Reservoir quality in the Jurassic sandstone reservoirs located in the Central Graben, North Sea

Marine and Petroleum Geology 102 (2019) 439–454

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Research paper

Reservoir quality in the Jurassic sandstone reservoirs located in the Central Graben, North Sea

T

Ali Mustafa Khan Niazia,b,∗, Jens Jahrena, Irfan Mahmoodc, Haris Javaidb a

Department of Geosciences, University of Oslo, Norway Petroleum Exploration Private Limited, Islamabad, Pakistan c Department of Meteorology, COMSATS University Islamabad, Pakistan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Central Graben Petrophysics Jurassic sandstones Quartz cement Carbonate cement Porosity

This study investigates the diagenesis and reservoir quality of Upper Jurassic Sandstones of Ula Formation from the Central Graben. Petrophysical and Petrographical studies have been done on cored interval from well 2/1-6 of Ula Field. Precipitation of quartz cement is the main porosity destroying process in deeply buried, quartz-rich sandstone reservoirs of the North Sea. Quartz cement precipitates in the form of syntaxial overgrowth over detrital grain of quartz. Grain coatings, like micro-quartz and illite, are the main reasons for preservation of porosity in sandstones as they cover the grain and inhibit the quartz overgrowth. Petrographical data in this study clearly indicates that grain coatings are present in the studied samples. Micro-quartz grain coating is the most common grain coat in the Upper Jurassic Sandstones of Ula, which is generated from the transformation of siliceous sponge spicules known as Rhaxella Perforata. Clay grain coats like illite and chlorite are also present. Relation between Intergranular Volume (IGV) versus Matrix and Quartz Cementation versus Porosity have also been studied. IGV is strongly affected by mechanical compaction, grain size, grain shape, quartz, and carbonate cements. Sandstones with high amount of matrix and fine grains have high IGV as compared to coarse grains because coarse grains are compacted more when they are subjected to mechanical compaction. Grain shape also has a pronounced effect on porosity. Angular grains tend to lose porosity easier as they are subjected to stress.

1. Introduction Sandstones constitute 60% of all petroleum reservoirs in the world. Sandstone is composed of sand grains with a diameter 1/16 and 2 mm (Bjørlykke and Jahren, 2010). Porosity and permeability are most important reservoir properties, but petroleum production can also be influenced by pore geometry and wetting properties of the mineral surfaces. Reservoir rock properties are continuously changing from time of deposition to burial at greater depth. Reservoir properties at any given depth are the factor of initial composition of sandstones at shallow depths, burial temperature and stress (Bjørlykke and Jahren, 2010). This research work is a collaboration between Det norske oljeselskap ASA (DETNOR) and the Department of Geosciences, University of Oslo. The study area is located in the Central Graben within the North Sea in block 7/12, 2/1, and 1/3 belonging to Ula, Gyda and Tambar fields respectively (Fig. 1). These blocks are in Cod Terrace which are affected by Triassic salt tectonics (Gowers et al., 1993). These blocks are in southern part of the North Sea (Fig. 1). The aim of this research is to increase the understanding of the distribution and quality

∗

of deeply buried Jurassic Sandstone Reservoirs located in the Central Graben. The main objective of this study is to characterize the cored reservoir interval of well 2/1-6 of Gyda Field. The objective also includes providing valuable and essential information on reservoir quality as a function of quartz cementation and porosity preserving mechanisms in Upper Jurassic Sandstones in the Central Graben which are buried to depths > 4 Km. This will be done by integration of methods on two levels of investigation; 1) Well correlation and petrophysical evaluation and 2) Petrographic analysis of thin sections (Optical Microscopy and Scanning Electron Microscope (SEM)). Cementation is the main cause of the drop-in reservoir properties of Jurassic sandstones from the North Sea which are buried deeper than about 3000 m (70–100 °C). Cementation is a process that is strongly controlled by temperature and kinetics. In deeply buried reservoirs (> 4000 m/140 °C), a good understanding of the factors controlling the cementation exists since normal quartz cementation would normally lead to limited reservoir properties at similar depths. Reservoir quality in the deeply buried sandstone prospects therefore depend on factors preventing or delaying the quartz cementation. These factors include

Corresponding author. Petroleum Exploration Private Limited, Islamabad, Pakistan. E-mail address: [email protected] (A.M. Khan Niazi).

https://doi.org/10.1016/j.marpetgeo.2019.01.001 Received 25 June 2018; Received in revised form 5 December 2018; Accepted 2 January 2019 Available online 07 January 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Structural element map of the study area. Black dots indicate the well location (Map modified from Norwegian Petroleum Directorate).

2018a). Sandstone reservoir experience loss of porosity due to diagenetic modifications over long geological history, thereby increasing tortuosity and number of disconnected pores (Lai et al., 2018a). Depositional porosities are reduced due to the process of compaction, both mechanical (grains rearrangement, pseudo-matrix formation) and chemical (pressure dissolution) (Freiburg et al., 2016; Lai et al., 2018a). In early diagenesis, rapid decrease of pore spaces occurs due to cumulative effects of compaction (Zou et al., 2012). Detrital grains repacking, and pseudo-plastic deformation of rock fragments are caused by mechanical compaction induced by lithostatic pressure (Wei et al., 2015; Freiburg et al., 2016). Chemical compaction becomes an increasingly dominant factor in reducing porosity with increasing temperature and pressure because of dissolution of intergranular pressure (Freiburg et al., 2016). Carbonate cements which are important diagenetic constituents in sandstones can both decrease porosity by filling pore spaces or can limit mechanical compaction by increasing pressureresistance of sandstones (Wang et al., 2016). In deeply buried hydrocarbon reservoirs, quartz cements play an

the grain coatings like chlorite and micro-quartz (Bjorlykke, 2010). The term diagenesis refers to physio-chemical processes which begins from deposition and continues through compaction, cementation and so on but stopping short of metamorphism (Zhang et al., 2015, 2017). In diagenesis compaction and cementation of minerals occur (Kassab et al., 2014; Zhang et al., 2017). Diagenesis can also degrade quality of reservoir via cementation (Becker et al., 2017). To understand the production in sandstones, it is important to analyze how reservoir quality is impacted by diagenetic alterations (Lai et al., 2018a). Tight sandstones undergo extensive diagenetic modifications and thereby are characterized by low porosity and permeability, strong heterogeneity and complex core structure (Wang et al., 2017; Lai et al., 2018a). Burial depth, composition and texture of sandstone and authigenic cements effects the reservoir quality of sandstone (Higgs et al., 2007; Morad et al., 2010; Zhang et al., 2015; Lai et al., 2018a, b). Studies by Cui et al. (2017) explain that the formation and distribution of tight spots can be determined by diagenetic facies. Sweet spot distribution and formation is determined by diagenetic facies (Lai et al., 440

