Paleoproductivity and paleoredox condition of the Huai Hin Lat Formation in northeastern Thailand

Abstract

The petroleum exploration has been conducted in the Khorat Plateau since 1962 and two gas fields have been discovered and commercially produced. The lacustrine facies of the Huai Hin Lat Formation is believed to be one of the main source rocks of the gas. Therefore, investigation and analysis of the Huai Hin Lat shale for understanding the paleoenvironment and petroleum source rock are carried out in this study. Petrographical study and geochemical analysis of shale samples were performed to explain the paleoproductivity and past redox condition. The palynofacies assemblage comprises abundant AOM, acritarchs, phytoclasts, and very small amount of spores and pollen. Geochemical analysis was used to determine the total organic carbon (TOC) and the concentration of major, trace, and rare earth elements. The paleoproductivity proxies are composed of palynofacies, TOC, excess SiO2, Ba/Al, and P/Al. They reflect a high paleoproductivity except the middle of the lower part (bed 3) and the lower bed 13 of the upper part. Bed 3 shows the highest peak of TOC and the lower bed 13 exhibits a relatively lower TOC, which can be explained by the excellent and the poorer preservation condition, respectively. The paleoredox proxies consist of U/Th, V/Cr, NiCo, (Cu+Mo)/Zn, Ni/V, and Ce anomaly. They are used to establish the depositional environments, to characterize the organic matter content, and to assess the source rock potential. They reflect many high peaks and predominantly high values of paleoredox proxies except the middle part and the lower bed (lower bed 13) of the upper part. They indicate that the section was mainly under anoxic or reducing condition, which is supported by the high Ce/Ce* (>0.8) and V/Cr (>2.0) values. The middle of the lower part (bed 3) shows lower productivity but it contains the highest peak of TOC, which is conformed to be the excellent preservation of organic matters in the strong reducing condition. The middle part, which shows high productivity, contains relatively lower TOC as it possesses a less reducing condition compared to the more reducing intervals. The lower bed 13 of the upper part shows a less reducing condition and a lower TOC, which conforms to a lower productivity. The organic matters of the Huai Hin Lat Formation consist mainly of AOM and acritarchs and possess good to excellent TOC (2%–7%). They belong mainly to type I and type II kerogens with some mixture of type III as indicated by the presence of phytoclasts, spores, and pollen. The organic matters of the Huai Hin Lat Formation, based on the kerogen type and the thermal history, have already generated significant amount of oil and some gas to the Sap Phlu Basin.