METHANE GENERATION FROM COALS OF THE MUARAENIM FORMATION SOUTH SUMATRA BASIN

• Main Author: Dwiantoro, Mulyono
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/54571
• Institution: Institut Teknologi Bandung

The methane generation of a series of coal samples from Muaraenim Formation has been analyzed using a multi - method approaches, including petrographical, chemical compositions, organic geochemistry, and kinetic reaction. An exploratory drilling was performed by Geological Research and Development Centre Bandung in order to investigate the gas potential and its characteristics at Muara Lawai area, Merapi sub-district, Lahat regency, South Sumatra province. The drilling intersects Kasai Formation and Muaraenim Formation with a total depth of 450 m. The former comprises mostly volcano-clastic sediments such as tuffaceous sandstone, tuff, and tuffaceous claystone. The latter is composed by shaley coal, sandstone, claystone, and multiple of coal beds. Core samples were collected from the drill core and underwent a series of desorption measurements which allows assessment of in-situ gas content. The desorption measurement results exhibit that the uppermost of three shaley coals named as Niru, Lematang, and Babat at depth from 87.5 m to 129.5 m, shows the absence of gas content. Meanwhile, Kebon shaley coal exhibits low of gas content eventhough this bed occurs at 175 m depth (0.02-0.04 cm3/g). The gas content values became more significant for the following coal beds: Benuang, Mangus, and Suban at depth of 201 m to 284.5 m. A very low amount of gas content is also found at the lowermost of three coal beds (Petai, Merapi, and Kladi) at depth of 388 m to 404 m (0.05-0.10 cm3/g). Based on gas composition analysis, all samples were dominated by methane (65.60-97.91 %) with lesser amount of oxygen (0.33-31.24 %) and carbon dioxide (0.23-3.96 %). Some development presumptions on gas content variation have been arised such as the role of organic matter, thermal maturity and kinetic reaction. Three hypotheses are proposed: 1) the huminite maceral is the main key on gas content variation, 2) the gas content increases in line with increasing the thermal maturity, 3) based on kinetic parameters, heating rate and temperature of gas generation can be determined. The current study has identified that all samples were dominated by huminite maceral group (72.4-92.6 vol.%) than liptinite (5,0-19,2 vol.%) or inertinite (1,2-15,4 vol.%). Analysis result shows that the higher huminite content will produce higher gas amount during coalification. Huminite can produce and store more gas than liptinite or inertinite because huminite is more porous and therefore has a higher surface area to desorb gas. The reflectance (Rmax) value of coals range between 0.24 to 0.31% indicating maturity of lignite stage. The study results suggest that coal rank gives an important effect to the gas generation, especially for three major coal beds (Benuang, Mangus and Suban). The negative correlation between gas content and rank values is indicated for three lowermost coal beds (Petai, Merapi, and Kladi). Apparently the rank does not contribute to gas content generation in coals. These coal beds have been degraded intensively due to oxidation processes during peatification, resulting relative higher amount of inertinite and lower amount of huminite. The proximate analysis results, particularly for total moisture, ash, volatile matter, and fixed carbon on coal samples show no significant value with increasing depth of drill hole. It indicates that there is a small changes for those chemical composition downward the drill hole. It also indicates that all samples occurs under diagenetic stage that the increases of temperature and pressure are small during coalification. Total organic content results show that the lower amount of its value will lead to poor gas content generation in coals. The variaton of TOC content is mostly affected by oxidation process which indicated by the variation of oxygen index values. The higher level of oxidation will lead to imperfect of organic matter preservation process during peatification. It correlates to low amount of gas generation in coal. TOC analysis result gives a usefull information in providing answer to the low gas content of coal beds (Petai, Merapi, and Kladi). For further charaterisation of all samples quality, a pyrolysis-GC was used, which provides details insight into the macromolecular of organic matter. From these pyrograms it can be seen that the most abundant pyrolysate compound detected upon pyrolysis-GC are low molecular weight (nC1-5), especially for methane (C1), than medium (C6-14) or high molecular weight (C15+). Quantitatively, this investigation is very usefull for answering the gas content variation in coals. Another important methods are to determine the heating rate and temperature of gas generation of analyzed samples. Based on the kinetic parameters (activation energy and frequency factor) result during pyrolysis experiment, the heating rate value of gas generation can be calculated. The result shows that the geological heating rate of hydrocarbon generation reached 15oC/Ma. Furthermore, according to kinetic parameters data and the value of geological heating rate, the beginning of hydrocarbon generation can be define as well. The hydrocarbon generation of Kebon, Benuang, Mangus, and Suban were suggested to the geological temperature of 82oC-107oC. The current study provides a new understanding of the gas generation history in the western part of South Sumatra Basin. Besides that, it is very substantial key as a guide for the next coal bed gas exploration leading to the area which is suggested to have larger amount of gas content.