Abstract
Gamma ray logs are most useful in identifying subsurface lithology and interpreting depositional environments. This study highlights the use of outcrop gamma-ray logs along with outcrop observations and total organic carbon (TOC) analysis to provide the stratigraphic framework of the organic-rich rocks of Huai Hin Lat Formation in central Thailand. The study reveals five sedimentary facies including (1) structureless sandstone, (2) structured sandstone, (3) interbedded sandstone and siltstone, (4) interbedded mudstone and siltstone and (5) calcareous mudstone. These facies can be grouped into two facies associations; mudstone-dominated and sandstone-dominated facies associations. The depositional environment was interpreted as lacustrine basin-fill subdivided into deep lacustrine environment and sublacustrine fan associated with the turbidity currents. The total gamma-log characteristics are closely related to the lithologies controlled primarily by clay mineral compositions. Whist, the use of spectral gamma-ray can reveal more details on depositional environments and conditions. In this study, U concentrations is proven to be useful in highlighting organic-rich rocks in low K and Th concentration successions due to its ability to be fixed in clay minerals and organic materials under an anoxic condition. Thus, the U spectral gamma ray is suggested to combine with conventional gamma ray log for depositional environment and recognition of organic-rich rocks.
Highlights
Identification and reconstruction of depositional environments are important in petroleum exploration and development as they provide the necessary framework for delineation and prediction of petroleum system components in the basin, especially in frontier exploration areas
To evaluate petroleum source rock, the gamma-ray responses should be used with caution as it is dominantly influenced by mineral compositions (Myers and Wignall, 1987; Rider, 1996)
The summary of descriptions and interpretation of processes in each facies is provided in Table 1
Summary
Identification and reconstruction of depositional environments are important in petroleum exploration and development as they provide the necessary framework for delineation and prediction of petroleum system components in the basin, especially in frontier exploration areas. Spectral gamma ray logs provide more information on the identification of organic-rich intervals, especially the U response (Myers and Wignall, 1987; Lüning and Kolonic, 2003). Outcrop gamma-ray log has been investigated for many years as it is useful for documenting changes in strata patterns, stratigraphic correlations and sequence stratigraphic interpretations (Myers and Bristow, 1989; Slatt et al, 1992; Davies and Elliott, 1996; Evans et al, 2007) It provides a link between lithological features observed in outcrop and subsurface which improves the understanding of the subsurface data (Slatt et al, 1992, 1995; Aigner et al, 1995)
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