POSSIBLE EXPLANATIONS OF THE INCREASING COAL RANK
AT THE PINANG DOME AREA, SANGATTA, EAST KALIMANTAN
AT THE PINANG DOME AREA, SANGATTA, EAST KALIMANTAN
Oleh
Herudiyanto
Sub Direktorat Konservasi – DIM
Herudiyanto
Sub Direktorat Konservasi – DIM
ABSTRACT
The Pinang Dome has been known for containing the good quality of coals. The average calorific value of the Pinang Dome coals is closed to 7000cal/gr, much higher than those of the typical and common calorific value of the same Neogen coals that ranging from 4320 – 5900 cal/gr. It has been widely accepted that the Pinang Dome was formed as a result of claystone and mudstone diapirism associated with an underlying over-pressured marine shale zone. The process was accompanied by a high geothermal gradient, locally caused by a heat convection cell from the underlying overpressured zone, thus causing an increase in coal rank. This theory was also applied by some oil companies to explain the mechanism of hydrocarbon expulsion, a function of the coincidental relative positions of the oil window zone and over-pressured zone in the Mahakam Delta, Kutei Basin.
At least 12 coal outcrop samples and core samples taken from shallow drill holes of the Pinang Dome and additional 53 cutting samples taken from the deep oil exploration wells of the Sangatta area were analysed for their rank (Rv). The results show that the Pinang Dome coals in general, show significantly higher rank than those of coals that having the same age/formation and the rank decreases gradually away from the dome. There was an abrupt changes in geothermal gradient at the S-3 well that probably was caused by thermal effect of a magma activity somewhere at the deeper part of the basin. This may be used to explain the possibility of an increasing rank in the Pinang Dome coals that consistent with the formation of the Pinang Dome as a result of an igneous activity in the Sangatta area.
1. Introduction
Geologically, Sangatta lies within the Kutei Basin, one of the most prolific oil-and gas-producing basins in the island of Kalimantan, Indonesia. The majority of oil and gas fields occur in the Neogene sequence of the Mahakam Delta area. This sequence was deposited in deltaic environments, ranging from upper delta plain to prodelta (Rose and Hartono, 1978). Apart from oil and gas, coal deposits also occur extensively in the Kutai Basin during Early Miocene to Upper Miocene. Coals in the form of lignites, are also found in younger sediments that ranging from Pleistocene to Recent in age (Allen, 1976; Marks et al., 1982).
The Sangatta area consists of two blocks, Block III (Pinang Dome or Sangatta and Lembak area) and Block IV (North Mahakam area). Comparing to those of other Indonesian Neogen coals, the Pinang Dome is known to have higher coal rank. The average calorific value of West Pinang Dome coals is 6962 cal/gr, ranging from 6411 – 7426 cal/gr, whereas those of Lembak area coals range from 6210 – 7380 cal/gr. On the other hand, the typical calorific value of Neogen coals, such as for example those represented by Sungai Lilin coals (South Sumatera Basin) that ranging from 4320 – 5900 cal/gr. Interestingly, the higher coal rank in general occurs only in the Pinang Dome area, whereas coals that occur away from the dome usually have lower rank than Pinang Dome coals. There are some factors may be applied to explain this feature.
The main objective of this research is to determine rank variation in the Sangatta area and to define the possible causes of an increasing rank of the Pinang Dome coal, based on mainly microscopic approach. Related aspects, such as geological background, analysis method and sample acquisition are also discussed briefly.
2. Location
The area, geographically is situated between 1o00’00” and 0o30’00” N and between 117o15’00” and 117o45’00” EL, covering an area of nearly 800,000 ha (Figure 1). A network of roads in the southern half of the area is created by logging and oil exploration activities and small settlement, whereas the northern half extends into rugged area with widespread tropical karst topography and the timber and oil exploration tracks. Transportation to the Sangatta field can be made by light plane to Pertamina airstrip at Sangatta or by helicopter from Balikpapan to Sangatta. A coastal track connects Sangatta to Bontang and Samarinda further south, but the traditional transportation has been by boat.3. General Geology
3.1 Tectonic SettingThe Kutei Basin is probably the largest and deepest Tertiary basin in Indonesia, covering an area of approximately 165,000 km2, adjacent to the continental shelf. The basin is estimated to contain some 10,000 m of sequence, although the basin basement has never been penetrated by exploratory drills (Williams, 1986).
