The Low Vitrinite Reflectance Observed in the Miocene Formations in S.E. Asia- Suppressed or Cooler Thermal History?

Kazuo Nakayama

The vitrinite reflectance data in the Miocene formations in S. E. Asia are found too low relative to the measured bottom hole temperatures and lower than the vitrinite reflectance values predicted by basin modeling. Such observation appears on the data from Khmer Trough, Vietnam offshore, South China Sea, and Japan Sea. Two possibilities are accounted for: 1) the vitrinite reflectance is suppressed under the certain conditions such as over pressure or the existence of generated oil, and 2) the paleo-heat flow in the area was significantly lower than we postulated as a rift basin.

The latter possibility has been rarely considered, however, one of the sites indicates that only suppression cannot recover the differences, but also the effect of low paleo-heat flow is necessary. It is considered the heat flow was high at the initial stage of forming a rift and it exponentially decreased, then it increased recently to indicate a relatively high heat regime at present stage. Introducing such assumption could solve the observed low value of vitrinite reflectance. Although this conclusion must be verified by other inorganic source such as Apatite Fission Track Analysis or X-ray diffraction of clays (smectite/ illite transformation), the thermal history with low heat flow would affect significantly the timing of oil/gas generation and expulsion. Thus the observation of ess mature vitrinite lead us a new exploration strategy for the area.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California