REVISION OF THE MAHAKAM COAL SERIES: ROCK-EVAL AND RANK(S) RELATIONS

Neogene sequences in the Mahakam delta, Indonesia, contain many coals for which abundant analytical and Rock-Eval data exist. It is essential that raw analytical data are adjusted for the effects of mineral matter, including pyrite, before they are used to calculate atomic H/C and O/C ratios; these ratios are required in order to present the coal series on a van Krevelen diagram.
In this paper, a method of adjustment is proposed which allows the definition of a well-constrained Mahakam coal band; this is comparable to the Late Cretaceous and Tertiary New Zealand coal band and the Japanese Palaeogene coal band. The relationships between: (i) coal rank (using the Rank(S) scale) and Rock-Eval parameters; and (ii) coal rank and vitrinite reflectance for the Mahakam coals are generally similar to those for the New Zealand coals.     

RELATIONSHIPS BETWEEN HYDROCARBON GENERATION, COAL TYPE AND RANK FOR MIDDLE EOCENE COALS, BULLER COALFIELD, NEW ZEALAND

The Buller Coalfield in the northern portion of the Paparoa Trough (NW South Island, New Zealand) contains a middle Eocene bituminous coal-bearing succession that exhibits marked variations in both coal rank and type. The across-basin rank changes result from differential late Palaeogene subsidence of the Paparoa Trough and subsequent inversion. Superimposed upon coalification trends are down-seam variations in coal type, evidenced by isorank variation in conventional chemical parameters. These type variations are not a consequence of changes in maceral group proportions, which are dominated by vitrinite. Forty-eight coal samples from fifteen drillholes through the Brunner Coal Measures have been examined for vitrinite reflectance, proximate analysis, specific energy, and sulphur content. Total Organic Carbon (TOC), Rock-Eval properties and the bulk composition of the bitumen were also determined. All the coals analysed lie within the high volatile bituminous B to medium volatile bituminous coalification stages. Variation in analytical properties within this suite, and more specifically within serial samples, provides insights into the nature of the organic matter comprising the coals. While TOC varies systematically with ash content, reflecting proximity of the depositional mires to fluvial systems, values increase with rank (68–86% ash free). The Hydrogen Index (HI) decreases from 334 to 190 mg hydrocarbons/g TOC over a range of 429 to 470 d?C Tmax, analogous to the volatile matter content and vitrinite reflectance, respectively. The more perhydrous coal samples at a given rank are characterised by depressed Tmax/vitrinite reflectance and elevated Hi/volatile matter contents. Bitumen and sulphur contents are not the primary control onperhydrous characteristics. The bitumen content crudely corresponds to the S1 peak plus the initial portion of the S2 peak liberated under low temperatures during Rock-Eval pyrolysis; however, the bulk of the generative potential is associated with the residual kerogen fraction. Suggate (S) Rank and maximum palaeotemperature appear to be the best indicators of coal rank. A period of significant hydrocarbon generation and release begins at Tmax～440d?C for the Buller coals. The main oil “window” is defined by the interval 440–455d?C Tmax or 0.8–1.1% vitrinite reflectance (Ro_max), and by correlation to maximum palaeo-temperatures, 125–155d?C. This is further substantiated by mass balance considerations, which indicate that hydrocarbon generation (Petroleum Generation Index (PG1) > 0.1) was occurring in the coals at a maturity level of O. 7–0.9 % Ro_max (Tmax 440–444 d?C) with an increase in PGI between ～0.9 and 1.1% Ro_max (～Tmax 445–455 d?C). Between PGI 0.1 and 0.4, the expulsion efficiency rapidly increased, presumably due to generation and expulsion of the bulk of the oil.     

