Depositional Characteristics and Petroleum Exploration Potential of the Upper Miocene Sedimentary Sequence in East Sabah, Malaysia.

Transcription

1 Depositional Characteristics and Petroleum xploration Potential of the Miocene Sedimentary Sequence in ast Sabah, Malaysia. Sanudin Tahir, Junaidi Asis and Baba Musta Faculty of Science and Natural Resources Universiti Malaysia Sabah Abstract This study is to document facies associations of the Miocene rock units recorded from the various locations in Sabah, Malaysia, namely; the Belait Formation, the Meligan Formation, the Kapilit Formation, the Sandakan Formation and the Bongaya Formation. Sections of the rock units were measured in detail to reveal their sequential variations and depositional environment as well as petrographic analysis to evaluate their reservoir characteristics. The measured sections are predominantly composed of interbedded sandstone and mudstone with shallow marine fossils and plant remains. The interbedded sandstone and mudstone regularly show sedimentary structures which are indicative of wave and tidal processes, such as swaley, hummocky, trough and planar cross stratifications. Bioturbation is common that include range of inchnofacies Skolithos to Cruziana, indicative of shoreface to inner neritic depositional environments. The major depositional systems, concluded as storm-dominated near shore package that includes five depositional facies, namely: tidal inlet facies, lagoon facies, upper to lower shoreface facies and inner shelf facies. This range setting from high tide level to below storm wave base is located between shoreline to inner shelf framework. The sequence grades up-section into facies sets of increasing tidal energy and terminate with the deposition within the tidal inlet setting. The arrangement of the facies suggests a pattern of normal regression of sedimentary successions in this part of Borneo during Miocene. Petrographic analysis of different sandstone samples from different facies were used to demonstrate the diagenetic maturity of the rock units. Some selected sandstones for petrograpic analysis gave evidence of texturally and mineralogically mature quartz arenite with good sorting. However, the presence of unstable lithic grains and feldspars during diagenetic processes contributed to the reduction of porosity, giving irregular porosity due to history of sedimentation, and susceptibility to chemical alteration of labile minerals. Most observed porosity in the sandstone sequence is secondary, developed from dissolution of both carbonate cement and unstable framework grains. Lithic fragments, feldspars and authigenic cements were altered to form diagenetic mineral suites, which tend to occlude porosity; besides, dissolution and chemical reactions of some of these rock forming particles also enhanced secondary porosity development. Reservoir quality was also influenced by the subsequent diagenesis of the sandstones, which was driven by overburden pressure and could be culminated by continuously increasing temperature and pressure during the history of proceeding depth of burial. Mean porosity suggested trace of decreasing trend with increasing depth of burial. However, detailed examination of several localities revealed that the major secondary porosity development was strongly influenced by local factors. Thus, the Miocene shoreline deposits in Sabah can be concluded as fair sandstone reservoir potential and could be a potential petroleum system which is very similar to the proven petroleum system in the adjacent area, the Kutei Basin. It is therefore recommended that further and more detailed studies to be conducted in this sedimentary sequence. Keywords: Sabah, Sedimentary facies and Petroleum Potential. 1

2 Introduction Miocene sequence is probably an attractive frontier for the key area to the petroleum exploration potential in Sabah. Previous worked and our recent study suggested that the area could contain potential petroleum systems. The area represents the major Neogene basins as identified onshore, the Sandakan Basin. quivalent rock unit is widely exposed in Dent Peninsula, Sandakan Peninsula and northeast Sabah (Figure 1). the Sandakan Formation and the Bongaya Formation. Sections of the rock units were measured in detail to reveal their sequential variations and depositional environment as well as petrographic analysis to evaluate their reservoir characteristics. Material and Methods The surface data and literature review of the area were used for this study. Sampling stations and measured sections were located within the traverse area and were chosen to provide the data for this study. In every station, field data support the lithofacies to do interpretation for every selected outcrop. Among the important data is the lithology, geometry, grain size, sedimentary structures, thickness of the rock layer and trace fossil distributions. The lithology logs were then analysed and interpreted with respect to the sedimentary facies and other characteristics to determine the depositional environments. Fresh rock samples were collected for petrographic analysis. Geologic Setting and Stratigraphy Figure 1. Geology of eastern Sabah showing the Miocene to Pliocene clastic sedimentary units. The geology of Sabah was established by the Geological Survey of Malaysia (Jabatan Mineral dan Geosains Malaysia), and excellent reports with high quality geological maps have been published that reveals the Miocene lithostratigraphic units. xtensive investigations for oil and minerals have been conducted in the offshore as well as onshore areas by PTRNAS and international oil companies since 1970s to give better views. With those credits, researchers gain excellent baseline data for the continuous collection of geological data to improve the understanding of the stratigraphy of Sabah. This study is to simplify the association of the Miocene rock units those were recorded from various locations in Sabah, Malaysia, namely; the Belait Formation, the Meligan Formation, the Kapilit Formation, 2 xtensive fieldwork data has been used to develop palaeogeographic reconstructions of Sabah (Figure 2), which have in turn been used to assess the sedimentary sequences of equal setting during the late Middle Miocene to Pliocene. The rock units those exposed in Sabah with these equivalent ages, are the Belait Formation, the Meligan Formation, the Kapilit Formation, the Sandakan Formation, the Tanjong Formation and the Bongaya Formation are dominated by clastic shoreline deposits, the Tabanak Formation, the Sebahat Formation, The Ganduman Formation and the Togopi Formation with locally influenced by channel feeding to the offshore setting. The rock units are sitting on top of the Middle Miocene unconformity that marked as deep regional unconformity (DRU) revealed by offshore seismic data. This unconformity marked the top erosion limit of the deep marine Palaeogene sequences of the Bornean and wide spread to the neighbouring stratigraphy. The Middle Miocene DRU is

3 marked by tremendously thick Mélange and volcanic facies all over onshore Sabah and none of the outcrops that differentiating between the mélange and the volcanic facies show any hiatus. Seismic interpretation deduced thick Tertiary sedimentary sequence and estimated that the upper section of the stratigraphic units are interpreted to be hydrocarbon potential. The potential reservoirs are expected to be within the structural and stratigraphic traps of the clean thick sandstone layers. orogenic belt. This evidence is proven by the geographical distribution that occupies the tips of the peninsulas in east Sabah (Figure 3). Figure 4 shows the simplified stratigraphy of the area. Figure 3. Geological map of Sabah (modified from Yin 1985). The Miocene Sequence east Sabah Figure 2. Paleogeographic reconstruction of northwest Borneo during Tertiary (Sanudin and Baba 2007). These post-tectonic sedimentary units occurred within the remnant of an accretionary prism of the orogenic belt forming shallow marine basins. The post-magmatic sequence was marked by the deposition of detritus within the Late Neogene circular basins of major areas in east Sabah (Figure 3). The sediments of the circular basins were unconformably deposited over deformed Palaeogene sequences, which indicate intra-arc basin. This was followed by the late Middle Miocene to Pliocene sediments of which are probably related to fore deep basin sedimentation that developed at the front of the This study documents shallow marine facies recorded from the Miocene basins in east Sabah. The rock units identified include; the Sebahat Formation, the Ganduman Formation, the Tabanak Formation the Sandakan Formation, the Bongaya Formation and the Togopi Formation. Sections of the rock units exposed onshore in eastern part of Sabah were examined and measured in detail to reveal their sequential variations and depositional environment. The successions of the sequence are exposures of major shoreline to deltaic deposits of the region during the late Miocene. The sections are predominantly composed of various thicknesses of interbedded sandstone and mudstone with indicative sedimentary structures, shallow marine fauna and trace fossils. The sandstone usually shows sedimentary structures which are indicative of storm, wave and tidal processes, such as swaley, hummocky, trough and planar cross stratifications (Figure 5). Figure 6 reveals the vertical section of the interpreted shelf sequence from the selected composite sections. Bioturbation is common and include range of inchnofacies Skolithos to Cruziana that indicate shoreface to inner shelf depositional environments. The sequence grades up-section into facies packages of increasing tidal energy 3

