ABSTRACT: Organic Geochemistry of Mae Sot Basin Oil Shales, Thailand--Implications for Depositional Setting and Basin Reconstruction

CURIALE, JOSEPH A., Unocal Science and Technology Division, Brea, CA, and MARTIN R. GIBLING, Dalhousie University, Halifax, Nova Scotia, Canada

Mae Sot basin is located in western Thailand along the border with Myanmar (formerly Burma), and is one of several elongate Cenozoic basins in the region. During the late Miocene through Pleistocene, oil shales were deposited episodically, in freshwater to brackish water lakes. As part of an effort to understand the generation and expulsion behavior of freshwater lacustrine oil source rocks, twelve shales samples within an 8 m thermally immature (VR = 0.36-0.48% Ro) interval are examined using techniques of molecular and isotopic organic geochemistry.

Surprisingly wide geochemical variability is evident within this narrow sampling interval. Total organic carbon and extractable organic matter (EOM) contents are 0.89-22.40% and 848-33,908 ppm, respectively. Carbon isotope ratios of kerogens, EOM and EOM fractions range over 3 percent; sulfur contents in the EOM range from 0.9 to 7.6%. The extent of n-alkane "waxiness" in the EOM and the kerogen pyrolyzates varies extensively. Variability is also evident in the distribution of tetra- and pentacycle hydrocarbons. For example, although oleanoids appear to be absent, and the steranes in all samples are dominated by 5a(H) and 5b(H)14a(H),17a(H)C27-29 components, ethylcholestane isomers range from 46 to 75% of total C27-29 steranes. In contrast, except for a single sample containing 25% al inite, the kerogen is entirely (95-100%) fluorescent amorphinite. The wide lateral extent of individual oil shales interbedded with marlstones, and coupled with the unexpected variability in organic character (over a compacted vertical extent of only 8 m), suggest extensive and rapid changes in physio-chemical parameters during deposition, occurring in concert with (and ultimately causing) fluctuating lake level and size. These parameters include salinity (varying concentrations of perhydro---carotene are present) and climate (TOC differences may be a function of productivity variance over time). The molecular characteristics of the Mae Sot oil shales are compared and contrasted with those of fresh-water lacustrine organic-rich shales in the Oligocene of Kishenehn basin (Montana) and the Miocene-Pliocene of Ridge basin (California).

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)