SOURCE ROCK EVALUATION OF COALS FROM THE LOWER MAASTRICHTIAN MAMU FORMATION, SE NIGERIA

The Lower Maastrichtian Mamu Formation in the Anambra Basin (SE Nigeria) consists of a cyclic succession of coals, carbonaceous shales, silty shales and siltstones interpreted as deltaic deposits. Sub‐bituminous coals within this formation are distributed in a north‐south trending belt from Enugu‐Onyeama to Okaba in the north of the basin. Maceral analyses showed that the coals are dominated by huminite with lesser amounts of liptinite and inertinite. Despite high liptinite contents in parts of the coals, an HI versus T_max diagram and atomic H/C ratios of 0.80‐0.90 and O/C ratios of 0.11‐0.17 classify the organic matter in the coals as Type III kerogen. Vitrinite reflectance values (%R_r) of 0.44 to 0.6 and T_max values between 417 and 429°C indicate that the coals are thermally immature to marginally mature with respect to petroleum generation. Hydrogen Index (HI) values for the studied samples range from 203 to 266 mg HC/g TOC and S₁+S₂ yields range from 141.12 to 199.28 mg HC/ g rock, suggesting that the coals have gas and oil‐generating potential. Ruthenium tetroxide catalyzed oxidation (RTCO) of two coal samples confirms the oil‐generating potential as the coal matrix contains a considerable proportion of long‐chain aliphatics in the range C_19‐35. Stepwise artificial maturation by hydrous pyrolysis from 270°C to 345°C of two coal samples (from Onyeama, HI=247 mg HC/g TOC; and Owukpa, HI=206 mg HC/g TOC) indicate a significant increase in the S₁ yields and Production Index with a corresponding decrease in HI during maturation. The Bitumen Index (BI) also increases, but for the Owukpa coal it appears to stabilize at a T_max of 452‐454°C, while for the Onyeama coal it decreases at a T_max of 453°C. The decrease in BI suggests efficient oil expulsion at an approximate vitrinite reflectance of ～I%R_r. The stabilization/decrease in BI is contemporaneous with a significant change in the composition of the asphaltene‐free coal extracts, which pass from a dominance of polar compounds (～77‐84%) to an increasing proportion of saturated hydrocarbons, which at >330°C constitute around 30% of the extract composition. Also, the n‐alkanes change from a bimodal to light‐end skewed distribution corresponding to early mature to mature terrestrially sourced oil. Based on the obtained results, it is concluded that the coals in the Mamu Formation have the capability to generate and expel liquid hydrocarbons given sufficient maturity, and may have generated a currently unknown volume of liquid hydrocarbons and gases as part of an active Cretaceous petroleum system.     

Organic Geochemical Evaluations of Bituminous Rock and Coals in Miocene Himmetoglu Basin (Bolu, Turkey)

The studied area is a lake basin located in Bolu basin in Turkey. In the basin, from Upper Cretaceous to Upper Miocene 3,000-m thickness sediments were deposited. Upper Miocene Himmetoglu formation consisted of sandstone, claystone, and marl. To the middle level of the formation are located coal, bituminous limestone, and bituminous shales. In the basin, there are two coal beds whose thicknesses range from 1 to 13 m. The coals are easily breakable and black in color. In the coal beds exists some bituminous limestone and bituminous shales, and their thicknesses are between 5 and 45 cm. The amount of organic matter of the bituminous rocks from the Upper Miocene Himmetoglu formation are between 6.83 and 56.34 wt%, and the amount of organic matter of the bituminous limestone from the formation are between 13.58 and 57.16 wt%. These values indicate that these rocks have very good source potential. According to hydrogen index (HI), S₂/S₃, HI-T_max, and HI-OI (oxygen index) parameters, kerogen types of the bituminous rocks and coals belonging to Upper Miocene Himmetoglu formation are Type I, Type II, and Type III. In accordance with HI, S₂/S₃, HI-T_max, and HI-OI parameters, the bituminous rocks and coals from the Upper Miocene Himmetoglu formation are mostly immature.     

