Sandakan Formation Research Articles

Some mechanical characterization of the Sandakan formation’s sandstone, Sabah, Malaysia

This work defines the mechanical characteristics of slightly weathered sandstone belonging to sedimentary Sandakan formation in Sandakan, Sabah. The mechanical properties of uniaxial compressive strength, shear strength (includes basic and peak friction angle and cohesion) was determined by the uniaxial compressive strength test, direct shear test, tilt test and triaxial compressive strength test. The slightly weathered very fine sandstone gave higher strength but lower in basic friction angle than slightly weathered fine sandstone. The basic friction angle for very fine and fine sandstone are 28° and 33°, respectively. The value of peak friction angle and cohesion of intact rock by the triaxial compressive strength test (52-58° and 2-4 MPa) is higher than direct shear test (16-20° and 0.25-4.00 MPa) of fine sandstone rock mass (for planar surface).

Linking the influence of diagenetic properties and clay texture on reservoir quality in sandstones from NW Borneo

Reservoir studies in the Sandakan sub-basin NW Borneo are challenging and inexplicable due to complex diagenetic effects such as compaction, cementation, dissolution and depositional means including grain size, sorting and packing with reference to clay texture. These diagenetic effects together impart significant amounts of uncertainty on reservoir quality assessment. In this regard, we documented the mineralogical, diagenetic and reservoir properties of sandstones from the Sandakan Formation, NW Borneo through thin sections, SEM, XRD, XRF, He-porosimeter and poroperm. In order to analyze the influential parameters, we used matrix, grain rearrangement and clay coating around detrital grains for the reservoir quality. Our results show that the sub-litharenites to quartz arenite that dominate in the sandstones are drawn from recycled orogenic sand. Furthermore, reservoir properties of the sandstones were ascertained to be controlled by diagenesis, micro quartz with clay coatings and pressure dissolution of feldspar with the alteration of lithic fragments. The result shows that the physical compaction, rearrangements and ductility of grains were the reasons that reduced porosity and permeability. The main reason for porosity reduction in the present study is pressure dissolution that resulted in the form of chemical compaction induced by micas with grain contacts. Nonetheless, through SEM and petrography, clay coatings around the grains and dissolution of feldspar were found to optimally contribute to the preservation of reservoir properties. Dissolution of feldspar and kaolinization resulted in the formation of secondary porosity that allows meteoric water to percolate through the open and detachment spaces of quartz or other grains. This percolation is also responsible in enhancing the pore throat radii in the reservoir media of the analyzed sandstones. These traits are obliging to diagenetic effects on porosity and permeability, which are linked with the physical compaction and clays coating to reduce and preserve the reservoir properties respectively of same type of reservoirs globally.

Sedimentological characterization, petrophysical properties and reservoir quality assessment of the onshore Sandakan Formation, Borneo

Sedimentological parameterization of rock heterogeneity and prediction of shallow-marine siliciclastic reservoir quality remains a challenge globally to date, especially in regions with complex depositional histories, in that, variations in petrographic and petrophysical properties can occur within the same sedimentary sequence of similar age and rock type. Because exploratory drilling strategies and reservoir models can significantly be augmented through direct observation of outcrops that are representative of equivalent reservoir stratigraphic intervals in the offshore zone, outcrop sedimentological characterization can help improve the understanding of subsurface reservoirs of similar strata in surrounding petroliferous basins. To this end, we analyzed well-exposed successions of the mid Mio–Pliocene shallow-marine sandstone deposits of the Sandakan Formation, Borneo, through conventional field investigation, petrographic and petrophysical studies of different sandstone facies types to predict reservoir quality and heterogeneity within different depositional settings. On the basis of these evaluations, the studied sandstone was grouped into three qualitative reservoir rock classes: Class I (Ø = 18.10–43.60%; K = 1265.20–5986.25 mD), Class II (Ø = 17.60–37%; K = 21.36–568 mD) and Class III (Ø = 3.4–15.7%; K = 3.21–201.30 mD). The petrographic and petrophysical results reveals a consistent rise in both gamma ray and permeability values due to a change in depositional environment from proximal lower shoreface to upper shoreface of some successions within the sequence. The change in depositional setting for each sandstone type establishes certain variation in classes within the same facies type. For instance, the permeability may indicate low values due to the presence of bioturbation and increased mud content owing to the burrows with bimodal grain classification, regardless of change in gamma ray readings. This phenomenon signifies that there exists no strict correlation between gamma ray and permeability profile despite containing good quality reservoir rocks and appropriate classification. Our study is of paramount significance to the development of initial exploration strategies in regions where similar siliciclastic strata can be potential reservoirs because it serves as analogs to offshore hydrocarbon bearing packages.