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prolonged extensional history that began in Permo-Triassic and further continued during Jurassic and Early Cretaceous, followed subsequently by respective thermal cooling and subsidence stages (Ravnås et al., 2000; Zanella and Coward, 2003). The Central Graben is the southern arm of the triple junction between Viking Graben, the Central Graben and the Moray Firth basins (Fig. 2). The Central Graben is more symmetrical in character as compared to the Viking Graben and the Moray Firth basin which are asymmetrical in character (Zanella and Coward, 2003). According to Gowers et al. (1993); development of Central Graben can be divided in to three different stages. These stages are; 1) Late Triassic to middle Jurassic flexural uplift, 2) Late Jurassic to early Cretaceous fragmentation, 3) Late Cretaceous to Tertiary flexural subsidence. Late Jurassic is rightly considered as the most important period in evolution of the North Sea petroleum system because Kimmeridge Clay Formation and its equivalent were deposited which are the major oil source rocks in the North Sea (Cornford, 2009). The complex tectonic history of Central Graben created vast majority of hydrocarbon traps, which were to be filled with hydrocarbons upon maturation of Kimmeridge Clay and its equivalents later. Therefore, Late Jurassic becomes the single most significant period in the overall development of petroliferous North Sea basin (Fraser et al., 2002). According to Fraser et al. (2002), the petroleum play of Central Graben is characterized by two

important role in deteriorating the reservoir quality (Islam, 2009; Xi et al., 2015; Makeen et al., 2016; Worden et al., 2018). Mechanical compaction is chiefly responsible for porosity loss at shallow depths whereas at high temperatures, quartz cements are major factors for loss of primary porosity (Dutton and Loucks, 2010). Studies have shown exponential increase in quartz cement precipitation with elevated temperatures (Lai et al., 2017, 2018a; Saïag et al., 2016). Both external and internal sources of quartz cements are present with chemical compaction as a significant source. Transformation of clay minerals have associated authigenic quartz. Quartz cementation also results from smectite to illite transformation. Therefore, it is commonly observed that authigenic clay minerals and quartz cements coexist. Dissolution pores are frequently filled by overgrowth of quartz. According to Islam (2009); neighboring mudstones and fluid flow controlled by presence of faults are external sources of quartz cements.

2. Geological setting Central Graben is in the middle and southern part of the North Sea, is 70–130 Km wide and 500 Km long and forms the southern arm of trilete rift system (Nguyen et al., 2013). Structural configuration of North Sea is predominantly controlled by Late Jurassic to Early Cretaceous rifting events (Fig. 2). The North Sea rift systems has a

Fig. 2. Regional structural map of North Sea modified after Domínguez (2007). Black square represents the study area. 441

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types of reservoirs: Coastal Shelf Sandstones and Deep Sea SubmarineFan Sandstones in the form of basin floor fans. Following rifting, sea level rose rapidly during Jurassic time which caused development of an extensive coastal shelf depositional system. This type of depositional system resulted in high reservoir quality shallow marine sands at the basin margins. These sands include Emerald, Fulmar, Heno, Hugin, Piper, Sognefjord and Ula Formations. In Oxfordian and Kimmeridigian times (Late Jurassic), the depositional pattern was progressive retrogradation of coastal shelf depositional system (Fraser et al., 2002). These shallow marine sandstones are though different in ages, but their physical characteristics are similar. They have been extensively bioturbated so internally they are structureless. In Central Graben, good quality sandstones are lying in upper parts of upward-coarsening progradational cycles which were deposited in high energy environment, influenced by storm waves. In deeper parts porosity is preserved by high overpressures along with some secondary porosity (Fraser et al., 2002).

∅d =

∅a =

∅d + ∅n 2

Archie Equation was used to derive the Water Saturation (Sw) using the average porosity. Water Saturation (Sw) was calculated using the values for a = 1, m = 2 and n = 2. Default values (for a, m and n) were taken from Pickett plot. The corresponding formation water resistivity (Rw) found was ∼0.03ohmm.

Sw = [

a × Rw 1/ n ] ∅m × Rt

Cut-offs selected were Vsh < 45%, ∅ > 6%, and Sw < 55%. Petrophysical Analysis helped in identifying and correlating high and low porosity zones. Well log data was also used to generate different cross plots (Supplementary Figs. 1–4). These plots were very helpful in matching the identified low and high porosity zones on the well log data with the petrographical data.

3. Methodology The study is based on an integrated approach where open hole logs and core data were utilized to investigate the diagenesis and reservoir quality of Upper Jurassic Sandstone from the Central Graben. The methodology which has been followed is 1) Well Correlation and Petrophysical Evaluation and, 2) Petrographic Analysis using Optical Microscope and Scanning Electron Microscope (SEM). Open hole log data of three wells 2-1/6 (Gyda Field), 1/3-9S- (Tambar Field), and 712/2 (Ula Field). Well -1-3/9-s and 7-12/2 were used only for well correlation with 2-1/6. Well 2-1/6 was taken as the reference well in the study and no rock samples from cores of this well were studied.

3.3. Petrographic Analysis Petrographic analysis was done using Optical Microscope and SEM on the core samples of well 2-1/6. SEM analysis has been done on samples using JEO2 JSM-6460LV Scanning Electron Microscope (SEM) with a LINK INCA Energy 300 Energy Dispersive X-Ray (EDX) system. Two types of samples were studied under SEM. This includes carbon coated thin sections and freshly fractured samples from core material which are mounted over gold coated stubs. 20 samples mounted over stubs were studied. 10 samples were chosen for carbon coating in a way that it covered the high and low porosity zones. Point Counting was also done using optical microscope on 20 thin sections from well 2-1/6. 300 points were counted on each thin section. It was done on Nikon Optiphot-Pol petrographic microscope in PPL (plain polarized light) and XPL (cross polarized light). Following parameters were determined; 1) Quartz 2) Feldspar 3) Rock Fragments 4) Matrix 5) Mica 6) Carbonate Cement 7) Quartz Cement 8) Primary Porosity 9) Authigenic Kaolinite 10) Illite 11) Secondary Porosity. Point counting was carried out to get an overall idea of composition of samples and their porosity. Grain size distribution and sorting was also observed. Degree of sorting was estimated by following Longiaru (1987) (Fig. 3). According to Longiaru (1987); sorting can be divided into well sorted, moderately sorted and poorly sorted. An overview of samples observed from their specific depth is shown in the Supplementary Table 1. Data collected from Point Counting was used to calculate IGV (Inter granular volume). IGV is used to measure compaction in sandstones. It is equal to sum of intergranular space, intergranular cement, and depositional matrix (Paxton et al., 2002). IGV in sandstones is on average ranging from 40 to 45 vol percent (IGV at the time of deposition). IGV usually varies with sorting and particle grain size.

3.1. Well correlation A regional NNW – SSE well log correlation was generated. The main objective was to correlate Ula Formation through Gyda, Tambar and Ula Fields located in North Sea. Ula Formation is divided into subparts based on sequence stratigraphy and porosity (Ramm et al., 1997). The well 2/1-6 of Gyda Field was taken as type well and correlated with the wells 1/3-9S of Tambar Field and 7-12/2 of Ula Field towards the NNW direction. Well Correlation was performed using PETREL software. Density log and Neutron Porosity logs were used for correlation of the units (Supplementary Figs. 1–4). The sequence stratigraphic units recognized by Ramm et al. (1997) are classified based on high and low porosity zones within the Ula Formation. These units are stratigraphically correlateable based on log motif throughout the Gyda, Tambar and Ula Fields. The correlation ascertains, the Ula formation is sub divided into five units Ula A, Ula B, Ula C, Ula D and Ula E on the above mentioned criteria. 3.2. Petrophysical evaluation Open hole logs of 2/1-6 well in LAS format were loaded and QC was carried out, followed by preliminary analysis which involved the selection of zones in the defined units of Ula Formation. Detailed Petrophysical evaluation was performed using PRIZM module of GeoGraphix Discovery. The log suite utilized for evaluation included Gamma Ray Log, Resistivity Logs, Neutron and Density Logs, Sonic Logs. The workflow initiated with the calculation of Volume of Shale determined using Gamma Ray log:

Vsh =

ρma − ρb ρma − ρfl

4. Results The study focuses on the investigation of petrophysical and petrographical properties of the cored intervals in the study area. Petrographical data from well 1/3-9 S and 7/12-2 will not be interpreted as this study focuses on the well 2/1-6 but LAS Files of these wells will be used for well correlation and petrophysical analysis. Main purpose of petrographical analysis is to mark the porous and nonporous zones in the cored interval of well 2/1-6 and to have the knowledge of the regional extent of these zones using regional well correlation.

GRlog − GRmin GRmax − GRmin

Density porosity was derived from the bulk density log, using matrix density (ρma) = 2.65 g/cm3 for sandstone and fluid density (ρfl) = 1.1 g/cm3. Average porosity was estimated using Density (∅d ) and Neutron porosity (∅n ) logs. 442

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Fig. 3. Degree of sorting presented by Choh et al. (2003) and modified after Longiaru (1987).

porosity decreases with increasing cementation. Thin sections from well 2/1-6 were grouped semi-qualitatively visually in the microscope according to the degree of sorting as poor, moderate or well sorted as shown in Table 4. Most of the samples are well sorted. It should be noted that samples with low porosity are medium grained, moderately to well sorted and sub-rounded. While high porosity samples are fine grained, well sorted and angular. Fig. 6b shows a linear relationship between Depth and Quartz Cementation. It is relatively clear that as the depth increases quartz cementation also increases. IGV also decreases with increasing quartz cement as shown in Fig. 6c.

4.1. Petrophysical interpretation Petrophysical interpretation of well 2/1-6 was carried out on PRIZM, a log analysis tool of Geo Graphix Discovery Suite. Only well 2/ 1-6 was selected for this interpretation because it is the key well in the study. Cut offs and petrophysical parameters used for petrophysical interpretation are mentioned in Table 1. 4.2. Well correlation Well Correlation helps us in providing the lithostratigraphic framework for the cores and samples under study. Large amount of data can be derived from wire line logs but here these logs will be used to correlate the low and high porosity zones varying through the study area. GR (Gamma), DT (Sonic), NPHI (Neutron Porosity) and RHOB (Density) Curves were used for well correlations. Fig. 4 shows the well correlation between 2/1-6, 1/3-9 S and 7/122. Table 2 shows the names of identified zones based on their porosities. It should be noted that Ula B and C zones weren't recognized in well 7/ 12-2. According to Table 2, there are only two zones Ula A and C which are high porosity zones. Ula B, D, and E are low porosity zones.

4.3.2. Inter Granular Volume (IGV) Inter Granular Volume (IGV) was calculated and is presented in Fig. 7. The average IGV of the well 2/1-6 is 31.5% and range between 14% and 44%. We get the minimum of IGV in sample 4 at depth of 4254.6 m depth. Reason for low IGV is that the sample is compacted and has very low matrix content. The highest IGV is 44% at depth of 4353 m. 4.3.3. Scanning Electron Microscope (SEM) Twenty samples, from well 2/1-6 have been examined under SEM. Purpose of study of samples under SEM was to identify grain coatings, mineral associations and quartz overgrowth. Grain coatings which were recognized were micro-quartz, illite, chlorite grain coats. Smectite was not recognized in the samples. All minerals identified in study were identified by Energy Dispersive Spectrometer (EDS). A wide range of minerals have been identified which include: quartz, feldspar, illite, chlorite, apatite, mica, calcite, dolomite, ankerite. Backscatter Image and Cathode Ray Iluminesence were used together to identify the quartz overgrowth. Almost all the samples studied under SEM had some degree of grain coats. Fig. 8 gives typical examples of micro-quartz and clay coats present in the study area. Fig. 9 clearly illustrates that how grain coats have prevented quartz grains to grow and have preserved the reservoir qualities.

4.3. Petrography 4.3.1. Point counting Model Analysis (300 points per sample) was performed on twenty different thin sections from well 2/1-6. The sampled sandstone from the well was predominantly sourced from the pre-rift sedimentary rocks that were uplifted in Late Jurassic time in the Central Graben. Therefore, it was expected that sandstones would be mature (high quartz content). Point counting of the samples indicate that majority of the samples are Quartz Arenites and thereby confirms the compositional maturity that was expected. Five of the samples fall into the category of Subfeldspathic Arenites because of higher feldspar content (Fig. 5). Results of the petrographical analysis are shown in Table 3. Fig. 6a shows the cross plot showing a relationship between Porosity and Quartz Cementation. From this cross plot, it should be noted that

4.3.4. Grain coats Micro-quartz is the main grain coat observed in almost all the samples (Fig. 10). Various amounts and developments of microcrystalline quartz overgrowths with the detrital quartz were also recorded (Figs. 10 and 11). Micro-quartz grain coating is usually extensive covering all the surface of grains. But where it is not covering the whole grain, quartz overgrowth does not stop, which resulted in destroying the reservoir qualities. Sponge spicules e. g Rhaxella, were also observed in samples 2-15, 2-18, 4, and 5 (Fig. 12). Clay coats were also observed in these samples. Illite was the most commonly observed clay coat. Chlorite clay coat was rarely found in the samples (Fig. 13). Chlorite clay also shows honeycomb morphology as observed in Fig. 13. Though clay grain coats were present in the observed samples, but micro-quartz was the most common observed grain coat which resulted in preservation of good reservoir qualities.

Table 1 Petrophysical Parameters used in petrophysical interpretation. Petrophysical Parameters Volume of Shale (Vsh) Porosity Cut Off Saturation of Water Cut Off (Sw) DT Fluid DT Matrix Rw (Resistivity of water calculated by Pickett Plot Method) a (Tortuosity Factor) m (Cementation Factor) n (Saturation Exponent)

< 40% > 6% 55% 189 μs/ft 57 μs/ft 0.03 Ω m 1 2 2

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Fig. 4. Well correlation of the three well's from Tambar, Gyda and Ula fields.

sandstone at early stages of mechanical compaction, later resulting in high IGV.

Table 2 Names and porosities of units of Ula formation. Sr. No

Units of Ula Formation

Porosity

Classification

1 2 3 4 5

Ula Ula Ula Ula Ula

15% 9% 14% 10% 9%

High Low High Low Low

A B C D E

5. Discussion This study shows that grain coatings like micro-quartz and clay coats are present in the area and are of primary importance in preserving porosities at depths greater than 4 km. From the results, it has also been assumed that grain size and shape has also played an important role in preserving porosities. Ula Formation is divided in zones based on high and low porosities. Five zones were recognized including Ula A, B, C, D and E. Only samples from Ula A, B, C and D were studied. Ula B and upper part of Ula D are the zones with low porosity while other zones are of very high porosity. The evolution of sandstones reservoir quality is controlled by diagenetic processes (compaction, cementation and dissolution) (Lai et al., 2018a). Three types of genetic modifications occur; destruction, preservation and generation of porosity (Islam, 2009; Nabawy and Géraud, 2015). Deep sandstone reservoirs are formed by combination of diagenetic and depositional processes at shallow depth marked by increased temperature conditions (Stricker and Jones, 2018). Several deep hydrocarbon basins have high reservoir porosity including Central Graben, North Sea and Indus Basin, Pakistan (Berger et al., 2009; Nguyen et al., 2013; Cao et al., 2014; Grant et al., 2014). Scanning Electron Microscope (SEM) results show that micro-quartz grain coating is present in all the twenty samples in study. North Sea is well known for presence of micro-quartz grain coating in the Jurassic sandstone reservoirs (Aase et al., 1996). As discussed, micro-quartz grain coats are the major reason of preservation of high porosities at depth greater than 4 km in the upper Jurassic sandstones of the North Sea (Aase et al., 1996). But from the results, it is found that Ula Formation has zones with high and low porosities even though all samples have micro-quartz grain coating in them. Ula B and upper part of Ula D

4.3.5. Quartz overgrowth Quartz grain overgrows where grain coats like micro-quartz and clay coats are absent. Quartz overgrows through spiral-growth. From Fig. 14, it is clearly visible that all of the porosity is lost by quartz overgrowth in the pore space. In the middle there is mica with sheet of illite in between its flakes. Quartz overgrowth can also be seen using Back Scatter Image and Cathode Ray Iluminesence together. In Fig. 15, note that comparison of two pictures show that quartz grain was deposited in a sub angular shape (a-2) while later it related to other quartz grain by quartz cementation (a-1). Similarly, in Fig. 15 b-1 and b-2, it is clearly visible that quartz cementation and overgrowth have reduced the porosity and in turn destroying the reservoir quality. Overgrowth was also observed in the form of several prismatic crystals (Fig. 16) which grow on detrital quartz covered with microquartz. As grain coats like micro-quartz covers the grain, the overgrowth of detrital quartz grain stops. But as silica saturations rises to higher level, micro-quartz grain starts to overgrow (Fig. 16). Note that small crystals of micro-quartz have overgrown in to a bigger size of quartz crystals. In this figure also note that quartz over growth has occurred where micro-quartz has not covered the grain. In most of the samples studied, carbonate cement was observed (Table 3, (Supplementary Fig. 5). This carbonate cement (mostly dolomite) occurs by filling the pore space which might have cemented the 444

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Fig. 5. Classification of the samples following Pettijohn et al. (1987). Box shows zoomed in data of plotted samples. Composition is also given in Table 3.

compaction and cementation (quartz, calcite, authigenic clay minerals), whereas dissolution of grain and fracturing are responsible for generation of porosity (Dutton et al., 2012; Yuan et al., 2015; Lai et al., 2016, 2018b). The presence of micro-quartz grain coatings indicates that an amorphous silica precursor was present earlier in the sedimentary/diagenetic history of the sandstone (Maliva and Siever, 1988). These silica precursors are most likely sponge spicules as shown in Fig. 12. Micro-quartz coat is caused by transformation of siliceous sponge spicules known as Rhaxella Perforata (Ramm and Forsberg, 1991; Hendry and Trewin, 1995; Aase et al., 1996). Thus, micro-quartz occurrence totally depends on the sedimentary environment and sediment age. Few

are the zones with low porosity while Ula A and Ula C are high porosity zones (Fig. 4, Table 5). From all these zones micro-quartz coating was recorded. Reason for high porosity in Ula A and Ula C is obvious that micro-quartz and other clay coatings preserved the porosity. But here the question arises that what caused the Ula B and D to have low porosities? Reason for low porosity could be related to the grain size and grain shape. Samples from Ula B are mainly of medium grain size and sub-rounded while samples from Ula A and C are fine grained and angular sandstones (Table 4). Tickell and Hiatt (1938) reported that angular grains tend to have more porosities as compared to spherical or sub-rounded grains. This could be the possible reason for high porosities in Ula A and Ula C. The main porosity destruction factors are Table 3 Results of petrographic analysis. Sample

Depth

Avg Grain Size

Framework Composition Quartz Feldspar Rock Fragments

2–12 2–14 2–15 2–16 2–17 2–18 2–19 2–21 3 4 5 6 7 9 10 11 12 13 14 16

4205.8 4212.55 4218.5 4222.6 4227.35 4231.8 4233.7 4240.6 4252.5 4254.6 4303.65 4309.6 4318.67 4321.7 4324.9 4327.6 4333.1 4336.95 4344.3 4353

0.25 0.0175 0.0175 0.015 0.015 0.0175 0.01 0.02 0.02 0.025 0.255 0.2525 0.0175 0.25 0.0175 0.02 0.015 0.0125 0.0125 0.0125

88.04 90.85 84.98 87.85 89.06 94.28 91.72 93.94 88.04 94.31 91.11 94.18 95.36 94.79 91.44 95.41 96.45 85.53 97.15 92.74

11.96 9.15 15.02 12.15 10.94 5.72 8.28 6.06 11.54 4.94 8.89 5.82 4.64 5.21 8.56 4.06 3.55 14.47 2.85 6.65

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.42 0.74 0.00 0.00 0.00 0.00 0.00 0.53 0.00 0.00 0.00 0.60

445

Matrix

Porosity

Quartz Cementation

Calcite Cementation

15.6 7.6 10.6 15.3 18 7 8 6 6.3 2.3 9.6 9.6 8 19 11.3 22.3 17.3 18.3 14 29.3

16.3 15 15 15.6 15.6 14.3 14.3 13.3 9.6 5 8.6 5.3 4.3 15 9.3 12.3 11.3 17.6 14.3 7.3

1.3 6.3 4.3 3 2 5 1.6 1 4.6 3.3 5 4 2.6 2.6 4.3 4 4 2 1.3 2.3

4 3.3 3 4.3 2.6 4 3 1.3 5.3 3 1.6 4.6 3.3 0.6 1.6 2.6 6 2.6 6.3 5.3

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buried sandstones, high porosity is present because sponge spicules, with burial and increasing temperatures dissolve to precipitate as microquartz overgrowth on quartz grains which inhibits normal macroquartz cement to develop. Equilibrium concentration of pore water silica with microcrystalline quartz (1-2 ppm) are higher than the equilibrium concentrations with the quartz. These concentrations inhibit the macroquartz cementation by effecting the normal supply of silica from dissolution of quartz along stylolites (Aase et al., 1996). At burial depths which are normally considered uneconomic, high porosity and permeability preservation in sandstones occur due to the shallow arrest of compaction in areas with high pore fluid pressure and relatively low vertical effective stress (Stricker and Jones, 2018). The presence of continuous diagenetic clay and microquartz grains in quartz rich sandstones inhibit cementation of macroquartz (Aase et al., 1996; Berger et al., 2009; Ajdukiewicz and Lander, 2010; Ajdukiewicz and Larese, 2012). Significant porosity enhancement can be provided by diagenetic dissolution of feldspars, and mechanical compaction has hardly any effect on these secondary pores (Nabawy et al., 2009a; Bowen et al., 2011; Yue et al., 2018). Secondary porosity is created through the process of dissolution (Saïag et al., 2016). The presence of high porosity in deeply buried sandstones is an indication that the processes of dissolution and leaching out have compensated for the effects of compaction and cementation (Nabawy et al., 2009b).

5.1. Effect of clay coats on reservoir quality Study have also shown that in deeply buried sandstones, clay coatings are responsible for preserving porosity (Aase et al., 1996). Aase et al., 1996 reported unusually high porosities (20-27%) and permeability (100-1000 md) in > 4 Km deep upper Jurassic sandstones of Central Graben area, North Sea. 0.1–2 μm thick microcrystalline quartz continuous grain coatings are responsible for 5-15% higher than expected porosities. Siliceous sponge spicules (Rhaxella) controls the distribution of these microcrystalline quartz coatings. Development of chlorite which is an important grain coating clay inhibits cementation of quartz and preserve porosity (Ajdukiewicz et al., 2010; Nguyen et al., 2013; Zhu et al., 2017; Lai et al., 2018b). Overgrowth of pyrites, gypsums and feldspars in sandstone reservoirs is volumetrically less important. Results show that the most common and frequent clay mineral (grain coat) found in twenty samples is Illite. Illite was also observed with chlorite clay grain coating. According to Bjørlykke and Aagaard (1992); the most common clay mineral observed in the study area is Illite. Two diagenetic processes can lead in the formation of illite in the reservoir, either by illitization of kaolinite or from a smectite precursor (Bjorlykke and Aagaard, 1992). Morphology of illite supports the second diagenetic process as the main cause of Illite grain coating in the study area. Smectite was not observed in the samples as smectite is only stable till 70 °C. All twenty samples are at depth greater than 4 km with temperatures of about 140–150 °C (Aase et al., 1996; Bjorlykke, 2010). After 70 °C, smectite converts into illite or chlorite. Chlorite clay coatings were minor but wherever they are present, they show honeycomb morphology (Fig. 13) which supports that the coatings have a possible smectitic precursor. Chlorite development preserver porosity by inhibiting quartz cementation (Zhu et al., 2017; Lai et al., 2018b). Microquartz is the most abundant and usually present grain coat in the studied samples and that is the reason that effect of illite coats on porosity has been hard to be estimated. These clay coats are usually thin and therefore may not compete with micro-quartz in preserving reservoir porosity even if clay coats were abundant. That is the reason that in study area, illite and chlorite are considered as the secondary contributor in the preservation of porosity by inhibiting quartz cementation. Moreover, illite coats could be of more importance locally.

Fig. 6. Cross plot of a) Porosity versus Quartz cementation, b) Depth versus Quartz cementation, c) IGV versus Quartz cementation.

sponge spicules or remnants of it were observed in the studied samples. This is in accordance with Vagle et al. (1994) who observed that sponge spicules were subjected to rapid decomposition. So, before deposition spicules were reworked because of decomposition. But Rhaxella spicules and micro-quartz together are reported from a wide variety of depositional environments which indicates that spicules were transported in those settings by sedimentary reworking (wave processes, tidal processes, gravity transport). So, it is difficult to interpret the specific settings in which Rhaxella Perforata is to be found but it seems that their presence is always linked to shallow marine environment. The major source of crypto and micro crystalline quartz is suggested to be amorphous silica sponge spicules suggesting crystallization at rapid rate from locally saturated silica solutions (Williams et al., 1985; Aase et al., 1996). At 25 °C, solubility of sponge spicules (120-140 ppm) is 15–20 times higher than the solubility of quartz (6-10 ppm). Recrystallization of Rhaxella spicules to microquartz and fine quartz crystallites lining of moldic pores causes little to no net reduction of porosity in deeply buried sandstones (Aase et al., 1996). In deeply

