Potential deposited source rocks of the former sub-system can be predicted. These are, in order of deposition, lacustrine shales developed during the rifting phase, and transgressive shales and coals of the deltaic systems deposited during the cratonic or margin phase. Finally, during the divergent margin phase and depending on environmental conditions marine source rocks could deposit, although the burial depth in Southeast Asia will be generally insufficient to consider those in prospect evaluation.

Potential reservoirs of the latter sub-system include the non-marine sandstones in the rifting phase, deltaic bars/channels and shallow marine offshore bars in the cratonic phase, carbonate reefs and shoals in the cratonic phase, and offshore bars to mass flow deposits during the margin phase. Next to these sedimentary reservoirs, the substratum should be taken into consideration, reservoir properties of which being the result of fracturing, hydrothermal alteration or weathering during exposure. Potential traps can be structural (tectonic inversions), morphological (juxtaposition or reefs), or stratigraphic.

Intense exploration during the last five years have resulted in a wealth of information, which is used here to test the theoretical petroleum system model outlined above.

The initial rift stage development of the Con Son Basin is mainly found south of a principle East-West faulting system. Even though not clearly dated by key micropaleontological markers, this series is assumed to be mainly of Oligocene age. The larger part of the sedimentary section corresponds to a series deposited during the cratonic phase. The lower part of this phase corresponds to a thick elastic series passing over all the structural domains of the area, with significant tectonic readjustment during the Middle Miocene. This activity reduces at the end of the Miocene and the series deposited during the Pliocene/Pleistocene corresponds to a thick wedge thickening east- and southward. In contrast the Mekong Basin exhibits well developed riff and cratonic stages, but no passive margin phase, as the oceanisation aborted much earlier than in the Con Son Basin. Further tectonic activity was less pronounced than in the Con Son Basin.

Tenuous indications for the presence of a lacustrine source rock in the Con Son Basin deposited during the rift phase are derived from geochemical analysis of oils and cutting samples. On the other hand numerous and widespread coal seams were encountered in the prograding/retrograding deltaic cycles of the sedimentary sequence of the cratonic phase, with a cumulative thickness varying between 20 and 60 m. In keeping with the well developed rift stage, lacustrine source rock are omnipresent in the Mekong Basin, overlain by a elastic series including coal seams representative of the cratonic phase.

Within the oil suite from the Con Son Basin, none of the oils can be assigned a “pure” lacustrine origin, such as encountered in the Mekong Basin. Results from one of the Block 11-1 wells further strengthen the possibility of the existence of lacustrine source rock in the Con Son Basin, as extracts of cutting samples from the deepest section just above the substratum have a typical Type I signature, whereas the cratonic phase sedimentary section has a clear Type III signature. However, the putative lacustrine source rock interval of the concerned well suggest, at least locally, poor quality and quantity. Also its geographic extension seems to be restricted, based on the limited occurrence of oils with a lacustrine contribution.

Maturity parameters, such as vitrinite reflectance (Ro) and Rock Eval Tmax, place the start of the oil window between 3000 and 4000 m depending on the position in the basin. Extrapolation of these trends place the end of the oil window between 4300 and 5300 m. These maturity constrains provide the possibility of generation and expulsion of hydrocarbons from the cratonic and rift sedimentary sequences in the Con Son basin, but restricts the generation and expulsion in the Mekong Basin to the rift sequence only.

In the Con Son Basin margins the presence of clastic reservoirs deposited during the cratonic phase has been confirmed by numerous exploration wells. Effective vertical sealing is provided by syn-depositional shales. These elastic reservoirs are considered to be mostly offshore stacked bars deposited during successive series of 50-100 m second order deltaic cycles. Fair to good petrophysical characteristics are proven by well data down to approximately 4000 m. The presence of numerous rock fragments within these sands leads to an anomalously high compaction level with depth which has a very detrimental effect on the preservation of their characteristics below 4000 m. Transgressive carbonate reservoirs are encountered mainly to the southern and the eastern parts of the basin, seawards, and develop mainly after the main elastic reservoirs.

The Mekong basin is characterized, next to similar type of elastic reservoirs, by important fractured basement reservoirs, accounting for most of the current oil production. Differences between the Mekong and Con Son basins with regard to this exploration objective seem to be related to different tectonic activity and sealing efficiency.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah