CHAPTER 1 INTRODUCTION
Miri is the birthplace of Malaysian petroleum industry. The oil exploration began in 1909 and the first exploration well, Miri-1, was drilled in 1910. Six years later, by February 1916, 50 wells had been drilled and the production reached a cumulative volume of 1.25 million barrels with a daily production rate of 2700 BOPD.
However, water and sand problems affected the field development in between 1916 and 1919. These problems were resolved and by 1929, some 500 wells had been drilled with peak production of over 15,000 BOPD. In the 1930s, activities started to decline. Production dropped to less than 11,000 BOPD in 1931, 7000 BOPD in 1933, and 2400 BOPD in 1941, shortly before the Japanese invasion.
During the war, the Japanese produced a total of 700,000 barrels. After the war, it took 2 years to bring the field back on stream. The post-war history of the Miri Field was characterized by two conflicting forces: (i) the uphill struggle to keep the dying field alive; and (ii) the necessity to release increasing areas of the field for town expansion and development. Despite efforts to boost production, including field rehabilitation and water injection projects, the field continued to decline. By the end of 1971, the field was producing only some 675 BOPD, with more than 10 times as much water from 98 wells. The battle was finally lost and the Miri Field was totally abandoned on 20 October 1972 (Tan et al., 1999). However, geologically Miri remains one of the most interesting and challenging place to be resolved stratigraphically and structurally.
2 1.2 The Study Area
The location of this research is in around the Canada Hill area in the northeastern part of Sarawak, Malaysia (Figure 1.1). It is located between latitudes 4o20’28’’ N and 4o26’16’’ N and longitudes 113o57’03’’ E and 114o03’01’’ E.
Geologically, the area is called Miri Formation and composed of only less than one third of the total Baram Delta Province which is known as the Miri Field.
1.3 Previous Studies
The studies on Miri Formation began in 1909 by Josef Theodor Erb who evaluated the prospectively of the various locations in the Miri Field and identified a number of structures of oil traps. Erb mapped the structure and recognized the Miri Hill as the top of an asymmetric anticline, with a gentle northwest flank and a steep overturned southeast flank. Erb expected disclosure in the deeper level of the anticline.
Later drilling results proved the stratigraphy and Erb’s structural model based on single anticline was unable to explain the occurrence of oil traps in other parts of Miri.
Large reservoirs were discovered in the plains at the foot of the hill near Miri town center. These reservoirs appeared to be narrow elongate strips extending away from the anticlinal axis towards the way down to the Miri River. Although folding as represented by asymmetric anticline did exist, closure appeared to be due to the fault system which intersected the structure (Tan et al., 1999). Thus L.C. Artis (in Schumacher, 1941) conducted a paleontology study which established a biostratigraphic zonation based on benthonic foraminifera. The study solved many correlation problems and showed that the southern part of the field had consistently been mis-correlated to the northern part by Erb where each sand in the south had been correlated to the higher sand in the north. Based on these two studies, Schumacher (1941) conducted a detail structural study and produced the current structural model of the Miri Field. Liechti et al. (1960) developed a detailed onshore lithostratigraphy of the Miri Formation. As a further development model of the previous study by Liechti et al., Wilford (1961) divided the Miri Formation into a lower and upper part based on lithological differences and small benthonic foraminifera.
Figure 1.1 – (A) Study area with exposures of Miri Formation at the northeastern part of Sarawak, Malaysia. (B) Satellite image of the Canada Hill.
In 1986, Hamid Mohammad assigned a Middle Miocene age to the Miri succession then produced a geological map of Miri which shows a subdivision into several producing reservoir units (Ecomedia, 2000). The structural study was followed by M.K. Shuib in 2003, which considered that the Miri structure evolved under a single deformation by NNE trending dextral strike-slip tectonics event. It was concluded because the evidence for a systematic thickening of the hanging wall block which considered the Miri structure (Shell Hill fault and an associated normal fault) to be a growth fault was absent. Shuib also analyzed that this dextral strike-slip deformation could have commenced from Middle Miocene right up latest Miocene to Pliocene times. Completing the structural studies, van der Zee and Urai (2005) developed a model based on the main structural elements that were formed during the early stages of fault development in the Miri Field. This study observed more than 450 segmented faults of the Airport Road outcrop which contains clay smear or lenses.
These clay smears or lenses are useful for bedding correlation. The numerical model shows that the deformation in lenses inside the fault zones can be expected to be higher than outside the zone.
The stratigraphy of the Miri Formation was defined into 10 different facies by Abdul Hadi (in Tan et. al., 1999) based on the lithology, bed geometry, sedimentary structures and bioturbation. These facies are: (i) medium-scale trough cross-bedded;
(ii) small-scale trough cross-bedded; (iii) herringbone cross-bedded; (iv) flaser- bedded; (v) wavy-bedded; (vi) sand-clay alternation; (vii) lenticular-bedded; (viii) mud crack surfaces and associated mudstones; (ix) hummocky cross-stratified sandstone; and (x) massive coarse sandstone facies. On the basis of the stratigraphic succession of these 10 facies, two facies associations namely: tidal facies and wave- and-storm-dominated facies are recognized (Tan et. al., 1999).
Another sedimentological and facies study was carried out by Abeida (2006) on five outcrops namely Padang Kerbau, Hilltop Garden, Airport Road, Riam Road, and Miri Hospital Road outcrops. Abeida recognized 12 facies based on the lithology, sedimentary structures, fossil traces, bed geometry and thin section information.
These facies are: (i) trough cross-stratified sandstone with mud drapes; (ii) parallel stratified sandstone with mud drapes; (iii) wavy-bedded sandstone; (iv) rhythmic
stratified sandstone and mudstone; (v) lenticular bedding; (vi) homogenous coarse grained sandstone; (vii) swaley cross-stratified sandstone; (viii) thick amalgamated hummocky cross-stratified sandstone; (ix) fine grained bioturbated sandstone; (x) interbedded to bioturbated siltstone and fine sandstone; (xi) bioturbated siltstone; and (xii) mudstone interbedding with parallel stratified to hummocky cross-stratified sandstone. These facies then was grouped into 2 facies associations, namely tide- dominated estuary facies and storm-and-wave-dominated facies.
