Geochemical and palynological characteristics of tertiary oil shales and lignites of the Mae Moh Basin, northern Thailand

Le, Van Minh (1994) Geochemical and palynological characteristics of tertiary oil shales and lignites of the Mae Moh Basin, northern Thailand. Masters thesis, Memorial University of Newfoundland.

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Abstract

The sources of organic matter, the depositional conditions during lacustrine/swamp phases, the degree of post-depositional thermal alteration are examined for a Tertiary intermontane basin in northern Thailand, the Mae Moh Basin. Alternating oil shale and lignite layers in this basin also preserve a record of changing temperature and precipitation patterns, and variable contributions from swamp flora and lake primary producers. -- Variations in molecular and isotopic geochemical characteristics (including compound-specific stable carbon isotope compositions) and spore and pollen assemblages (including thermal alteration index) across a 300 meter section of the Middle Mae Moh Formation are investigated. Samples collected are classified into: group I, lignite; group II, argillaceous lignite; and group III, oil shale. The saturate fraction for all samples is dominated by n-alkanes from C₁₅ to C₃₃. Group I is characterized by a high abundance of the high molecular weight (HMW) C₂₆ to C₃₃ (up to 82%), and a low abundance of the low molecular (LMW) C₁₅ to C₁₉ (3 to 8%). Group III has a lower abundance of the HMW alkanes (29 to 66 %), and a greater abundance of the LMW homologues (13 to 43 %). Group II has n-alkane distribution that is intermediate between these end members. Although the dominance of higher plant contributions is indicated throughout the section, the oil shales clearly received substantial contributions from lake primary producers (algae). These molecular trends are consistent with the δ¹³C values determined on bulk samples, which vary from -22.5 to -26.8‰. The δ¹³C of n-alkanes in the HMW range, which are mostly lighter than -30‰, also substantiate a dominantly higher plant source. Algal contributions are suggested by the relative abundance of LMW alkanes with carbon isotopic compositions (about -24 to -30 ‰), which are somewhat enriched compared to the HMW homologues (-28 to -35‰). -- Decomposition and settling conditions of organic particulates are inferred from the pristane/phytane ratios (Pr/Ph). Oil shale samples, having Pr/Ph ratios close to unity (0.95 to 1.58), were likely deposited during oxic-anoxic fluctuations on the bottom of lakes and bogs within the basin. Deposition and degradation of lignitic organic matter probably occurred in prevailing oxic conditions as indicated by their Pr/Ph values that range from 1 to 3.53. -- The degree of thermal maturation given by thermal alteration index (TAI) values ranging from 2.0 to 2.4 and carbon preference index (CPI) from 0.63 to 3.77 (most values are > I) indicate immature to early mature stages in terms of oil generation for Mae Moh source rocks. The spore and pollen assemblages are mainly palm (Spinizonocolpites, Oncosperma, Calamuspollenites, and Cyrtostachys), fern (Verrucatosporites sp. and Polypodiidites sp.), deciduous trees (Quercoidites, Alnipollenites, and Trivestibulopollenites), and other specimens of unknown affinity: Crassoretitriletes sp., Trichanthera sp., Monosulcites, CupuUferoipollenites, Biretisporites huonensis and Roipites sp.. Their temporal variations suggest a dominant tropical paleoclimate with possible cooler conditions in the early stage of basin evolution.

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