DIFFERENCE ANALYSIS OF HYDROCARBON GENERATION IN THE SOUTHERN PART OF THE WESTERN CONTINENTAL MARGIN OF THE SOUTH CHINA SEA
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摘要:
南海西部大陆边缘南段发育万安盆地和湄公盆地，二者仅以昆仑隆起相隔，万安盆地以生气为主，湄公盆地以生油为主，造成二者油气差异性的原因有待探究。在已有的油气勘探资料和现有认识的基础上，通过对湄公盆地和万安盆地的热流背景、构造格局、沉积环境和生烃母质等特征进行剖析和类比，探讨2个盆地油气形成差异性的原因。研究认为，造成二者油气差异性的主要原因在于热流背景、构造格局、沉积环境以及生烃母质的不同。受断裂活动影响，万安盆地地温梯度和大地热流值均高于湄公盆地。万安盆地主力烃源岩时期是渐新世—早中新世，主要发育断拗型盆地湖沼-海湾相沉积，渐新世生烃母质主要为湖沼环境的藻类、水生植物以及高等植物的混源，早中新世生烃母质主要来自红树林等陆源高等植物，以Ⅱ2—Ⅲ型干酪根为主，生气为主。湄公盆地主力烃源岩时期为渐新世，主要发育断陷型盆地湖相沉积，生烃母质主要来源于中深湖相的藻类和水生植物，以Ⅰ—Ⅱ型干酪根为主，生油为主。因此，断裂活动的差异控制了热流背景，构造格局的差异控制了沉积环境，沉积环境的差异控制了生烃母质，生烃母质的差异控制了油气类型。
Abstract:Two basins, the Wan'an and Mekong basins, occur in the southern part of the western continental margin of the South China Sea (SCS), separated by a swell called Con Son. The former contains natural gas, whereas the latter produces oil. In order to study the cause of the difference as such, heat flow background, structural pattern, sedimentary environment and hydrocarbon kerogen of the two basins are reexamined and compared in this work, based on existing geologic data and references. Research results indicate that in addition to the differences in contained hydrocarbon, there are differences in heat flow, structural pattern, sedimentary environment and hydrocarbon kerogen as well between the two basins. Affected by fault activities, the geothermal gradient and heat flow value of the Wan'an Basin are obviously higher than that of the Mekong Basin. In the Wan'an Basin, the major source rocks include the Oligocene to Early Miocene lacustrine-estuarine sediments deposited in a fault-depressed basin, and the kerogen of the Oligocene source rock mainly comes from lacustrine algae, aquatic plants and some higher plants, while the kerogen of Early Miocene mainly from terrestrial higher plants such as mangrove forest, dominated by type Ⅱ2-Ⅲ of kerogen, which largely generates natural gas. In the Mekong Basin, however, the major source rock deposited in Oligocene is dominated by lacustrine sediment of rift type, with hydrocarbon generation material coming mostly from algae and aquatic plants, and the kerogen of type Ⅰ-Ⅱ that generates mainly oil. Therefore, it is concluded that the difference in structural pattern controls the sedimentary environment, the difference in sedimentary environment controls the hydrocarbon generation material, and the difference in hydrocarbon generation material controls the type of hydrocarbon generated.
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