1.4 Scientific Problems
The Miri Formation is a siliciclastic sequence consisting of a succession of clay-sand packages that are coarsening upwards where its sand member were very important oil and gas reservoirs in the early production in Malaysia. The outcrop localities in the Miri Field have been reported by many authors on various aspects, however the definite structural and stratigraphic model of this formation is still questionable until today. The outcrops have shown a very puzzling geology both structurally and stratigraphically. For example one of the outcrops, i.e. the outcrop at Miri Hospital Road 2, shows a very big contrast of a thick vertically dipping section with a sub-horizontal or gently dipping sequence situated side by side. This was interpreted by Schumacher (1941) as the Canada Hill Thrust which is marked by the sudden change in topography and bedding orientation. However, the high angle (
55o) fault plane in the area is not indicative of thrust fault. The absence of the significance folded fault plane in the field and the evidence of a very short contact zone of a very big contrast between a thick sequence of vertically dipping section with a sub-horizontal or gently dipping sequence situated side by side at the Miri Hospital Road 2 outcrop suggest there is a serious weakness in the early theory of the Miri deformation processes proposed by Schumacher (1941), which was based on two phases of deformation; extension followed by compression.
Thirty years after the oil wells abandonment, many studies for capturing new geological information from the Miri Formation outcrops are still being conducted, simply because new outcrops are exposed resulting from earth works during the latest
urban development in the Miri Town. These outcrops particularly the Boulevard 1, Boulevard 2 and Miri By-pass Road outcrops reveal new structural and stratigraphic information that indicate more detailed study to be conducted. The presence of hydrocarbon seepages in several outcrops in the area indicates that there may still be accumulations of hydrocarbon in the reservoir also warrant further investigation.
1.5 Objectives of the Study
The primary objective of the present study is to provide additional interpretation on the stratigraphy and structural geology of the Miri Formation in the Miri Field based on the new information gathered from new outcrops in the area. Four main sub-objectives are developed as follows:
(1) To describe the facies characteristics of the Miri Formation, and to identify the depositional environment within the investigated facies.
(2) To develop the facies model of the Miri Formation based on the facies association.
(3) To identify and analyze the bedding and faults orientation particularly in the investigated outcrops of the Miri Formation.
(4) To develop the structural framework of the Miri Formation based on the structural patterns of the investigated outcrops and its correlation to the regional structure of Miri, Sarawak.
1.6 Thesis Outline
This thesis consists of five chapters. Chapter 1 is the introduction including the scientific problems and objectives of the study. Including in this chapter is overviews of the previous works were done in the Miri Field. Chapter 2 presents the literature review on the geological setting of the Miri Formation and fundamental concept of geology applicable on Miri. Chapter 3 presents the material and methods that were used in this research. Including in this chapter is overview on the outcrops
description. Chapter 4 explains the results of field observation, discussion on facies, structural geology, and the structural framework of the Miri Formation based on the structural patterns of the outcrops and its correlation to the regional tectonic of Miri.
Chapter 5 includes the conclusions and recommendations for further study.
CHAPTER 2 LITERATURE REVIEW
This chapter is brief and focused on: (1) geological setting of the Miri Formation, and (2) fundamental concept of geology which applicable in Miri. The geological setting of the Miri Formation reported by the previous workers on this field is a very important literature to be reviewed. This information is very fundamental in understanding the geology of the study area, and valuable for guiding the field investigation and analysis conducted in this study. Due to the large scope of fundamental geological concept which may applicable in Miri, thus only concepts which relate to the objectives of the research were selected to be reviewed in this chapter.
2.2 Geological Setting of the Miri Formation
Regionally, Miri Formation represents the lower part of the Sarawak Basin which is Late Eocene to Recent in age (Mazlan, 1999). Sedimentation in the Sarawak Basin was mainly in the coastal to shallow marine environments during the Oligocene and Early Miocene. It was a foreland basin in collisional setting during the Oligocene to Early Miocene subjected to active extensional and strike slip tectonic, but later underwent a phase of coastal-shelf progradation and passive continental margin during the Middle Miocene to Recent accompanied by phases of deformation involving extensional, wrench faulting and thrusting (Mazlan, 1999).
Although regionally the Miri Formation is part of the Sarawak Basin but geologically, it is situated as an extended onshore part of the West Baram Delta which is roughly triangular in shape, with its apex occurring onshore and centered in Brunei and the northeastern coastal area of Sarawak (Tan et al., 1999). The Baram Delta depocentre developed throughout the early Miocene as faults controlled depression, formed at the intersection of two major crustal-scale faults; the West baram Line and the Jerudong-Morris Fault (Figure 2.1). The appearance of Baram Delta which is limited between major crustal-scale basement faults is an example of a continental embankment founded by extreme sediment loading on transitional-oceanic crust.
Following an Early Miocene tectonic event, uplift and erosion were accompanied by the deposition of a thick pile of clastic sediments which prograded seaward throughout Neogene times. Relatively coarse sediments, predominantly sand, were deposited in coastal plain, deltaic and coastal environments. Sedimentation was strongly influenced by tectonic activity and very thick sequences accumulated in sub- basin (Mazlan, 1999; Tan et al., 1999)
Since Middle Miocene, the Baram Delta has been subsiding relative to the more stable Central Luconia and Balingian provinces to the west. Within the Baram Delta, major increases in sedimentary thickness occur across growth fault, which generally trend NE-SW in the main depocenter but swing towards the NW-SE direction, on trend with the West Baram Line to the west (Berbeito, 2003). The West Baram Delta comprises up to 9-10 km long of Miocene to Recent siliciclastic sediments derived from the south-southeast, along the trend of the present-day Baram River, and from the west and southwest across the West Baram Line. The distal part of the Baram Delta is situated on the continental slope and extends into the Sabah Through (Tan et al., 1999).
2.2.1 The Miri Formation
Rock successions outcropping around the city of Miri, which stratigraphically belonging to the Miri Formation of Middle to Late Miocene, are the uplifted part of the oil-bearing reservoirs in the Miri Field. The stratigraphic relationship between the Miri Formation and surrounding Formations is shown in Figure 2.2.
Figure 2.1 – Baram Delta Province structural map (modified from Mazlan, 1999).
Figure 2.2 – Schematic Stratigraphic successions and correlations of Neogene formations in Miri area (Tan et al., 1999).
Artis (in Schumacher, 1941) established a biostratigraphic zonation based on benthonic foraminifera, resulting in the lithostratigraphic scheme (Table 2.1) which is still in use today. The study solved many correlation problems encountered by the previous workers. Liechti et al. (1960) described the formation consists predominantly of sandstones with shale and clay restricted mainly to the lower part.
The base of Miri Formation forms a conformable transitional contact with the argillaceous Setap Shale and Lambir Formations. The predominated arenaceous Miri Formation is conformably overlain by the Seria Formation.
