Abstract

An extensive intra-cratonic rift system, comprising more than seventy Tertiary basins, extends some 2000 km from northern Thailand across the Gulf of Thailand, southeastwards between peninsular Malaysia and the Indo-China margin to the Natuna Ridge. It includes significant hydrocarbon provinces in the Malay, West Natuna, Pattani and Phitsanulok basins. Demarcation of this rift system is by a series of major intra-cratonic faults and sutures. Its origins are related to movements along these discontinuities, the onset of which can be correlated to the commencement of collision between the Eurasian and Indo-Australian plates.

Previously the genesis of basins comprising this rift system had been viewed in isolation or, in the case of the Thailand basins, in the context of simple east-west extensional opening in response to a northwest-southeast directed dextral shear. However these views failed to acknowledge evidence of the widespread occurrence of rifting and of subsequent Miocene and Pliocene transpressional deformation including structural inversion. Moreover, detailed field studies and examination of the stress fields operative since the onset of Eurasian Indo-Australian plate-collision indicate that major intra-plate discontinuities experienced displacement reversals. Reversals of the corresponding inferred shear couples are incompatible with a history of simple rift basin formation.

A new model of basin development is proposed based on extrapolating present day stress patterns of the Eurasia Indo-Australia plate collision back through the Tertiary. In this model the present northwest-southeast dextral shear regime became established during the Neogene following a 5-10 m.y. period of structural quiescence, which in turn was preceded by a Paleogene phase of northwest-southeast directed, sinistral shear. Changes, and reversals, in shear direction reflect the deflection of the stress field ahead of the northern collision front of the Indian indentor as it penetrated into the Eurasian Plate.

Some key implications of this model are that it predicts a multi-phase history of basin development. Reversal of motion along the major intra-plate discontinuities commenced in the south and progressively moved northwards through Indo-China since the beginning of the Miocene. This model implies that the reversal in fault displacements coincided with structural inversion and formation of major compressionally related traps. Structural inversion involved the creation of escarpments along compressional segments of major fault boundaries. These escarpments provided both local provenance areas for the deposition of potential reservoir sequences and acted as barriers to drainage patterns, creating lakes such as the Phitsanulok Lake which became major sites for subsequent lacustrine deposition. Displacements along major faults traversing basement, particularly where composed of magmatics, provided fracture enhanced "basement" reservoir potential, the exploration significance of which can be seen in areas such as at the Bach Ho Field in the Cuu Long Basin, offshore Vietnam.

This new model thus provides a framework in which compressionally induced traps, "localised" lacustrine source sequence distributions and enhanced fracture related pre-Tertiary ("basement") reservoirs can now be viewed in a broader regional context, a context which has application to the refinement of petroleum exploration strategies throughout Indo-China.