CHARACTERISTICS AND EXPLORATION POTENTIAL OF LACUSTRINE TURBIDITIC SANDSTONE IN LOWER CONGO BASIN OF WEST AFRICA
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摘要: 下刚果盆地在古近系和新近系海底扇砂岩中发现了大量油气，其典型的海底扇沉积特征已被熟知。然而，近年来的勘探实践证实，在下刚果—刚果扇盆地盐下下白垩统Djeno组也发育重力流形成的厚层湖相浊积岩，该套湖相浊积复合体的厚度较大，可达370~1 280 m。由古生物和岩性特征推测，研究区在Djeno时期为深湖相沉积环境。从砂岩矿物组分分析认为，Djeno组浊积砂岩为岩屑杂砂岩—长石质岩屑杂砂岩，以岩屑杂砂岩为主。根据粒度概率曲线和C-M图，推测该套浊积复合体砂岩为重力流沉积作用下的产物，提出了湖相大型浊积体的发育模式，并在其内部识别出多套完整和不完整的鲍玛层序。通过物性分析，认为鲍玛层序的A、B、C段砂岩物性最好，是优质储层发育层段。基于在下刚果—刚果扇盆地盐下Djeno时期识别出的大型湖相浊积复合体和优质储层段，为该盆地盐下Djeno砂岩未来的勘探开发指出了方向。Abstract: Substantial oil and gas have been discovered in the Tertiary submarine fan in the Lower Congo Basin. However, large lacustrine turbiditic sandstones have been discovered by oil exploration wells in the pre-salt Lower Cretaceous Djeno Formation of the Lower Congo-Congo Basin in the past few years. The lacustrine turbiditic complex is quite thick, ranging from 370 to 1289 meters. The Djeno Formation was deposited in a deep lake as ostracod assemblages and lithology demonstrate. It varies from lithic greywacke to feldspar lithic greywacke dominated by the former. Both grain size probability curve and C-M plot suggest that the Djeno lithic greywacke-feldspar lithic greywacke is a kind of gravity flow deposited in a deep-water lake. The deposition model of the Djeno lacustrine turbiditic sandstone is discribed in the paper. Both complete and incomplete Bouma Sequences are identified from the core data of the Upper Djeno. High quality reservoir sandstones are well developed in the A and B and C members of the Bouma Sequence. They are favorable targets and focuses in the future exploration and development.
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