Abstract:

 Curie point isotherm is the temperature isotherm where ferromagnetic minerals in the rocks change from ferromagnetism into paramagnetism because the temperature increases to the Curie point. Below this isotherm, the rocks are thought to be almost non-magnetism because their temperature is higher than the Curie point. Sino-US two-ship expanding spread profiles eastern transect (ESPE) in 1985 and Sino-Japan Ocean Bottom Seismography profile in 1993 (OBS93) crossed the major structural belts and basins in the northeastern South China Sea. Integral gravity, magnetic, heatflow and seismic data were acquired along these profiles. Magnetic Curie point isotherm was obtained by inversion of the magnetic anomalies, and thermal Curie point isotherm (the 575℃ isotherm) was obtained according to the geothermal temperature field calculated from the heatflow survey data. The results showed that the two Curie point isotherms derived respectively from the prior two methods were inconsistent with significant differences in many regions. After analyzing their calculation principle, the authors believed that the reason for these differences was that the prerequisite calculating the stable-state geothermal temperature filed was untenable in some areas with possible unknown underground heat events, then the thermal Curie point isotherm derived from this method would not show the actual Curie point isotherm in such areas. In order to explain these differences more clearly, the authors introduced the concept of thermal equilibrium, which showed that the two types of Curie point isotherm were consistent in thermal equilibrium region and inconsistent in thermal disequilibrium region. As a result, they could achieve the magnetic and thermal Curie point isotherm first in some areas and distinguish the unknown underground heat events from their differences, which has important instructional significance and reference value for the study on regional tectonics, oil and gas geological conditions and resource predictions.

Key words: Northeastern South China Sea, Thermal Curie point isotherm, Magnetic Curie point isotherm, Thermal equilibrium mode, Petroleum geological significance