Facies and Reservoir Characteristics of the Ngrayong Sandstone in the Rembang Area , Northeast Java ( Indonesia )

The Rembang area is a well-known prospective region for oil and gas exploration in Northeast Java, Indonesia. In this study, the reservoir characteristics of the Ngrayong Sandstone were investigated based on outcrops in the Rembang area. Petrological, mineralogical, petrophysical and sedimentological facies analyses were conducted. These sandstones are grainand matrix-supported, and composed of very fine to medium, sub-angular to poorly-rounded, moderatelyto very well-sorted sand grains. These sandstones are mainly composed of quartz, orthoclase, plagioclase, and micas with minor amounts of clay minerals, and therefore are predominantly classified as sub-lithic arenite and sub-felds pathicarenite. Petrographic observations and grain size data indicate that these sandstones are texturally quite mature, based on their good -sorting and the occurrence of minor amounts of matrix clays. Common clays in the samples include illite, smectite, kaolinite, and gibbsite. The porosity of the Ngrayong sandstones ranges from 25.97% to 40.21%, and the permeability ranges from 94.6 to 3385 millidarcies. Thus, these sandstones exhibit well to excellent reservoir qualities. Eight lithofacies were identified from five measured stratigraphic sections, and are dominated by foreshore and tide-dominated shoreface facies. The Ngrayong sequence shows a single transgressiveregressive cycle. Cross-bedded sandstone and massive sandstone are identified as the most promising potential reservoir facies based on their characteristics in outcrops, their lateral and vertical distributions, their sedimentological characteristics and their petrophysical properties.


Introduction
The Northeast Java Basin is a Tertiary sedimentary basin in Indonesia that covers an area of approximately 50 km 2 in the eastern part of Central Java, East Java, Madura and the areas adjoining the East Java Sea and Madura Straits.It consists of a relatively stable northern platform and a series of deep basins to the south.
The Northeast Java Basin has a long history of petroleum production.Early hydrocarbon exploration was mainly conducted in the onshore portion of the basin.
The Ngrayong Formation is one of the most prolific reservoirs in the Northeast Java Basin; hydrocarbon has been produced from its reservoir since the 19th century [1].A cumulative 150 million barrels of oil have been extracted from the Ngrayong Sandstone, which represents about 75% of the total hydrocarbon production of the basin [2].The Ngrayong Sandstone exhibits a complete regressive-transgressive sequence that comprises coarse-grained sandy clastics with thin layers of limestones at the top of the formation.The Ngrayong Sandstone is regarded as a clean, quartzose, fine-to medium-grained, well-sorted, and commonly cross-bedded sandstone unit.This sandstone has been interpreted as deposited in various sedimentary environments, such as inner and outer neritic and bathyal settings, and was influenced by a wide variety of sedimentary processes.Consequently, interpretation of the depositional environment of the Ngrayong Sandstone is still under debate.The observed complex variation in sedimentary facies has been attributed to the complexity of the depositional environments.Therefore, the reservoir varies both qualitatively and quantitatively as the sandstone varies geographically throughout the basin (Figure 1).The Rembang area is a prospective region for oil and gas in the Northeast Java Basin.The generative reservoir rock, the Ngrayong Sandstone, crops out in several localities in this area.Occurrences of oil and gas seepage observed in this sandstone unit represent a potentially viable petroleum system in the subsurface; therefore, this area may be promising for future exploration.For such future exploration to be successful, a more precise understanding of the characteristics and geological conditions of the Ngrayong Sandstone is required.This study was therefore conducted to investigate the quality of the siliciclastic reservoir rock, especially the Ngrayong Sandstone, based on observations of sedimentary facies distribution, variation in depositional environments, and internal characters.
This detailed research was conducted to characterize the reservoir geology and petrophysical properties of the Ngrayong Sandstone in the Rembang area of Northeast Java, Indonesia.

