The stratigraphic column must be included information, such as rock units, the thickness of lithology, sedimentary structure, grain size, sorting, geological structure, dip and strike, coordinate, composition, photo of the outcrop, and sample number. Importantly, it is required to note the date, place, and specialty of formation to easily recognize the recorded information after finishing geological fieldwork. In this study, stratigraphy measurement was carried during geological field investigation in Bokor Formation by observing the outcrop of the sedimentary rock. The data collection will be summarized in sedimentary template by using Corel drawing program. The equipment that was used in this field including: Notebook or sedimentary template, GPS, Compass, Jacob staff, Hand lens and geological hammer. Stratigraphy measurement revealed the different types of sedimentary structures at Bokor formation are recognized as planar cross-bedding, cross-bedding, parallel lamination, normal grading, massive, wavy and reverse graded-bedding. Lithological characteristics of each rock unit and associated sedimentary structure (Figure 2).

According to stratigraphy measurement, there are two lithofacies, sandstone and claystone at Bokor formation. Phnom Bokor is one part of Kampong Som basin deposited by post-rift Jurassic to Cretaceous sediments mainly sandstones and clay sediment formation [4] [7] . Such interesting locations for hydrocarbon exploration, a geological fieldwork also conducted to Phnom Bokor location to clearify more evident to such as lithofacies, sedimentological structure and stratigraphy of sedimentary rock at there. As conduction geological fieldwork, sedimentary facies, structure and stratigraphy have been recorded as shown in (Figure 2). The sedimentary outcrops at Phnom Bokor having depth extension around 140 meters starting from location’s coordinate 401207; 1174169 with planar strike N2010 and dip 330. Lithology at their composed mainly sandstone

inter bedded with some strongly weathered sandy clays and clay sediments (Figure 2). Most sandy clay to clay sediments have total depth less than 10 meters. Some of clay sediments having gray color with wavy sedimentary structure (depth 2.0 m to 2.3 m) while other clay sediments have white to gray color that mostly massive structures and some are parallel lamination structures. There has no any geological structure was found on the sedimentary outcrop. Sandstone is the most abundant rock sediment of the study location. Finely to coarse grain and some highly matrix, that some having white, milky, gray and pink colors, sandstones under weekly to strong weathering condition. Most sandstone are occurred as massive, parallel lamination, planar cross-bedding, normal grading and some are reverse grading as primary sedimentary structures during deposit. Each sample having noticed clearly on out-crop locations.

For BKSS03, 18˚ planar cross-bedded structure, medium grain sandstone with light pink color containing clay lens along with 1 - 1.5 cm width. BKSS06 is fining grain upward as normal shorting, about 1 cm quartz grain, fine to medium grain size having white color sandstone. BKSS07 founded as fine grain mixed with less than 1 cm of subrounded, fined grain, well sorted, white-light pink, 2 m bed-set cross-planar bedding sandstone. BKSS09 is white pink medium grain size, massive structure with 0.4 to 0.7 m bed set, containing 0.1 to 1 cm grain size. BKSS10 is coarsening upward grading structure as coarse sand, grey white, thin layer mixed with fine sand. BKSS15 is fine-medium, moderate matrix, weathering color, light brown, laminated (287˚/15) while BKSS18 is fine grained sandstone, highly weathering, pink color, highly matrix with massive structure. BKSS19 indicated medium-coarse grain, white sandstone, moderated shorting, and subangular to subrounded. BKSS21 are mainly composed of white color, medium to coarse grain, subangular, poorly sorted, and mass structure.

Petrography is a method to analyze samples based on the color observation of minerals under the Optical Microscope. This method provides qualitative information about mineral composition, texture, relative porosity, and the diagenetic process. There are nine sandstone samples (BKSS03, BKSS06, BKSS07, BKSS09, BKSS10, BKSS15, BKSS18, BKSS19, and BKSS21) were cut into the standard petrographic sample are prepared for petrography analysis. They were glued with a glass slide thin section using struts specifics resin, and blue-dyed powder mixed with a curing agent in a ratio of 10:1. Thin section samples were analyzed with a transmitted light microscope using Cross Polarized Light (XPL) and Plane Polarized Light (PPL) under Optical Microscope (Model: ECLIPSE CI-POL.B) and determine mineral percentage base on comparison chart estimation.

Petrophysics is carried out to determine the rock’s ability to store the fluid and allow for their movement within the reservoir, which were presented as porosity and permeability, respectively. Seven core sandstone samples (BKSS03, BKSS06, BKSS07, BKSS09, BKSS10, BKSS15, and BKSS18) were prepared to test both porosity and permeability for the measurement under the TPI-219 helium porosimeter and TKA-209 nitrogen permeameter that represents the reservoir quality at Bokor formation.

There are three sandstone samples are prepared for scanning electron microscope analysis. Scanning electron microscope (SEM-EDS) is a method that provides qualitative information about pore geometry, locating and identified mineral, particularly clay mineral that use to determine based on their microstructure properties. SEM-EDS works by using high energy focus on beam electron. When beam electron hits the sample, it causes secondary electrons to be released from sample which are detected to provide an image based on the topography of the surface. In addition, EDS is also evaluated with SEM which is called SEM-EDS (JMS IT 500) analysis. In this study, thin section of sandstone samples was prepared and coated with platinum to determine type of clay mineral and also define pore size, pore type and pore connected in bulk sample. In addition, SEM-EDS analysis has been conducted at the Nanostructure and Chemical Analysis laboratory, Institute of Technology of Cambodia (ITC), Cambodia.

Based on the result of petrography analysis, sandstones from the Bokor Formation are classified into Quartz Arenite (Figure 3), Pettijohn et al’s classification

[8] . The sandstones are mainly composed of different minerals such as quartz and rock fragment. The abundant of clastic grain is quartz which is composed of both monocrystalline and polycrystalline that components ranging from 95% to 97% with an average composition of about 96%. The monocrystalline quartz is dominated in the sandstone samples rather than polycrystalline quartz which is almost 95% of monocrystalline quartz grain. The monocrystalline of quartz grains varies from sub-angular to sub-rounded grain shape (Figure 4(c) and Figure 4(d)) and (Figure 4(e) and Figure 4(f)), partially low to high spherical, and poorly to well sorted, which have a grain size ranging from 100 μm to 300 μm. Polycrystalline quartz grains are composed of two or more crystals (Figure 4(g) and Figure 4(h)). The crystal boundary of polycrystalline quartz often be obscured due to recrystallization, but extinction characteristics generally indicate the polycrystalline nature. The boundary between the crystals of polycrystalline quartz may be straight or sutured [1] (Figure 4). A few quartz grains have numerous vacuoles as indicated by their cloudy appearance. Photomicroscope also indicates that the amount of quartz grains slides past each other and packed tighter or penetrated one another causing concavo-convex grain contact and sutured contact (Figure 4). Furthermore, some quartz grain shows grain fracturing with a small amount of clay filling.

Lithic grains (rock fragments) are found to be the most abundant to feldspar among the detrital components. The range of total lithic grains is 2% to 4% with an average of 3% which are presented as fine-grained material with grey to white color in XPL and light brown color in PPL (Figure 4). Lithic sedimentary grains are found to be more abundant than lithic metamorphic grains (Figure 4). Very few traces of lithic volcanic grains are found. The lithic sedimentary grains consist of mainly sand/siltstone fragments with little chert and argillite shale (Figure 4). Among the lithic metamorphic grains, schist mica-schist and graphite-micaschist are common. In this case, it indicates that it may be eliminated by the dissolution that is replaced by the clay matrix during the diagenesis [1] [9] .

Clay matrix is recognized based on the fine-grained material deposited between larger particles, such as quartz, feldspar, and rock fragment. Under the microscope, the matrix commonly has dark grey color in XPL and dark brown in PPL (Figure 4). Clay matrix is also seen to be present in Bokor sandstone which is identified by its fine-grained material and bright color in XPL. There was less amount of matrix contained in the sandstones which could be considered as clean sandstone with less than 15% of the matrix [8] . This less amount of matrix is indicated that the pore throats are less resulting in the loss of interparticle pores influenced by matrix [10] . Matrix is the second dominant component in the sandstone samples in composition.

However, it also presents deformed mica-flake as bending between quartz grain contacts (Figure 4(a) and Figure 4(b)). The quartz grains and mica flake in the center of the view are surrounded by numerous small crystals with low relief and show first order grey interference colors. Thus, this is evidence of mechanics

and pressure dissolution that occurred during the diagenesis phase caused by a loss in the primary porosity of sandstone [11] . Silica dissolved during the process may be precipitated as cement away from grain contacts, leading to the destruction or occlusion of porosity (Figure 4) [1] . Mica generally occurs along partings, laminae, or bedding planes and frequently shows bending due to compaction (Figure 4(g) and Figure 4(h)).

Pore relative porosity in sandstones could be analyzed under petrography analysis using photomicrograph in plane-polarized light (PPL) and cross-polarized light (XPL) views (Figure 4), for qualitative interpretation of the reservoir rock properties. The observation under the thin section microscope is essential for the identification of pore type, pore connectivity, and pore size which could be used to identify the factor controlling reservoir quality. Based on the results of the sandstone reservoir from the Bokor Formation indicated the pore size ranges from 50 μm to 500 μm with interparticle pore type (Table 1). The pore spaces are primary pores that contribute to the increased porosity and their pore throats are well connected between one pore to another pore, thus the texture is tended to produce high permeability for hydrocarbon reservoirs. In addition, the pores are also unconnected with each other that is corresponding to authigenic clay precipitated by filling in the pore, which leads to reduce the permeability of the fluid to flow and porosity for the hydrocarbon reservoir.

The results of porosity obtained from the helium porosimeter shows the seven sandstones from Bokor Formation contain a porosity value between 6.55% to 13.19% with an average of 9.80%, which there are four sandstones indicated that poor reservoir rock, and three sandstones showed fair reservoir rock (Figure 5). Based on these data and the classification of North, 1985, it is classified into fair porosity [12] . On the other hand, the absolute permeability measuring from the

core samples are range from 10 - 60 mD with an average of 36 mD. Based on these data and the classification of Koesoemadinata, 1980, the permeability is classified as fair to good reservoir quality for fluid flow [13] (Figure 6). The differences of permeability values of Bokor formation may be influenced by grain packing, grain sorted, and clay mineral content which occurred during diagenetic stage.

The observation three samples such as BKSS03, BKSS06, and BKSS09 under SEM analysis is important for the identification of pore type, pore connectivity, and pore size which influences the reservoir quality and heterogeneity. Moreover, SEM analysis can also identify the structure of clay minerals that mostly fill

into inter-particle pore or coating on detrital grain mineral. Based on the results, most of pore type are inter-particle pore that are mostly connected and unconnected (Table 1). Moreover, there are three types of clay mineral, such as kaolinite, chlorite, and illite (Figure 8), which partly formed in pores and coated on quartz grain (Figure 7).

Based on the result of petrophysics analysis, the relation between porosity and permeability is made in the context of various aspects to find a reliable relation between porosity and permeability. (Figure 9) shows the porosity and permeability relation of sandstones at Bokor formation. Regard to this plot, it shows a low correlation for all sandstones at Bokor formation which is interpreted as poor to fair porosity for reservoir rock and fair to good permeability for fluid

flow [12] [13] . This is because these sandstones have different properties such as pore size, pore connectivity. According to the result of porosity and permeability, the reservoir properties of the Bokor formation have low porosity and permeability, averaging 9.80% and 36 mD, respectively (Figure 9). Therefore, the particle size and properties of clay minerals impact the relationship between porosity and permeability. In addition, the porosity and permeability values are suggested to be a fair reservoir quality. This is because most of the pores are mostly contributed from the isolated pores which their pore throats are interparticle pore and unconnected to well connected with pore size between 50 μm to 500 μm (Figure 4 and Table 1).

