Characteristics of a sublacustrine fan in a half-graben rift lake basin and its petroleum prospects: Case study on the second member of the Tenggeer Formation, Saihantala Sag, Erlian Basin

Abstract

Analysis has been carried out on the petrology, context and structure of sublacustrine fan sand bodies in the second member of the Tenggeer Formation. The sublacustrine fans are well developed, and are characterized by the Bouma sequence resulting from typical turbidite deposition and the presence of AAA and ABABAB sequences of non-typical turbidite systems. The depositional textures feature grain distributions typical of turbidite deposits and contain abundant turbidite deposit structures. The sublacustrine fans mainly develop in the lower part of the fault slope break formed by syn-sedimentary faulting in the fan delta front of the gentle slope in the sag and in the troughs in front of offshore subsea fan deposits in the down-thrown areas at boundary of the basin, controlled by an abrupt slope change. The formation of the sublacustrine fan is closely related to the long period of syn-sedimentary faulting activities, and the scale is controlled by the fault activity, slope gradient of palaeogeomorphology and the sediment supply. The sublacustrine fan sand bodies are adjacent to source rocks, and easily form lithologic traps and are favorable for petroleum migration and accumulation, which is significant for the petroleum prospecting in this area. 摘要: 以二连盆地赛汉塔拉凹陷为例, 通过对腾格尔组二段湖底扇砂岩体的岩石学、结构、构造等特征的分析, 认为赛汉塔拉凹陷湖底扇比较发育, 具有典型浊流沉积所形成的鲍马序列以及非典型浊积岩“AAA”、“ABABAB”序等沉积特征, 在沉积结构上具有典型浊流沉积的粒度分布特征, 并具有丰富的浊流沉积构造现象。湖底扇主要发育在凹陷缓坡带扇三角洲前缘同沉积断裂形成的断裂坡折带下部, 以及陡坡带控盆边界断裂下降盘的近岸水下扇沉积前端的洼槽区。湖底扇的形成与同沉积断裂的长期活动密切相关, 规模大小受断裂活动、古地貌坡度、物源供应等因素控制。湖底扇砂岩体具有毗邻烃源岩, 易形成岩性圈闭, 油气运移和聚集条件好等优越条件, 对油气勘探具有重要的意义。 Key words: half-graben rift lake basin, sublacustrine fan, depositional characteristics, depositional model, petroleum prospecting, Saihantala Sag