The difference between the Lower and the Upper Miri is not clear in order to be mapped based on the boundary on lithology alone. The Lower Miri, is composed of interbedded sandstone and shale that grades downwards into Setap Shale Formation. The Upper Miri is more arenaceous. This unit is composed of more numerous and irregular shale sandstones alternations, with sandstones beds passing gradually into clayey sandstone and sandy or silty shale (Wilford, 1961). From the identification of marine microfauna and lithological characteristics, Liechti et al.
(1960) concluded that these sediments were deposited in a litoral to inner neritic shallow marine environments.
Table 2.1 – Stratigraphic framework of the Miri Field (Hutchison, C.S., 2005 based on Tan et al., 1999).
12 2.2.2 The Miri Structure
A structural model of Miri was developed after the identification of the fossils by paleontologists was introduced by L.C. Artis (in Schumacher, 1941). Schumacher (1941) then combined the results of the new correlation with the results of various detailed fault studies (Figure 2.3a). This map that was improved by the Sarawak Shell shows a subdivision of several producing reservoir units (Figure 2.3b).
Figure 2.4 is the cross sections through the Miri Field (Schumacher, 1941). It illustrates the following elements of the structure:
(1) a set of steep normal faults hading to the northwest, one of which (the Shell Hill fault), with a vertical displacement of thousands of feet.
(2) a set of flat normal faults, hading to the southeast with a combined throw of some 1000 ft.
(3) a set of merging reverse faults, hading to the northwest (the Canada Hill Thrust)
(4) an asymmetric, slightly overturned, anticlinal fold.
The development of the present day Miri structure was interpreted to be attributed to two separate periods of deformation (Schumacher, 1941):
(1) an early period of extension, indicated by two sets of normal faults.
(2) a later period of compression, indicated by the reverse faults and the asymmetric anticline.
The early period of extension, which prevailed during and shortly after the deposition of the sediments (Late Miocene) gave rise to three features according to the interpretations by Schumacher (1941):
(1) a large normal fault (the Shell Hill Fault) with a throw up to 2500 feet.
(2) a set of more or less parallel normal faults, which tend to merge with the Shell Hill Fault at greater depth, but with much smaller displacement.
(3) a set of normal faults (antithetics) which head in the opposite direction and may have formed as reaction to the space created during deformation.
( a )Figure 2.3 – (a) Miri Field structural map (modified from Tanet al., 1999). (b) The Sarawak Shell geological map of the Miri Hill, showing the subdivision of the hill into different reservoir producing units (after Ecomedia, 2000).
( b )
Figure 2.4 – The NW-SE cross section of the Miri Field and the stratigraphic- structural position of the different reservoir units according to the Shell interpretation (modified from Tan et al., 1999).
Similar composite fault systems have often been related to growth faulting.
The Shell Hill fault, also is called a growth fault, evidence for thickening of the sediments in the down-thrown blocks is not obvious, but the regional context supports the idea that the Miri structure does not stand on its own but as a part of a system of similar structures with more or less the same orientation, extending all over the Baram province. This fault system is thought to be induced by a regional system of tensional stress which may also have been responsible for the very high rates of subsidence in the Baram Delta and perhaps for the Baram Delta Basin itself (Berbeito, 2003).
Much later, in Pliocene times after the Miri Formation had been buried below the Seria Formation, tectonic conditions changed. Compressional forces pushed the previously stretched Baram Basin and buckled the sediments in a row of more or less parallel anticlines some 10 km apart. Most oilfields in the Baram Delta province, including the Miri Field, appear to be situated on the intersection of extensional growth faults and compression anticlines (Berbeito, 2003).
This later (Pliocene) period of compressional movements modified the original Miri structure: the whole package of sediments was folded and the first generation of faults was rotated anti-clockwise, so that the West headers were steepened and the East headers flattened (Figure 4.1). Locally the formation was too competent to buckle and break thus, creating the thrust faults (eg. Canada Hill thrust fault) at the back of the Hill (Berbeito, 2003).
2.3 Fundamental Concept of Geology Applicable to Miri
A sedimentary facies is defined as a sediment (or sedimentary rocks) that displays distinctive physical, chemical, and/ or biological characteristics that make it readily distinguishable from the associated facies in the locality (Stow, 2005). As the product of the deposition, a sedimentary facies can simply express the characteristic of a particular depositional environment or a particular depositional process. Thus facies model have been proposed to show the lateral and vertical relationships between facies (Tucker, 2001).
Structural geology is the study of the architecture of the earth—especially of the Earth’s crust. The word ‘structure’ means ‘that which is built or constructed’. In a specific point of view, the objective of structural geology is to improve the understanding of the internal architecture of the crust, of how that form came into existence, and of how it has been modified (Spencer, 1988). The structural geology study thus leads to consider the ‘setting’ or the structural framework of the surrounding region. Simply, the study on structural framework attempts to fit the mapped beds, folds, and/ or faults of the study area into the pattern of folded or faulted blocks of the regional area.
Based on studies of modern and ancient sedimentary environments, processes and facies, generalized facies models have been proposed to show the lateral and vertical relationships between facies (Tucker, 2001). These models
facilitate interpretations of sedimentary formations and predictions of facies distribution and geometries. However, facies models are just snap shots of an environment; sedimentary systems are dynamic and a facies model may only relate to a particular state of relative sea-level change (Tucker, 2001). The importance of the vertical succession of facies was first appreciated by Johannes Walther at the end of the nineteenth century in his ‘Law of the Correlation of Facies’: different facies in a vertical succession reflect environments that originally were adjacent to each other, providing there were no major breaks in sedimentation (Tucker, 2001). In other words, only those environments that are laterally associated to each other geographically may become associated in a vertical sequence. Thus the study of vertical profiles means also the study of lateral facies relationship through time (Reineck and Singh, 1980).
184.108.40.206 Sedimentary Environments
Selley (1996) defined a sedimentary environment as a part of the surface of the earth that is physically, chemically and biologically distinct from adjacent areas. A sedimentary environment may be a site of erosion, non-deposition (equilibrium), or deposition. The third type of environment is the environment of depositions that occurs in continental, shoreline and opens marine settings. It is the depositional environment that is the principal concern and interest to sedimentologist, because this is the environment that actually generates sedimentary sequences that are preserved in the stratigraphic column (Selley, 1996).
There are some classifications of depositional sedimentary environments.
Furthermore most environments can be divided into sub-environments. Here are the overview of the main sedimentary environments, sedimentary processes and the resulting sedimentary structures for the selected environments which are related to the outcrops detailed further in this thesis. The selected environments are tidal depositional system, deltaic, and shallow marine depositional system.