Based on petrography analysis, most of the sandstones at Bokor formation are identified by interparticle pores that have a pore size range of ~50 μm to 500 μm, and pore throat between grains are poorly connected to well-connected pores (Figure 4). This is because grain sizes are medium to coarse grain, well-sorted with sub-angular to sub-rounded. During the diagenetic stage, the grain packing is well arranged and produced more interparticle pore in bulk samples. Therefore, the porosity and permeability values in Bokor sandstones are increased when grain packing is well arranged by producing more interparticle pores [15] . The effects of diagenetic processes on the sandstone of the Kampot formation include the reduction of porosity by compaction and cementation as well as, the enhancement of porosity by dissolution [16] . The best quality rocks are characterized by medium to coarse grain size, good sorting with high percentages of detrital quartz, and low percentages of matrix and cement (quartz and calcite). Most quartz in sandstones occurs as subangular to subrounded monocrystalline fragments. Grain contact changes from dominantly floating to the point, long contact, and concavo-convex shapes due to the progressive burial and compaction effect. The detrital quartz grain contacts are transformed from point to long and concavo-convex contacts due to severe mechanical compaction, which reduces permeability [17] . In addition, the large amount of authigenic clay mineral, kalinite, illite, and chlorite in isolated pore, pore lining and pore throat also reduced porosity and permeability (Figure 7 and Figure 8), However, there are some cases which large amount of clay in pore-throat has reduced permeability. Clay minerals in sandstone commonly altered from quartz grains, feldspar grains dissolution and rock fragments. Clay minerals are usually considered detrimental to the quality of sandstone reservoirs, as they can clog pores and some clay minerals advocate chemical compaction [18] . Most clay minerals formed where rocks are in contact with water, air, or steam in many conditions such as weathering, erosion and also diagenesis [19] . Hence, the properties of pore size, pore type, and pore network and clay mineral are influenced on both porosity and permeability. Monocrystalline quartz under the influence of increasing pressure and temperature, uncorrelated single-crystal quartz gradually transforms into corrugated quartz, polygonal quartz (quartz exhibiting well-defined extinction bands with sharp boundaries), and finally changes to polycrystalline quartz [20] . Presence of secondary quartz overgrowths around primary quartz grains will make reduction in pore space due to pressure solution [21] . Grain contact types also reveal some diagenesis mechanism reflecting influence of pressure solution suggested the key evidence of compaction and consolidation on the sandstone [14] . Long grain contact, concave-convex and sutured contacts are identified on the thin section’s photos (Figure 4). Long grain contact and concave-convex grain contact indicate the sandstones have undergone substantial compaction indicate intermediate depths whereas Sutured contacts indicate diagenesis and deep burial [14] . The concave-convex and sutured contacts are mostly representative of intense compaction and pressure dissolution process during deep burial diagenesis [14] . The effect of detrital grain size and grain sorting on permeability and helium are indistinct comparing to other data point [22] . clay minerals in these samples may trapped in the small necks of pores provide increased fluid pressure to maintain flow, reducing the overall permeability of porous systems. The occurrence of quartz overgrowth is influenced on the lost of primary porosity due to the development of new crystalline and widespread close to inter-granular contact [2] . In some cases, quartz overgrowth can retard or prevent porosity lose from mechanical compaction by increasing grain size [23] . In the Bokor formation, there are more positive correlation between quartz overgrowth and primary porosity. Most of sandstones at Bokor formation have quartz overgrowth which grain size are medium to coarse grain (Figure 4(e) and Figure 4(f)) and have been tightly compacted (Figure 4). In this case, Bokor formation sandstones may be affected by high temperature of geothermal gradient and high pressure from overburden pressure. Thus, the positive correlation of quartz overgrowth and primary porosity reveal that porosity can be increased with increasing of quartz overgrowth. The compaction is prevented by quartz overgrowth; that causes primary porosity to be stable. Deposition of quartz overgrowths is recognized in many studies as a mechanism for porosity loss in sandstones.

The depositional environment of the sandstone samples in Bokor formation have been interpreted based on the detail study of the stratigraphic log, and petrography analysis. The sandstone lithofacies at Bokor formation consist of sedimentary structures, such as planar cross-bedding, cross-bedding, parallel lamination, normal grading, massive, wavy and reverse graded-bedding, which indicates these lithofacies may be deposited in shallow marine environment [24] [25] . Moreover, the result of petrography analysis showed sandstones are quartz arenite which are dominant of quartz mono and polycrystalline over 95%. The highly content of this quartz is suggested as shallow marine sandstone. Furthermore, sandstone in Bokor formation contain grain size from medium to coarse-grained, which is caused by high energy and rapid flow condition in shallow marine environment [25] [26] [27] . In addition, the characteristic of cross-bedding with normal and revers graded are related with increasing current and wave velocity that occurred near the surface [24] [26] [28] . On the hand, quartz overgrowth of sandstone facie at Bokor formation usually formed in shallow marine diagenetic environment and in the early diagenetic stage under temperature greater than 100˚C [20] , it occurred as syntaxial overgrowth by growing continue from quartz grain, and the result is given quart grain more euhedral crystal face as well as the increase in grain size [16] . Clay minerals in sandstone commonly altered from feldspar grains dissolution, quartz grains and rock fragments. The dissolution occasionally occurred at shallow depth by meteoric water [24] .