Different Types of Organic-Rich Geological Markers in Surat, Central-North of Pattani Basin, Gulf of Thailand

Abstract

The lower to middle Miocene organic-rich sediment in the Surat area, central Pattani Basin, Gulf of Thailand has been studied using conventional core with the integration of wireline and petrographical data. The results suggest three types of organic-rich sediments, which are non-marine coaly mudstone, marginal marine coaly mudstone, and estuarine shale. Non-marine coaly mudstone is indistinguishable from floodplain sediments on wireline due to their similar characteristics. Marginal marine coaly mudstone with abundant pyrite nodules has low density, and high gamma well-log characteristics and a widespread distribution. Thin estuarine shale is difficult to distinguish on wireline only, however; associated marginal marine coal with low resistivity and high gamma shale can be recognized and correlated. The other associated facies in the succession are interpreted as dominantly fluvial deposits with tidal influence. The recognition of estuarine shale suggests that major marine transgressions occurred in the lower and middle Miocene. The presence of a thin transgressive section indicates the low basinal slope of the Miocene of Pattani Basin allowed the marine shoreline to migrate to the central-north of the basin. The evidence of marine incursions provides a better understanding of the lower to middle Miocene stratigraphic framework in the Surat area and Pattani Basin, and it is an important controlling factor for possibly tide-influenced fluvial sand reservoirs in the southern area.