446

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Table 4 Thin sections from well 2/1-6 showing degree of sorting. Sample

Average Grain Size mm

IGV %

Sorting

Grain size

Grain Shape

2–12 2–14 2–15 2–16 2–17 2–18 2–19 2–21 3 4 5 6 7 9 10 11 12 13 14 16

0.25 0.0175 0.0175 0.015 0.015 0.0175 0.01 0.02 0.02 0.25 0.255 0.2525 0.0175 0.25 0.0175 0.02 0.015 0.0125 0.0125 0.0125

37.2 32.2 33.9 39.2 38.2 30.3 26.9 21.6 25.8 13.6 25.8 23.5 18.2 37.2 26.5 41.2 38.6 40.5 36.9 44.2

Well sorted Moderately sorted Well sorted Well sorted Well sorted Well sorted Moderately sorted Poorly sorted Moderately sorted Moderately sorted Moderately sorted Well sorted Well sorted Poorly sorted Poorly sorted Well sorted Well sorted Well sorted Well sorted Well sorted

Medium grained Fine grained Fine grained Fine grained Fine grained Fine grained Fine grained Fine grained Fine grained Medium grained Fine to medium grained Fine to medium grained Medium grained Medium grained Fine grained Fine grained Fine grained Fine grained Fine grained Fine grained

Angular Angular Sub-angular Angular Angular Angular Angular Angular Sub-angular Sub-rounded Sub-rounded Sub-rounded Sub-rounded Sub-rounded Angular Sub-rounded Angular Angular Sub-rounded Angular

cementation grown in to the pore space and destroys the porosity (Islam, 2009; Xi et al., 2015; Worden et al., 2018) until or unless grain is covered by some grain coating like micro-quartz or clay coats. Quartz overgrowth was recognized easily by comparing backscatter images and CL images. CL images easily differentiate between detrital quartz grain and quartz overgrowth caused by cementation around it (Fig. 15) (Götze et al., 2001). Quartz overgrows through spiral growth, but this type of overgrowth was not observed in the studied samples. Though, normal quartz overgrowth was observed as shown in Fig. 14. This type of quartz overgrowth is caused by low (< 5%) silica saturations (Jahren and Ramm, 2000). Source of quartz cement in Ula formation can be a pressure-solution along stylolites. Contacts between illite clay or mica and quartz grain are the preferred sites of dissolution. These contacts are called stylolites (Fisher et al., 2000). 5.3. Reservoir quality: A regional scale perspective Three different wells from Ula, Gyda, and Tambar fields were correlated based on porosity variation trends in the area (Fig. 4). Well correlation was done by following studies by Ramm et al. (1997). High porosity zones recognized in the study are Ula A and Ula C. While low porosity zones are Ula B, Ula D and Ula E. Fig. 4 shows that the most important and high porosity sandstone Ula A is easily correlate-able in Gyda and Tambar Fields. In well 7/12-2 (Ula Field) we have only one low porous zone which is the upper part of Ula A. Ula B, Ula D and Ula E are low porosity zones. Ula C is also absent in well 7/12-2. From the correlation (Fig. 4), we can say that porosity preservation mechanisms in the Tambar and Ula Fields are the same as in Gyda Field (well 2/1-6) which are presence of micro-quartz and clay coatings. The reason for low porosity zones in Tambar and Ula fields is possibly the same as in Gyda Field which is that sub-rounded grains have less porosity as compared to angular grains. To confirm these findings, authors would suggest looking in to the samples from Tambar and Ula Fields.

Fig. 7. a) Cross plot of IGV against Matrix, b) Inter Granular Volume from well 2/1-6. IGV is on X-axis and depth is on Y-axis. It shows variation in IGV with depth.

5.2. Quartz cementation 5.4. Effect of matrix, grain size, grain shape and sorting on reservoir quality In deeply buried sandstone reservoirs, the main porosity controlling factor is quartz cementation (Hansen et al., 2017). Quartz cement in samples of well 2/6-1 ranges between 1 and 6.3% (Fig. 6, Table 5). Majority of the samples have less than 4% quartz cementation. Samples which have ≥4% quartz cementation are generally fine grained (Table 5) which implies that large surface area promotes quartz cementation. Scanning Electron Images (Fig. 14) show that quartz

Porosity and permeability of sandstones is largely influenced by factors such as; 1) Compaction (mechanical or chemical), 2) Grain shape, size and sorting, 3) diagenetic cements (amount, type, location), and 4) secondary porosity generation (Nguyen et al., 2013; Lai et al., 2016). Grain size and sorting are strongly correlated to quality of reservoir because small changes in these parameters can cause loss of porosity due to compaction (Bjørlykke, 2014; Lai et al., 2016). Study by 447

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Fig. 8. Showing typical examples of grain coats like micro-quartz (mQz) and illite (Il) present in study area. (a) Quartz overgrowth (Qzo), (b) Quartz cementation over quartz grain. Picture (a) was taken from sample 11 and (b) was taken from sample 3.

Fig. 9. Showing preservation of porosity in sandstones by the grain coats. (a) Micro-quartz (mQz) and Illite (Il) are covering whole quartz grain, (b) Fibrous pore filling Illite (Il). Picture (a) was taken from sample 11 and picture (b) was taken from sample 12.

Fig. 10. Showing extensive micro-quartz (mQz) coating over quartz grain with quartz overgrowth over it. Picture was taken from Sample 2-17.

compaction bands and is inhibited in sandstone formations which are more porous and poorly sorted (Cheung et al., 2012). Intergranular volume is the initial space between sand grains determined by matrix content, grain shape, size and grain sorting (Paxton et al., 2002; Ajdukiewicz and Lander, 2010). Reason for high IGV is large amount of matrix as observed in thin section. Most of the samples have IGV above 30%. The reason for high IGV in most of the samples is high matrix content. IGV is strongly influenced by composition and

Cheung et al. (2012) shows that the development of compactions bands is strongly influenced by the distribution of grain size. Well sorted quartzose sandstones have porosities from 22 to 25% (Ajdukiewicz and Lander, 2010; Fossen et al., 2011; Cheung et al., 2012). Development of compaction bands is attributed to uniformity of grain size distribution suggesting that, sorting is also an important attribute which influences compaction band formation (Wang et al., 2008; Cheung et al., 2012). Grain size distribution is relatively narrow in sandstones that develop 448

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Fig. 13. Pore filling Chlorite clay (Ch) showing its characteristic honey comb morphology. Micro-quartz (mQz) is also covering another quartz grain. This picture is taken from sample 11.

