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Element Geochemical Characteristics and Its Paleo-environmental Significance of Permian Carbonate in Khorat Basin, Thailand


The Khorat basin is a stable intracratonic basin, which located mainly in northeastern Thailand and composed of an initial rift sequences of Carboniferous to Triassic sediments, and a “sag” sequence of Late Triassic to Cretaceous sediments. Hydrocarbon exploration in Khorat basin began in the early 1960s and only two commercial discoveries have been made in limestones of Pha Nok Khao Fm of Permian. No accurate assessment of source rocks has yet been made in this basin. Although the Carboniferous mudstones of Si That Fm are considered to be major source rocks, It has't been confirmed by well drilling. Recently, four new wells were drilled in the central of Khorat Basin, and cores acquired from Permian carbonate rocks were analyzed by a series of testing of ICP-AES and stable isotope mass spectrometry and TOC analysis, etc. The core samples were examined for the compositions of trace elements, C and O isotopes, and organic geochemical characteristics. Additionally, the study for Palaeo-sedimentary environment and its influence on carbonate source rocks were carried out, which helps understanding the development and distribution of source rocks in this basin. The results show that foreslope facies of carbonate platform is typically developed in the central of Khorat basin, which could be divided into shallow water foreslope sub-facies and deeper water foreslope sub-facies. Sedimentary environment in shallower foreslope sub-facies typically occurs above sea wave base and characterized by fresh-brackish water, relatively weak water dynamic condition and weak oxidation-weak reduction environment, which is not good for preservation of organic matter. Deeper foreslope sub-facies generally developed below the sea wave base with a relatively deeper water depth, weak water dynamic and brackish-saline water condition, anoxic and reduction environment. It's favorable for preservation of organic matter and considered to be conducive to development of carbonate source rocks. Black muddy limestones samples, taken from layers of deeper foreslope sub-facies, typically have a TOC of 0.16-2. 8% (avg of 1.6%) and Ro of 4.76%. Organic matter is II1 type and characterized by sapropelic matter with no terrestrial organic materials. Generally, this type of the black muddy limestone layers could be one of the potential source rocks in the central of Khorat basin, due to its widely distribution, relatively large thickness and favorable oil-generating indexes.