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Stratigraphic Evolution of Oligocene Fluvial Systems, Gulf of Thailand: Seismic Workflows, Depositional Patterns, Shallow Analogs, and Exploration Significance

A. S. Madof, S. C. Lang, and H. W. Posamentier
Chevron Energy Technology Company, Houston, TX, USA

Abstract

The fluvial Oligocene "O" sands constitute voluminous and prolific oil-bearing reservoirs, located in the Pattani basin, central Gulf of Thailand. The basin formed through 90 km of late Oligocene to middle Miocene extension, accommodated on numerous north-striking normal faults, which served as the margins for well-developed eastward-thinning half grabens. Although subsequent "O" sand deposition was previously interpreted as transversely-oriented and shed from syn-rift foot wall highs, we conclude that the seismically observable fluvial systems were the product of axial transport towards the south, and developed during post-rift stratigraphic healing. This conclusion, which hinged on the use of advanced 3D seismic interpretation and visualization techniques, proposes that the "O" sand architecture was dominated by avulsion, with individual channel belts migrating into antecedent fault-controlled depocenters. The fact that topographic inheritance, as opposed to active deformation, served as the primary control on fluvial architecture in the Pattani basin, suggests that the importance of post-rift gradient is commonly under-represented and misunderstood.

Three seismic facies associations constitute the "O" sands in the Pattani basin: channel belt, floodplain, and undifferentiated. The channel belt seismic facies assemblage is characterized by laterally discontinuous, ribbon shaped, high reflectivity strength reflections, with evidence of truncation below and onlap onto the event. These reflections cut into both the floodplain and undifferentiated facies associations, which are characterized by laterally discontinuous to continuous, nondescript reflections. The primary difference between these two associations is in the reflectivity strength: the floodplain assemblage is associated with high strength, whereas the undifferentiated association has low strength.

In addition to seismic, the availability of well log and core data allowed a direct comparison between reflectivity, petrophysical response, lithofacies type, and stacking pattern. Although discrete vertically stacked upward fining channel belts were readily observable from core, and discernable on gamma ray log, the limit of separability (1/4 wavelength) and detectability (1/16 wavelength) precluded imaging of less than 4-5 or 1-2 deposits, respectively. Therefore, two cycle events (peak and trough, or vice versa) belonging to the channel belt facies assemblage are interpreted as numerous and amalgamated deposits.

Aside from the integration and calibration of seismic, petrophysical, and lithologic data, our study relied on the creation and utilization of a new interpretation workflow: reflection auto-picking on a shifted horizon slice. This technique, which requires picking a laterally continuous high strength reflection, shifting to the zone of interest, extracting amplitude, and picking directly on the previously interpreted surface, has proved extremely useful in areas exhibiting significant amounts of brittle deformation. It is this workflow that allowed the identification of 16 sets of channel belts, which systematically drifted 75 km through one cycle of east-west lateral migration. This conclusion, along with the use of shallow analog data, significantly helped identify and characterize new areas of exploration in the Pattani basin.

AAPG Search and Discovery Article #90206 © AAPG Hedberg Conference, Interpretation Visualization in the Petroleum Industry, Houston, Texas, June 1-4, 2014