17 A. Tidal Depositional System
Tides result from the gravitational attraction exerted on oceanic or lake waters by the moon and the sun, with the moon having more than twice the effect of the sun.
Although tides have less effect on the transport of sediment and on coastal morphology than waves, they affect coasts in two important ways. They govern: (i) the strength and flow pattern of the regularly fluctuating tidal currents; and (ii) the amount and timing of tidal rise and fall. Tidal currents are most effective when operating in conjunction with waves (Reading, 1996).
Siliciclastic sediment is commonly deposited along marine shorelines in beaches, barrier islands, tidal flats, estuaries and the shoreface-shallow offshore (Selley, 1996). Detail environments classified by Tucker (2001) are described below:
(1) Beach-barrier island and strandplain system, develop in microtidal to mesotidal areas, generally where tidal range is less than 3 m. A lagoon is located behind a barrier island and connected to the open sea via tidal inlets (Figure 2.5). In microtidal areas, tidal channel (inlet) are widely spaced along the barrier, but in mesotidal locations it is prominent and tidal deltas are usually developed at the ends of the inlets. Mud flats and marshes are common around the lagoon, especially on the landward side (Figure 2.5). Where there is an abundant supply of sand, high wave energy and low tidal range, a strandplain of beach ridges forms.
(2) Tidal flats reach several kilometers in width and occur around lagoons, behind barriers, and in estuaries and tide-dominated deltas. Tidal flat can be separated into two main zones – the supratidal, above high-tide level and intertidal, between high and low tide levels (Reineck & Singh, 1980).
Intertidal can be divided into three zones which are mud flats, mixed flats, and sand flats (Figure 2.6). There is usually a decrease in sediment grain size from sand in the low intertidal zone to silt and clay in the higher part.
Common sedimentary structures of the mid upper tidal flat (mix-muddy flats) are various types of ripple, usually showing interference patterns, and then give rise to flaser, wavy and lenticular bedding (Figure 2.7). In sand flats and mixed flats, small-current ripples and wave ripples-mainly asymmetrical. Cross-bedded sands with some herringbone structure are most abundant (Figure 2.8)
(3) Estuaries, usually shaped as an open funnel of a river in the sea which is really influenced by tidal movement and mixing between river and sea waters. Estuaries vary according to whether tides dominate over waves, or the reverse. Mud flats and swamps also occur in estuaries (Tucker, 2001).
The characteristic feature of estuarine deposits is its position in the transition from fluvial to marine deposits in a transgressive sequence, both laterally and in a vertical sequence. Once the supply of sediment from river becomes dominant, the estuary changes into delta and a progradational delta sequence is produced, on the top of a transgressive sequence (Reineck & Singh, 1980)
B. Deltaic Depositional System
Deltas are complex environments with characteristics that are determined by the nature of the river system supplying the sediment, coastal processes and climate.
Deltas can be divided into several parts (Figure 2.9). The delta plain refers to the area landward of the shoreline, and an upper delta plain, dominated by river processes, is distinguished from a lower delta plain where there is some marine influence, mainly in tidal inundation. The delta front includes the mouth bars, distal bars in front of the distributary channels, and the prodelta in the deeper offshore region (Tucker, 2001).
The upper delta plain is the area where fluvial, lacustrine and swamp sediments occurs. Both braided and meandering streams can occur, although the latter are more common in upper delta plains. Branching of the main river channel may occur to give smaller channels separated by floodplain. Also present are shallow lakes mostly filled with fresh water, although it may be hipersaline if the climate is arid.
The lakes are site of mud deposition derived from overbank flooding of the distributary channels. Frequently a river will break its banks by crevassing and a small delta will build into a lake. A coarsening upward unit from silt and clay passing up into sand is common, and this thin lacustrine deltaic unit would be cut through by feeder channel (Tucker, 2001).
Figure 2.5 – Subenvironments for a beach-barrier island and lagoon shoreline system (Tucker, 2001).
Figure 2.6 – Block diagram of a typical siliciclastic tidal flat (Dalrymple, 1992).
Figure 2.7 – Tidal bedding changes from (C) lenticular through (B) wavy and (A) flaser bedding as the proportion of mud decreases, seaward (Reineck and Singh, 1980)
Figure 2.8 – Diagrams showing common types of cross bedding; (a) planar cross- bedding, with planar bedding surfaces and tabular to wedge-shaped cross-bedded units, (b) trough cross-bedding in horizontal, transverse and longitudinal sections, (c) herringbone cross-bedding showing foreset lamina dipping in opposite direction, (d) tabular, planar and trough cross-bedding interbedding (Reineck and Singh, 1980).
On the lower delta, channels become more numerous as they divide into smaller distributaries. Leeve are well developed and interdistributary bays occur at the shoreline between channels. Bays between channels are large in some deltas and these are gradually filled by sediments crevassing from major distributaries. (Tucker, 2001).
The delta front is the region where the sediments carried by distributaries is deposited. At the distributary channel mouth, the flow expands, mixes with sea water and deposited its bedload, mostly sand, at the mouth bar. Dunes giving cross bedding and ripples giving cross lamination and flasser bedding occur on the mouth bar, but modification takes place if wave action or tidal current are operative. Fine sediments accumulate farther offshore in a distal bar, where fine sand and mud give laminated and lenticular bedding. Bioturbation is common here. The delta front is the area of progradation. Deposition on the mouth and distal bars results in a seaward building of the delta front so that coarser sediments of the mouth bars come to overlie finer sediments of the distal bar and prodelta. The thick, coarsening-upward unit so produced is the characteristics feature of the deltas (Tucker, 2001). Modern deltas can be subdivided on the strength of the fluvial, wave and tidal input into three categories;
river-dominated deltas, wave-dominated deltas and tide-dominated deltas (Tucker, 2001).
Figure 2.9 – The subenvironments of a lobate and elongate (bird’s foot) delta.
Progradation of lobate delta give rise to a laterally extensive delta front, whereas a linear sand body is generated by an elongate delta (Tucker, 2001).
C. Coastal and Shallow Marine Depositional System
The coast separates continents from seas and may develop in various geomorphic and sedimentological variants. Shallow marine environment away from the coastline with water depths ranges from 10 to 200 m occur on continental shelves.
Coastal and shallow marine environments are characterized by the interplay between chemical or biogenic grains and clastic sediment transported into the system by rivers of variable current strength, and the marine processes in the form of wind-generated waves, storm, tides and the fluctuations of relative sea-level (Reineck & Singh, 1980).
