Eocene to Oligocene Structural History and Paleogeography of the North Malay Basin; The Key to Formation 0 Exploration

Robert C. Shoup, Padsakorn Suwanruji, and Christopher K. Morley
PTTEP, Bangkok, Thailand

Several wells in the North Arthit Concession in the Gulf of Thailand have penetrated a thick sequence of syn-rift Formation 0 sediments that appear to have been deposited in lacustrine fan deltas. The key to prospecting for these fan-delta reservoirs is to delineate the rift-valley lakes into which they were deposited. This in turn, requires a more detailed understanding of the syn-rift structural history and paleogeography of the North Malay Basin.

The syn-rift section appears to be of Late Eocene-Oligocene age. Regionally, the orientation of the main rift trend in the North Malay basin is NW-SE, with a number of well- developed branches that have a more ‘N-S’ orientation. The ‘N-S’ trends probably follow pre-existing fabrics in basement. In NW Arthit, some of the older half grabens have been reactivated as thrusts (either Eocene or early Oligocene). The main extensional bounding fault appears to have developed along the western-most former thrust, but the eastern-most thrust was not reactivated. The pre-existing structures have impacted the terminations and orientations of later normal faults.

The change in the rift orientation from NW-SE to ‘N-S’ resulted in the formation of a series of en echelon half-grabens. Lacustrine development occurred along the axis of these half-grabens. Fan delta deposition will occur preferentially along the lake margin defined by the main extensional bounding fault.

Paleogeographic reconstruction of the early rift history has allowed us to predict the general location of the alluvial fans being shed off the highlands, lacustrine fan delta reservoirs and turbidite reservoirs, and lacustrine source-prone shales. From this information, reservoir trend maps were constructed to define the play fairways for focusing seismic mapping.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas