STRATIGRAPHY AND RESERVOIR POTENTIAL OF THE MESOZOIC KHORAT GROUP, NETHAILAND .1. STRATIGRAPHY AND SEDIMENTARY EVOLUTION

In this paper, the sedimentology, petrography and biostratigraphy of t he Nam Phong Formation and the overlying Mesozoic Khorat Group in NE T hailand are described. Data were gathered during extensive studies in the field (about 100 outcrop localities were investigated) and from la boratory analyses of over 500 surface samples. The Nam Phong Formation is dated accurately for the first time as late Norian to Rhaetian, wh ile the Phra Wihan, Sao Khua, Phu Phan and Khok Kruat Formations (of t he Khorat Group) are reassigned to the Early Cretaceous (Berriasian to Aptian) on the basis of new palynological data. The age of the interv ening Phu Kradung Formation, which is conventionally assigned to the L ower Jurassic, is probably Late Jurassic or Early Cretaceous, although this cannot be determined with confidence. Changes in provenance betw een the Nam Phong Formation and the overlying Phu Kradung Formation su ggest a possible sedimentary hiatus. Palaeocurrent data suggest that s ediments were mainly sourced from the north and east. Part I of this p aper describes the sedimentology and biostratigraphy of these units; P art 2, which is scheduled to appear in the April, 1996 issue of this J ournal, reports on their diagenesis and reservoir quality.