quartz surface morphology of tertiary rocks from north east sarawak, malaysia: implications for...

RESEARCH PAPER

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 41, Issue 6, December 2014 Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2014, 41(6): 761–770.

Received date: 02 Feb. 2014; Revised date: 25 Aug. 2014. * Corresponding author. E-mail: [email protected]; [email protected] Copyright © 2014, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Quartz surface morphology of Tertiary rocks from North East Sarawak, Malaysia: Implications for paleo-depositional environment and reservoir rock quality predictions

Abdullah Musa Ali*, Eswaran Padmanabhan Department of Geosciences, Faculty of Geosciences and Petroleum Engineering, Universiti Teknologi PETRONAS (UTP), Tronoh, Perak, Malaysia

Abstract: The relationship between quartz surface textural defects (derived from weathering and diagenesis), palaeo-depositional envi-ronment and reservoir quality were studied using Tertiary outcrop rock samples obtained from the Belait and Lambir formations of the Sarawak Basin, Malaysia. Thin sections were used for mineral identification and to make observations regarding grain size and texture. Morphological characterization of the samples was performed using scanning electron microscopy (SEM) attached with energy-dispersive X-ray spectrometry (EDX) system, to show variations in quartz surface texture. The SEM images of Belait conglomerates reveal euhedral quartz crystals characterized with prominent mechanical weathering defects (such as straight and conchoidal fractures and striations). Conversely, the analysis of the Lambir sandstones identified chemical weathering features (such as chemical etchings, pitting, solution pits and notches). On the basis of petrology, SEM and CT scan images, evaluation results of reservoir quality indicate that the Lambir Formation in this study area is high-energy coast deposit, with apparent tide-dominated features; while Belait Formation is neritic-delta deposit, with obvious wave-dominated features; reservoir quality of the Belait Formation and Lambir Formation are poor, but the porosity of the Belait Formation is relatively higher than that of the Lambir Formation.

Key words: Sarawak Basin; quartz morphology; scanning electron microscopy (SEM); microtextures; paleo-depositional environment; reservoir quality

Introduction

The impact of rock evolution processes on the surface morphology of quartz has been further studied over the past few years[1]. Related studies[2−3] have predicted palaeo-depo-sitional environments from which sediments were originally derived based on sand-grain surface textures. Others think that grain compositions and textures are related to weathering processes[4]. In addition, Xia et al.[5] carried out combined textural and kinetic study of pseudomorphic mineral replace-ment reactions to show the role of fluid phase on distinctive surface textures. These studies are significant because they elucidate the relationship between provenance, mineral com-position and grain surface features. This correlation is impor-tant because quartz morphology has been shown to control petrophysical properties, capillarity and sorption[6−13].

The Tertiary rocks from North East Sarawak have been much studied over the past years. Although studies have been

carried out to determine its hydrocarbon distribution[14], field characteristics[15], and facie and reservoir characteristics of Tertiary sediments[16], no specific study has investigated varia-tions in surface textural defects of these Tertiary samples and their relation to reservoir quality. In their studies on the im-pact of fabric variability on layered Fe-oxide deposits in Mid-Late Miocene sedimentary formations, Padmanabhan & Kessler[17] suggested that there were more changes in the en-ergy levels and processes involved than what has been earlier known from usual fabric analyses, as shown by the variations in the fabric within concretions. As a result, they asserted that there is a major gap in the understanding of the composition of mixed solid-phase minerals, their sizes, morphology and arrangement in the matrix and possible interactions with pore solutions and the environment of deposition[17]. Therefore, this study aims to characterize the distribution of microtextures on quartz grain surfaces for the selected study area to supplement what is already known, predict the palaeo-depositional environments from these grain surfaces, and determine

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Abdullah Musa Ali et al. / Petroleum Exploration and Development, 2014, 41(6): 761–770

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ronments from these grain surfaces, and determine consequent weathering and diagenetic features that can control the reser-voir quality.

1 Overview of the study area

The study area is located in the Sarawak Basin of Malaysia (Fig.1). The Sarawak Basin is most widely believed to origi-nate as a foreland basin formed after the collision of the Lu-conia Block with the West Borneo Basement during the Late Eocene, as a result of NW-SE trending right lateral fault movement during late Oligocene to Pliocene times[18]. How-ever, the rapid subsidence in the early stage of this basin indi-cates a strike-slip origin. This notion is also supported by the findings of regional seismic stratigraphic study, which sug-gests that the whole onshore area of Sarawak and northern Borneo was subjected to strike-slip tectonism during Tertiary Period[18].

The sediments were from the deformation and uplift areas of Rajang Group accretionary prism, which formed the Ra-jang Fold Thrust Belt[19]. The samples used for this study were obtained from outcrops of Belait and Lambir formations in onshore Sarawak basin, located in North Eastern Sarawak as indicated by B and L respectively in the map of the study area (Fig. 1). Previous studies have described the Miocene sedi-ments of the Belait Formation as intervals of thick bedded, barren, massive, pure white, and medium-coarse grained sandstones interbedded with claystones and alternating shales with minor limestone and marl in some places[18−20] from dif-ferent sedimentary environments associated with relatively large delta[22] or fluvial sediments[18]. The Lambir Formation in Middle Miocene mainly consists of fine-coarse grained sandstone, shale and some limestone, cemented by iron ox-ides[20]. Provenance studies carried out on the Lambir sedi-ments by Liechti et al., (1960) suggested shallow marine to deltaic sedimentary environment[20], while others have put forward diverse depositional settings ranging from distal tur-bidite through to upper distributary channel[32]. The Lambir Formation grades into the Belait Formation as observed in the stratigraphic column of Fig. 2.

The Belait outcrop samples collected for this study pre-dominantly consist of clean, fine-medium grained sandstone,

Fig. 1 Sketch map of the study area (modified from reference documents [18]−[21])

Fig. 2 Generalized stratigraphic column for the Belait and Lambir formations (modified from reference document [20])

occasionally intercalated with mudstone. Its major deposit structures include planar cross-bedding, trough cross-bedding, herringbone cross-bedding, wave ripples and thin parallel lamination (Fig. 3). These features indicate that the deposi-tional environment of the Belait Formation was tide-do-minated fluvial environment. The Lambir outcrop samples mainly consist of massive medium-grained to coarse-grained sandstone, intercalations of sandy clay with bioturbation and

Fig. 3 Sedimentological log of (a) Belait Formation outcrop characterized by tide-dominated parallel laminations, planar cross-bedding, herringbone cross-bedding, wave ripples and mud drapes; (b) Lambir Formation outcrop characterized by simpler features that includes planar cross-bedding, parallel laminations and massive sand deposits.


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ophiomorpha. Indurated clayey ironstone and pyritic nodules with Fe-bearing sandstone were also observed in other parts of the outcrop.

Outcrop observation showed that there are apparent differ-entiating characteristics between the Belait and Lambir for-mations, although they have many similarities in general from in the outcrop areas. For instance, presence of asymmetrical ripple marks on the Belait Formation indicates fluvial envi-ronment, whereas the much more developed symmetrical rip-ple marks in the Lambir Formation, indicate offshore transi-tional to shallow marine environment[15]. To evaluate reservoir quality, the porosity of the rock samples were quantified via pore network generation and extraction from CT scan images using the Avizo software.

2 Petrology features of samples

Two samples from the Belait Formation and two samples from the Lambir Formation were analyzed for this study. Thin sections of the samples were studied at different magnifica-tions using a polarizing microscope. The morphological char-acterization and composition analysis of the samples were performed using a high-resolution field emission scanning electron microscope (FESEM: Carl Zeiss Supra 55VP FESEM; operated at 5 to 20 kV) attached with an energy-dispersive X-ray spectrometry (EDX) system. Carbonate coatings and ferruginous contamination were left unmoved to study the interaction between quartz and secondary mineral composi-tion. Thereafter, the ferruginous sample of the Lambir Forma-tion was cleaned for a more detailed study of the quartz sur-face. By following the SEM atlas created by Mahaney[2], the environmental implications of various types of microtextures were identified. To evaluate reservoir quality, the porosity of the rock samples were quantified via pore network generation and extraction from CT scan images using the Avizo software.