Using the Geographical Information System (GIS) Methods in Determination of Landuse Changes at Batu Sapi, Sandakan, Sabah, Malaysia

The study area is in the southwestern part of the Sandakan city center bounded by latitude 05° 48’ until 05° 52’ N and longitude 117° 58’ until 118° 03’ E and covers the area of 100km2. The study area comprises the Sandakan Formation which is overlain by the Quaternary Alluvium. The landuse changes is done through the Geographical Information System (GIS) method by using the ILWIS (Integrated Land and Water Information System) software. There are four main stages involved in the GIS method to produce the thematic maps. Landuse map of the year 1980 and 2010 have been used to study the changes at the study area. The results show the percentage of urban area coverage have increased, while the village area have the highest percentage of land diminution. So, with the model of the evolution of landuse, it calls for landuse planning in the study area is more sustainable in the future.

Ichnofacies and sedimentary structures: A passive relationship with permeability of a sandstone reservoir from NW Borneo

Permeability network is influenced by ichnofacies in the surface and subsurface sedimentary media, which modifies textural heterogeneity of reservoir rocks by the filling of burrows with the surrounding sediments. In situ measurements of rock permeability from surface analogues of proven subsurface hydrocarbon traps demonstrate the influences of ichnofacies in primary sedimentary structures and resultant high variability within reservoir zones. This work emphasizes on the evaluation of three ichnofacies (i.e., Psilonichnus, Skolithos and Cruziana) that are interdependent and characterized by dense ichnofabrics with sedimentary structures in the exposed rocks of Sandakan Formation. The sedimentary structures associated with ichnofacies are parallel to subparallel, multidirectional trough cross-stratification with current cross laminae and low-angle, undulatory, hummocky cross-stratification (HCS) and oscillation ripple laminated sand with the alteration of silty or sandy mudstones. Results show that the Sandakan Formation was deposited in a transitional environment from a back to foreshore setting and extends to very distal fringes of a proximal lower shoreface or distal delta front to pro-delta. The effective permeability network in the bioturbated horizons of sandstone and interbedded mudstone is very high (250–950 mD). The integration of ichnofacies and sedimentary structures with high permeability profile suggests that the Sandakan Formation has good reservoir potential with the textural heterogeneity. The implication is that the presence of ichnofacies in sedimentary reservoir horizons can substantially increase secondary porosity and enhance pore connectivity; thus, making them valuable and lucrative subsurface hydrocarbon reservoirs.

High resolution facies architecture and digital outcrop modeling of the Sandakan formation sandstone reservoir, Borneo: Implications for reservoir characterization and flow simulation