The northern boundary is the Mangkalihat Peninsula, a basement high which separates the Kutei Basin from the Tarakan Basin to the north. The eastern edge of the basin is bounded by the continental shelf which extends along the Makassar Strait. The Paternoster Shelf and Meratus Range are regarded as the southern boundary of the basin, whereas to the west, the basin is bounded by the pre-Tertiary Kuching High. These boundaries, together with the Sunda Shield, Tarakan Basin, Paternoster Shelf and North-West Borneo Basin are considered to be the major structural elements used when defining the geological evolution of Kalimantan basins.
According to Rose and Hartono (1978), the formation of the Kutei Basin, Meratus Graben and Makassar Strait were due to the break up of eastern Sundaland during the latest Cretaceous to Early Paleogene. The intervening rifts were filled with clastic sediments derived from the uplifted highs. They further noted that obduction, accompanied by the northwesterly rotation in the northern part of the Kutei Basin, resulted in the formation of the Kuching High and the deposition of a second generation of regressive sediments in the north and south. Subsequent uplift of the Kuching High provided the impetus for the gravitational folding that formed the structures that contain the bulk of oil in the Northwest Borneo Basin. The Kutei Basin developed as a flanking basin. During the Oligocene and Miocene, to the east of the Kuching High, the Kutei and Barito Basins were connected. The Late Miocene to Pliocene Meratus Uplift partially separated these basins.
In the Middle Miocene to Pliocene, it has been proposed that bed parallel faulting along the base of the basin caused the movement of a large body of clay of Late Oligocene-Early Miocene age. This clay was subjected to diapiric forces and the so-called Pinang Dome formed (Muggeridge, 1987).
3.2 Structure
In general, major structures of the Sangatta area trend N-S and NNE-SSW (Fig. 1), similar to those of the Mahakam Delta area. However, as described by Sikumbang et.al (1981), the structural trend in the northern part of the area is somewhat different to that on the southern part. The main fold and fault trends in the northern part of the Sangatta area are in a NE-SW direction with some minor normal faults cutting the main structures in SE-NW and E-W directions. Locally, in the Mangkalihat Ridge region, the structural trends change towards an EW direction. In the southern part of the Sangatta area, the structural trends have N-S and NNE-SSW directions and are characterised by wide synclines. These synclines are separated from one another by steep zones of thrust faults and asymmetrical anticlines. The main anticline to the north becomes the Melawan and Sangatta Anticlines, whereas further north is Pinang Anticline.
3.3 Stratigraphy
The stratigraphy of the Upper Tertiary Kutei Basin was first defined by Rutten (1914, cited in Land and Jones, 1987) and his work was revised by Leupold and van de Vlerk (1931) using assemblages of larger foraminifera to characterize successive stages of the Indonesian Letter Classification. One problem relating to this classification of the stratigraphy of the Kutei Basin, according to Land and Jones (1987), is due to a lack of representative sections and useful index fossils. Moreover, thick beds, caused by rapid sedimentation, could also create difficulties for stratigraphic studies.
Based on a detailed geological study of the Samarinda-Balikpapan area, Land and Jones (1987) proposed a new stratigraphic succession. This revision was possible because they had access to continuous core over 3000 m long and reports from detailed examination of these cores made available to them by exploration drilling. However, the stratigraphic subdivisions of Land and Jones appear to be restricted to the southeastern area near Samarinda and Balikpapan. Sedimentation within the Kutei Basin progrades toward the east where many hydrocarbon accumulations are found offshore in the younger sedimentary units, so that their stratigraphic succession may not be applied regionally. In addition, the area studied covered only a small portion of the Kutei Basin.
A compilation of the stratigraphic succession of the Sangatta and Bungalun area up to the South Mangkalihat Peninsula was produced by Sikumbang et al. (1981). It was based on geological reports that had been previously published and geological investigation in the Sangatta-Bungalun area under a joint cooperative agreement between Pertamina and GRDC. Stratigraphic subdivisions generally were based on palaeontology (chronostratigraphic units). The later version of this stratigraphic subdivision was given by Fukusawa et al. (1987) for the Sangatta area, and is applied in this paper (Fig. 2). It is commonly used by Pertamina geologists to define the sequence within the Sangatta and Bungalun wells.
4. Research Method
This study, in general, is based on an organic petrology approach, a microscopic study that allows identification of three important aspects of organic matter – the type, abundance and the level of maturity (or rank) – of organic matter, but to support the main objective of this paper, the method will be based more specifically on the data of vitrinite reflectance measurement.