Fig. 11. Showing micro-quartz grain coating (mQz) and size variation of microquartz grains. Overgrowth (Qz) of euhedral quartz grain within the microquartz grain coating probably because grain coat didn't cover all of the grain. Picture was taken from sample 2-18.

compaction involves rearrangement of grains, ductile bending of grains and breakage of grains. Though ductile deformation was not observed in the study; all these processes are most likely to reduce the porosity and in turn IGV of the sandstones. As sandstones are subjected to mechanical compaction, grain frame is locked. Porosity starts to decrease because of grain crushing, reorientation and deformation. Sandstones which have clay/matrix in between, show little subsequent grain reorientation because of soft grain contacts. Chuhan et al. (2002, 2003) proved that coarse grained sandstones are compacted more as compared to fine grained sandstones. This study is in analogy with Chuhan et al. (2002, 2003) as Ula A and C which are fine grained sandstones (Table 4) have high IGV as compared to Ula B which is medium to coarse grained sandstone. Mechanical compaction doesn't necessarily lead to chemical compaction in sandstones that are quartz rich, clean and well sorted whereas, extensive compaction is expected in sandstones with ductile grains such as shale clasts (Ajdukiewicz and Lander, 2010). All the samples of the sandstones of Ula Formation from well 2/1-6 are medium to fine grained and are both poorly and well sorted. Seven samples from Ula B, which is the low porosity zone of the well 2/1-6,

texture (Ajdukiewicz and Lander, 2010). With depth, overburden increases causing decrease in IGV as grains become closely packed. Average IGV in the study is 31.5% and ranges from 14% to 44% (Fig. 7). Relatively low IGV and porosity is observed in sandstones with poor sorting (Lai et al., 2016; Bjørlykke, 2014). Mature sandstones have higher intergranular primary porosity whereas feldspar rich sandstones have higher secondary porosity (Lai et al., 2016). Fig. 7a show that a very nice correlation exist between IGV and matrix. With increasing matrix, IGV also increases. We have high percentage of IGV in samples from Ula A and Ula C as these zones have high amount of matrix content and porosity (Table 4). Here matrix is defined as depositional clay and silt size particles which fill the space between framework grains. Grains of sandstones have been deformed in different styles and were observed in thin section study (Figs. 15 a-1). This deformation involves the fracturing of grains, grain crushing, and compaction of grains. Grain crushing, and deformed grains were observed in the study of thin sections under optical microscope. Grain deformation is done by sliding and reorientation of the grains. This observation agrees with Bjørlykke (1998, 1999, 2003) who proposed that the mechanical

Fig. 12. Showing sponge spicule (Sp) lying over a quartz grain coated with micro-quartz (mQz) grain coating. Uncoated surface (uQz) of quartz grain is also visible. EDX spectrum showing peaks of Silica and Oxygen at sponge spicule. Picture taken from sample 2-15. 449

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Fig. 14. Typical spiral quartz overgrowth pattern in absence of grain coats. Il = Mica, Il = Illite clay. Spectrum 1 = Illite, Spectrum 2 = Mica, Spectrum 3 = mica. (picture taken from sample 3).

are medium to fine grained and are mostly moderately sorted. Samples from Ula A and Ula C are well sorted and fine grained (Table 4). Though coarse-grained samples were not observed so medium and fine-grained sandstones were compared. Experimental compaction done by Chuhan et al. (2002, 2003) proved that well sorted coarse grain sandstones compact more and loose porosity as compared to the fine grained and poorly sorted sandstones. This is the reason that samples from Ula B (medium grain size) show less porosity on thin sections. While samples from Ula A and C (fine grain size) show high porosity (Table 5). Therefore, this study is in consistent with findings of Chuhan et al. (2002, 2003).

Cleaner sandstones are abundant in feldspar and detrital quartz, better sorted and are coarse grained (Lai et al., 2016). Studies have shown that grain coating chlorites preserve primary porosity by inhibiting secondary quartz cementation on detrital quartz grains (Ajdukiewicz and Lander, 2010; Nguyen et al., 2013; Lai et al., 2016). Loss of depositional porosity majorly through compaction than cementation is largely caused by small amounts of detrital quartz grains. Grains contact changes is responsible for reduction in primary intergranular porosity (Lai et al., 2016). Lai et al. (2016) suggests that primary porosity destruction factor by mechanical compaction is caused by the presence of planar and concave-convex grain contacts as well as 450

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Fig. 15. Showing quartz cementation over a quartz grains destroying reservoir quality. Grain crushing in a-1) is also visible. a-1) and b-1) are backscatter images. (a2) and (b-2) are Cathode Ray Illuminesence images. Picture was taken from samples 2-16 and sample 2-19.

Fig. 16. Showing overgrowth of micro-quartz grains due to high saturation of silica. mQz = Micro-quartz, Qz = Quartz overgrowth (picture was taken from sample 2-12).

IGV of medium grained samples is 26% while fine grained samples have 34% IGV (Table 4), which implies that coarse/medium grained sandstones have less IGV as compared to fine grained sandstones because coarse/medium grained sandstones are more crushed and compacted as compared to fine grained sandstones. Samples from Ula A and C are

mud clasts. Average grain size in the studied samples ranges from medium to fine grain. Samples from Ula B are medium grain size and samples from Ula A and C are fine grained. As no coarse grain sandstone was observed in study, so medium and fine-grained samples were compared. Average 451

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Table 5 Point counting data. Sample

Depth

Quartz

Feldspar

Rock Fragments

Matrix

Calcite Cement

Quartz Cement

Authigenic Kaolinite

Illite

Spicules

Mica

Primary Porosity

Secondary Porosity

IGV

Clay Index

2–12 2–14 2–15 2–16 2–17 2–18 2–19 2–21 3 4 5 6 7 9 10 11 12 13 14 16

4205.8 4212.55 4218.5 4222.6 4227.35 4231.8 4233.7 4240.6 4252.5 4254.6 4303.65 4309.6 4318.67 4321.7 4324.9 4327.6 4333.1 4336.95 4344.3 4353

48.6 55.6 54.3 50.6 51.3 59.3 62 71.3 63.3 76.3 64.6 69.6 74 54.6 56.6 54 54.3 47.3 54.6 46

