with state-of-the-art modeling techniques to treat the asset as a whole unit rather than as isolated silos, which had been the traditional approach. Introduction The Samarang field is located in Subblock 6S-18 northwest of Labuan in eastern Baram Delta province. The production history of the field, first discovered and developed in the 1970s, is discussed in detail in the complete paper. A full-field review was carried out from 2004 to 2007 with the objective of identifying redevelopment opportunities. A field-development plan (FDP) was then established before an alliance was signed in 2010 to redevelop the field. The redevelopment team has drilled more than 17 infill wells, obtained new 3D- seismic data, carried out several studies including fine tuning of the enhanced-oil-recovery (EOR) development plan, and executed an extensive program of production enhancement and surveillance activities to boost the field production. This has given the team further insight into the Samarang field potential as well as the untapped near-field potential. The proposed EOR process is gas-assisted simultaneous water-and-gas (GASWAG) injection. This process consists of simultaneously injecting water updip in the gas cap and gas downdip in water-swept zones. The numerical reservoir-simulation model is the primary tool used for the EOR evaluations, optimizations, and forecasts. Implementation of the EOR scheme requires strategically positioning infill and injection wells. The recovery mechanisms targeted by this EOR scheme include Displacing attic oil back to existing well completions Sweeping the remaining oil toward new wells Repressuring the compartments Improving vertical sweep by using assistance gravity segregation Reducing residual oil in water by injecting gas in water-swept layers Traditionally, planning, forecasting, and optimization have been performed by the conventional approach of considering the reservoir, surface-network, and process-system models as being relatively isolated from one another. In this regard, many simplifications were taking place to translate the shared production limits, and account for surface backpressures, for each of the isolated models. These simplifications have negatively influenced production forecast for long-term predictions. The objective is to use an IAM approach to analyze and improve the production process for long-term planning.

Effect of bioturbation on reservoir rock quality of sandstones: A case from the Baram Delta, offshore Sarawak, Malaysia

Abstract With the Baram Delta in Malaysia as research subject, the effect of bioturbation on porosity and permeability of reservoir sandstones is evaluated and analyzed based on core and thin section analysis, EDX (energy-dispersive X-ray), FESEM (Field Emission Scanning Electron Microscope), mercury porosimetry and spot permeametry measurement. Samples are from cored intervals of two wells in the Baram Delta, W-1 and W-6. Analysis results indicate that the cored intervals in well W-1 are dominated by Diplocraterion ichnofabrics, intensely bioturbated, sediment packing activity is observed, and fine grade materials (clays and organic matter) from the host sediment are incorporated into burrow fills and linings, thereby decreasing isotropy and sorting of the sediments and reducing the local porosity and permeability in the burrows. The cored intervals in well W-6 are dominated by Ophiomorpha ichnofabrics, highly to intensely bioturbated, sediment cleaning activity by burrowing organisms results in cleaner and well sorted burrow fill materials, and clays and mud are cleaned from the burrow fill and host sediment and concentrated in the burrow linings. Porosity and permeability of reservoir rocks in the Baram Delta have therefore either been enhanced or reduced by bioturbation depending on the type of burrow, fill material and burrowing activity.

Organic geochemical and petrographic characteristics of Neogene organic-rich sediments from the onshore West Baram Delta Province, Sarawak Basin: Implications for source rocks and hydrocarbon generation potential

The Neogene succession of the onshore petroleum prolific West Baram Province of North-West Borneo contains organic-rich sediments particularly within Sarawak Basin. Bulk and quantitative pyrolysis results, coupled with organic petrographic characteristics of Neogene organic-rich sediments have enabled investigation of the source rock characteristics and evaluation of the hydrocarbon generation potential. The results were used to assess the quantity of organic matter, kerogen composition, thermal maturity and type of petroleum that could possibly be generated from these Neogene sediments.The total organic carbon (TOC) contents of the Neogene sediments range between 1.06 and 11.11 wt. % and Hydrogen Index (HI) values are largely below 105 mg HC/g TOC, suggesting that the sediments are organically rich and contain gas-prone Type III kerogen. This kerogen type is supported by the abundant terrestrial materials (i.e., vitrinite phytoclasts, spores and pollen), low atomic hydrogen-to-carbon atomic (H/C) ratio and high abundance of aromatic hydrocarbons in Pyrolysis GC pyrograms and would likely generate mainly condensate and gas. The analysed samples have vitrinite reflectance in the range of 0.39–0.48 %Ro and pyrolysis Tmax in the range of 401–434 °C which indicate that these Neogene sediments contain immature to very early-mature organic matter. This implies that these onshore sediments have not been buried to a sufficient depth, however the offshore stratigraphic equivalent of these sediments are known to have been buried to deeper depth and could therefore act as an effective petroleum source rock in particular for natural gas.