The reflectance of vitrinite is the most widely-used parameter in assessing the rank and maturity of organic matter. Vitrinite is commonly the most abundant maceral in coal. As vitrinite reflectance increases consistently with depth, it can be used to estimate the paleogeothermal gradient which in turn is used to interpret the burial history of a sedimentary basin.
With advances in microscope technology, the type of organic matter can now be characterised accurately, employing a combination of white light and fluorescence mode microscopy. In particular liptinite macerals, fluoresce when irradiated with ultraviolet and blue light. The fluorescence technique can also be used to estimate qualitatively the rank or level of maturity but this is less precise than vitrinite reflectance.
4.1 Data Acquisition
Core and outcrop samples of coal were supplied by the PT. Kaltim Prima Coal, the operators of the Sangatta coal mine. Kaltim Prima Coal carried out an extensive drilling program in the Sangatta area and drill holes selected for this study were mostly from the Pinang Dome area. The depths of the coal core samples taken from drill holes are relatively shallow, ranging from 5 m to 190 m and penetrating the Balikpapan Formation of Middle to Late Miocene age.
Additional samples consist mostly of cuttings from two Pertamina petroleum exploration wells in the Sangatta fields, S-1 and S-3. S-1 is located in the Pinang Dome area, while S-3 is located further south of the dome. Sampling was carried out after examination of well logs (geophysical and geological logs). The lithology, availability of the sample and the abundance of organic matter (as indicated visually by the presence of shaly coal fragments) were taken into consideration when selecting samples.
Overall, 12 coal core and outcrop of the Pinang Dome samples and another 53 cutting samples of the Sangatta area were analysed.
The depths of available boreholes ranged from 900 to 3000 m, penetrating the upper to lower Miocene sedimentary rocks of the Balikpapan Formation to the Pamaluan Formation in the Sangatta area. Data were also provided by reports, maps and diagrams which had been prepared by Pertamina, although for confidential reasons, they have not been published.
4.2 Analytical Procedure
4.2.1 Sample Preparation
Preparation of coal samples has been discussed in detail by numerous authors, but this study adopts the methods outlined in the Australian Standard (AS-2856, 1986; AS-2486, 1989). Non-coal samples, such as claystone, siltstone, sandstone, volcanic rocks and carbonate, were prepared in the same manner as coal samples. Many samples, such as claystone and siltstone, were moist as a result of sweating after prolonged storage in plastic bags. Such samples were dried under sunlight or in an oven at 40o to 60oC for about half to an hour.
Basically, the procedure involves crushing, splitting, mounting, grinding and polishing the sample. In order to avoid errors in the reflectance measurements, the samples should be ground flat with little or no relief on polished surface. Moreover, the samples should be free from scratches.
4.2.2 Microscope Examination
The study that basically involving reflectance measurement and maceral characterisation was carried out by using Leitz Orthoplan-Pol, MPV2 microscopes.
Procedures for measuring reflectance have been described by Ting (1978), Davis (1978), Cook (1982), Bustin et al. (1983), Robert (1988), and the Australian Standard (AS 2486, 1989). The methods used in this study were adapted from these. The microphotometer was calibrated against synthetic yttrium aluminium garnet (0.917%), spinel (0.42%) and gadolinium gallium garnet (1.726%) standards. Reflectance measurements were made in incident light of 546 nm wavelength and using an immersion oil with a refractive index of 1.518 at a room temperature of 23oC + 1oC.
Vitrinite is usually considered to be a pseudocrystalline material (Murchison et al., 1985). As rank increases, the degree of anisotropy - the property characteristic of a uniaxial negative material - also increases. In this type of 'crystal' two significantly different reflectances can be measured, the maximum and minimum reflectance (Ting and Lo, 1978). For a surface cut perpendicular to the bedding, the maximum reflectance typically can be obtained where the incident light is polarised parallel to the bedding. On a surface cut parallel to the bedding the maximum reflectance occurs in all direction (Davis, 1978; Murchison et al., 1985).
The bedding in coal generally is easy to recognise as it is nearly always represented by vitrinite bands, microstructure within vitrinite or the edges of vitrinite phytoclasts. With the polariser set at 45o, the maximum reading can be obtained more quickly by positioning the grain with bedding parallel to the vibrational direction of polariser, in this case at 45o.
Under the Australian standard, reflectance measurements are made, preferably, on telovitrinite. This maceral occurs a relatively large, homogenous and abundant phyterals in most samples and is also normally free from inclusions of mineral matter and liptinite macerals that come anomalous results. Normally, for each reading, two maximum values approximately 180o apart, are taken. The readings were averaged to give the mean maximum reflectance. Pairs of readings, that differ by more than 5% relative, are discarded. At least 25 to 30 measurements are taken for each sample except for those samples where the vitrinite population is small. In this study, a minimum of 30 reflectance readings for each sample were taken (AS-2486, 1989), unless the vitrinite population was too small to allow this.