Shallow marine depositional system includes environments from beach (foreshore) and shoreface, through inner and outer shelf settings as shown in Figure 2.10. The backshore represents the upper part of a beach which remains normally dry, except under unusually high water conditions, when it can be flooded and acted upon by waves and weak currents (Reineck & Singh, 1980). Within foreshore and shoreface environments, sediment transport is driven by waves. The waves are able to move sediment on the sea floor at a maximum depth of about half their wavelength.
Shoreface is characterized by day to day sand transport above fair-weather wave base.
This environment is sand-dominated. The base of the shoreface can be defined at the point where sandstones-mudstones pass upward into relatively clean sandstones.
While offshore is characterized by mud-dominated. Wave and storm may produce some common sedimentary structures in shoreface and shallow marine, such as ripples and dunes, hummocky cross-stratification, and swaley cross-stratification (Reineck & Singh, 1980).
Figure 2.10 – Generalized shoreline profile showing subenvironments, processes and facies (Reading, 1996).
23 220.127.116.11 Methods for Environments Diagnosis
There are many different techniques which can be used to determine the depositional environment of a sedimentary rock. These vary considerably according to whether the study is based on surface or sub-surface information. The techniques of environmental analysis can most conveniently be discussed under the five defining parameters of a facies: geometry, lithology, sedimentary structures, palaeocurrent patterns, and fossils (Selley, 1985). Sedimentary facies is the result of various processes that have operated in the environment in which the rock was deposited, such as physical, chemical and biological processes (Table 2.2).
Table 2.2 – The relationship between the sedimentary environments and sedimentary facies (Selley, 1985).
A facies is a three dimensional body of rock having geometrical shape that reflect its pre-depositional topography, the geomorphology of the depositional environment and its post-depositional history (Selley, 1985). Similarity facies geometry could be produced in one of several environments, for example channels could be fluvial, deltaic, tidal or submarine. Geometry of sedimentary facies is not diagnostic of sedimentary environment. Determination of geometry is relatively simple where is exposed clearly on the outcrop surface. Tracing a sedimentary unit provides information on lateral continuity, thickness and changing in characters.
24 B. Lithology
Simply, lithology means the description of the physical character of a rock. In term of a sedimentary rock, lithology is considered as a function of transportation processes on land of the type of rock from which it was originated. Rock texture is referred to be as part of lithology study that holds many important clues to its depositional processes and environments (Tucker, 2001). The basic descriptive texture element of all sedimentary rocks is the grain size. Grain size of sediments is a sign of a hydraulic energy of the environment, where the finer sediments are transported by slower-flowing currents and tend to accumulate in quiet environments, whereas the coarser sediments are transported and deposited by faster-flowing currents. The spatial relationship between grain size distributions in a rock is often described in terms of sorting. In a well sorted rock all the grains are about the same size and shape, whereas a poorly sorted rock contains grains with different size and shape. Sorting is one of the most useful parameters because it gives an indication of the effectiveness of the depositional medium in separating grains of different classes (Tucker, 2001). Three aspects of grain morphology are the shape, sphericity and roundness. The shape of a grain is measured by various ratios involving the long (L), intermediate (I) and short (S) axes. Sphericity is a measure of how closely the grain shape approaches that of a sphere. Roundness is concerned with the curvature of the corners of a grain. Grain fabric in a sedimentary rock refers to the grain orientation and packing, and to the nature of the boundaries between the grains (Tucker, 2001).
C. Sedimentary Structures
Sedimentary structures are the larger-scale features of sedimentary rocks and include the familiar cross bedding, ripples, flute and load cast, dinosaur foot prints and worm burrows. The majority structures are formed by physical processes, before, during and after sedimentation, whereas some are the results of organic and chemical processes. Sedimentary structures particularly those formed during sedimentation have a variety of uses: (1) for interpreting the depositional environment in term of processes, water depth, wind strength, etc.; (2) for determining the way-up of a rock succession in an area of complex folding; and (3) for deducing the palaeocurrent pattern of the palaeogeography (Tucker, 2001).
Sedimentary structures can be classified into primary and secondary classes (Selley, 1988). Primary structures are formed by physical processes in sediments during or shortly after deposition such as cross bedding, , ripples, flute marks, slumps, etc. Primary sedimentary structures are divisible into inorganic structures including those already mentioned and organic (biogenic) structures such as burrows, trails, etc., while the secondary structures that formed sometime after sedimentation is the result of chemical processes, which generally caused diagenetic formation of concretions.
D. Palaeocurrent Patterns
Many sedimentary structures can be used to determine the sense, and sometimes the flow direction of the current from which they deposited the sediment.
Palaeocurrent analysis is very important for mapping the palaeogeography of sedimentary basins, and as an aid to interpreting depositional environment.
Furthermore it is a very powerful tool in predicting the geometry, and trend of mineral deposits and petroleum reservoir characteristics in sedimentary rocks. The structures that may be used to interpret palaeocurrent range in scale from channels, down to the orientation of fossils and sand grains (Selley, 1996).
Fossil and trace fossils are regarded as one of the most important methods of identifying the depositional environment of sediment. Fossils is related to the remains of animal and plant in sediments such as shells, leaves and fish scales, while the sedimentary structures that formed by organism, the way in which fossil lived, behaved and their influence by their environments are known as trace fossils (Selley, 1985). A group of fossils that occur at the same stratigraphic level are largely controlled by certain related parameters that reflect their environment. such as gradients in water temperature, salinity, dissolved oxygen, turbulence, sedimentation rates and current activity are the main parameters controlling the occurrence of various fossil types. Trace fossils such as tracks, trails, burrows and borings may often be the only evidence of life in sediment if body fossils were not preserved.
26 2.3.2 Structural Geology
The primary goal of structural geology mapping is to use measurements of present-day rock geometries to uncover information about the history of deformation (strain) in the rock and ultimately to understand the stress field and link it into important events in the regional past geologic processes. On a large scale, structural geology is the study of the three dimensional relationships of stratigraphic units to one another within geological regions as a result of deformation processes.
18.104.22.168 Deformation Processes
Deformation is often described as strain. Strain is the measure of the deformation of a body. It involves the displacement of the parts relative to each other that is a change in shape or in volume, or both. Changes in shape are called distortion, and in volume are called dilatation. When forces are applied to the external surface of a body, they set up internal forces within the body, which is then called as stress (Hills, 1975). The response of a rock to stress depends on the type of stress, the amount of pressure, the temperature, the type of rock, and the length of time the rock is subjected to the stress. Based on Hills (1975), there are three types of stress:
(1) Compressional stress is forces which directed toward one another that tend to decrease the volume of a material.