Research Methods
This research is based on field investigations involving facies analysis of five measured stratigraphic sections and supported by petrographic study, granulometric analysis, X-ray diffraction (XRD) analysis and petrophysical analysis.
Stratigraphic sections were measured at five different locations along the road between Blora and Rembang.Attributes of the rock strata and geological structures, such as strike and dip, joints, folding and faults, were measured with a Brunton compass.The lithology, texture, sedimentary structures, fossil content and tectonic deformation were observed and recorded during measurements.
Petrographic study was performed to determine the composition, texture and fabric of the sandstone as well as the characteristic of the pores spaces.Fifteen samples were prepared for this petrographic analysis.All of the prepared thinsections were impregnated with blue epoxy to expose the voids or pore spaces in the samples.The proportions of the constituent mineral grains were estimated visually to determine the mineral composition of the sandstone.
The minerals and specific types of clay present in the sandstone were identified using XRD.This analysis included both powdered bulk rock and clay fraction analyses.Three samples were prepared for XRD analyses.Air-drying and ethylene glycolation were employed to treat the clay fraction.To confirm the presence of smectite clay, the swelling of the diffraction peak d-value was checked with the ethylene glycol (EG)-treated samples.
To investigate the textural maturity of the sandstones, granulometric analysis of fifteen samples was performed to determine the quantitative distribution of grain sizes and the sorting of the grains.The classification scheme and terminology of Pettijohn et al. [3] was used, including class intervals for grading.Percentile value or phi values (such as Ø5, Ø16, Ø25, Ø50, Ø75, Ø84 and Ø95), applied in the calculations, were deduced from a binary plot of the cumulative weight percent versus the phi value.Parameters such as the geomeric graphic mean, graphic standard deviation, skewness, and kurtosis were applied for defining grain size, sorting, and skewness.Statistical analyses were carried out to quantify the grain size, sorting, roundness and skewness of the analyzed sandstone using these percentile values and following the standard formulas.
The petrophysical analysis included determining the porosity and permeability to evaluate the quality of the Ngrayong Sandstone as a reservoir.Nine outcrop samples of the Ngrayong sandstone were selected and analyzed for porosity and permeability.
Petrographic microscopy, XRD analysis, and granulometric analysis were performed at the laboratory of the Geological Engineering Department, Faculty of Engineering, Gadjah Mada University, Yogyakarta (Indonesia).Porosity and permeability were measured at the laboratories of the Petroleum Engineering Department, Universitas Pembangunan Nasional (Veteran), Yogyakarta (Indonesia) and at the Laboratory of National Oil Institute Legimas (Lembaga Minyak Dan Gas Bumi), Jakarta (Indonesia).

Petrographic Analysis
The modal compositions of the sandstone samples were determined petro-

X-Ray Diffraction Analysis
The results of the XRD analysis indicate that quartz and orthoclase are the main

Sieve Analysis
Based on the result of the sieve analysis, the Ngrayong sandstones are very fineto 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).The granulometric data indicate that these sandstones are texturally quite mature.The result of the granulometric analysis is shown in (Table 1).

Petrophysical Analysis
Petrophysical analysis includes determining porosity and permeability.The results of petrophysical analysis of these sandstones are shown in (Table 2).
Adopting the classification scheme of Koesoemadinata [5], the porosity of the analyzed sandstones is excellent in quality, and the permeability ranges from good to excellent in quality.The higher percentage of porosities in the sandstones of different facies would be probably due to the clean nature (matrix poor and loosely cemented) of the sandstones.Higher porosities and permeabilities were also probably favored by the outcrop nature of the sandstones which experienced extensive weathering.This would lead to the secondary porosity by the dissolution of the clay particles or matrix within the sandstone.

Stratigraphic Sections and Lithofacies
Eight lithofacies were identified from five measured stratigraphic sections (Figure 7, Figure 8) based on the characteristics of the sedimentary units, such as their lithology, grain size, color, thickness, sedimentary structures, and biogenic content [6], combined with the results of laboratory analyses such as petrographic microscopy and granulometric analysis.The identified lithofacies are a crossbedded sandstone facies (Facies A), a carbonaceous mudstone facies (Facies B), a laminated shale facies (Facies C), a thin-bedded sandstone facies (Facies D), a The cross-bedded sandstone facies (Facies A) is dominated by sand-sized grains.
In outcrop, these sandstones are whitish, yellowish and reddish brown in color, very fine-to medium-grained, and thick-bedded to massive.Sandstones in these intervals are moderately to very well-sorted, sub-angular, and grain-supported.
The characteristic sedimentary structures are cross-bedding (planar and trough), and herring-bone structures.Facies A was deposited in a tide-dominated shallow marine shelf (upper shoreface) environment, as indicated by the cross-bedding and herring-bone cross-stratification [8].
The carbonaceous mudstone facies (Facies B) is brown to dark gray, massive, and characterized by mud cracks and carbonaceous material.Scattered clasts of amber (up to 3 cm in diameter) were observed.Fossil leaves and carbonaceous plant remains were found in the basal parts of these beds.Based on the sedimentary characteristics of this facies, it was likely deposited in a lagoon environment characterized by low-energy conditions [6].
The laminated shale facies (Facies C) is generally light gray to dark brown, thinly-laminated to thin-bedded, and fissile, and contains minor silty shale.
Gypsum flakes are commonly intercalated within the laminated shale.This facies gradually grades into the thin-bedded sandstone facies designated Facies D in the upper part, which is characterized by intercalated sandstone and shale.Based on the sedimentary structures and characteristics of this facies, it was likely deposited in a tide-influenced shelf (upper shoreface) environment [6].
The bioturbated sandstone facies (Facies F) is also dominated by sandstone.In outcrop, these sandstones are yellowish to reddish in color, fine-grained, and grain-supported.The grains are moderately-to well-sorted and angular.The most characteristic feature of this facies is the presence of burrow structures.The ichnotaxon Ophiomorpha is common and typically filled with sedimentary iron oxides.This facies is intensely bioturbated.Based on the petrographic analysis, these sandstones have been identified as sub-feldspathic arenite.According to the characteristic vertical burrow structures, it can be inferred that this facies was deposited in a beach (foreshore) environment [6].
The fossiliferous sandstone facies (Facies G) is dominated by sandstone.In outcrop, sandstones of this facies are yellowish to reddish brown and finegrained.They are moderately to well-sorted and grain-supported with angular grains, and contain abundant fossils and fossil fragments of bivalves, gastropods, bryozoans and foraminifera Based on petrographic analysis, this sandstone is defined as sub-lithic arenite.The characteristics of the sandstone and the abundant fossil content imply that this facies was likely deposited in a coastal (foreshore) environment [6].
This limestone is characterized by bioclasts of an assemblage of large foraminifera dominated by Cycloclypeus sp., Lepidocyclina sp., Miogypsinoides sp., Discocyclina sp., and Planorbulinella sp.Based on the occurrences of larger foraminifers' assemblage, the age of the Ngrayong Formation is assigned as Middle Miocene (Figure 9).The greater porosities of the sandstones of certain facies are likely the result their clean nature (i.e., matrix-poor and loosely-cemented).The high porosities and permeabilities of certain samples were also likely enhanced by extensive weathering of the exposed outcrops from which they were collected.This weathering would lead to secondary porosity associated with dissolution of the clay particles and/or matrix in the sandstone.

Conclusions
Based on data from field investigations and petrographic, mineralogical, sedimentological and paleontological analyses, the characteristics of the Ngrayong Sandstone in the Rembang area can be summarized and interpreted as follows.
These sandstones are generally either grain-or matrix-supported, and are composed of very fine-to medium-grained, angular to poorly-rounded, moderately to very well-sorted sand grains.These sandstones are predominantly classified as sub-felds pathic and sub-lithic arenites, are clean in nature (matrix/ cement-poor), and show some degree of textural maturity; therefore, these rocks may serve as a good reservoir for hydrocarbon accumulation.
Eight lithofacies were identified from five measured stratigraphic sections:

Figure 1 .
Figure 1.Location map of the Rembang area, Northeast Java, Indonesia.
Figure 5(b)).However, most of these sandstones are fine-grained, very wellsorted and grain-supported.The porosities of the sandstones were estimated from petrographic study, and range from 5% to 30%.

Figure 8 .S
Figure 8. Lithofacies log of five measured stratigraphic sections (Section A, B, C, D and E).
cross-bedded sandstone (Facies A), carbonaceous mudstone (Facies B), laminated shale (Facies C), thin-bedded sandstone (Facies D), massive sandstone (Facies E), bioturbated sandstone (Facies F), fossiliferous sandstone (Facies G), and bioclastic limestone (Facies H).In general, the sedimentary successions of the measured stratigraphic sections are dominated by shallow marine shelf (tidedominated) sandstone facies (Facies A and E), which are overlain by a shallow marine (middle inner neritic) limestone facies (Facies H).The Ngrayong sequence in the Rembang area shows a single transgressiveregressive cycle.Of the eight lithofacies observed in the Ngrayong Formation, five principal facies have been identified as potential reservoir facies: the crossbedded sandstone facies (Facies A), the thin-bedded sandstone facies (Facies D), the massive sandstone facies (Facies E), the bioturbated sandstone facies (Facies F), and the fossiliferous sandstone facies (Facies G).The cross-bedded sandstone facies (Facies A) and the massive sandstone facies (Facies E) were identified as good potential reservoirs rocks based on their characteristics in outcrop, their lateral and vertical distributions, and their sedimentological characteristics.

Table 1 .
Result of granulomatric analysis of Ngrayong Sandstones.

Table 2 .
Summary result of petrophysics analysis of different facies of Ngrayong Sandstones.