Advances in photogrammetry have eased the acquisition of high-resolution digital information from outcrops, enabling faster, non-destructive data capturing and improved reservoir modeling. Geocellular models for flow dynamics with in the virtual outcrop in siliciclastic deposits at different sets of sandstone facies architecture remain, however, a challenge. Digital maps of bedding, lithological contrast, spatial-temporal variations of bedding and permeability characteristics make it more easy to understand flow tortuosity in a particular architecture. An ability to precisely model these properties can improve reservoir characterization and flow modeling at different scales. Here we demonstrate the construction of realistic 2D sandstone facies based models for a pragmatic simulation of flow dynamics using a combination of digital point clouds dataset acquired from LiDAR and field investigation of the Sandakan Formation, Sabah, Borneo. Additionally, we present methods for enhancing the accuracy of outcrop digital datasets for producing high resolution flow simulation. A well-exposed outcrop from the Sandakan Formation, Sabah, northwest Borneo having a lateral extent of 750 m was chosen in order to implement our research approach. Sandstone facies and its connectivity are well constrained by outcrop observations, data from air-permeability measurements, bilinear interpolation of permeability, grid construction and water vector analysis for flow dynamics. These proportions were then enumerated in terms of static digital outcrop model (DOM) and facies model based on sandstone facies bedding characteristics. Flow simulation of water vector analysis through each of the four sandstone facies types show persistent spatial correlation of permeability that align with either cross-bedded orientation or straight with more dispersion high quality sandstone (porosity 21.25%–41.2% and permeability 1265.20–5986.25 mD) and moderate quality sandstone (porosity 10.44%–28.75% and permeability 21.44–1023.33 mD). Whereas, in more heterolithic sandstone (wavy- to flaser-bedded and bioturbated sandstone), lateral variations in permeability show spatially non-correlated patterns over centimeters to tens of meters with mostly of low quality sandstone (porosity 3.4%–12.31% and permeability < 1 mD to 3.21 mD). These variations reflect the lateral juxtaposition in flow dynamics. It has also been resulted that the vertical connectivity and heterogeneities in terms of flow are mostly pragmatic due to the interconnected sandstone rather than the quality of sandstone.

GEOHAZARDS IN SANDAKAN TOWN AREA, SABAH, MALAYSIA

The geology of the Sandakan town area provides a favourable setting for geomorphological hazards occurrences. The exposed rocks in the study area and its surrounding vary in types and ages, from Late Eocene-Early Miocene Neogene’s clastic sediment of the Garinono Formation, the Sandakan Formation and Volcanic Facies to vary recent Quaternary alluvial materials which are still being deposited. These rock units are dissected by numerous lineaments with complex structural styles developed during series of regional Tertiary tectonic activities. Rapid urbanization activities have caused changes in the land use. Urbanization processes may contribute to the exposure of impervious surface, loss of vegetation cover, modification of slope gradient and drainage systems. The last decade has seen slope failure, flash flood and erosion events, which have claimed lives, damaged properties and increased the cost of maintenance. The main factors causing geomorphological hazards occurrences in the study area are natural (geology, meteorology, topography and drainage system) and human factors (lack of proper planning, human activities and community’s attitude). Serious efforts to increase the community’s awareness to geomorphological hazards occurrences and reconstruction of natural ecosystem must be taken. To handle this issue, both prevention and mitigation are necessary. At the planning level a multi disciplinary approach to zoning, risk assessment and design with construction practices is recommended. This is to ensure that future activities will not cause further damage to natural environmental condition or ecosystem.

Facies Distribution and Petrophysical Properties of Shoreface-Offshore Transition Environment in Sandakan Formation, NE Sabah Basin

Newly exposed outcrop of Miocene shallow marine sandstone in Sandakan Formation, allows characterization of the facies distribution and petrophysical properties of shoreface to offshore transition environment. Six facies are defined: (1) Poorly bioturbated Hummocky Cross Stratified (HCS) sandstone (F1), (2) Moderately bioturbated HCS sandstone (F2), (3) Well bioturbated HCS sandstone (F3), (4) Poorly bioturbated Swaley Cross Stratified (SCS) sandstone (F4), (5) Interbedded HCS sandstone with sand-silt mudstone, (6) Heterolithic mudstone. The sedimentary successions were deposited in upper to lower shoreface, and offshore transition environment. Facies F3, F4 and F5 shows good reservoir quality with good porosity and fair permeability values from 20% to 21% and 14 mD to 33 mD respectively. While Facies F1 exhibits poor reservoir quality with low permeability values 3.13 mD.