Impact of Bioturbation on Reservoir Quality: A Case Study of Biogenically Reduced Permeabilities of Reservoir Sandstones of the Baram Delta, Sarawak, Malaysia

Impact of Bioturbation on Reservoir Quality: A Case Study of Biogenically Reduced Permeabilities of Reservoir Sandstones of the Baram Delta, Sarawak, Malaysia

Sequence stratigraphic modelling and reservoir architecture of the shallow marine successions of Baram field, West Baram Delta, offshore Sarawak, East Malaysia

The Baram Delta Province is located in the northern part of Sarawak (West Baram Delta) and extends north-eastward into Brunei and further into the south-western part of Sabah (East Baram Delta). The delta is a Neogene basin which developed over an accretionary wedge implying Cretaceous to Eocene sediments during the Late Eocene to Late Miocene times (Tongkul, 1991; Hutchison et al., 2000; Morley et al., 2003; Sapin et al., 2011).Facies and well log analyses were carried out on core and well data for the Late Miocene successions of Baram field, a medium-sized oilfield located in the north-eastern flank of the Baram Delta Oil Province, offshore Sarawak. A numerical model of sea-level fluctuations and progradational basin-fill was generated using the Clastic Modeling Program (Hardy and Waltham, 1992a and 1992b; Waltham, 1992) software to evaluate the possible controls of sea-level changes in the development of the siliciclastic successions and their bounding surfaces. This model was based on four lines of evidence, namely core data, fieldwide wireline logs correlation, seismic sections and average thickness variations across the field.Cored intervals of the Upper Cycle V (Late Miocene) display reservoir successions dominated by thick swaley cross-stratified (SCS) sandstones, thin hummocky cross-stratified sandstones and other shallow marine, wave and storm-dominated facies, interbedded with laminated shelfal mudstones. The vertical facies organisation suggests deposition during shoreface progradation associated with a fall of relative sea level.Analysis and correlation of well logs reveal stacking patterns comprising three scales of depositional cyclicity: the parasequences (∼10–∼30 m thick), the parasequence sets (∼45–∼130 m thick) and the major cycles (∼600–800 m thick).Field-wide, dip-oriented seismic sections show very well-developed horizontal to slightly upward convex layers traceable over great distances, which suggests a ramp-type margin, in which the basin floor dipped gradually seaward and lacked a distinct shelf-slope margin.The evidences gathered demonstrate that the deposition and build-up stratigraphy of the Late Miocene sedimentary successions could have been strongly controlled by superimposed short-term, medium-term and long-term sea-level changes.The simulated sea level and sedimentary basin-fill model, generated by the Clastic Modelling Program, match to the well log correlation. This model illustrates that high frequency sea-level fluctuations enable sands to be distributed over large areas within a shallow, low gradient shelf. Our study shows that integrated studies incorporating cores, well logs, seismic sections and simulated models can be employed as important tools for correlation and reservoir modelling.

Geochemical Variations in Hydrocarbon Components Distribution in a Prograding Deltaic Sequence: A Case Study of the Baram Delta, Offshore Sarawak Basin, Malaysia

