A subsurface interpretation using three-dimensional seismic methods of a portion of the Erawan gas/condensate field, Gulf of Thailand

Abstract

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.