Abstract

The Nakhon Thai Ranges fold-belt is a late Palaeozoic play in Northern Thailand. Blocks L7/50 and L13/50 cover the mountain ranges and are composed of outcropping Mesozoic and Palaeozoic rocks between the Tertiary Sukhothai and Phtchabun/Wichan Buri Basins. The Nakhon Thair ranges fold-belt is a western extension of the Phorat Plateau Basin complex where producing gas fields are contained in Permian carbonate reservoirs. The present day structural closure and proven gas column of the Sin Phy Horm gas field is much larger than the palaeo-closure when the gas was generated during the Mesozoic. This indicated recent recharge by gas released during Tertiary uplift and inversion. There was a relatively low relief paleo-structure to the Sin Phy Horm feature after the Triassic Indosinian Orogeny but the present day alrge surface fold was formed during the Tertiary. Different sets of surface folds and faults formed during the Tertiary. Magnetic anomaly image indicates Triassic volcanics not present north of the Phichit Line. Permian and Lower Carboniferous age carbonates (potential reservoirs) are exposed around Block L13/50 and are thought to be part of large platforms that in the subsuface extend across Block L7/50 under the Nakhon Thai Ranges fold-belt. Permian carbonate outcrops on western side of the Phetchabun Basin are "lozenges" within fault zones (reactivated with left lateral strike-slip motion in the Tertiary). The "lozenges" of Permian carbonates now lie structurally higher that the surrounding Triassic volcanoclastics due to Tertiary transpressional uplift. A wide zone (8 km x 200 m) of silicification and hydrothermal leaching of Permian carbonates along an old fault (Permo-Triassic age, but reactivated in the Tertiary) is thought to provide an analogy to the hydrothermal alteration of the reservoir systems in the Sin Phu Horm and Dong Mun gas fields. Sample of silicified and leached Permian carbonates from the Kho Somphot fault zone. The hydrothermal fluids are believed to be related to the nearby extrusion of a large volcanic field during the Triassic rather than during the Tertiary strike-slip reactivation of the fault system. Regionally gas and oil prone source rocks are known from Lower Carboniferous to Permian age marine formations and Triassic lacustrine basins. Prior to licensing the only seismic survey in the area was a reconnaissance grid of 8 poor quality vibroseis lines acquired in 1997. After licensing and field tapes were available, reprocessing of the DMR97 series seismic lines jointly with MOECO provided better imaging and conformed a sedimentary section was present below the Triassic age Indosinian Unconformity. Prior to reprocessing some structural features were interpreted below the Indosinian Unconformity, but with limited confidence. After reprocessing tilted fault blocks were clearly present below the Indosinian U/C and onlap of Khorat Group indicated they were Palaeo-highs. Quality of imaging of the DMR97 seismic survey still remained relatively poor after reprocessing in 2008. Initial "quick look" processing of a line acquired by Twinza in early 2010 (parallel to the previous DMR97 survey line) was disappointing. Reprocessing by GeoTrace of the Twinza acquired 2009-10 seismic survey has significantly improved imaging of both the Mesozoic Khorat Group and the underlying Palaeozoic section. A large Palaeozoic sedimentary basin has been imaged below the cover of the Mesozoic Khorat Group red-beds. Only a few of the faults in the area were reactivated during the Tertiary, indicating the top seal is generally intact.

Keywords: Uttaradit • Lowi • Phitsanulok • Lom Sak • Phichit • Phetchabun • Sin Phu Horm gas field • Nam Phong gas field • Salamander well • PTTWP well • Tatex well