2.1 Analysis of micro-sections

Thin sections of the study samples were analyzed in plane polarized light. The minerals and components identified in the thin sections include: quartz, carbonates (organics), iron ox-

ides, and rock fragments. The thin section of Belait conglom-erate shows predominant fine grained quartz mineral with clay cement (Fig. 4A). The visibly interlocked grains indicate very low sensitivity to weathering and diagenesis. Its occurrence in the basal part of the Belait Formation suggests short distance of transport and close proximity to source area.

Conversely, the Belait sandstone consists of detrital terri-genous quartz with a high amount of authigenic clay mineral and carbonate cement as well as authigenic quartz overgrowth (as shown in Fig. 4B). Also typical intergranular pores were also observed in the samples. However, the Belait sandstone formation has lost part of its porosity through the compact deformation of the ductile grains, accompanied by grain to grain contact induced silica precipitation and authigenic pre-cipitation of clays within the pore spaces, which in turn fur-ther reduced the porosity of this formation.

The Lambir sandstones comprise of single detrital quartz crystal along with secondary iron oxide mineral and organics (clay minerals, shown in Fig. 5). The thin sections show ex-tensive fracturing and weathering of quartz with moderate cementation and substantial pore infilling by iron oxide and clay minerals.

2.2 SEM analysis

2.2.1 Belait conglomerate

The surfaces of the Belait conglomerate reveal euhedral angular quartz crystals with well-defined faces and polygonal texture (Fig. 6). Identifiable structural features include con-choidal fractures, serrated edges, striations and granulations, flat cleavage planes, brecciated quartz grains, granulations, and irregular ridges. Flakes of silica precipitates are also ob-served in the samples, which are possibly resultant features of pressure-induced dissolution (by collision between grains) or chemical dissolution by reactive pore fluid.

The observed quartz grain fragmentation and brecciation are resultant features of silica solution penetration along micro fractures and dislocation defects[23−24]. As a whole, the quartz crystals exhibit irregular grain surface due to intense me-

Fig. 4 Thin sections in plain polarized light of (a) Belait conglomerate showing fine grained quartz (Q) and (b) Belait sandstones show-ing quartz (Q) and carbonate cement (C)


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Fig. 5 Thin sections in plain polarized light of (a) Lambir sandstones showing fractured quartz (Q) with minor iron and organics impu-rities and (b) Lambir sandstones showing weathered quartz (Q) with iron oxide coating and infilling

Fig. 6 SEM images of Belait conglomerate. (a) crystalline quartz grains with distinct polygonal texture showing normal (F) and con-choidal fracture (CF), cleavage planes (CP) and striations (S); (b) Closer look of crystalline quartz grains at 1000x magnification showing striation marks (S), serrated edges (SE), normal fracture (F) irregular ridges (IR); (c) brecciated quartz and (d) granulations

chanical weathering, and their intact inherent euhedral indi-cate low degree of chemical weathering. The EDX obtained from the sample show predominant silica composition with trace amounts of clay (Fig.7).

2.2.2 Belait sandstone

In contrast to the clusters of euhedral grains of Belait con-glomerates (Fig. 6), Belait sandstones depict platy minerals loosely attached to the grain walls and appears to rest on the surface of the detrital quartz, suggesting that the quartz was first deposited (Fig. 8). The images show notch and pits de-velopment along with phyllosilicate intrusions (Fig. 8).

The quartz morphology is also characterized by discernible chemical weathering features such as incomplete coatings, pitted surfaces, dissolution pits and etchings, which indicate an interaction between quartz and phyllosilicate. Adhering

particles in mechanical/chemical origin were also observed. These features have been attributable to the catalytic action of clay phyllosilicates[25]. This involves the ion-exchange be-tween silica and phyllosilicates through the substitution of

Fig. 7 EDX of Belait conglomerate showing predominant silica composition with trace amounts of clay


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Fig. 8 SEM images of (a) notch (N) and pits development on quartz surface, (b) pitted surfaces from dissolution, (c) adhering particles (AP), striations (S), etchings (E) and notches (N), and (d) phyllosilicate intrusions (PI) and etchings (E)

Fig. 9 EDX of Belait conglomerate showing predominant silica composition with trace amounts of clay

tetravalent Si4+ by trivalent Al3+, thus creating uncompensated charges in the quartz crystal structure[26−27]. To maintain elec-trical charge balance, monovalent H+, Li+, Na+, and/or K+ from surrounding leachate or connate water intrude into the quartz tetrahedral structure in interstices and cavities[27]. These foreign ions create dislocations and lattice defects in quartz, providing easy access for physiochemical attack. The EDX obtained for the sample show predominant silica composition with trace amounts of clay (Fig. 9).

2.2.3 Lambir sandstone

Two samples of Lambir Formation were analyzed. The first sample consists of quartz coated with iron oxides and authi-genic cement. The quartz crystal is hardly visible as a result of crystallographic degradation by iron oxide coating (Fig. 8). Chemical features identified are notches and surface etching as observed in Fig. 8. Depending on the mode of origin, these chemical features are formed as a result of chemical dissolu-tion and precipitation. The notches formed by the chemical dissolution on quartz surface are in the form of micro circular

pits (Fig.10). The EDX obtained for the sample show consid-erable amounts of iron oxides and sulfur because of extensive quartz coating by these minerals (Fig. 11).

In order to properly study quartz surface microtextures of the Lambir sandstone, the Fe-oxide and organics coating should be removed. Firstly, about 20 g of the sample was

Fig. 10 The SEM micrographs Lambir sandstone showing car-bonate mineral (C), quartz (Q), notches (N), iron oxide coating (IC), surface etchings (SE)

Fig. 11 EDX of Lambir sandstone


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ultrasonified in cold HCl solution of 0.1 mol concentration to remove the carbonate coatings and ferruginous contamination. The washed samples were soaked with 35% hydrogen perox-ide (H2O2) to remove the organic debris and adhering iron coatings. Lastly, the quartz grains were washed several times in distilled water and dried in oven at 60 ºC for 24 hours. In-dividual quartz grains with grain size ranging between 200 and 400 μm sizes were selected randomly, to study the depo-sitional features of various environments[2]. The resultant im-ages show more distinctive and more discernible surface fea-tures like the etch pits (Fig. 12).

2.3 Genesis of micro-structure on quartz surface

The quartz surface textures observed are listed in Table 1. A total of 13 microtextures were identified, and then classified based on their origin into three main groups: mechanical, me-chanical/chemical, and chemical. Six microtextures were classified as mechanical, three as mechanical/chemical, and four as chemical origin. Chemical origin includes microtex-tures developed by dissolution and precipitation.

3 Discussions

3.1 Depositional environment

3.1.1 Belait Formation

The depositional system and facies distribution of the Be-lait Formation suggest tide-dominated environment based on the distinctive combination of sediment supply, structural features and relative change in sea-level. Several studies have identified tide-dominated sub-environments that include tidal channel, muddy tidal flat and mixed tidal flat sub-en-vironment. However, the scope of this study is limited to the tidal channel sub-environment. The proportion of sediments preserved in the tide-dominated environment is controlled by the variation in energy of the tidal current and the type of sediments available for transportation. The sandstone depos-ited in tidal channel predominately consist of clean, fine-me-dium grained sandstone with trough cross-bedding, planner cross-bedding, wave ripples, herringbone cross-bedding con-tain mud intraclasts, mud drapes and carbonaceous material (Fig. 3). The base of each tidal channel is marked by an ero-

Fig. 12 Cleaned Lambir quartz grain showing etch pits

Table 1 Microtextures observation results on quartz grain surface

Microtextures Belait conglomerate Belait sandstone Lambir sandstone Origin

Conchoidal fractures Present Absent Absent Mechanical

Serrated edges and irregular ridges Present Absent Absent Mechanical

Striations Present Absent Absent Mechanical

Granulations Present Absent Absent Mechanical

Flat cleavage planes Present Absent Absent Mechanical

Brecciated quartz grains Present Absent Absent Mechanical

Silica ‘plastering’ Present Present Absent Mechanical/chemical

Notches Present Present Present Mechanical/chemical

Adhering particles Present Present Present Mechanical/chemical

Incomplete coatings Absent Absent Present Chemical

Pitted surfaces Absent Present Present Chemical/dissolution

Dissolution pits Absent Present Present Chemical/dissolution

Etchings Absent Present Present Chemical/dissolution


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sive surface that cuts downward into sandstone to create mud intraclasts (1−3 cm). The tidal channel base is also character-ized by mud drapes with planner cross-bedding and parallel thin lamination structures. In some rare cases, tidal channel sandstone was identified with the structure of herringbone cross-bedding, which was about 15 cm thick with an angle of 40o from the bedding plain (Fig. 3A). The herringbone cross-bedding is the characteristic of tidal sedimentation, al-though it does not occur in all instances.

