Abstract: The Member 6 of Triassic Yanchang Formation is one of the key contributors to the oil reserves and crude oil production in Ordos Basin, and it is also the earliest oil production reservoir in the continental areas of China. This study aims at revealing the formation mechanism and distribution pattern of tight oil in the Member 6 of Yanchang Formation. Based on analyzing the data of drilling, mud logging, well logging, and core samples from 1 505 prospecting wells in southeastern Ordos Basin, the paper systematically studies the formation mechanism and enrichment laws of tight oil in the Member 6 of Yanchang Formation, and presents three palaeo-geomorphic units, including steep and gentle paleoslopes, and paleolake bottoms, as well as more than ten types of secondary palaeo-geomorphic units. Based on the comprehensive analysis of palaeo-geomorphology and hydrocarbon accumulation conditions, in combination with the physical simulation experiments of crude oil migration and accumulation, four typical accumulation assemblages have been identified, i.e., the dual-source hydrocarbon supply with the medium to poor reservoirs at the end of distributary channels on gentle slopes, the single-source hydrocarbon supply with the poor to medium reservoirs under scattered gravity flow at the lake bottom, the dual-source hydrocarbon supply with the medium to good reservoir assemblages under gravity flow at the bottom of steep slopes, and the single-source hydrocarbon supply with the damaged and adjusted good to medium reservoirs on steep slopes. The genesis mechanism and accumulation process of tight oil reservoirs in delta front and gravity flow are revealed. Then it is clarified that the scale of high-quality reservoirs in near-source effective traps is the key to the enrichment and high yield of tight oil. Further, the paper proposes the differential distribution and accumulation mode of tight oil be controlled by the heterogeneous source-reservoir-cap assemblages under diverse paleogeomorphology setting, improves the hydrocarbon exploration strategies applied in typical oilfields, and puts forwards the unconventional oil exploration strategies such as the expanding exploration along the bottom of steep slopes, three-dimensional exploration at the lake bottom, detailed exploration of multiple wells in gentle slopes, and effective traps exploration in the structurally adjusted areas of steep slopes. Finally, the research focuses on exploring the four basic geological conditions for the formation of large-scale continental tight oil fields, i.e., tectonic action, provenance, paleo-geomorphic units, and preservation condition. In the Member 6 of Yanchang Formation in southeastern Ordos Basin, the cumulative geological reserves of tight oil amount to 1.885 522×10⁸t, the cumulative crude oil production in the past three years is 16.72×10⁴t, and the first 100 million-ton-level integrated tight oil field, i.e., Huangling oilfield, has been discovered and established. It is expected that the research results have a positive impact on the development of geological theories in terms of unconventional petroleum accumulation, as well as the exploration and development practices in the terrestrial lacustrine basin of China.

Key words: Ordos Basin, Member 6 of Yanchang Formation, tight oil, formation mechanism, distribution pattern, accumulation mode, exploration strategy

摘要： 三叠系延长组6段(长6段)是鄂尔多斯盆地石油储量与原油产量的主力贡献层段之一,也是中国陆上最早进行石油开发的层段。为揭示鄂尔多斯盆地东南部长6段致密油的形成与分布规律,基于1 505口探井的钻井、录井、测井及岩心样品等资料分析,系统研究了致密油的形成机理与富集规律,刻画出古陡坡、古缓坡和古湖底3种古地貌单元以及坡底、沟道、洼地等10余种次级古地貌单元,建立了"二坡一底、南北分区"的新构造模型。综合古地貌、成藏条件与原油运聚物理模拟实验,明确了缓坡双源供烃+分流河道末端中—差储层型、湖底单源供烃+零星重力流差—中储层型、陡坡坡底双源供烃+重力流中—优储层堆积型、陡坡单源供烃+优—中储层破坏调整型4种结构差异的成藏组合,揭示了三角洲前缘型致密油藏和重力流型致密油藏的成因机理与过程,明确近源有效圈闭中优质储层规模是致密油富集高产的关键。提出多元古地貌背景下异构的生-储-盖组合控制了致密油的差异分布和富集成藏模式,优化了典型油区的勘探对策,提出陡坡坡底扩边勘探、湖底立体兼探、缓坡多井型精细勘探和陡坡构造调整区有效圈闭勘探等非常规石油勘探对策,探讨了大型陆相致密油田形成的4项基本地质条件(构造作用、物源、古地貌单元和保存条件)。鄂尔多斯盆地东南部长6段累计落实致密油地质储量为1.885 522×10⁸t,近3年生产的原油量累计为16.72×10⁴t,发现并建成长6段首个亿吨级整装致密油田——黄陵油田。研究成果以期对中国陆相湖盆的非常规石油成藏地质理论发展与勘探开发实践起到促进作用。

关键词: 鄂尔多斯盆地, 延长组6段, 致密油, 形成机理, 分布规律, 成藏模式, 勘探对策