Ranong Fm

Named after Ranong fault which cuts obliquely across the Thai-Malay Peninsula. Type section at B-1 well from the interval 8,030-11,710 feet. Polachan (1988) revised the “Ranong formation” of Nakanart and Mantajit (1983) into 2 formations: Ranong Fm consists mainly of sandstone, and Yala Fm comprises mostly of shale with minor sandstone. Parent unit: Mergui Gr.

Synonym: หมวดหินระนอง

[Figure 1: Map of the Mergui Basin showing the main structural features including basin depocentres, based on an extensive grid of 2D seismic reflection data. (From C K Morley & A Racey, 2011)]

Massive or thick bedded, variable color sandstones and conglomerate in the lower part of the formation; grey sandstone, siltstone and shale with occasional thin coals in the middle part; and predominantly fine-grained, calcareous greenish grey massive/ thick bedded sandstones in the upper part. Thickness: 1,104 m at type locality to 230 m at the Payang well.

[Figure 2: Stratigraphy and cross-section through the Mergui Basin, Andaman Sea, based on data in Polachan & Racey (1994) and Andreason et al. (1997). (a) Oligocene–Recent stratigraphy of the Mergui Basin; (b) stratigraphy of the syn-rift section (Oligocene–early Miocene) schematically illustrating the east–west variation in stratigraphy across the basin and the effect of rift-basin highs (e.g. Central High horst block) on large-scale distribution of sedimentary facies, modified from Andreason et al. (1997); (c) east–west cross-section through the Mergui Basin illustrating the typical half-graben geometries of the basin (modified from Polachan & Racey 1994). (From C K Morley & A Racey, 2011)]

Upper Oligocene to Lower Miocene

Fluviatile environment in its lower part, followed by a shallow marine environment in the middle and upper parts.