119 / 1971-01-01 00:00:00

Palaeoproductivity And Palaeoredox Condition Of The Huai Hin Lat Formation In Northeastern Thailand

fluvio-lacustrine facies,primary productivity,Ban Nong Sai section,acritarchs,Sap Phlu Basin,trace elements,organic carbon preservation,dysoxic to anoxic enviro

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, the study of the Huai Hin Lat shale as a potential source rock of petroleum is carried out.
The Huai Hin Lat Formation consists of fluvio-lacustrine sediments of Late Triassic (Norian) age. Chonglakmani & Sattayarak (1978) suggested that the Huai Hin Lat Formation was deposited in half-grabens and consisted predominantly of clastic sediments which belong to alluvial fan, restricted lacustrine, and fluviatile facies. It is exposed along the margin of the Khorat Plateau and is present in the sub-surface basins beneath the Khorat Group of Rhaetian to Late Cretaceous age. The Kuchinarai Group is the equivalence of the Huai Hin Lat Formation occurred in the sub-surface (Booth, 1998). The studied section is located in the Sap Phlu Basin, which is a small basin among the Late Triassic basins of this region. It is about 14 m thick and consists mainly of calcareous shale, calcareous mudstone, marlstone, and limestone, which can be correlated to a part of the Dat Fa Member of a deep lacustrine facies (Chonglakmani and Sattayarak, 1987).
The petrographical study and geochemical analysis of shale samples of the studied section were performed in this study to explain the past redox condition and palaeoproductivity. The biotic diversity and its concentration of palynofacies were analyzed and calculated using light microscopy. The palynofacies assemblage comprises in average abundant AOM (51.7 %), acritarchs (26.7 %), phytoclasts (21.6 %), and very small amount of spores and pollen. The geochemical analysis was used to measure the values of total organic carbon (TOC) and the concentration of major, trace, and rare earth elements. The palaeoproductivity proxies are composed of palynofacies, TOC (1.9-7.1 %), excess SiO2 (1-13.43 %), Ba/Al (11.85-49.55(10-4)), and P/Al (0.009-0.023(10-4)). The higher palaeoproductivity is indicated by higher values of TOC, Ba/Al, P/Al, acritarchs, and phytoclasts. They reflect a high palaeoproductivity especially in the middle part of the section. The sediments of high palaeoproductivity were accumulated adjacent to terrestrial source as indicated by mixing of abundant phytoclasts and spores and pollen. This palynological assemblage (including AOM) is associated with organic matter predominantly of Type I and Type II kerogen.
The palaeoredox proxies consist of U/Th, V/Cr, NiCo, (Cu+Mo)/Zn, Ni/V, and Ce anomaly. They were used to establish the depositional environments, to characterize the organic matter content and to assess the source rock potential. The V/Cr (1.5-3.3), (Cu+Mo)/Zn (0.52-1.04), Ni/V (0.09-0.20), and Ce anomalies (0.84-1.19) are good indicator for redox condition. U/Th and Ni/Co values also show the same trend through the section. The average value of V/Cr is greater than 2 indicating that it is above the oxic-dyosxic boundary (Jones and Manning, 1994) and Ce/Ce* exceeds 0.8, which is above the oxic-dysoxic and anoxic cutoff (Shen et al., 2012). The lower and the upper parts of the studied section as shown by their values indicate that they were deposited under the depleted oxygenation and good preservation condition. They can be referred to the typical black shale facies of dysoxic to anoxic environment. The middle part is lighter grey in color and may represent the more oxic condition of bottom water. The shale samples of the Huai Hin Lat section of the Sap Phlu Basin in this study are, therefore, interpreted to have been deposited in a dysoxic to anoxic environment.