Textural Characteristics and Depositional Environment of Ngrayong Sandstone (Middle Miocene) from Rembang Area, Northeast Java, Indonesia

Granulometric analysis of Ngrayong sandstone samples from Rembang area of NE Java (Indonesia) has been carried out to examine the textural characteristics and elucidate depositional environment. The result data from the grain size analysis indicates that the Ngrayong sediments are classified as sand or muddy sands. They are very fine-to medium-grained (phi values: 1.67 to 3.35), moderately-to well-sorted (standard deviation: 0.19 to 0.57), strongly fine-skewed to strongly coarse-skewed (skewness values: 4.82 to −5.97), and very platykurtic to very leptokurtic (kurtosis values: 0.36 to 2.41). Sandstone samples show unimodal grain size distribution. The sediments are interpreted to be transported in all three modes-traction, saltation and suspension, however, suspension and rolling are the major processes during transportation. Discriminant functions indicate diversity in the depositional environment for the sandstones. However, shallow marine is regarded as the dominant depositional environment. The preponderance of fine-grained sediments and lack of coarse sands suggest low to moderate energy conditions during deposition of Ngrayong sediments.


Introduction
Grain size is a unique descriptive property and used as an important textural character in classifying the siliciclastic sedimentary rocks. The distribution of grain size and textural characters in the clastic rocks generally reflects the physical changes in transporting media, the depositional processes and environment. Granulometric or sieve analysis is a technique widely applied in studying sandstones and sandy rocks by sedimentologists. Hence, systematic presentation and analysis of grain size data provide basis for the interpretation of nature of sediments, transportation process, hydrodynamic conditions, and depositional environment.
Grain size data have been widely applied in deciphering various depositional environments and hydrodynamics condition of clastic sediments ( [1]- [17]).
The Ngrayong Sandstone (Middle Miocene) is one of the important reservoirs in the Northeast Java Basin of Indonesia and hydrocarbon production from its reservoir has been carried out since the 19th century [18]. A total of 150 million barrels of oil have been produced from this reservoir sandstone, reaching about 75% of the total production of petroleum from the NE Java basin [19] [20]. The Ngrayong Sandstone is widely exposed in the Rembang area, Northeast Java, Indonesia ( Figure 1). These sandstones are classified as sub-feldspathic and sub-lithic arenite and have exhibit excellent reservoir properties [20]. In the present study, an attempt has been made to undertake a detailed grain size analysis of sandstone samples from the Rembang area. Statistical parameters such as mean, median, standard deviation, skewness and kurtosis have been deduced from the grain size analysis data. These statistical data have been utilized to infer the textural characteristics of the clastic sediments. Bivariate plots have also been constructed and utilized in the interpretation of the depositional pattern of the sedimentation and depositional environment.

Regional Geology
The research area is located in the Rembang Zone [21] of Northeast Java Basin. The Northeast Java Basin was developed as a back-arc basin behind the active volcanic arc of the Central Java. It consists largely of a foreland shelf dipping gently southward. It is comprised of a large number of east-west trending anticlinorium, alternating with alluvial plains. Lithology was dominated by mix siliciclastic of shallow marine with shelf carbonate and clastics derived from continent. The Rembang Zone sequence was strongly influenced by continental-derived sediment. Seven major lithostratigraphic units have Middle Miocene to Quaternary age's occurr in the Rembang Zone. They are, from oldest to youngest; the Tawun, Ngrayong, Bulu, Wonocolo, Ledok, Mundu, Piciran and Lidah Formations. The Ngrayong Sandstone (Middle Miocene) was the main petroleum reservoir in the Rembang Zone (e.g. Cepu Oil Field). Total thickness of Ngrayong Formation was varied, with an average more than 300 m. Regional geological map of Northeast Java area is shown in Figure 2

Materials and Methods
During field investigation, about fifty representative sandstone samples were collected from different layers of Ngrayong Sandstones from different locations ( Figure 1) in the Rembang Area. Of these samples, fifteen sandstone samples have been selected and subjected to grain size analysis. The selected sandstone samples were disaggregated and 100 grams of clean and dry loose sands were sieved for at least 30 minutes using Retsch vibratory sieve shaker for proper separation of each grade. Different grain sizes of the samples were screened by conventional sieving technique and sieves used in the analysis follow the US standard sieve mesh system (i.e. 10, 18, 35, 60, 120, 230, and the remainder in the pan). The phi (ϕ) values of the sieves mesh size represent the grain diameter for each sieve fraction.
The sand grains retained by each different sieve were taken, weighed and weight percentage calculated. Subsequently, the frequency for cumulative weight percent and corresponding phi data was applied to plot on a log-linear graph. Percentiles values (5,16,25,50,75,85 and 95) were recorded on the cumulative curves and calculate statistical measures like graphic mean (Mz), inclusive graphic standard deviation (σ 1 ), inclusive graphic skewness (SK 1 ) and kurtosis (K G ) were calculate using the formulae provided by [2] [22]. The depositional processes and environments of the Ngrayong Sandstone were deduced from the linear discriminant functions proposed by Passega [3] [4] and Sahu [10]. C-M plot of Passega [4] is carried out to determine different depositional processes, mechanism of sedimentation and hydrodynamic condition of the transporting medium. Granulometric analysis was performed at the laboratory of Geological Engineering Department, Gadjah Mada University, Indonesia.

Result and Discussion
In the Rembang area, Ngrayong Sandstone is generally exposed along the Blora-Rembang Car Road, in which outcrops are well-exposed at the quarries. Sandstone samples were collected from these outcrops ( Figure 1). Detailed description on measured stratigraphic sections, lithostratigraphic facies and sample distributions are fully described in [20]. Samples A16 and A17 were collected from the cross-bedded sandstone in the upper part of the stratigraphic section whereas sample A2 was collected from the bioturbated sandstone in the lower part of the section. Samples B5 and B6 were collected from laminated sandstone and shale in the middle part of the stratigraphic section. Samples C2 and C5 were collected from massive sandstone outcrop in the middle part of the stratigraphic column whilst samples C8 and C9 were collected from cross-bedded sandstone in the lower part of the column. Samples D2 and D3 were collected from fossilliferous sandstone in the upper part of the stratigraphic section whereas sample D7 and D8 were collected from massive sandstone in the lower part of the section. Sample E6 was collected from laminated sandstone in the middle part of the geologic section and E7 from cross-bedded sandstone in the upper part of section.

Textural Characteristics
The result data from sieve analysis indicates that the study samples are dominated by sand-size sediments ( Table 1). The sand content of the samples varies from 57.97% to 96.57%. Sample D7 has the lowest sand concentration (57.97%) whereas Sample A2 has the highest percentage of sand (96.57%). The silt content in the samples ranges from 1.47% to 30.01%. The lowest silt percentage is observed in sample E6 (1.47%) and the highest content is seen in samples D7 (30.01%). The clay content varies from 1.42% to 12.25%. The clay content is lowest at sample A2 (1.42%) and highest at B5 (12.25%). A2 has the lowest clay content and the highest sand content, while D7 has the lowest sand content and the highest silt content. The overall percentage composition of grain size distribution showed that the studied samples are sand dominated. The distribution of higher percentage of sand-size sediments in the studied samples from Ngrayong Sandstone is probably due to the prevailing high-energy environment during deposition of these clastic sediments. Figure 3(a) [23] and Figure 3(b) [24] show the textural classification of sediments based on sand/silt/clay ratios. The Ngrayong sediments are classified as sands (except one sample which falls in the clayey sand) (Figure 3(a)), whereas the majority of the Ngrayong samples are classified as muddy sands ( Figure  3(b)).

Statistical Parameters
Statistical parameters deduced from grain size data for the sandstone samples and interpretation for classes of class, sorting, roundness and skewness are shown in Table 2.
Graphic Mean (Mz) The mean value of the graph is the average grain size of the overall data. Graphic means corresponds very closely to the mean as computed by the method of moments, yet it is much easier to find. It is calculated based on three points and gives a better overall picture. Graphically, the calculation of mean value is calculated by the following formula [2] [22]: Open Journal of Geology  [22] was used in order to classify the sand size grain from the mean value. Graphic mean values of the sandstones range between 1.67 and 3.35 and fall in the size class very fine to medium ( Table 2). The average mean value (2.7) of the studied samples indicates the dominance of fine-grained sand. The size of the sand can be used as to infer the medium of transportation and average kinetic energy that prevailed during deposition of sediments. Coarse sands are transported as bed loads, medium sand are carried by saltation and finer grains are transported via suspension. The variation in grain size in the studied samples indicates the variable kinetic energy at the time of deposition. However, the dominance of fine-grained sediments in the Ngrayong sandstone samples implies low energy condition during the deposition of these sediments.
Inclusive The graphic kurtosis represents a measure of the peakedness of curves and a valuable test normality of a distribution. According to Folk and Ward [2], it is mathematically expressed as:  Table 2). The lowest value is seen at sample E6 where the sediments are very plakykurtic and highest at D7 where the sediments are very leptokurtic. Variation in kurtosis values of the sandstone samples indicates that variation in the flow characteristics of the depositional medium occurs at the time of deposition.

Relationship between Grain Size Parameters
The relationship between different grain size parameters help to understand, the nature of sediments, transportation processes, energy level of depositing agents and depositional environments of siliciclastic sediments ( [30]).
The bivariate plot between mean and standard deviation (Figure 4(a)) [30] indicates that sandstones are moderately-to very well-sorted. Moderately to very-well sorting are likely due to winnowing action of depositing agent under moderately to high energy level conditions.
The plot between mean and skewness also shows a wide range of plot from strongly fine skewed to strongly coarse skewed zones (Figure 4(b)) [30]. Sandstones having very fine-skewed and coarse-skewed natures indicate that these sandstones were deposited under low to high energy conditions. Mean versus kurtosis scatter diagram shows variation in kurtosis from very leptokurtic to very platykurtic (Figure 4(c)) [30].
Variation in these values expresses the reflection of the flow characteristic of the depositing medium [31] [32]. Platykurtic show low energy and mesokurtic show moderate energy flow whereas leptokurtic show high energy flow. This fact indicates that Ngrayong sediments were deposited under extreme environments characterized by low to high energy flow environment.

Frequency Diagrams
Modal distribution of the sediments is described by the frequency diagrams whereas the description of the transportation medium is generally given by the curves.
Histograms: The histogram distribution of different grain size for some selected samples given in Figure 5. All samples show uni-modal distribution (except sample Sp-D7 which shows bimodal distribution). Bimodal distribution is likely to be the sub-populations. In most cases, modal class lies in the range of phi value 2 to 3.
Cumulative curves: and cumulative weight percent versus phi (ϕ) value of all studied samples were shown in Figure 6(a) and Figure 6    plotted on the probability ordinate scale do not form continuous straight line except few samples curves which show two, three or more straight line segments. Each segment carries different slopes which indicate the presence of more than one population of grains. Different population is likely to be related with different modes of transportation-traction, saltation and suspension [13] [33] [34]. Cumulative curves indicated that the processes such as saltation, traction and suspension have played an important role during deposition of sandstones.

Linear Discriminant Functions
The statistical parameters (mean, inclusive graphic standard deviation, skewnessand graphic kurtosis) are also used to decipher the depositional environment following discriminant function analysis (Table 3) [10]. The following equations are applied in order to distinguish the different depositional environments: where Mz, σ 1 , SK and KG represents mean grain size, standard deviation, skewness and kurtosis respectively. Equation (1) is applied to distinguish between shallow agitated water and beach environment of sediment deposition. In order to differentiate between beach and shallow marine environment, Equation (2)  Therefore, these sediments were mainly deposited in shallow marine environment by beach and wave processes.

Passega Diagram
The Passega diagram (C-M Pattern) (Figure 9) is commonly applied to determine the environmental conditions of sediment deposition. Passega diagram is based on parameter C (one percentile value) and M (the Median: 50 percentile value). The relationship between C and M helps to understand the nature of sediment types and energy of transporting medium [4] [9]. The diagram involves several fields, pelagic suspension (T field), uniform suspension (SR field), graded    suspension (QR field), suspension and rolling (QP field), rolling and suspension-(PO field), rolling (ON field) corresponding to the various transports and sedimentation conditions in the marine, littoral or fluvial regimes. On the Passega diagram (Figure 9), eight samples plotted in the suspension and rolling zone (QP zone). Two samples plotted near the graded suspension (QR) zone, a sample plotted around the uniform suspension (SR) zone, and other three samples plotted outside the featured fields. The C-M diagram reveals that the sediments from Rembang area were deposited under diverse conditions and transportation. However, transportation by suspension and rolling is the dominant process.

Conclusions
Present research investigates the grain size distribution, statistical parameters and interrelationship of Ngrayong Sandstone from Rembang area, NE Java of Indonesia in order to explore the textural characteristics and depositional environment of the sandstone based on granulometric analysis.
The following summary conclusions were made from the results of granulometric analysis. The Ngrayong sandstones show the dominance of fine to very fine grained particles. The sediments are classified as sands or muddy sands. They are moderate to very well-sorted due to winnowing action of depositing agent. Most samples show uni-modal distribution. The modal class lies between 2 -3ɸ (0.125 to 0.25 mm) in most samples. The median value ranges from 0.3 to 3.75. Mean size of the study ranges between 1.67 and 3.35. This high variation in grain size indicates the variation in the kinetic energy at the time of deposition. Due to fluctuation in the kinetic energy there was mixing of sediments of various subpopulations transported in different modes. Samples are positively, negatively as well as nearly symmetrical.
The sediments were transported in all three modes-traction, saltation and suspension. However suspension and rolling remain the major process of transportation. The preponderance of fine grained sediments and lack of coarse sands suggest low to moderate energy conditions of deposition Bivariate analysis indicates diversity in the depositional environments. However, shallow marine is regarded as the dominant depositional environment.