Abstract: Following the discovery of Bayan oilfield with substantial hydrocarbon accumulations in deep strata (3 300-5 800 m)of Linhe depression of Hetao Basin, significant discoveries have been made in ultra-deep clastic strata (deeper than 6 000 m). However, the mechanisms governing hydrocarbon accumulation and the enrichment patterns remain unclear. Drawing on the latest drilling, mud-logging, wireline-logging, and geological laboratory data, this study systematically investigates the tectono-sedimentary evolution of the Paleogene Linhe Formation in Linhe depression and the principal controls on ultra-deep hydrocarbon accumulation and enrichment patterns. The results show as follows. (1)Linhe depression was an asymmetric fault depression with a steep western flank and a gentle eastern flank. The large shallow-water braided river delta to saline lacustrine deposition system predominated in Paleogene Linhe Formation. In terms of sequence stratigraphy, the succession can be subdivided into lowstand, lacustrine transgressive, and highstand system tracts. Overall, it is characterized by a gentle-slope, shallow-water sedimentary setting. (2)Deposition of Linhe Formation predominantly occurred under semi-humid to semi-arid evaporative lacustrine conditions, favouring the development of high-quality saline-lacustrine source rocks characterized by early maturation, early expulsion, and high hydrocarbon yields. Late-stage rapid deep burial accelerated source-rock maturation and triggered explosive hydrocarbon generation over a short period, providing a substantial hydrocarbon supply for reservoir formation in the subsag areas. (3)Reservoir development in Linhe Formation was primarily controlled by weak diagenesis, with early hydrocarbon charging and ultra-high overpressure preserving porosity and enhancing permeability. Effective reservoirs are predicted to exist at burial depths exceeding 8 000 m. (4)Late-stage intense extension and strike-slip deformation gave rise to multiple favorable structural belts in the central-northern Linhe depression. Several hydrocarbon accumulation models are proposed:strong sealing and ultra-high overpressure accumulation model characterized by intra-source evaporate involvement and down-dip detachment; intra-source enrichment model characterized by strong hydrocarbon supply and strong lateral sealing by antithetic faults; and high-point enrichment model characterized by near-source limited hydrocarbon supply and irregular lateral sealing by antithetic faults. These models provide a strong basis for continued large-scale discoveries in Linhe depression. The improved understanding of ultra-deep hydrocarbon accumulation and the significant breakthrough in risk exploration in Linhe depression provide valuable insights for ultra-deep clastic reservoir exploration in comparable basins.

Key words: Hetao Basin, Linhe depression, Paleogene Linhe Formation, ultra-deep clastic rocks, reservoir formation mechanism, accumulation model

摘要： 河套盆地临河坳陷自其在深层（3 300～5 800 m）发现规模富集的巴彦油田后，尽管在超深层（＞6 000 m）碎屑岩领域也取得了重要勘探发现，但其油气成藏机制与富集规律尚不明朗。基于最新钻井、录井、测井和地质实验等基础资料，针对临河坳陷古近系临河组的构造-沉积演化特征、超深层油气成藏主控因素与富集规律开展了深入研究。研究结果表明：①临河坳陷古近纪为西陡东缓非对称的断坳型沉积坳陷，其临河组主要发育大型浅水辫状河三角洲—盐湖沉积体系，层序上可划分为低位域、湖侵域、高位域，整体具有坡缓、浅水的沉积特征；②临河组沉积期，临河坳陷主要为半湿润—半干旱蒸发环境，发育咸湖相优质烃源岩，具有早熟早排、高产烃率的特征，同时，晚期快速深埋使得烃源岩在短期内快速熟化并爆发式生烃，为临河坳陷近洼槽区的油气藏形成提供了重要资源基础；③临河组具有“弱成岩主控、油气早充注和超高压保孔增渗”的成储机理，预测其有效储层埋深超过8 000 m；④晚期强伸展与走滑改造使得临河坳陷中—北部发育多排有利构造，“源内膏盐岩卷入+顺向滑脱强封闭超高压型”、“源内强供烃+反向断层强侧封源内富集型”和“远源弱供烃+反向断层侧向随机封挡高点富集型”等多种油气成藏模式的提出可为临河坳陷持续规模发现提供有力支撑。临河坳陷超深层油气成藏认识的深化与风险勘探的重大突破对同类盆地超深层碎屑岩油气勘探具有重要借鉴意义。

关键词: 河套盆地, 临河坳陷, 古近系临河组, 超深层碎屑岩, 成储机理, 成藏模式