4 and terminate with the deposition within the upper flow regime shoreline settings deltaic and tidal inlets. Silty samples collected from one of the circular basins in the Dent Peninsula area contains nannofossils of at least Middle to Late Miocene age, namely Cyclococcolithus macintyrei, Helicosphaera carteri, Cyclococcolithus leptoporus, Sphenoliythus abies, and Recticulofenestra pseudoumbilica (Rangin et al. 1990). AG PLISTCN P L I. M I C N Lower RCK UNIT R S T Hc Togopi Fm Bongaya Sandakan Middle Tb Gn... Sebahat...Hiatus. Volc. Fs. Melange Lower Lst. Fs. L I G Lower C N Labang Fm Middle Figure 4. Stratigraphy of the ast Sabah (Sandakan Basin): R potential reservoir, S potential seal, T potential trap system, and HC potential hydrocarbon. The major depositional system, reconstructed as partly storm-dominated nearshore package that 4 includes four major depositional facies, namely the tidal inlet sandstone facies, lagoonal mud facies, upper to lower shoreface sandstone facies and inner shelf interbedded sandstone and mudstone facies. Some depositional events that prevailed during the early stages of the deposition of the shallow marine sequences indicate instability of the area during deposition, as evidenced by numerous slumps and submarine mass flow deposits grading conformably into little deformed sediments. The argillaceous is characterized by dark gray, thickly bedded mudstone intercalated with thin beds of siltstone and claystone, with ferruginous partings and concretions of calcareous argillite and marl. Isolated sandstone, marlstone and conglomerate are other rock type s equivalent in age. The proportion of the rock types is variable, but the claystone are the dominant type. The sequence is rich in foraminifera, mollusks, plant remains with amber, coral and algae. From the faunal analysis, the depositional environment is between intertidal to several meters depth. The structureless and thickly bedded or massive claystone in most section indicates a calm environment, probably lagoon. This sequence of the lower part grades on to the upper section which is more sandy and grades into sandy facies. This part of the sequence comprises of thick sandstone with claystone interbeds. Throughout the sequence, the lithology is rather uniform with common small to large trough and hummocky cross-bedding. Sedimentary characteristics The field characteristics of the clastic sequence were studied by using facies analysis method and interpreted to have formed within the shoreface and equivalent environments. Field characterisation of sedimentary structures to delineate to the depositional environment of the shallow marine deposits in Sabah has not been reported. The sedimentary structures are relatively recognised feature of storm influenced shoreline deposits. The presence of hummocky cross-stratification (HCS), swaley crossstratification (SCS), deep depositional scours,

5 and intraformational conglomerates indicates that storms were important depositional agent during the accumulation of the sedimentary Fine-grain sandstone, lower section Coarsening upward sequence, upper section The sedimentary structures found can be interpreted as a product of a combination of both bed load transport and sediment falling off from suspension. The clastic facies composed of mainly thick bedded sandstone interlayered with medium to thin bedded shale and is represented by well-sorted, fine to coarse grained arenite. The coarse sandy deposits, mainly upper section of the sequence are characterised by hummocky and swaley cross stratification. Sandstones are texturally and mineralogically mature quartz arenite with good sorting Physical Characteristics Sandstone Samples Hummocky cross bedding Swaley cross bedding Figure 5. Typical sedimentary structures indicative of shoreline deposits from selected sections of the Miocene outcrops. m ud Litholog Facies nvironment Interpretation Sequent nvironment d si lt san Amalgamated and low angle planar cross-stratified shoreface coarse grained sandstone m Hummocky-swaley cross-stratified Lower shoreface fine to coarse grained sandstone Shelf Rare hummocky Shoreface - shelf cross-stratified transition fine grained sandstone-siltstone interbeds Figure 6. Sequence of the Miocene shallow marine deposits. sequence (Figure 5). Combining information from trace fossils and sedimentary structures allow the subdivision of the sequence into deposits of the upper shoreface, lower shoreface and distal lower shoreface. of Selected Petrographic analysis of sandstone samples from selected locations were used to demonstrate the diagenetic maturity of the rock units. Some selected sandstones for petrographic analysis gave evidence of texturally and mineralogically mature quartz arenite with good sorting. However, the presence of unstable lithic grains and feldspars during diagenetic processes contributed to the reduction of pore spaces, giving irregular porosity due to history of sedimentation, and susceptibility to chemical alteration of labile minerals (Figure 7). Most observed porosity in the sandstone sequence is secondary, developed from dissolution of both carbonate cement and unstable framework labile grains. Lithic fragments, feldspars and authigenic cements were altered to form diagenetic mineral suites, which tend to occlude porosity; besides, dissolution and chemical reactions of some of these rock forming grains also enhanced secondary porosity development. Reservoir quality was also influenced by the subsequent diagenesis of the sandstones, which was driven by overburden pressure and could be culminated by continuously increasing temperature and pressure during the history of proceeding depth of burial (Sanudin Tahir 2003). Mean porosity suggested trace of decreasing trend with increasing depth of burial. However, detailed examination of several localities revealed that themajor secondary porosity 5