Studies on hydrocarbon distribution have evolved from basic reservoir characterization to complex studies today involving the interactions between oil components and clay minerals and sequential extraction studies on hydrocarbon extracts in reservoir rocks. Findings from such studies include the discovery of variations in oil fractions in reservoirs such as adsorbed oil and free oil. The theory that first oil charge preferentially interacts with clay minerals occurring in pores and as coatings in reservoirs was also proposed by some researchers. Despite, all these studies some aspects of variations in the composition of hydrocarbons in reservoir rocks still need to be investigated further. This study has been carried out particularly because the qualitative and quantitative composition of aromatic and aliphatic components of hydrocarbons in terms of the presence and quantities of hydrocarbon functional groups and how they relate to hydrocarbon migration have not been exhaustively discussed. This study uses Ultra-Violet visible light (UV-vis) and Fourier Transform Infra Red (FTIR) to characterize variations in hydrocarbon distribution in reservoir quality sandstones from three fields namely BD01, BD02 and BD03 in the Baram Delta, offshore Sarawak and to deduce how these variations relate to differential migration patterns in hydrocarbons. Hydrocarbon extraction was done in the ultra vilolet visible (UV-vis) experiment using 0.1M sodium pyrophosphate as solvent whereas in the Fourier Transform Infra Red (FTIR), the experiment was done on very fine powdered samples of the sandstones. Results from both the UV-vis and FTIR experiments indicate a dominance of aromatic functional groups in the samples. Most of the samples have E4/E6 ratio of more than 1 which indicates a high degree of aromacity. The BD01 field sandstones with a maximum porosity of 32% has the highest average E4/E6 ratio of 1.21, followed by the BD02 field sandstones with a maximum porosity of 29% and average E4/E6 ratio of 1.19 and the BD03 field with a maximum porosity of 20% and an average E4/E6 ratio of 1.09. The dominance of aromatics in the samples is interpreted as a possible indication of episodes of migration of aliphatics in the past leaving the aromatics behind with the variations in E4/E6 ratio reflecting the micro heterogeneities in the samples.

Storm‐dominated shelf‐edge delta successions in a high accommodation setting: The palaeo‐Orinoco Delta (Mayaro Formation), Columbus Basin, South‐East Trinidad

AbstractPliocene age deposits of the palaeo‐Orinoco Delta are evaluated in the Mayaro Formation, which crops out along the western margin of the Columbus Basin in south‐east Trinidad. This sandstone‐dominated interval records the diachronous, basinwards migration of the shelf edge of the palaeo‐Orinoco Delta, as it prograded eastwards during the Pliocene–Pleistocene (ca 3·5 Ma). The basin setting was characterized by exceptionally high rates of growth‐fault controlled sediment supply and accommodation space creation resulting in a gross basin‐fill of around 12 km, with some of the highest subsidence rates in the world (ca 5 to 10 m ka−1). This analysis demonstrates that the Mayaro Formation was deposited within large and mainly wave‐influenced shelf‐edge deltas. These are manifested as multiple stacks of coarsening upward parasequences at scales ranging from tens to hundreds of metres in thickness, which are dominated by storm‐influenced and wave‐influenced proximal delta‐front sandstones with extensive, amalgamated swaley and hummocky cross‐stratification. These proximal delta‐front successions pass gradationally downwards into 10s to 100 m thick distal delta front to mud‐dominated upper slope deposits characterized by a wide variety of sedimentary processes, including distal river flood and storm‐related currents, slumps and other gravity flows. Isolated and subordinate sandstone bodies occur as gully fills, while extensive soft sediment deformation attests to the high sedimentation rates along a slope within a tectonically active basin. The vertical stratigraphic organization of the facies associations, together with the often cryptic nature of parasequence stacking patterns and sequence stratigraphic surfaces, are the combined product of the rapid rates of accommodation space creation, high rates of sediment supply and glacio‐eustasy in the 40 to 100 Ka Milankovitch frequency range. The stratigraphic framework described herein contrasts strikingly with that described from passive continental margins, but compares favourably to other tectonically active, deltaic settings (for example, the Baram Delta Province of north‐west Borneo).

Review of pore-pressure prediction challenges in high-temperature areas

Pore-pressure prediction relies heavily on interpretation of seismic and well attributes such as velocity, resistivity, and density which capture porosity changes during shale compaction under vertical loading. Relationships such as Eaton (1975) were developed in the Gulf of Mexico using a relatively simple lithological mix of geologically young sandstones and shale mudrocks at relatively low temperatures. An alternative approach using data from the same region was more deterministic°also with vertical stress (e.g., Hottman and Johnson, 1965; Bowers, 1995). Another approach using mean stress, developed by Harrold et al., (2000) used similar sand and shale sequences at relatively low temperatures from data in southeast Asia basins. All the above approaches can be shown to provide acceptable prediction of pore pressure in shale mudrocks in young, rapidly deposited siliciclastic sequences, such as the Baram Delta, Brunei (Tingay et al., 2009) and along the West African margin (Swarbrick et al., 2011). The results can be calibrated, with careful attention to evidence for lateral drainage or lateral transfer, using data from their associated reservoir. However, in higher-temperature environments (e.g., Malay Basin, Hoesni 2004) these methods fail to deliver predictions, which may be accurate enough for effective well planning and safe drilling. This paper reviews the above methods and how they can be modified for use at elevated temperatures (e.g., above 100°C). However, the review also draws attention to the wide uncertainty inherent in conventional approaches to prediction in some regions, which may require an entirely different approach to pore-pressure prediction prior to drilling.