(2) Tensional stress is stretching stress that tends to increase the volume of a material, and
(3) Shear stress is forces that is parallel but in opposite directions, resulting in displacement of adjacent layers along closely spaced planes.
As a response to stress, strain on a rock may be taken as two types of deformation (Hills, 1975):
(1) Elastic deformation where strain is proportional to stress. Rock will return to original volume/shape if stress is removed.
(2) Plastic Deformation is a permanent deformation caused by flowing and folding at stresses above the elastic limit at high confining pressure and/or temperature. Warm rocks tend to deform plastically.
Once the elastic limit is surpassed, the rocks behavior can be different. Rocks will deform plastically if the rock is ductile—called ductile deformation, or they will fracture (rupture) if the rock is brittle—called brittle deformation. Rocks at or near the surface (cold, low pressure) tend to deform by brittle rupture.
22.214.171.124 Geological Structures of planar Type
Geological mapping is one of the most important sources of structural data which is usually dealing with outcrops. The rocks at the vast majority of outcrops posses some kind of planar structure. A frequently used way of describing the attitude of a planar structure is to measure and record the strike, dip and direction of dip.
Geologists use the concept of strike and dip to describe the orientation of deformed rock layers
In most sedimentary rocks a planar structure known as bedding is visible. This is a primary feature formed at the time of deposition and layering is characterized by compositional, textural or grain size variations (Leyshon and Lisle, 1996). Other planar geometry in rocks is joint, fault and fold planes which the elements of deformation product. Joints are fractures in rocks along which little or no movement has taken place. Assessment of the orientation of joints is an essential part of any stability analysis. Joints can be caused by compression or tension. Compressional stress can produce joints in the area of a fold axis. Whereas faults are planes along which the rock on one side is displaced relative to the other. The directions of principal stress axes at the time of faulting can be estimated from the orientation of fault planes (Leyshon and Lisle, 1996). Joints and faults are the features of brittle deformation. Folds are structures produced when an originally planar surface becomes bent or curved as a result of deformation (Park, 1997). Folds are produced by ductile deformation during compressive stress. Axial plane of fold is another planar structure which bisects the angle between two limbs of a fold. Fault and fold will be the main point discussed later in this thesis.
28 A. Faults
Faults are the results of brittle deformation where the breaks move relative to each other. The forces that are acting in the block can be compressional, tensional or shear stresses. Several classifications of faults may be used to give a more complete description of faults (Spencer, 1988). The most commonly used terms are based on the apparent relative movement of the fault blocks as follows:
(1) Normal fault: the hanging wall (block above an inclined fault) is down relative to the foot wall (block below the fault). Commonly normal fault have steep dips (± 60o), but in many cases the dips become less at depth and can become sub horizontal.
(2) Reverse fault: the hanging wall is up relative to the footwall. Reverse faults dip at steep angles—commonly 45o or more.
(3) Thrust fault: a reverse fault with a low average angle of dip. Lateral displacement is generally much greater than the vertical displacement.
Dips may vary from horizontal to steep along the same fault.
(4) Strike-slip fault: lateral movement of the blocks parallel to the strike of the fault.
Normal faults, is form in response to extension. The fault zones associated with them are quite different from thrust or strike slip zones. Most normal faults have steeply dipping fault plane, but many are curved and have shallower dips at depth and some curve until they become parallel to bedding at depth. The following are some common structural patterns along the normal faults classified by Spencer, 1988:
(1) Grabens, down-dropped blocks bounded by more or less parallel normal faults that dip toward one another, and horsts, up thrown blocks bounded by normal faults, are characteristic features of lateral extension.
(2) Antithetic faults are a minor, secondary fault, usually one of a set and have an orientation opposite to its associated major and synthetic faults.
Antithetic-synthetic fault sets are typical in areas of normal faulting.
(3) Reverse drag is distortion of layering near normal faults caused by drag of the layering along the fault as the result of frictional forces set up in the fault zone. Drag on the downthrown side of the fault usually in the opposite direction from the block movement orientation (Figure 2.11)
(4) Listric fault is a normal fault where the dip of the fault decreases with depth (Spencer, 1988). Listric fault may be accompanied by an accommodation fold, known as a rollover anticline. The accommodation may take place by the formation of a set of antithetic faults, which have the effect of extending and thinning the hanging wall (Park, 1997).
Figure 2.11 – Features associated with normal fault zones. (A) Rotation of one block on a curved fault. (B) Reverse drag. (C) A small graben formed along the fault zone as a result of movement on antithetic faults (Spencer, 1988).
Figure 2.12 – Cross section of St. Genevieve fault zone, Ozora, Missouri, showing drag along one of the reverse faults in the zone (Spencer, 1988).
A reverse fault is one on which the hanging wall moves up relative to the footwall. This type of fault is the result of compressional stress. Generally reverse fault have a high (45o or more) average dip. This distinguishes them from thrust faults which have low average dips and generally involve large amounts of lateral movement of the rocks above the fault zone (Spencer, 1988). The following are some common structural patterns along the reverse faults classified by Spencer (1988):
(1) Synthetic and antithetic faults usually accompanied a major reverse faults.
(2) Extensional features are another structural pattern formed along the reverse fault. As the uplifted block rises, a high relatively unconfined surface is formed along the margin. This surface tends to bend and expand, and the resulting extension leads to the development of grabens and step faults oriented parallel with the block margin.
(3) Drag is likely resulted by a deformation of stratified rocks along reverse fault. This may appear on either the upthrown or the downthrown side of the fault; it may appear on both sides and only a single side (Figure 2.12).
The term thrust fault is applied to faults of low average dip (< 30o) on which the hanging wall has moved up relative to the footwall (Spencer, 1988). This fault is also the result of compressional stress. Folding that usually accompanies thrusting, especially of thin bedded sedimentary layers within and adjacent to thrust fault, is probably due to drag produced as the overthrust sheet moves laterally. Development of folds presumably progresses from open to asymmetric and finally to overturned form. The next step in this progression would presumably be a thrust fault forming parallel to the bedding and cutting through the layers on the overturned limb of the asymmetric fold.
Strike slip fault is also called wrench faults because they involve lateral shearing. Where they cut local structures, they may be called tranverse faults, but many strike slip faults are parallel to the regional trends of the deformed belts in which they occur (Spencer, 1988).
Another type of fault which is related with the topic in this thesis is growth fault. Growth fault is a type of fault on which there were displacements at the same time as the sediments on either side of the fault were accumulating (Spencer, 1988).