A RE-CONSIDERATION OF THE "OIL WINDOW" FOR HUMIC COAL AND KEROGEN TYPE III SOURCE ROCKS

Based on natural data-sets and several hydrous-pyrolysis experiments on immature humic coals, the conventional "oil window" (0.5–0.6%R₀ to 1.3–1.35%R₀) for humic coals is re-considered. A worldwide coal data-set comprising coals of Carboniferous - Tertiary age and having vitrinite reflectances from 0.32%R₀ to 3.6%R₀ indicates that initial liquid hydrocarbon generation commences at a vitrinite reflectance of 0.5%R₀ to 0.6%R₀, and that a significant liquid hydrocarbon build-up occurs from approximately 0.6%R₀ to 0.85%R₀. The start of the "oil window" corresponds to a vitrinite reflectance of 0.85%R₀; however, the start of the "oil window" can vary significantly for individual coal series, and it may range up to a vitrinite reflectance of 1.15%R₀. Liquid hydrocarbon generation is negligible at a vitrinite reflectance of approximately 1.8%R₀, and at 2.0%R₀ the hydrocarbon generative potential is exhausted. A vitrinite reflectance of 1.8%R₀ corresponds to the end of the "oil window", and for general use a vitrinite reflectance range of 0.85–1.8%R₀ is suggested to define the "oil window" for humic coals and kerogen type III. The end of the "oil window" at 1.8%R₀ is supported by published data, indicating a much higher thermal stability of nC₁₅₊ hydrocarbons than is generally accepted, and therefore limited oil-to-gas cracking at 1.3%R₀. The re-considered "oil window" between 0.85–1.8%R₀ has significant implications for evaluating the prospectivity of basins dominated by terrestrial organic matter, such as coals.     

HYDROCARBON SOURCE POTENTIAL AND MATURATION IN EOCENE NEW ZEALAND VITRINITE-RICH COALS

The results of traditional methods of coal characterisation (proximate, specific energy, and ultimate analyses) for 28 Eocene coal samples from the West Coast of New Zealand correspond well with biomarker ratios and Rock-Eval analyses. Isorank variations in vitrinite fluorescence and reflectance recorded for these samples are closely related to their volatile-matter content, and therefore indicate that the original vitrinite chemistry is a key controlling factor. By contrast, the mineral-matter content and the proportion of coal macerals present appear to have had only a minor influence on the coal samples' properties.
Our analyses indicate that a number of triterpane biomarker ratios show peak maturities by high volatile bituminous A rank; apparent maturities are then reversed and decline at the higher medium volatile bituminous rank.
The Rock-Eval S1+ S2 yield also maximizes by high volatile bituminous A rank, and then declines; however, this decline is retarded in samples with the most hydrogen-rich (perhydrous) vitrinites.
These Rock-Eval and biomarker trends, as well as trends in traditional coal analyses, are used to define the rank at which expulsion of gas and oil occurs from the majority of the coals. This expulsion commences at high volatile A bituminous rank, and persists up to the threshold of medium volatile bituminous rank(c. 1.1% Ro ran. or 1.2% Ro max in this sample set), where marked hydrocarbon expulsion from perhydrous vitrinites begins to take place.     

REFLECTANCE RETARDATION (SUPPRESSION) AND SOURCE ROCK PROPERTIES RELATED TO HYDROGEN-ENRICHED VITRINITE IN MIDDLE JURASSIC COALS, DANISH NORTH SEA

Middle Jurassic high volatile bituminous A coals from the Danish North Sea may yield vitrinite reflectance values retarded by 0.10–0.14 %R_r The coals are low in liptinite content, and the reflectance retardation is related to the vitrinite composition. Vitrain (assumed to be pure vitrinite) was hand-picked from eight samples and characterised by means of element analysis, Fourier Transform infrared spectroscopy and Rock-Eval pyrolysis. In contrast to the vitrinite from samples yielding "true" reflectance values, the vitrinite from three of the samples yielding retarded reflectances is perhydrous in nature (5.57–5.79 wt-% (d.a.f.) H) and has H/C atomic ratios of 0.80–0.83. Vitrinite from another reflectance-retarded sample is enriched in sulphur. The hydrogen enrichment is inherited from oxygen-deficient, waterlogged and marine-influenced conditions in the precursor mires. FTIR spectra demonstrate that the perhydrous vitrinites have a lower aromaticity as measured by aromatic hydrogen vibrations in the region 900–700 cm^-1 compared to the "normal" vitrinites. This suggests a reduced maturation rate of the hydrogen-enriched vitrinite. S_1(Vitr), S_2(vitr) and HI_vitr values from the vitrinite concentrates indicate that the vitrinite is a significant contributor to the overall generative potential; this is particularly the case for the perhydrous vitrinite. Activation energies for the vitrinites are centred at 260 and 268 kJ/mole. It is not possible to detect a measurable difference in E_a between the perhydrous and "normal" vitrinites. This may suggest more-or-less similar generation characteristics, or it may indicate that Rock-Eval kinetics do not replicate Nature in this regard.     

