北康盆地古地热场与油气远景

PALEOGEOTHERMAL FIELD AND PETROLEUM PROSPECT OF THE BEIKANG BASIN, SOUTH CHINA SEA

采用现今盆地地壳厚度作为约束条件,应用Mckenzie模式与构造沉降曲线拟合计算古地幔热流值,并考虑地壳和沉积层放射性衰变所释放的热流,研究盆地古地温场及其演化史,然后据此研究烃源岩的热成熟度史,结合盆地油气地质条件,评价油气资源潜力。结果表明,中始新世末盆地基底热流为45～66mW/m2,随后盆地基底热流略有上升,至早渐新世末为51～66mW/m2,现今盆地基底热流为51～61mW/m2,并且坳陷的基底热流比隆起的基底热流高。所计算的现今表面热流与实测的表面热流一致。中始新统烃源岩在早中新世末处于成熟阶段,现今处于高成熟-过成熟阶段;上始新统-下渐新统烃源岩在晚中新世末处于成熟阶段,现今处于高成熟阶段;上渐新统-下中新统烃源岩现今处于成熟阶段,局部达到生烃高峰。油气资源评价认为北康盆地具有良好的油气资源潜力。

The Beikang basin lies in the middle of the Nansha sea area and its geotectonic location belongs to the southwest boundary of the Nansha massif. The north part of the basin is separated from Nanweixi and Nanweidong basins by faults, 3 000 m isopach and island reefs. The Tingjia fault bounds the southwest of the basin, and the northwest edge fault of the Nansha Trough acts as the east boundary of the basin. The basin covers an area of 6.2×10^(4) km^(2) and contains mid-Eocene to Holocene sediments of up to 11 km thick and can be divided into three depressions and two uplifts on basis of marked different structural styles and stratigraphy.Thermal histories and subsidence were accounted for by uniform lithosphere stretching model (Mackzie) based on the studies of geology and tectonics of the Beikang basin. The results show that there is good agreement between modeled and observed present surface heat flow formed by radioactive substance in sediments and crust. The heat flow of the basin basement ranged from 45 to 66 mW/m^(2) in the end of middle Eocene, and the heat flow of the basin basement increased and ranged from 51 to 66 mW/m^(2) in the end of early Oligocene. The heat flow of the basin basement ranged from 51 to 61 mW/m^(2) now. Also the Ro of source rocks is calculated from the thermal histories with David A Wood method.The results indicate that source rock of middle Eocene is mature in the end of early Miocene and high mature-overmature now in the western and eastern depressions, and source rock of late Eocene-early Oligocene is mature in the end of late Miocene and high mature now and source rock of late Oligocene-early Miocene is mature now.