Most growth faults are normal faults. A growth fault is characterized by the strata on the hanging wall side of the fault tend to be thicker than those on the footwall side (Figure 2.13). It is commonly formed in the delta front or prodelta region. With time, offsets of marker beds increase with depth, and sediment thickness increases abruptly across the fault. The fault angle decreases with depth and commonly associated with it are rollover folds or reverse drag structures (Tucker, 2001).
Figure 2.13 – Features of growth fault shows the thickening strata on the hanging wall.
The direction of stress that causes faulting is illustrated in Figure 2.14. Three sets of conditions in which the stresses act as recognized by Hills (1975):
(1) Maximum stress horizontal; mean stress horizontal; minimum stress vertical. This produce reverses faults. The section of faulted rock is shortened in the direction of maximum compression (Figure 2.14-A).
(2) Maximum stress horizontal; minimum stress horizontal; mean stress vertical. This produce complementary strike-slip faults (Figure 2.14-B).
(3) Maximum stress vertical; minimum and mean stress horizontal. This produces normal faults dipping at more than 45o (Figure 2.14-C).
Figure 2.14 – Initial stress distribution on the mechanism of faulting (Hills, 1975).
The main elements of the geometry of the fold shape are, firstly, the hinge (or closure), which is the zone of maximum curvature of the surface, and secondly, the limbs which are the areas between hinges (Park, 1997). A single fold comprises a hinge and two limbs which enclose the hinge. In a series of folds, each limb is common to two adjacent folds. If it is possible to define a line along which the maximum curvature of the fold takes place, this line is called the hinge line. Under normal conditions where the bedding becomes younger upwards, a fold will contain older rocks in its core is called anticline. Thus the term anticline strictly applies only to a fold with older rocks in its core. Conversely, a syncline is a fold that contains younger rocks in its core. Other common term related to fold is dome. It is an up- warped structure with a circular or elliptical outcrop pattern. Beds dip away from center of structure suggests the oldest rocks are at center.
Most folds, when viewed in a section perpendicular to the axial surface, may be described as symmetrical or asymmetrical on the basis of the symmetry of the limbs disposed about the axial surface. Depending on the inclination of the axial surface, folds (Figure 2.15) may be described as upright, inclined, overturned or recumbent (Spencer, 1988).
Figure 2.15 – Example of various types of fold symmetry as viewed in cross sections normal to the fold axis (Spencer, 1988).
Other components that can be determined in a fold are limbs, interlimb angle, fold axis and axial plane. The interlimb angle expresses the tightness of a fold. As the name suggests, it is the angle between the two fold limbs. Once calculated, according to Leyshon and Lisle (1996), the interlimb angle allows the fold to be classified as shown in Table 2.3.
Table 2.3 – Fold classification based on the interlimb angle (Leyshon and Lisle, 1996)
Class Interlimb angle
Gentle fold (180-120o)
Open fold (120-70o)
Close fold (70-30o)
Tight fold (30-0o)
In analyzing folds, the orientation attributes of folds (plunge and axial surface) allows to be classified as in tables 2.4 and 2.5.
Table 2.4 – Fold classification based on plunge (Leyshon and Lisle, 1996)
(0 -10o) non-plunging
(10-30o) gently plunging
(30-60o) moderately plunging (60-80o) steeply plunging
(80-90o) vertical fold
Table 2.5 – Fold classification based on dip of axial surface (Leyshon and Lisle, 1996)
(0 -10o) recumbent
(10-30o) gently inclined (30-60o) moderately inclined (60-80o) steeply inclined
(80-90o) upright fold
The mechanism of folding and it relationship with stress and strain can be examined in laboratory using a slab material of uniform internal structure. When it is bent into a fold, it may be seen by using a square grid, or lines of circles impressed on the side of the slab before folding. The distribution of tension and compression within the bent slab can be analyzed (Hills, 1975). The outer, convex side is subject to an extension parallel to the circumference of the fold, while the inner, concave side undergoes compression (Figure 2.16).
Figure 2.16 – Distribution of tension and compression within a bent slab (Hills, 1975).
126.96.36.199 Stereographic Projection Techniques in Structural Geology
A stereographic projection is a special kind of azimuth projection that was developed and refined by cyclographers (Marshak and Mitra, 1998). The idea of stereographic projection is to project the structural line or plane onto a sphere (Leyshon and Lisle, 1996). Figure 2.17 shows the projection of line and plane as they are observed in the field. The final result of this projection is to produce a representation on a flat piece of paper of three-dimensional orientations i.e. attitudes of bedding planes, fault plane, etc. (h, i).
Besides providing a means of representing three-dimensional orientations, the stereographic projection is used as a tool for solving a large variety of geometrical problems such as to calculate:
(1) The plunge and plunge direction of the axis of folding of bedding planes.
(2) The angle between any pair of bedding planes and the interlimb angle of the folds.
(3) The orientation of the axial plane of the fold (the plane which bisect the angle between the limbs of the fold).
(4) The net slip of faults and estimating the fault trends of an area, etc.
The stereographic net is the device used for these (above) constructions. Types of stereographic net are stereographic (Wullf) equatorial net, equal-area (Lambert/
Schmidt) equatorial net, equal-area polar net, and Kalsbeek counting net. In practice, Wullf net and Schmidt net are almost having a same concept except that Wullf projected directions in the centre of stereogram, thus area-distortion effect may happen.
Leyshon and Lisle (1996) recommended simple rules on the use of the nets:
(1) Whenever the densities of plotted directions are important, the equal area (Schmidt net) projection must be used.
(2) For all other applications, including the geometrical constructions, either projection can be used.
(3) Some constructions which involve drawing small sircle may be more conveniently carried out using the Wullf net.
Figure 2.17 – A scheme shows the idea of stereographic projection of the structural line and plane (Leyshon and Lisle, 1996).
The polar equatorial net is a tool which simplifies the construction of a stereogram, because by polar net, it is unnecessary to rotate the net during the procedure for plotting lines or planes. While Kalsbeek counting net is a tool which simplifies the densities of plotted directions into contouring features. Contouring is a way of showing the density of plotted planes or lines on a stereogram and the variation of density across the stereogram. The effect of contouring is to produce a smoothed representation of the data which emphasizes the properties of the assemblage of points rather than of individuals (Leyshon and Lisle, 1996). Figure 2.18 is a scheme that shows the idea of density contouring on stereogram.
Figure 2.18 – (a) The poles of planar or linear structures plotted using a Schmidt net.
(b) The Kalsbeek counting-net. (c) The number of points occurring in each hexagon, and (d) the resulting contour lines (modified from Leyshon and Lisle, 1996).