Present‐day stress orientations and tectonic provinces of the NW Borneo collisional margin

Borehole failure observed on image and dipmeter logs from 55 petroleum wells across the NW Borneo collisional margin were used to determine maximum horizontal stress (σH) orientations; combined with seismic and outcrop data, they define seven tectonic provinces. The Baram Delta–Deepwater Fold‐Thrust Belt exhibits three tectonic provinces: its inner shelf inverted province (σH is NW‐SE, margin‐normal), its outer shelf extension province (σH is NE‐SW, margin‐parallel), and its slope to basin floor compression province (σH is NW‐SE, margin‐normal). In the inverted province, σH reflects inversion of deltaic normal faults. The σH orientations in the extension and compression provinces reflect deltaic gravitational tectonics. The shale and minibasin provinces have been recognized in offshore Sabah. In the shale province, σH is N010°E, which aligns around the boundary of a massif of mobile shale. Currently, no data are available to determine σH in the minibasin province. In the Balingian province, σH is ESE‐WNW, reflecting ESE absolute Sunda plate motions due to the absence of a thick detachment seen elsewhere in NW Borneo. The Central Luconia province demonstrates poorly constrained and variable σH orientations. These seven provinces result from the heterogeneous structural and stratigraphic development of the NW Borneo margin and formed due to complex collisional tectonics and the varied distribution and thicknesses of stratigraphic packages.

Present‐day stresses in Brunei, NW Borneo: superposition of deltaic and active margin tectonics

ABSTRACTThe Baram Delta System, Brunei, NW Borneo, is a Tertiary delta system located on an active continental margin. Delta top regions in many Tertiary delta systems (e.g. Niger Delta) are thought to exhibit a normal‐fault stress regime and margin‐parallel maximum horizontal stress orientations. However, unlike in passive margin Tertiary delta systems, two present‐day stress provinces have been previously identified across the Baram Delta System: an inner shelf inverted province with a margin‐normal (NW–SE) maximum horizontal stress orientation and an outer shelf extension province with a margin‐parallel (NE–SW) maximum horizontal stress orientation. Before this study, there were few data constraining the inverted province other than in the vicinity of the Champion Fields. New data from 12 petroleum wells in the western inner shelf and onshore west Brunei presented herein confirm the margin‐normal maximum horizontal stress orientations of the inverted province. A total of 117 borehole breakouts, all documented in shale units, and one drilling‐induced tensile fracture (in a sandstone interval) reveal a mean maximum horizontal stress orientation of 117 with a standard deviation of 19°. This orientation is consistent with contemporary margin‐normal maximum horizontal stress orientations of the inverted province described previously in the vicinity of the Champion Fields that have been linked to basement tectonics of the Crocker–Rajang accretionary complex and associated active margin. However, stress magnitudes calculated using data from these 12 petroleum wells indicate a borderline strike–slip fault to normal fault stress regime for the present day; combined with the absence of seismicity, this suggests that the studied part of the NW Borneo continental margin is currently tectonically quiescent.

Reservoir properties prediction in the West Baram Delta through data integration constrained by rock physics

We present the methodology and results that allowed conditioning the geological model in Siwa Field in Malaysia's Sarawak Basin using seismic-derived properties. Synergic integration of advanced petrophysical techniques, rock physics analysis/modeling, and seismic inversion, combined with a detailed structural modeling and depth conversion, allowed us to successfully incorporate the seismic-driven petrophysical properties and achieve a more complete description of the reservoir than could be done through conventional statistical techniques based on wells and seismic maps. The high structural and stratigraphic complexity, lack of shear-wave velocity information, and high levels of noise in the data represented the biggest challenges to deriving seismic properties based on amplitude variation for conditioning the reservoir model.

