Geological and geophysical data were combined to
conduct a highly detailed interpretation of the southern
third of the Erawan gas/condensate field in the Gulf of
Thailand. Data included three-dimensionally migrated
seismic lines, horizontal time slices through the 3-D data
volume, time and amplitude maps generated on an interactive
computer interpretation system, and borehole data.
Using these data, six seismic horizons were mapped in the
study area: (1) top of acoustic basement (Paleozoic or
Mesozoic rocks); (2) near top of Cycle I (within the lower
Miocene); (3) an arbitrary reflector within Cycle II
{within the lower Hiocene); (4) near top of Cycle II (within the lower Miocene); (5) near the middle Miocene unconformity, near top of Cycle III (within the
upper Miocene); and (6) a very continuous reflector within
Cycle IV (within the upper Miocene). The regional
geologic rlistory included several different tectonic
settings from the Cambrian to the Recent. Subduction from
the east commenced in the Silurian to Lower Devonian and
continued until continental collision occurred in the
Triassic. Subduction from the west commenced in the
Jurassic and has continued to the present. Associated
with the subduction was the formation of large granitic
bodies which were intruded throughout most of the region.
The Tertiary section within the Pattani trough was
dominantly controlled by backarc spreading in the early
Tertiary which initiated subsidence along older zones of
weakness. Most of the faulting associated with the study
area developed in the Miocene as the result of increased
sediment load and basin subsidence. The configuration of the horizons mapped demonstrates the presence of a complex, faulted graben. Fault blocks
of less than a quarter mile (half a kilometer) in width
were mapped. Faults generally strike north-south in the
northern part of the study area and trend eastward in the
southern half. The structure contains major east-dipping
faults which cut basement and associated west-dipping
antithetic faults. The axis of the graben shifts from the center of the study area in the north to east of the area in the southern part. In the soutlnvest corner of the
area, the west-dipping flank of another graben structure
is present. Stratigraphic mapping performed on and
between these horizons revealed a complex stratigraphy.
Most sand bodies are thin and lenticular with limited
lateral extent. Most of the section is fluvial in origin
with only minor amounts of marine influence. Reservoir
beds are mainly meandering channel sandstones and bars,
rather than extensive sheet sands with each fault block
acting as a separate unit in controlling hydrocarbon
accumulations, it is critical to encounter these small
faulted sand bodies in an optimum structural position.
The use of three-dimensional seismic methods have aided in
defining these structural positions.
Description:
Includes bibliographical references (pages 82-86)
Includes digitized image(s) in TIFF format.
California State University, Northridge. Department of Geology.