6 development was strongly influenced by local factors. Table 1. Analysed source rocks collected from some of the rock units exposed onshore. Sam ple Figure 7. Photomicrograph of a selected sandstone sample showing general components.(mo monocrystalline quartz, Po polycrystalline quartz, K altered lithic fragments, I altered feldspar and dark dirty appearance are voids. Most of the grain contacts are tangential and longitudinal types). The organic source rock is matured (Table 1) and is expected to be an intraformational mudstone migrated up dip from a source deeper in the basin. Potential hydrocarbon kitchen areas in the basin are expected to be late Middle Miocene argillaceous facies. Chemical analysis shown that terrestrial to shallow marine are the proper anticipated source rocks. Maturity window was tested by using ratios of oxygencarbon and hydrogen-carbon giving the result of mature migrated hydrocarbon. Hydrocarbon migration from a source to the trap is expected to be via permeable beds present in various stratigraphic levels and fracture system. Timing of trap formation is not considered critical in the basin since hydrocarbon generation from potential source rocks continued up to the present day. The regional top seal of the basin is anticipated to be the Plio-Pleistocene finegrained sequence that is thick andwidespread onshore outcrops. Local top seals would be provided by intra-formational clay stone associated with transitional to shallow water sequences. The potential traps present were formed mostly during the Plio-Pleistocene compressional phase although additional potential traps were formed during the post-rift. 6 Proximate Analysis Moist. % Ash % Volatile Matter % ST a ST b STc SP SP SP SK SK SK KP KP KP Ultimate Analysis C% H% % Total Sulph ur % Play Types There are three major petroleum plays, they are: Structural closures stacked with Late Miocene reservoirs; Late Middle Miocene reefs, and; Late Miocene stratigraphic closures. Conclusions and Recommendations Thick sands deposited by delta and shoreline complexes around Borneo during Miocene have proven to be prolific hydrocarbon reservoirs, e.g.bagram Delta and the Mahakam delta. The Sandakan Basin at the present day is fed by a relatively large river systems, e.g. Kinabatangan River and Segama

7 River could be the key evidence to the thick sedimentary sequence of the area. Seismic data indicates the presence of a large offshore regressive system and delta equivalent complexes (could be a combination of both rivers mentioned). ne of the main challenges for oil and gas exploration in the area, part of the Sandakan Basin, is therefore to understand the provenance, transport, deposition and distribution of sand within the basin, all of which have implications for reservoir quality. In addition, part of the basin has been affected by gravity tectonics, producing spectacular linked up dip extensional structures, all of which are likely to have influenced local depositional transport. High resolution seismic data and experienced analyst are necessary to map sedimentary systems within the Neogene Basin of the area to determine their interaction with the formation of sedimentary sequences. The results can be linked to onshore parts of the basin to give better view of the prospects. It may contain a potential petroleum system that is very similar to the proven petroleum system in the neighbouring areas, e.g. Kutei Basin and Baram Delta. It is therefore recommended that further and more detailed studies be conducted in the area the future addition hydrocarbon prospects in Sabah. References Rangin,C.; Bellon, H.; Bernard, F.; Letouzey, J.; Muller,C. and Sanudin,T Neogene arc-continent collision in Sabah, North Borneo (Malaysia). Tectonophysics, (183): Rangin, C; Muller, C; Letouzey, J; Stephan, J.F.; Chorowicz, J; Bernard, F; Butterlin, J.; Bellon, H.; Sanudin, T.; Baladad, D.; and Berrador, A Arccontinent collision in Sabah and Panay Island (Philippines): Some constraints on the Cenozoic evolution of Sulu Sea marginal basin. Proc. Int. Symp. Geodyn. volut. astern urasian Margin, Paris. Sanudin Tahir Geologi Petroleum. Penerbit UMS: Kota Kinabalu, 377 p. Sanudin Tahir and Baba Musta Pengenalan kepada stratigrafi. Penerbit UMS: Kota Kinabalu, 203 p. Yin,.H Geological map of Sabah. Scale, 1/500,000. Geol. Surv. Malaysia. 7