Moreover, the microtextures of the exposed quartz grains were also used to analyze the depositional environment. For the Belait conglomerate, the relative absence of chemical weathering defects on the quartz grain surface suggests stable chemical environment devoid of dissolution. The inherent morphological features like striations and fractures reflect processes such as grain to grain impact and mechanical abra-sion during weathering, transportation by wind as well as ele-vated concentrations of crystalline quartz. Likewise, these mechanical weathering features can also indicate wave action during fluvial transportation[28]. The conchoidal fractures are microtextures generally related to glacial sediments[29−31], al-though several studies have demonstrated that they are the most frequent mechanical features of sand grains derived from crystalline rocks, in a high energy environment (such as flu-vial or wave action of high tide in littoral environment)[2, 28], while the straight striations can be resultant features of trans-portation by wind[2]. The combination of these diverse micro-textures in a single grain suggests that the sediments are products of several sources or alternate events of weathering and provenance. Overall, the strike slip tectonism that oc-curred in the Sarawak Basin during Tertiary Period suggests that the sediments were reworked from older sediments and deposited in a high energy coastal environment[18].

The chemical weathering defects observed on the quartz surface (such as the etching and dissolution pits) are attribut-able to the penetration of authigenic clay minerals into detrital quartz and the formation of weathered grain boundaries by the catalytic action of clay phyllosilicates[25]. Alkalis can be cleaved off from the sides of phyllosilicate flakes into the pore water by incongruent dissolution thereby increasing mineral reactivity leading to quartz grain deterioration. The chemical features can also suggest mineral and sea water interactions, which indicate its marine environment deposition.

3.1.2 Lambir Formation

The Lambir Formation predominantly consists of massive medium-grained to coarse-grained sandstone, intercalations of bioturbated sandy clay along with indurated clayey ironstone and pyritic nodules The sandstone is characterized by sedi-mentary features that include planar cross-bedding, parallel laminations and ripples. This formation becomes less com-pacted and more friable towards the top, although indurations from iron oxide coating were observed. The sediments have been described as neritic-delta sediments[20], and its sedimen-

tation suggested being wave-dominated and more tidally in-fluenced in some local outcrops[32].

The Lambir sandstone grains contain large amount of iron coating and pyrite which made visual quartz identification difficult. The pyrite frambroids are generally associated with bacteria sulphate reduction, which concurs with the suggested marine sedimentary environment. Several mechanisms have been put forward to explain the physiochemical behavior and reactivity potential of Fe-oxides under different environments. The formation of Fe-oxide coating through redox reactions has been considered as a plausible cause of quartz deteriora-tion[33], where Fe-oxides can be reduced and mobilized under acid conditions by accepting electrons from water molecules, thus making the environment alkaline. Oxidation reverses the above reaction and H+ is released to the surrounding envi-ronment, making the precipitation zone acidic. As the Fe so-lubilizes, dissolution voids are created, thereby subverting the morphology of the enclosed quartz grains[34−35]. This results in chemical/dissolution features that include dissolution pits, etchings, pitted surfaces and incomplete coating.

3.2 Reservoir quality

The composition of the Belait conglomerate is mainly coe-site, capable of retaining low stress interval, but can lose po-rosity to grain fracturing due to overburden compaction. The quartz flakes formed from pressure-induced precipitation and brecciation inhibits quartz overgrowth. Moreover, the lack of Fe impurities and relatively minor amounts of clay minerals allow the transformation of the rock samples to more crystal-line products. In addition, the serrated interfaces of adjacent quartz grains promote dissolution, whereas, thermodynami-cally, a smooth boundary would be less advantageous. The exposed grain surface due to the absence of coating provides space for potential nucleation sites that promote the growth of quartz cement which subsequently limits compaction. The quartz cementation can reduce porosity, but preserve perme-ability.

In contrast, the authigenic clay and iron oxides that act as coating materials for Belait and Lambir sandstones inhibit or delay cementation, consequently preserving porosity. The presence of the carbonate intraclasts, pyrite frambroids and iron oxides in the Lambir sandstones reduces thermally-acti-vated quartz cement growth[36], while the authigenic quartz overgrowth and clay coatings act as pore lining minerals at the interstices between the grains. The clay minerals and adhering particles exhibit no apparent alignment relative to the frame-work grain surfaces, but partially replace detrital grains or fill voids (notches and pits) left by dissolution, and can preserve the textures of the host quartz grains they replace. The direct relationship between the degree of cementation and sandstone porosity at surface conditions suggests that grain coating re-stricts or hinders cementation and preserves porosity during deep bury and high temperatures, but decrease permeability at pore throats. Therefore, grain coating is a determining factor


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Fig. 13 Workflow for obtaining pore network model for the Belait sandstone

Fig. 14 Workflow for obtaining pore network model for the Lambir sandstone

for the porosity preservation potential of a formation[32]. To quantitatively analyze the impact of pore infilling, we

carried out micro-structural studies to investigate the porosity of the ferruginous sandstone sample. By means of CT scan and multi-scale visualization techniques (Avizo Fire), a CT scan imagery corresponding to a 3D linear X-ray attenuation pixel matrix was generated, by developing a 3D data set of stacked adjacent cross-sectional 2D images (Fig. 13 and Fig. 14). After performing serial sectioning and imaging, 320 CT scan slices were obtained for both samples. A 3D pore struc-tural model was developed by stacking the 2D CT scan im-ages. Image processing software (Avizo Fire) was used to generate a 3D mineral grid and reconstruct a pore network model. The process was performed in two main steps. (1) The first step involves the creation of 2D binary images using an image conversion process, to show the porosity in the image with pixel values of 0 and 1. Grayscale threshold value of 0−50 was used for porosity calculation and 50−200 was used for volumetric bulk material estimation. (2) A scale transfor-mation from pixels into actual dimensions was performed according to the image size and magnification scale. A 2D quantification module was utilized to quantitatively analyze the pore size distribution. The pores were extracted, and af-terwards the pore volume was calculated to give a value of 3.8% and 0.1% for the Belait and Lambir samples, respec-tively. The Lambir samples are characterized by lower poros-ity, possibly due to secondary ferrous impurity present.

4 Conclusions

The Lambir Formation and the Belait Formation are

time-equivalent ones, yet quartz grain morphology variations exist between these formations. The Belait Formation mainly composes of fine-medium pure sandstones, with minor mud-stone interlayers. Its major deposit structures include planar cross-bedding, trough cross-bedding, herringbone cross-bedd-ing, wave ripples and thin parallel laminations. The Lambir Formation mainly includes massive sandstones with bioturba-tion and ophiomorpha, generally showing regular alternations of sandstone and sandy clay; the sandstone is generally cross-bedded, medium-coarse grained and contains lenses with small quartz pebbly surfaces. Indurated clayey ironstone and pyritic nodules occur in argillaceous sediments, locally with Fe-bearing sandstone.

The quartz content and associated textural defects of the Belait conglomerate indicate high chemical stability. On the contrary, the images of Belait sandstones and Lambir sand-stones suggest the dissolution and replacement of silica by clay minerals and iron oxide. Therefore, interactions between clay, iron oxides and quartz play significant roles in sandstone porosity evolution. In addition, pore evolution by means of dissolution can be better monitored in Belait conglomerate because its original texture is well preserved, while the Belait and Lambir sandstones are characterized by more intricate grain defects. Based on petrology, SEM and CT scan image analyses, the reservoir quality indicate that the Belait Formation was deposited in a high energy coastal environ-ment, with apparent tide-dominated features; the Lambir Formation was deposited in neritic-delta facies, with obvious wave-dominated features. The reservoir quality of both for-


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mations is poorer, although the Belait Formation exhibits slightly better porosity than that of the Lambir Formation.

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