ABSTRACT: Stratigraphic architecture and intra-reservoir seals in Miocene fluvial to marginal marine reservoirs, Pattani Basin, Gulf of Thailand

Bracken, Bryan R., Christopher N. Denison, and Jack Livingston , Chevron Overseas Petroleum, San Ramon, CA

An integrated, structural-stratigraphic model of Oligocene and Miocene reservoirs in Block B8/32, central Gulf of Thailand, has been constructed to help understand the controls on hydrocarbon accumulation. Wireline log correlations have been tested by iterations with 3D seismic data. Sedimentological and biostratigraphical data have been integrated to generate a depositional model.

In the Benchamas field hydrocarbons occur in braidbelt and sandy, upper estuarine fluvio-deltaic sandstones. These laterally discontinuous reservoir sandstones are grouped into high-net-to-gross intervals. Intervening fine-grained parts of these successions are pedogenically reddened (oxidized) floodplain and slightly carbonaceous (reduced), lacustrine mudstones. The distribution of abundant and diverse terrestrial plant spores and pollen assemblages in these settings is strongly controlled by depositional environments, although similarity co-efficient and other statistical approaches promise an improved chronostratigraphic framework.

Reservoir-bearing intervals alternate with carbonaceous mudstones with relatively few reservoir quality sandstone beds. Sparse occurrences of marine palynomorphs and calcareous nannofossils provide evidence of short-lived marginal to shallow marine incursions. Some of these marine pulses are correlative regionally, but sparse microfossil populations do not provide age control.

Identification of marine units is the key to understanding overall stratigraphic architecture and hydrocarbon distribution. Fieldwide recognition of correlative marine units provides a chronostratigraphic framework that constrains correlation of intervening complex fluvio-deltaic reservoir intervals. The fine-grained marine mudstones probably acted as regional seals during hydrocarbon migration and now act as top-seals in fault compartments.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia