Geo Trivia

8. Thermal Evolution of source rocks in Khorat, Thailand.
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We will not have the oil/gas fields unless the source rocks have been subsided to a great depth at high temperature environment for a certain time.

7. Reservoir Rocks and Source Rocks of Permian limestones
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The gas fields in NE Thailand have been producing gas from the Permian limestone. From the geological map of Thailand, the Permian limestone, shown as orange color, is widely exposed in the western part of the mono-cline Khorat Plateau and continued dipping down Under the Khorat Plateau. This Permian limestone is the sources and reservoirs for hydrocabons. The Permian limestone in NE Thailand was studied in detailed by several geologists. The results of the study showed that during Permian, thick successions of coastal to open marine siliciclastic and carbonate sediments were deposited across NE Thailand. Most people thought that the Upper Permian time was the continuation of the Middle Permian and must be eroded away after the big uplifted tectonic event.
***************************************************************** After the petroleum exploration and development has been started in NE Thailand, more and more sub-surface data were collected. Abundant anhydrites were noticed in several wells drilled. These anhydrites indicate a shelfal environment in which oolitic and subkha deposits occurred. These formations could have been deposited during Lopingian (Upper Permian) and conformably overlain the Middle Permian. The Lopingian (Upper Permian) was not be preserved and was eroded away during the later tectonic events, but they could have been survived and preserved underneath the great Khorat Plateau.
*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*. Please be noted that the end of Permian is a big event of mass extinction. However, it is noticed that the end of Permian event is not a single event but a double-phased events, first at the Guadalupian-Lopingian boundary (G-LB) and second at the Lopingian-Induan (Permo-Triassic) boundary (Isozaki, et al., 2008). The globally G-LB was started when a sudden transgression in Guadalupian accompanied by a cooling system and an oceanic anoxia condition that led to an extinction of reef ecosystems and a mass extinction as a whole. Approaching the end of Guadalupian, volcanic and basalts eruption dominated followed by rapidly regression. Lopingian (Upper Permian) was the time for shore appearance and hot climate.

I would like to summarize the 6 sections posted last year before i stop posting. It is about the gas fields in NE Thailand.
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Gas in NE Thailand was found at the depth of 3 km in the carbonate rocks (limestone of Permian age) and is dry gas with minor condensate in limited areas. The H2S gas and water are found at a variable amount from field to field. Chloride and Carbon Dioxide are also found in the gas too.
H2S is extremely poisoning gas.
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I propose “Thermochemical sulfate reduction (TSR)” is the major mechanism for generating H2S because it can explain the presence of CO2 and water in the reservoirs.
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I want to emphasize that to ignite the “TSR”, (1) sulfates, (2) hydrocarbons and (3) high temperature are vitally needed.