6.6 5.6 9.6 7 6.3 3.6 5.6 4.6 8.3 4 6.3 4.3 3.6 3 5.3 2.3 2 8 1.6 3.3

0 0 0 0 0 0 0 0 0.3 0.6 0 0 0 0 0 0.3 0 0 0 0.3

15.6 7.6 10.6 15.3 18 7 8 6 6.3 2.3 9.6 9.6 8 19 11.3 22.3 17.3 18.3 14 29.3

4 3.3 3 4.3 2.6 4 3 1.3 5.3 3 1.6 4.6 3.3 0.6 1.6 2.6 6 2.6 6.3 5.3

1.3 6.3 4.3 3 2 5 1.6 1 4.6 3.3 5 4 2.6 2.6 4.3 4 4 2 1.3 2.3

2 1.6 1.3 1.6 2 2.3 2.3 1 0.6 3.3 3 1 0.6 2.9 0.6 0.6 2.6 2.3 4.6 3.3

3 4 1 1.3 1.6 3.6 1.3 0.9 0.3 1.6 0.6 1.3 3 1 9.3 0.6 2 0.6 2.3 0

0 0 0 0.3 0 0 0 0 0 0 0 0 0 0.6 0 0 0 0 0 1

2.3 0.6 0.3 0.3 0.3 0.6 1.6 0.3 0 0.3 0 0 0.3 0.7 1.3 0.6 0.3 1 0.3 1.6

16.3 15 15 15.6 15.6 14.3 14.3 13.3 9.6 5 8.6 5.3 4.3 15 9.3 12.3 11.3 17.6 14.3 7.3

0 0 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0

37.2 32.2 33.9 39.2 38.2 30.3 26.9 21.6 25.8 13.6 25.8 23.5 18.2 37.2 26.5 41.2 38.6 40.5 36.9 44.2

6.661728395 5.671942446 9.618416206 7.0256917 6.331189084 3.660708263 5.620967742 4.612622721 8.304739336 4.020969856 6.309287926 4.318678161 3.640540541 3.018315018 5.464310954 2.311111111 2.036832413 8.012684989 1.642124542 3.3

grain coatings are also present and Illite is the most common clay grain coating as compared to chlorite grain coating. Micro-quartz grain coating seems to be the main cause of preserving porosity at depths of > 4000 m in the Ula B zone. Micro-quartz is present in all samples in both high and low porosity zones. It is most likely that microquartz is present, in all zones because of sedimentary reworking of the sponge spicule Rhaxella Perforata. Grain shape has pronounced effect on porosity of the sandstones in the area. Angular grains loose porosity with mechanical compaction as they have small contact areas, and this promotes deformation resulting in porosity loss. Perhaps this is the reason that Ula A and Ula C have low porosities. Inter Granular Volume is very high in the study area (up to 44%). Reason for high IGV in most of the samples is high amount of matrix. IGV depends on many factors including mechanical compaction, grain sorting, grain size, and grain shape. IGV is higher in fine grained sandstones as compared to medium grained sandstones.

mostly well sorted. Samples from Ula B are poorly and moderately sorted. Studies like done by Rogers and Head (1961) and Beard and Weyl (1973) have proved that well sorted sandstones have higher IGV and higher porosity as compared to moderately and poorly sorted. These findings are in agreement with the study (Table 4). Samples from Ula A and C have high IGV and porosity while samples from Ula B have low IGV. Most common shape of grain in the study was the angular grains (11 samples). Sub-rounded grains were present as second majority (7 samples) but there were only two samples of with sub-angular grains. Average IGV of angular grains calculated through point counting is 34%. Average IGV of sub-rounded grins is 28%. While subangular samples have average IGV of 30%. This clearly indicates that samples with angular grains have high IGV while sub-rounded grains have less IGV as compared to sub-angular and angular grains. Lai et al. (2016) show that with increasing grain size, porosity increases; siltstones (< 0.1 mm) have much lower porosity than finegrained sandstones (0.1–0.25 mm). During progressive burial, porosity reduction and mechanical compaction will be less in case of well-sorted sandstones. Quality of reservoir improves with increasing grain size and better sorting (Lai et al., 2016). Mature sandstones that are rich in detrital quartz are potentially good reservoirs even at depth because they can withstand compaction (chemical and mechanical) (Lai et al., 2016). Coarse-grained quartz-rich sandstones have higher concentration of preserved primary pores whereas feldspar-rich sandstones have preserved secondary pores. Reservoir intervals which are fine grained, moderately well-sorted having low detrital clay and cement content, but high percentages of detrital quartz and feldspar are of good quality due to their high porosity and permeability (Lai et al., 2016). Another important parameter controlling the quality of reservoir is pore-throat size, which affects the important properties of rocks such as permeability and drainage capillary pressure (Xi et al., 2016). Storage potential and physical rock flow is represented by pore-throat size. Permeability is represented by pore-throat sizes while porosity is represented by pore spaces (Xi et al., 2016). Poor sorted sandstones have low initial porosity whereas sandstones with high grain angularity have higher porosities (Fawad et al., 2011). For well-sorted sandstones, increasing grain size causes increase in compaction and velocity (Fawad et al., 2011).

Acknowledgement The authors would like to acknowledge Det norske oljeselskap ASA (DETNOR), Norway and the Department of Geosciences, University of Oslo, Norway. Special thanks to Tom Erik Mast and Berit Løken Berg for their esteemed guidance, suggestions, discussion and professional support throughout the research work. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpetgeo.2019.01.001. References Aase, N.E., Bjorkum, P.A., Nadeau, P.H., 1996. The effect of grain-coating microquartz on preservation of reservoir porosity. AAPG (Am. Assoc. Pet. Geol.) Bull. 80, 1654–1673. Ajdukiewicz, J.M., Lander, R.H., 2010. Sandstone reservoir quality prediction: the state of the art. AAPG Bull. 94 (8), 1083–1091. Ajdukiewicz, J.M., Larese, R.E., 2012. How clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones: observations and experiments ClayCoat Experiments. AAPG Bull. 96 (11), 2091–2119. Ajdukiewicz, J.M., Nicholson, P.H., Esch, W.L., 2010. Prediction of deep reservoir quality using early diagenetic process models in the Jurassic Norphlet formation, Gulf of Mexico. AAPG Bull. 94 (8), 1189–1227. https://doi.org/10.1306/04211009152. Beard, D.C., Weyl, P.K., 1973. Influence of texture on porosity and permeability of unconsolidated sand. AAPG (Am. Assoc. Pet. Geol.) Bull. 57, 349–369. Becker, I., Wüstefeld, P., Koehrer, B., Felder, M., Hilgers, C., 2017. Porosity and permeability variations in a tight gas sandstone reservoir analogue, Westphalian D, Lower

6. Conclusion Micro-quartz grain coating is very common in the study area. Clay 452

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