Evidence is presented from the study area for the occurrence of a regional compressive tectonic event in the mid-Cretaceous (Aptian-Cenomanian). This is tentatively attributed to the effects of a distant continent-continent collision to the west. Pre-existing structural trends were reactivated parallel to palaeo-continental sutures to the northeast (Song Ma and Song Da) and west (Nan). The event interrupted the latest Jurassic to earliest Palaeocene subsidence and continental sedimentation of the Khorat Plateau Basin. The primary effects are shown by seismic data to be regional tilting, compressive folding, reverse faulting and inversion of the basin subsidence pattern. The data do not support the current view that such structuring was initiated during the Tertiary. However they do show that there was some reactivation of mid-Cretaceous structures during regional uplift in the Tertiary. This revision of timing of structuring suggests the hydrocarbon potential of the area may be greater than previously anticipated.
The Vientiane area of southern‐central Lao Peoples' Democratic Republic (Laos) includes an extension of the Khorat Plateau Basin, which covers most of neighbouring NE Thailand. This basin contains one producing gasfield and several other gas discoveries. The Vientiane area is in the initial stages of petroleum exploration, and no drilling has yet occurred. With the aid of new seismic data, the structure and stratigraphy of the Vientiane area are described in relation to the region's tectonic history. Some aspects of this history are still poorly understood. We suggest that the age of the regional base‐Khorat unconformity is Late Jurassic instead of Late Triassic as most previous studies have assumed. Recent studies have also shown that the structural inversion of the Khorat Group began in the mid‐Cretaceous and not in the Tertiary. With reference to published information from petroleum exploration in NE Thailand, new data from Laos are interpreted as showing that the Vientiane area has encouraging petroleum potential. Exploration risk, which is associated with the presence and reservoir quality of the Permian carbonates which form the primary target, is judged to be lowest in traps containing Permian palaeo‐highs.
The Pak Lay Foldbelt in the northwest of the Lao PDR is a product of the Indosinian orogeny, resulting from the collision of Shan-Thai and Indochina along a suture now marked by the Nan-Uttaradit ophiolite zone. Current hypotheses place the timing of this collision in either the Permian or the Triassic. Recent field work and subsequent laboratory analyses suggest that, in the area to the south of Pak Lay: pre-collision sediments are as young as Middle to Upper Jurassic; arc-related volcanism continued from the Triassic into the Late Jurassic; all these sediments and volcanic rocks form an imbricate zone; and the Cretaceous Khorat Group rests with marked unconformity above the imbricate wedge. From this we conclude that the Shan-ThaiIndochina suturing occurred in the Late Jurassic.
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