Organic geochemistry and palynology of coals and coal-bearing mangrove sediments of the Neogene Sandakan Formation, Northeast Sabah, Malaysia

The coals and coal-bearing mangrove sediments of Sandakan Formation have been evaluated for their source rock potential, paleoredox conditions and source of organic input, as well as the age and paleovegetation distribution using geochemical and petrographical methods. The studied samples were generally rich in organic carbon, which varied from 0.60 (massive mudstone) to 62.37wt% (coal). The studied samples have vitrinite reflectance (Ro) ranging from 0.31 to 0.49% and Tmax values ranging from 353 to 436°C indicating an immature to very early mature stage for hydrocarbon generation. The sediments were dominated by Type III kerogen, with some Type II/III kerogen, suggesting a significant land plant contribution to the organic matter. The bimodal distribution displayed by gas chromatogram, with a clear odd/even carbon number and skewed to higher molecular weight carbon (nC31), indicating a transitional depositional environment. The variation in pristane/phytane ratio, varied from 0.71 to 2.97, suggesting an alternating anoxic and oxic deposition conditions. The presence of marine dinoflagellate cyst in the mangrove and offshore mudstones were consistent with the relatively higher sulphur content in the studied samples which indicate marine inundations.Palynomorph recovered in this study, with abundant mangrove and freshwater types of pollen, suggesting a biogeographical distribution of ancient mangrove vegetation that expanded landward into the freshwater peat swamp setting and hinterland. The presence of marker species, viz. Florschuetzia levipoli, F. meridionalis and F. semilobata delineated an Early Miocene to Middle Miocene age for the Sandakan sediments.

English

English

Low energy, low latitude wave-dominated shallow marine depositional systems: examples from northern Borneo

The depositional environments of the wave-dominant successions in the middle to late Miocene Belait and Sandakan Formations in northwestern and northern Borneo, respectively, were determined based on grain size distributions, sedimentary structures and facies successions, as well as trace and microfossil assemblages. Generally, progradational shoreface successions in the Belait Formation were deposited in very low wave energy environments where longshore currents were too weak to generate trough cross-bedding. Shoreface sands are laterally continuous for several km and follow the basin contours, suggesting attached beaches similar to the modern Brunei coastline. In contrast, trough cross-bedding is common in the coarser Sandakan Formation and back-barrier mangrove swamp deposits cap the progradational succession as on the modern northern Dent Peninsula coastline, indicating barrier development and higher wave energy conditions than in the Belait Formation. The Borneo examples indicate that barrier systems that include significant tidal facies form under higher wave energy conditions than attached beaches with virtually no tidal facies. Also, Borneo’s low latitude climate promotes back-barrier mangrove which reduces tidal exchange and reduces tidal influence relative to comparable temperate climate systems. The results of the study indicate that depositional systems on low energy, wave-dominated coasts are highly variable, as are the sand bodies and facies associations they generate.

The sedimentology of Miocene shallow marine clastics of the Sandakan Formation of eastern Sabah

The Sandakan Basin lies on the east coast of Sabah and contains some excellent exposures, resulting in part from the steep topography of the region and from the recent development of housing estates and road construction. These often spectacular outcrops allow examination of the Miocene succession, which has not previously been feasible. A total of 4 months has been spent collecting sedimentological data throughout the c. 25 km by 15 km basin. Over 70 measured sections have been logged across the basin and this, in conjunction with numerous macrofossil, microfossil and ichnofacies studies, has allowed the differentiation of the interbedded sands and clays, previously described as a marginal marine succession, into facies associations ranging from continental to shallow shelfal in origin. Seven main facies groups are identified which range from mangrove deposits cut through by fluvial channels, through delta top with channels into facies deposited in a lagoonallinterdistributary bay environment. These pass into shoreface trough or planar crossbedded sands with common Ophiomorpha or Skolithos trace fossils and finally into offshore marine muddy facies. Palaeoseismicity is suggested by synsedimentary deformation, often confined in discrete horizons which can be traced over several kilometres. Detailed palaeontological studies ofthese rocks will allow development ofthis facies scheme and further division of these systems into sub-environments. These well exposed marginal marine facies present an exceptional opportunity to develop a sequence stratigraphic framework of the Sandakan Basin through the Miocene. Examination of the eustatic/ tectonic control of the system will then be applied to hydrocarbon-bearing strata in adjacent basins. Furthermore this study will constrain palaeogeographic reconstructions of the region.