The Cogollo Group and the oceanic anoxic events 1a and 1b, Maracaibo basin, Venezuela

ABSTRACTCarbonates of Cogollo Group (Apón, Lisure and Maraca formations) constitute the broader calcareous platform system originated during Aptian and Albian of Cretaceous in north-western South America, Maracaibo Basin, Venezuela. On the shallow shelf, a variety of calcareous sedimentary facies were deposited during marine transgressive and regressive cycles. Some of them developed porosity and constitute important hydrocarbon reservoirs. Due to some major marine transgressions, from early Aptian, the anoxic environment and characteristic facies of a pelagic environment moved from the outer slope and basin to the shallow shelf, during specific time intervals, favouring the sedimentation of organic matter-rich facies, which correspond to the oceanic anoxic events (OAEs) 1a and 1b. The source rock of Machiques Member (Apón Formation) was deposited during early Aptian OAE 1a (~ 120 Ma). The source rock of Piché Member, located at the top of the Apón Formation, was deposited during late Aptian OAE 1b (~ 113 Ma). Finally, La Luna Formation, from Cenomanian, that covers the OAE 2 (~ 93 Ma), represents the most important source rock in the Maracaibo Basin. In this way and based on sedimentological and organic geochemistry results from the determinations performed on 247 samples belonging to six cores in the Maracaibo Basin, we propose these two organic-rich levels, deposited on the shallow shelf of the Cogollo Group, as "effective source rocks", additional to La Luna Formation, with oil migration in relatively small distances to the porosity facies.

摘要：委内瑞拉西北部马拉开波盆地的科戈洛组（Cogollo Group）碳酸盐岩（涵盖阿蓬组、利索雷组与马拉卡组（Apón, Lisure and Maraca formations）），构成了白垩纪阿普第期至阿尔布期形成的广域碳酸盐台地系统。在浅海台地上，海侵与海退旋回期沉积了多种碳酸盐沉积相，其中部分沉积相发育孔隙度，成为重要的油气储层。受早阿普第期以来的多期大规模海侵影响，在特定地质时段内，远洋环境的缺氧环境与特征沉积相从外斜坡与盆地向浅海台地迁移，促进了富有机质沉积相的沉积，该类沉积相对应大洋缺氧事件（Oceanic Anoxic Events，OAEs）1a与1b。阿蓬组马基克斯段（Machiques Member）的烃源岩（source rock）形成于早阿普第期的大洋缺氧事件1a（约120 Ma）；位于阿蓬组顶部的皮谢段（Piché Member）烃源岩则形成于晚阿普第期的大洋缺氧事件1b（约113 Ma）。最后，覆盖大洋缺氧事件2（约93 Ma）的森诺曼期拉卢纳组（La Luna Formation），是马拉开波盆地最为重要的烃源岩。本研究基于对马拉开波盆地6个岩心的247个样品开展的沉积学与有机地球化学测定结果，提出除拉卢纳组之外，科戈洛组浅海台地上沉积的这两套富有机质层位同样为"有效烃源岩"，其油气可通过相对短距离运移至孔隙相储层中。