Type and abundance of organic matter were analysed in both reflected light and fluorescence modes. Terminology used in the maceral characterisation to identify organic matter or maceral follows the Australian Standard (AS-2856, 1986).
Two methods were used to calculate the abundance of organic matter in samples. An electronic SWIFT point counter was used to measure the volume of coal and shaly coal. The point counter was set to count + 500 points, covering most of the sample with standard deviation ranges from 1.0 to 2.2 %, depending on volume percentage of component. The second method was to estimate, semiquantitatively, the abundance of organic matter in claystone and other rocks with dispersed organic matter. This method is commonly used for petrographic analysis of cuttings and core samples from petroleum exploration wells. Since there is no standard microscopic analysis for calculating the volume of dispersed organic matter in sedimentary rocks, this is probably the best available method.
The procedure requires examination of the grains during traverses across the blocks and full coverage of the block is attempted. At least 50 to 100 grains in every sample have to be observed. Recorded data for each grain include the type of lithology, abundance and type of organic matter, fluorescence intensity and other important features. The abundance of iron oxides, pyrite and fossil fragments if any were also recorded.
The abundance of organic matter is grouped into the categories as follows :
| % ORGANIC MATTER | CATEGORY |
| o.m > 40% | Dominant |
| 10% < o.m < 40% | Major |
| 2% < o.m < 10% | Abundant |
| 0.5% < o.m < 2% | Common |
| 0.1% < o.m < 0.5% | Sparse |
| o.m < 0.1% | Rare |
The grain counting method is more applicable to claystone and carbonaceous shales or samples where non-coal grains are more abundant than coal grains. Point counting is less accurate where organic matter comprise less than about 10% of the sample. The weakness in the grain counting method is that estimation of the organic matter can be very subjective. To minimise the inconsistencies of estimation, this method should be used in conjunction with abundance estimation diagram.
Analysis results on the type, characteristics, composition and the abundance of organic matter are not part of the main objectives of this study, so that they will not be discussed in any detail.
5. Analysis Results
Results of the reflectance measurement of the Kaltim Prima Coal and Pertamina samples are summarized in Table 1 and Table 2 respectively.
| No. | SAMPLE No. | DEPTH (m) | Rvmean (%) | RANGE (%) |
| 1 | 24635 | O/C | 0.47 | 0.40 – 0.55 |
| 2 | 25013 | 17.72 – 23.24 | 0.66 | 0.60 – 0.73 |
| 3 | 25014 | 19.24 – 21.34 | 0.65 | 0.61 – 0.70 |
| 4 | 25015 | 76.28 – 82.12 | 0.65 | 0.58 – 0.74 |
| 5 | 25016 | 43.88 – 48.38 | 0.46 | 0.37 – 0.52 |
| 6 | 25017 | 42.26 – 46.63 | 0.50 | 0.40 – 0.64 |
| 7 | 25018 | 64.40 – 65.98 | 0.59 | 0.52 – 0.71 |
| 8 | 25019 | 123.60 – 129.90 | 0.58 | 0.53 – 0.65 |
| 9 | 25020 | 4.85 – 6.43 | 0.53 | 0.47 – 0.59 |
| 10 | 25021 | 111.90 - 114.00 | 0.61 | 0.51 – 0.67 |
| 11 | 25022 | 177.60 – 186.50 | 0.67 | 0.60 – 0.76 |
| 12 | 25023 | O/C | 0.65 | 0.57 – 0.70 |
Table 1
Summary of the Analysis Results of Coal Core and Outcrop Samples from the Pinang Dome Area (PT. KALTIM PRIMA COAL)
Coal rank of the Pinang Dome area in the Table 1 above shows a little variation, ranging from 0.47 to 0.67%, but the average reflectance value population most likely is > 0.55%. The variation probably is related to the nature of coal samples that originally come from at least five different seams with their own individual geological history. In addition, in the case of the Pinang Dome coals, the presence of significantly high content of resineous material in some samples may also cause the decrease in the vitrinite reflectance values. However, comparing to those of Indonesian Neogen coals, the Pinang Dome coals rank in general are still higher.
Analysis results of the Sangatta rock samples that shown in Table 2 consist of various lithology. Microscopically, coal occurs mainly at the upper part of the sedimentary sequence, whereas fine clastic rocks present at the lower part. With an increasing depth, coal gradually disappears at the depth greater than 1250 m and 1900 m in S-1 and S-3 well respectivelly and is taken place by mostly fine sedimentary rocks.
Technique to differentiate coal and non coal samples in the microscope examination is based on organic matter content as shown in the following table :
| ORGANIC MATTER (%) | LITHOLOGY |
| o.m > 70 | COAL |
| 40 > o.m < 70 | SHALY COAL |
| o.m < 40 | NON COAL |
6. Discussion
Application of microscopic study on organic matter in coal science as well as petroleum exploration nowadays becomes part of a data assessment. Vitrinite reflectance measurements in microscopic studies of petroleum source rocks are used to assess the maturity of organic matter, the term maturity is commonly used in source rock studies and is analogous to the degree of coalification or rank in coal.
Vitrinite reflectance probably is the most widely-used method for determining the maturation level of organic matter in petroleum source rocks assessment as well as coal rank in coal potential assessment. Maturation of organic matter or its analogous, the degree of coalification, is a geological process that caused by burial and consequent increase in temperature over a period of time.
However, problems are often faced during collection of vitrinite reflectance data when dealing with cuttings type of samples. Unless correct identification of the representative vitrinite population is made, the maturation concept is meaningless. Reflectance measurements are commonly carried out on well cuttings. The potential for discontinuities or irregularities in the vitrinite reflectance profile exists if samples contain cavings populations (Feazel and Aram, 1990). Caving populations do not give the true maturity of organic matter since they may have been derived from some other unit, usually from the overlying younger beds (uphole cavings). These data will lead to the underestimation of maturation level. Durand et al. (1985) and Barker and Pawlewicz (1986) stated that cuttings samples for reflectance studies can produce multiple populations of vitrinite and, therefore, bimodal histogram or histograms with multiple modes.
In most petroleum exploration drillholes such as those of Pertamina S-1 and S-3 wells, it is common for only cuttings to be available because of the cost of coring. In order to minimise the problem, it is essential to plot the reflectance results as a histogram for every individual sample (Davis, 1978; Waples, 1981; Durand et.al., 1985). Ideally, a histogram will have a single peak as usually shown by reflectance data for coal.
For samples that contain caving populations, the first generation of vitrinite, that is, the primary vitrinite population that represents the correct rank of vitrinite that is representative in the nominated horizon, will be grouped as one population and the cavings will be grouped separately, generally at a lower reflectance for younger cavings. However, from a histogram it is not always easy to identify the first generation vitrinite; it does however commonly help with the interpretation if samples above and below the specific sample are consulted. In the case of multi-populations, the range of reflectance values between the lowest and highest values for the whole vitrinite population usually is high; this is confirmed by a relatively high standard deviation. The standard deviation for reflectance values can be calculated from the reflectance data. For Cooper Basin (Australia) coals, Kantsler et.al (1986) calculated the standard deviation to be in the range of 0.05% to 0.075% for a range in reflectance values of 0.2% to 0.3%. This method is not always applicable because the range in reflectance values may vary from one place to another, particularly when dealing with cuttings samples.
Coal samples in the study area, although they are mostly drill cuttings gave a range for the standard deviation of 0.03% to 0.05%. Noncoal samples on the other hand, tended to give higher standard deviations, generally up to 0.09%. Probably, this is a result of the effect of variable oxygen levels in each of the environments in which the sedimentary rocks were deposited. For example, in a well-oxygenated environment, reflectance could well be above the expected or anticipated average because of increasing oxidation of the vitrinite. The environment could give rise to the development of a maceral transitional to vitrinite-inertinite; the reflectance would be higher than vitrinite but lower than inertinite.
It is important therefore, to recognise and select the first generation or primary vitrinite so that the true rank can be assessed. Primary vitrinite can be identified using reflectance values and the generally higher relative abundance compared to that of the caving populations. Uphole cavings usually posses significantly lower reflectance values than the primary vitrinite and generally are present within rare to sparse categories (<0.1-0.1%).
Less commonly, a sample contains a significant cavings population. Although this is perhaps more related to the technical drilling problems rather than to the analytical procedures, the true reflectance can still be obtained by comparing the mean reflectance of the sample with those of the overlying and underlying samples. Samples with abundant cavings should be rejected for reflectance analysis.