Geometry of an oblique thrust fault zone in a deepwater fold belt from 3D seismic data

Growth of a 12 km long, deepwater anticline during the late Pliocene–Recent is documented from 3D seismic reflection data across NW Borneo. The fold is part of a train of folds formed along the slope at the distal margin of the Baram Delta Province. Growth of the anticline involved fold lateral propagation and linkage of two thrusts formed in the anticline forelimb as either break thrusts or as imbricates ramping up from a master detachment (at ∼3 km depth). For the southwestern anticline, the northern tip of the NE–SW striking, SE-dipping thrust passes into an E–W striking oblique termination, which lies in the linkage zone between the two anticlines. The thrust termination is characterised by the following changes passing east towards the fault tip: 1) the fault zone dips steeply to the south, then 2) passes to a vertical segment (inferred to have oblique motion), and 3) furthest east the fault dips northwards with an extensional component of displacement. The fault zone terminates in a transtensional graben. This graben does not fit with a simple pull-apart geometry or simple oblique ramp geometry. In the future if the thrust faults propagate together and link this oblique fault zone may develop into an oblique ramp that acts as a transfer zone between the faults. However at present the oblique fault zone appears to be a region of 3D strain, where deformation at the fault tip, and the gravity effects of plunging folds have affected the shallow, weak sediments and given rise to a complex thrust termination at a early stage of thrust and fold development. The oblique structure may have developed in response to strains imposed by the encroachment of the fold and thrust belt on an uplifted basement or volcanic high that forms a pronounced topographic feature across a narrow part of the thrust front, 14 km NW of the most external (oceanward) fold.

Balancing deformation in NW Borneo: Quantifying plate-scale vs. gravitational tectonics in a delta and deepwater fold-thrust belt system

Recent GPS measurements demonstrate that NW Borneo undergoes 4–6 mm of plate-scale shortening a year, which is not accommodated by plate-scale structures. The only geological structure in NW Borneo described to accommodate on-going shortening is the Baram Delta System located on the outer shelf to basin floor. Delta toe fold-thrust belts are commonly thought to be caused by margin-normal compressional stresses generated by margin-parallel upslope gravitational extension. The Baram Delta System is divided into three neotectonic provinces: 1) an inactive onshore and inner shelf region of inversion superimposed on an older extensional deltaic province, 2) an outer shelf region of present-day deltaic extension, and; 3) a compressional delta toe. However, it is uncertain whether compression in the delta toe area is purely driven by gravity (i.e. shortening is less than upslope extension) or if there is a component of regional shortening involved, which can only be demonstrated if shortening is greater than upslope extension. In order to quantify the balance between the shortening related to the upslope gravitational extension and the shortening related to the regional geodynamic framework in the Baram Delta System, we have used a geomechanical code based on the Finite Element Method and conservation of mass and momentum, Dynel 2D. This enabled us to reconstruct the tectonic evolution of the delta and to demonstrate for the first time that the total shortening observed in the delta toe does not balance against the active extension in the delta top; with the system exhibiting >1.8% shortening overall (or ∼2.0 km). This additional shortening is therefore attributed to plate-scale shortening across NW Borneo produced by far-field compression. Using a convergence rate of 4 mm y −1, demonstrated by GPS measurements, the delta toe has accommodated far-field compression only in the last 0.5 My.

Present-day stress and neotectonic provinces of the Baram Delta and deep-water fold–thrust belt

Abstract: We present the first present-day stress orientation measurements across a delta and deep-water, delta toe fold–thrust belt. Stress and neotectonic data for NW Borneo reveal three discrete neotectonic provinces: an inverted, inner shelf province (where present-day maximum horizontal stress, σ H , is oriented margin-normal); an extensional, delta top province (σ H is margin-parallel), and a compressional, delta toe province (σ H is margin-normal). We interpret the delta top and delta toe data to reflect that compression in the fold–thrust belt is coupled to delta top extension and that the inverted, inner shelf province reflects the location of this system on an active margin.

