Establishing a Petroleum System Framework in the Northern Gulf of Thailand

Kelly Dempster¹, Barry Katz¹, and Greg Nelson^2
1Chevron Energy Technology Company, Houston, TX, USA
²Brunel Energy, Houston, TX, USA

The ability to establish the framework of a petroleum system helps define exploration risks, even in known producing areas. Very often in producing regions, the effective source rocks have not been penetrated and questions exist concerning stratigraphic and structural position. One such area is the northern portion of the Gulf of Thailand, where an active exploration program is underway. This work examines the methods used to develop an integrated basin study where significant gaps existed in the dataset.

The construction of the basin model required refinement of the regional stratigraphic framework. This refinement included an increase in the number of defined intervals and the introduction of multiple carrier beds representing interconnected, but impossible to correlate, sand bodies with increasing sandiness to the west. This synthetic stratigraphy was able to reproduce the discovery well’s accumulations, as well as calibration points at other fields as the basin modeling progressed.

Although prior models had been developed, questions had been raised concerning the overestimation of crustal stretching, and the temperature and vitrinite reflectance profiles because of complications associated with the reporting of data from deviated wells. In addition, estimates of sand and coal content were questioned, raising concerns with the thermal conductivities used in the model. This work examined the available raw data. Vitrinite reflectance data were screened for quality and used to calibrate a heat flow history at key wells. Resultant values were linked to other heat flow studies for confirmation of regional trends. The suggested rifting aspects used in some of the earlier models were shown to have minimal impact and that the faulting may have had a lateral component. Maturity trends derived from vitrinite data were found consistent with other geochemical maturity indicators obtained using oil and gas data from multiple locations.

Backstripping and comparison of interval thickness on opposite sides of faults were used to estimate periods of structural movement. This provided key information on migration along faults. Movement through time and the impact of structural evolution on migration has been tied to discoveries, and indicates laddering of hydrocarbons as they move predominantly westward.

Geochemical analyses of oils and gases provided indicators of productivity, lake level, marine influence, environment of deposition, algal productivity, and plant input. In conjunction with limits placed on source rock presence by drilling, the geochemical data developed into a picture of source rock character and the environment of deposition.

Empirical oil and gas windows were also derived and used to “triangulate” to a self-consistent stratigraphic source rock position.

All observed hydrocarbons were found to be consistent with an Oligocene lacustrine source rock. There appear to be three distinct ‘lakes’ with facies variation that may imply sub-basins that may occasionally coalesce. Geochemical analyses determine the source to be predominantly Type I in a deep productive lake setting. Types I/III mixed affinities occur in a shallower lake to the south, with perhaps swampy conditions. A third lake is also shallow but with different properties. In addition to defining the source character, the geochemical data indicated that in the extreme north and south of the study area, there is no evidence of thermal cracking of oils; however there appears to be cracking in the central region.

The resultant integrated basin models were able to identify more oil- or gas-prone areas as well as areas of migration focus. These data supported the location of new platforms and the acquisition of new exploration concessions, as well as the relinquishment of acreage.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.