In coal with a rank of brown coal to high volatile bituminous coal (up to Rv = 1.0%), liptinite fluorescence is a useful tool to assess the maturity of organic matter and therefore can be useful for eliminating cavings. In cuttings samples for example, the presence of strongly fluorescing liptinite in some grains where the vitrinite reflectance is 0.9% in other grains, would strongly indicate contamination from a cavings population, especially if confirmation of the reflectance can be obtained from adjacent samples.
It is sometimes possible to distinguish cavings populations from the primary vitrinite on the basis of maceral composition and maceral assemblages. For example, seams may be inertinite-rich or vitrinite-rich and the two coal types are easily identified. However, with Indonesian coals, all are generally vitrinite-rich and therefore coals from different seams are more difficult to distinguish. Another example, seams from the Pinang Dome consists of more than 80% vitrinite (mmf); thus it is difficult to distinguish one seam from another on the basis of maceral composition.
The clastic sediments also has vitrinite as the most abundant maceral group; vitrinite always occurs in much greater quantities than liptinite and inertinite. Thus cavings are unlikely to be distinguished on maceral composition alone unless vitrinite is misidentifed as inertinite. However, for the deeper parts of the formation the vitrinite reflectance approaches the inertinite reflectance and the amount of organic matter diminishes with depth. Thus very shallow cavings can be distinguished when in deeper samples.
Despite the above limitations, it is an advantage to apply petrographic analysis to raw or untreated samples as, in many cases, the cavings grains or grains with reworked vitrinite, can be readily excluded from those of the primary vitrinite. In addition, treated samples or organic matter concentrates for chemical analysis give the values for the bulk of organic matter, so that they do not give the real vitrinite population and information on the particular type of organic matter which can only be seen under the microscope.
Multiple peaks on the vitrinite reflectance histogram can also be obtained if :
· data for opaque nonfluorescing bitumen are included;
· presence of organic matter derived from turbidite sequences (Castano and Sparks, 1974)
· influenced by multiple facies on the samples; and
· the presence of reworked organic matter.
Reworked vitrinite or allochthonous organic matter usually shows anomalously high reflectance values, whereas alginite and bitumen, except impsonite which has a relatively high reflectance for a bitumen, generally depresses the reflectance value. Impregnation of bitumen may depress the true reflectance and thus unusually low Rvmax values are given for the vitrinite data as shown in some Sumatera and Sarawak coal samples (Daulay and Cook, 1988). The effect on the vitrinite values, due to the presence of alginite was shown by Hutton and Cook (1980) in their study on Joadja (New South Wales, Australia) coals and torbanites in which alginite content depressed the vitrinite reflectance values.
The use of lignite as a mud additive in an exploration drilling can also cause problems as found in some example where lignite was a contaminant that influencing the reflectance data for suite of samples from Cerro Prieto.
The histogram of the frequency distribution of reflectance data from the Sangatta area generally shows a single population with the only exceptions being for S-3 well. These apparently anomalous data can be attributed to any of the above causes. For example, turbidite sequences, as mentioned by Castano and Sparks (1974), were not found in the Sangatta-Bungalun sedimentary sequence, at least in the Miocene sedimentary formations (Sikumbang et al., 1981). Bitumen impregnation into the vitrinite probably occurs in some samples as indicated by a weak fluorescence from the vitrinite but this occurrence is rare; no reflectance measurements were made on this type of vitrinite because of its rare occurrence. Alginite was not recorded in any samples from the study area, so that the vitrinite suppression as discussed by Hutton and Cook (1980) is unlikely. Therefore, apart from caving populations, no such problems have been encountered in the analysis of the Sangatta samples.
Determining the relationships between reflectance and burial depth in exploration wells is the initial stage when reconstructing the thermal history of a sedimentary basin. The natural processes involving changes in organic matter, depend essentially on burial temperature and therefore heating of the sediments. Heat flows continuously from the deeper parts of the earth towards the surface; this heat is the bulk of the energy that heats buried sediments and promotes the organic and mineralogical reactions that control for example, the generation, migration and accumulation of oil and gas (McCulloh and Naeser, 1989). In other words, temperature is the main factor influencing coalification reactions or organic matter metamorphism, which according to Teichmuller (1987), normally depend on the depth to which a unit subsides, the geothermal gradient and the heat conductivity of underlying and accompanying rocks. It has been stated that time is also an important factor in the maturation of organic matter in sedimentary rocks (Teichmuller and Teichmuller, 1968; Shibaoka and Bennet, 1977; Teichmuller, 1987).
The reflectance profile for S-3 well in the Sangatta area is shown in Figures 3. Additional information for the following interpretations was also obtained from shallow wells and outcrop data from the Pinang Dome area.
The reflectance profile for S-3 well, up to approximately 2200 m depth is considered to be normal, showing a relatively straight line with a reflectance gradient of 0.15%/km. Below this depth, an abrupt change occur in the reflectance gradient which increases to approximately 0.62%/km. Scattered values occurring in some depths probably represent the presence of minor 'cavings populations' as some low reflectance values were included in the mean reflectance calculation (see Figure 4). The low reflectance values cannot be attributed simply to caving populations (uphole cavings) because the boundary between reflectance values for the upper and lower horizons is not distinct; the change in reflectance over the depth range values is gradational and there is a marked overlap of values in successive samples.
The mean reflectance is 0.78% at 2100 m depth and increases to 1.38% at the depth of 2400 m which is only over an interval of 300 m. At this sequence the reflectance value ranges from 1.38% to 1.91%, giving a standard deviation of 0.09 to 0.28 respectively. Possible explanations for these apparently anomalous profiles include the following. Koch (1974, cited in Robert, 1988) classified vitrinite reflectance profiles into four groups using the forms of the curves : linear, hyperbolic and two types of branching curves, with each curve corresponding to a specific geothermal history regime. According to the Koch classification, the reflectance profile for S-3 well is of the branching type. This feature probably is related to two periods of geothermal history that differ in geothermal regimes or, as an alternative interpretation, it may be resulted from an unconformity, a structure defined and discussed by Dow (1977). It is significant that higher reflectance values in S-3 well is found in the older formation, e.i the Pulau Balang Formation. In contrary, stratigraphic studies in the Kutei Basin do not support the idea of unconformity, previous interpretations even showed that the Balikpapan Formation conformably overlies the Pulau Balang Formation (see Fig. 2).
Histograms of the frequency distribution of the reflectance values in the Pulau Balang and Pamaluan Formations show populations with several peaks. Studies show that where two populations are present, as a general rule, the lowest reflectance population most likely is selected as the primary vitrinite population (Castano and Sparks, 1974; Tissot and Welte, 1978). The second and higher reflectance peak may represents reworked material, but this is not the case for the Pulau Balang and Pamaluan Formations in the S-3 well. So far, under the microscope examination no reworked vitrinite has been encountered in the Pulau Balang and Pamaluan samples.
For the Sangatta wells, the vitrinite population with anomalously high reflectance is most likely to be thermally-altered vitrinite, associated with coal that has been locally heated by an intrusive body which is at depth and not intersected in the drill hole. No record of an intrusion has been reported from either surface mapping or drilling in the Sangatta area. The only reported magmatic activity has been related to what was described by Weerd et.al (1987, cited in Pieters et.al, 1987) and Pieters et.al (1987) as volcanic intercalations in the Miocene sediments. Volcanism was contemporaneous with a widespread magmatic phase occurring within a belt of the Early Tertiary sedimentary outcrops which are known to occur western Kalimantan to eastern Kalimantan. This magmatic activity, in the form of small intrusions of granodiorite, minor diorite and granite have ages ranging from 30 Ma to 16 Ma (based on the K-Ar analyses). This the scenario for intrusions in the Sangatta area existed.
The coal measures succession of the Balikpapan Formation cover the Pinang Dome (Location Map Fig. 1). Detailed geological investigation in the Pinang Dome was carried out by Muggeridge (Muggeridge, 1987) as part of a coal exploration program of the Kaltim Prima Coal Company. Structurally, the Pinang Dome is interpreted as a dome based on indications from the geological data including strike and dip measurements of the surrounding strata. This sequence contains multiple coal seams of an export quality steaming coal within a fluvio-deltaic sequence. The sequence consists of at least ten economic coal seams, ranging from 1 to 14.0 m thick. Vitrinite reflectance values from the coal in this area range from 0.60 to 0.65% (Muggeridge, 1987).
Data collected for the present study, shows that vitrinite reflectance of the Pinang Dome coals from outcrop and shallow drill holes ranges from 0.40 to 0.76%, which is significantly higher than those of the surrounding strata. For example, in S-1 well, located approximately 3 km to the south-southeast of the Pinang Dome, vitrinite reflectance in the coals is 0.35% at a depth of 50-55 m. Reflectance value of 0.76% occurs at a depth of approximately 1240 m in S-1 well (Figure 4). Similar reflectance values occur at much greater depths away from the Pinang Dome.