State and origin of present-day stress fields in sedimentary basins

The present-day stress field provides fundamental insight into the forces driving plate tectonics and intra-plate deformation. Furthermore, Knowledge of the present-day stress field is essential in petroleum, geothermal and mining geomechanics applications such as the stability of boreholes and tunnels, and improving production through natural and induced fractures. The World Stress Map (WSM) Project has, for over 20 years, compiled a public global database of present-day tectonic stress information to determine and understand the state of stress in the Earth?s lithosphere. The WSM database has revealed that plate-scale stress fields are controlled by forces exerted at plate boundaries (e.g. mid-ocean ridges, continental collision zones), commonly resulting in regional stress orientations sub-parallel to plate motion. However, the state and origin of present-day stress fields at smaller scales, such as within sedimentary basins, remains poorly understood in comparison. Detailed analysis of present-day stresses from within 70 sedimentary basins commonly reveals significant and complex variations in the present-day stress orientation, both across basins and within fields. For example, borehole breakouts in the North German Basin, Nile Delta and the Baram Delta province of northwest Borneo indicate broad regional rotations in the horizontal stress orientation. The present-day maximum horizontal stress orientations in the Gulf of Thailand are approximately north-south at the basin-scale (perpendicular to plate motion) and are perturbed locally to be sub-parallel to fault strike. The North Sea and Permian Basin of Texas display widely varying stress orientations between fields, with some neighbouring fields exhibiting perpendicular stress orientations. Basin- and field-scale stress fields result from the complex combination of numerous factors acting at different scales, including far-field forces (e.g. plate boundary forces), basin geometry (e.g. the shape of deltaic wedges), geological structures (e.g. diapirs, faults), mechanical contrasts (e.g. evaporites, overpressured shales, detachment zones), topography and deglaciation.

Origin of overpressure and pore-pressure prediction in the Baram province, Brunei

Accurate pore-pressure prediction is critical in hydrocarbon exploration and is especially important in the rapidly deposited Tertiary Baram Delta province where all economic fields exhibit overpressures, commonly of high magnitude and with narrow transition zones. A pore-pressure database was compiled using wireline formation interval tests, drillstem tests, and mud weights from 157 wells in 61 fields throughout Brunei. Overpressures are observed in 54 fields both in the inner-shelf deltaic sequences and in the underlying prodelta shales. Porosity vs. vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the prodelta shales but have been generated by fluid expansion in the inner-shelf deltaic sequences. However, the geology of Brunei precludes overpressures in the inner-shelf deltaics being generated by any conventional fluid expansion mechanism (e.g., kerogen-to-gas maturation), and we propose that these overpressures have been vertically transferred into reservoir units, via faults, from the prodelta shales. Sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer, and hence, different pore-pressure prediction strategies need to be applied in the prodelta shales and inner-shelf deltaic sequences. Sonic and density log data detect overpressures generated by disequilibrium compaction, and pore pressures are accurately predicted using an Eaton exponent of 3.0. Sonic log data detect vertically transferred overpressures even in the absence of a porosity anomaly, and pore pressures are reasonably predicted using an Eaton exponent of 6.5.

Present-day stress and neotectonics of Brunei: Implications for petroleum exploration and production

The present-day state of stress in Tertiary deltas is poorly understood but vital for a range of applications such as wellbore stability and fracture stimulation. The Tertiary Baram Delta province, Brunei, exhibits a range of contemporary stress values that reflect the competing influence of the northwest Borneo active margin (situated underneath the basin) and local stresses generated within the delta. Vertical stress (v) gradients at 1500-m (4921-ft) depth range from 18.3 MPa/km (0.81 psi/ft) at the shelf edge to 24.3 MPa/km (1.07 psi/ft) in the hinterland, indicating a range in the shallow bulk density across the delta of 2.07–2.48 g/cm3. The maximum horizontal stress (Hmax) orientation rotates from margin parallel (northeast–southwest; deltaic) in the outer shelf to margin normal (northwest–southeast; basement associated) in the inner shelf. Minimum horizontal stress (hmin) gradients in normally pressured sequences range from 13.8 to 17.0 MPa/km (0.61–0.75 psi/ft) with higher gradients observed in older parts of the basin. The variation in contemporary stress across the basin reveals a delta system that is inverting and self-cannibalizing as the delta system rapidly progrades across the margin. The present-day stress in the delta system has implications for a range of exploration and production issues affecting Brunei. Underbalanced wells are more stable if deviated toward the hmin direction, whereas fracture stimulation in mature fields and tight reservoirs can be more easily conducted in wells deviated toward Hmax. Finally, faults near the shelf edge are optimally oriented for reactivation, and hence exploration targets in this region are at a high risk of fault seal breach.