38 CHAPTER 3 METHODOLOGY
This chapter explains the materials and research methodologies that were used in this study. A field work was conducted to investigate the sedimentology, stratigraphy and the structural geology of the Miri Formation. The research uses primary data collected from outcrops and secondary data from previous maps and structural and sedimentological studies of the Miri Field. The primary data from the field study were integrated to produce the facies model and the structural framework of the Miri Formation, in the Miri Field, Sarawak. In this research, features in mesoscopic and macroscopic scale were investigated.
In order to complete the field work, four field trips of accumulated 40 days were spent in Miri town and the surrounding area. In details are; reconnaissance study (1st-6th August 2007), field mapping (3rd - 17th November 2007), field mapping (21st April -1st May 2008) and field mapping (3rd – 8th January 2010). The field work focused on sedimentological features description, structural mapping and logging of the outcrops belonging to the Miri Formation.
3.2 Outcrop Descriptions
Eighteen outcrops were examined in detail on structural geology. In order to simplify the data correlation between the outcrops, studied outcrops were divided into three sections part: northern part (represented by 6 outcrops), middle part
(represented by 9 outcrops), and southern part (represented by 3 outcrops), as indicates in Figure 3.1. Among these eighteen outcrops, five outcrops were chosen also for detail sedimentology and stratigraphy studies. The choosing of these five outcrops were based on consideration that for stratigraphy and sedimentology purposes, an outcrop need to be well exposed, fresh, showing a thick sequences which make it proper to do a sedimentological logging, and/ or indicate a lithology contact.
While for structural geology studies, the measurable orientation of structural elements in the outcrop is the main consideration. Summary of the outcrops description of the fieldwork based on the section division are as follows:
3.2.1 The Northern Part
Location 1: Boulevard 1 outcrop
This outcrop located in front of the Boulevard Shopping Complex, on Jalan Miri Pujut is recently exposed because of construction activities in the area. It represents the northern flank of the Miri anticline. The hill cut here exposes thick beds of sandstone.
Stratigraphically, the rocks belong to the Pujut Shallow Sands, which is the most important reservoir in the Miri Field. The Boulevard 1 outcrop also exposes a series of faults, which can be followed along vertical and horizontal surfaces in over large distances. The three-dimensional view of these faults thus can be observed and analyzed.
Location 2: Boulevard 2 outcrop
This outcrop is also located along Jalan Miri Pujut, about two kilometers to the south of the Boulevard 1 outcrop. It is the most recent outcrop which exposed because of construction activities in the area. The hill cut here exposes thick beds of sandstone in relatively steeply dipping beds.
Location 3: Padang Kerbau 1 outcrop
This outcrop located behind a residential estate on Jalan Padang Kerbau and also situated near production well Miri-611. It is represents the southern flank of the Miri anticline. The hill cut here exposes thick beds of sandstone interbedded with thin muddy interval. Stratigraphically, the rocks here belong to the “105” Sands.
Figure 3.1 – Topographic map showing location of the eighteen outcrops. All outcrops were selected for structural studies (O), except for five outcrops ( ) which were selected for structural, and also stratigraphy and sedimentological studies.
41 Location 4: Padang Kerbau 2 outcrop
This outcrop is also located behind a private house along Jalan Padang Kerbau, about 750 meters to the northeast of the Padang Kerbau 1 outcrop. The hill cut here exposes thick beds of sandstone in relatively gentle dipping beds.
Location 5: Padang Kerbau 3 outcrop
This outcrop is also along Jalan Padang Kerbau, about 700 meters to the northeast of the Padang Kerbau 2 outcrop. The hill cut here exposes thick beds of sandstone which stratigraphically, the rocks here belong to the “Pujut Shallow Sands”.
Location 6: Canada Hill western flank 1 outcrop
This outcrop is located along the way to the top of Canada Hill (Grand Old Lady), not far from road junction where Jabatan Pengangkutan Jalan Miri is located. It is represents the western flank of the Miri anticline.
3.2.2 The Middle Part
Location 7: Canada Hill western flank 2 outcrop
This outcrop is located along Jalan Miri Pujut, beside the Shell petrol station, about a kilometer to the south of the Canada Hill western flank 1 outcrop. The hill cut here exposes amalgamated beds of sandstone.
Location 8: Canada Hill top outcrop
This outcrop is located behind the Miri Museum, on top of the Canada Hill. It is also situated near the Miri-1 discovery well (Grand Old Lady) represent the crest of the Miri anticline. The outcrop exposes sandstone in a gently dipping as a topographic capped-stone which formed the top of Canada Hill as plateau or flat terrain.
Location 9: Hillstone Utama outcrop
This outcrop is located in the Taman Hillstone residential estate, on Jalan Hillstone Utama, which also situated at the top of the hill cut of Hospital Road 1 outcrop. The
outcrop exposes sandstone in a very gently dipping beds, and stratigraphically the rocks here belong to the upper part of the “105” Sands.
Location 10: Hospital Road 1 outcrop
This outcrop is located behind the residential estate, on Jalan Cahaya (better known as Hospital Road) represents the east side of the Canada Hill which is characterized by an abrupt topographical drop due to the fault scarps of the Shell Hill fault and the Canada Hill thrust. The hill cut here exposes thick beds of sandstone interbedded with muddy sediments, in sub horizontal or gently dipping beds. Stratigraphically, the rocks here has been classified as part of the “105” Sands.
Location 11: Hospital Road 2 outcrop
Only a few meters to the southwest from the sub-horizontal of Miri Hospital Road 1 outcrop, exposed the Miri Hospital Road 2 outcrop as a thick vertical dipping layers.
The total measured stratigraphic thickness of this vertical beds section is in excess of 260 meters thick. The Miri Hospital Road outcrop, which reveals both sub-horizontal and vertical dipping beds, is a complicated facies stratigraphy and very puzzling structural geology where such as faults, fold nose and joints were exposed here. The correlation model between these outcrops is still questionable. The correlation ideas of these outcrops then were proposed at the last part of this chapter.
Location 12: Hospital Road 3 outcrop
This outcrop is also located along Jalan Cahaya, about two hundred meters to the east of the three junctions Jalan Miri Bintulu. It is a small outcrop in total length is 5 meters with 2 meters in height. The outcrop exposes sandstone in relatively gently dipping beds to the west.
Location 13: Hilltop Garden 1 outcrop
This outcrop is located in the Taman Hilltop residential apartment, which exposes thick beds of sandstone with intensely bioturbation imprint. Stratigraphically, the rocks here have been classified as part of the “No. 1” Sand. This was the first reservoir to be brought to production in Miri. It started to produce 83 bbl per day in December 1910 from the Miri-1 discovery well and maintained production until the