According to Muggeridge (1987), the Pinang Dome was formed as a result of claystone and mudstone diapirism associated with an underlying over-pressured marine shale zone. The process was accompanied by a high geothermal gradient, locally caused by a heat convection cell from the underlying overpressured zone, thus causing an increase in coal rank. If this concept is true, any anomalously high reflectace values in the Pinang Dome area would be attributable to this overpressuring rather than phenomena such as intrusions. The theory of an overpressured zone was applied by Oudin and Picard (1982) to explain the mechanism of hydrocarbon expulsion, a function of the coincidental relative positions of the oil window zone and over-pressured zone in the Mahakam Delta, Kutei Basin.
Changes in the reflectance values is a feature of the coalification process and implies changes in chemical and physical properties of organic matter, including an increase in vitrinite reflectance. The cause of coalification has been a subject of study by several researchers. Teichmuller (1987) stated that the main factor driving coalification reactions is temperature. Temperature is governed by the depth to which a stratum is buried during subsidence penecontemporaneous or after deposition, the geothermal gradient and the heat conductivity of underlying and accompanying rocks. According to Hilt's law, temperature increases with increasing depth of burial although it depends also on the type of lithology (Damberger, 1968 cited in Hacquebard and Donaldson, 1974). High thermal conducting rocks such as thick sandstone and conglomerate can cause lower temperature gradients. Another factor influencing coalification is heating time, at high temperatures; the influence of time is greatest (Teichmuller, 1987).
The role of pressure in coalification is still in debate. According to Teichmuller and Teichmuller (1968) the effect of pressure on the increase in coal rank is only physical, that is, compaction accompanied by reduction of volume and moisture in low rank coals and the development of anisotropy in higher rank coals. On the other hand, some tectonic structures can influence the rank and therefore, reflectance values, as well as changing the optical properties of vitrinites (Stone and Cook, 1979; Hower and Davis, 1981; Bustin, 1983 and Levine and Davis, 1984). Stone and Cook (1979) stated that biaxial optical properties in the Late Permian Bulli coals which were influenced by the stress field, associated with faults, in Southern Coalfield of the Sydney Basin. Bustin (1983) obtained anomalously high reflectance values for coals from near thrust faults in the Rocky Mountains although the values were restricted to very narrow zones immediately adjacent to or within the shear zone.
In the Pinang Dome coals, the bireflectance, or the difference between maximum and minimum reflectance, is relatively small because of the low rank; this is similar to coals in other parts of the Sangatta area except for the thermally-heated coals which have an elevated rank. Magmatic activity in the Sangatta area is the best explanation for the formation of Pinang Dome. The dips of the strata along the eastern part of Pinang Dome shows that Pinang Dome sedimentary sequence
extends and probably continues to the position of S-1 well. However, reconstruction of the stratigraphic position of the Pinang Dome seams would put them at depths of approximately 700 to 800 m in this well. Therefore, it is possible that the increase in vitrinite reflectance values could have resulted from thermal alteration by an intrusive rock, consistent with the Pinang Dome being formed as a result of an igneous activity in the Sangatta area. This interpretation is also supported by the presence of anthracitic coal in one part of the Kaltim Prima Coal concession area where vitrinite reflectance values range from 1.6% to 2.03%.
The effect of an intrusion varies depending on the distance from the igneous rock to the intruded sedimentary rocks, type and size of intrusion, temperature difference between the intrusive and sedimentary rocks, and the depth at which the intrusion takes place. This is seen in the Sangatta area where the effect of the intrusion is different at various localities. Although the nature of the intrusive rocks in the Sangatta area is not known, it can be postulated that in the southern part of the Sangatta area only the Pamaluan and Pulau Balang Formations would be affected whereas in the north it is the Balikpapan Formation near the Pinang Dome that would be affected.
7. Conclusions
Based on microscopic examination on 12 coal samples from the Pinang Dome outcrops and drill holes and another 53 cutting samples from an oil exploratory well, supported also by geological data from previous authors, it can be concluded that :
Coal and coaly sediment such as shaly coal or carbonaceous clay occur mainly at the upper part of the sedimentary sequence, whereas fine clastic rocks usually present at the lower part.
With an increasing depth, coal and coaly sediment disappear gradually at the deeper part of the sedimentary sequence (> 1250 m at S-1, and > 1900 at S-3).
Pinang Dome coals, in general, show significantly higher rank than those of coals that having the same age/formation.
The abrupt changes in geothermal gradient at the S-3 well probably was caused by thermal effect of a magma activity somewhere at the deeper part of the basin
The feature in S-3 well may be used to explain the possibility of an increasing rank in the Pinang Dome coals that consistent with the formation of the Pinang Dome as a result of an igneous activity in the Sangatta area.
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