DETERMINATION OF THE TEMPERATURE HISTORY FOR THE U THONG OILFIELD AREA (SUPHAN BURI BASIN, CENTRAL THAILAND) USING A REALISTIC SURFACE TEMPERATURE

The temperature history for the BPI-W2 well in the oil-producing Suphan Buri Basin, central Thailand, has been investigated using different surface temperatures (T_s). Two ID models using T_s values of 0°C and ∼22°C were able to fit the suppression-corrected vitrinite reflectance (VR) values and burial peak temperatures (T_peak) in the well. The geothermal gradient averaged over 3 km is ∼54°C/km for T_s= 0°C, whereas it is ∼42°C/km for T_s∼22°C. T_s= 0°C is, however, considered to be unrealistic and the ∼54°C/km gradient is therefore too high. Similarly, a previously determined geothermal gradient of 62°C/km is considered to be an overestimate. The geothermal gradient of ∼42°C/km is plausible compared to other geothermal gradients onshore and offshore Thailand, although it is at the low end. This may be due to a too low suppression correction for the measured VR values. The obtained temperature history can be used to predict measured present-day temperatures of T_s∼22°C and 77°C in the reservoir in the U Thong oilfield. The obtained temperature history associated with the geothermal gradient of ∼42°C/km seems realistic as it predicts that the onset of oil generation at 107°C will have post-dated reservoir and trap formation in Middle to Late Miocene times.     

EFFECT OF SOLVOLYTIC EXTRACTION AND PYROLYSIS PRETREATMENTS ON THE TOEMPRATURE STEAM GASIFICATION REACTTVITY OF COAL AND REDUCTION OF TAR FORMATION IN PRODUCT GASES

Pre-extraction of Talcher coal ( a bituminous coal) in anthracene oil was found to be a beneficial treatment for enhancing the reactivity for steam gasification of coal. Similarly, anthracene oil followed by liquid paraffin extraction of coal was also found to be a-better pretreatment for the steam gasification of coal. Anthracene oil extraction at 270°Cand steaming of coal at 650°C had almost equal effect on steam gasification of coal. Pyrolysis in steam of pre-extracted coals was found to further enhance the reactivity of coal for steam gasification. The studies were further confirmed by carrying out thermogravimetric analysis of pretreated coals in steam atmosphere. Degree of cross-linking in pretreated coals was studied by measurement of swelling of coal in quinoline. The formation of tar in gaseous product was reduced as a result of pre-extraction and steam pyrolysis of coal prior to their gasification in steam.     

Organic Geochemical Evaluations of Bituminous Rock and Coals in Miocene Himmetoglu Basin (Bolu,Turkey)

Abstract

The studied area is a lake basin located in Bolu basin in Turkey. In the basin, from Upper Cretaceous to Upper Miocene 3,000-m thickness sediments were deposited. Upper Miocene Himmetoglu formation consisted of sandstone, claystone, and marl. To the middle level of the formation are located coal, bituminous limestone, and bituminous shales. In the basin, there are two coal beds whose thicknesses range from 1 to 13 m. The coals are easily breakable and black in color. In the coal beds exists some bituminous limestone and bituminous shales, and their thicknesses are between 5 and 45 cm. The amount of organic matter of the bituminous rocks from the Upper Miocene Himmetoglu formation are between 6.83 and 56.34 wt%, and the amount of organic matter of the bituminous limestone from the formation are between 13.58 and 57.16 wt%. These values indicate that these rocks have very good source potential. According to hydrogen index (HI), S ₂/S ₃, HI-T _max, and HI-OI (oxygen index) parameters, kerogen types of the bituminous rocks and coals belonging to Upper Miocene Himmetoglu formation are Type I, Type II, and Type III. In accordance with HI, S ₂/S ₃, HI-T _max, and HI-OI parameters, the bituminous rocks and coals from the Upper Miocene Himmetoglu formation are mostly immature.