酒泉盆地青西凹陷下沟组混积层系致密油成藏机理与富集影响因素

碎屑岩与碳酸盐岩混合沉积层系中的致密油是一种重要的致密油类型，在中国及北美地区分布广、资源潜力大，研究混合层系中致密油成藏机理与富集影响因素具有重要意义。以酒泉盆地青西凹陷青南次凹下沟组（K₁g）混积层系为例，主要利用盆地模拟技术恢复下沟组埋藏史、热演化史及生烃史，研究异常高压形成与源储压差演化过程及相应的致密油充注孔喉下限，探讨混积层系中致密油充注渗流过程及富集影响因素。结果表明，早期高热流导致K₁g⁰烃源岩早期生烃（早白垩世末），K₁g²和K₁g³属晚期生烃（新近纪以来），K₁g¹则有早晚两期生烃过程。生油增压是青南次凹下沟组异常高压的主要因素。20 Ma以来，源储界面的剩余压力由2 MPa逐渐增大至现今的32 MPa。基于充注动力学平衡关系，计算得到源储界面充注孔喉下限可达6 nm。青南次凹下沟组混积层系致密油充注及短距离运移符合入侵逾渗方式，导致形成近源聚集和源内聚集的广泛分布的致密油。源储组合配置方式和内部构成特征是影响致密油富集程度的主要因素，源储一体、三明治型油气富集程度较下生上储型和薄互层型高。

Tight oil accumulation mechanism and controlling factors for enrichment in mixed siliciclastic and carbonate sequences in the Xiagou Formation of Qingxi Sag,Jiuquan Basin

Tight oil reservoir in mixed siliciclastic-carbonate strata is an important type of tight oil resources.This type of tight oil reservoir is widely distributed across China and North America,and is believed to have great resource potential.Study on tight oil accumulation mechanism and main controlling factors for enrichment in mixed siliciclastic-carbonate strata are of great significance.The mixed strata of the Xiagou Formation (K₁g),Qingnan subsag,Qingxi Sag,Jiuquan Basin,are the case of study.We restored its burial history,thermal evolution history and hydrocarbon generation history via basin simulation techniques,investigated the formation of abnormal overpressure,pressure difference evolution between reservoir and source rock,and corresponding lower limit of pore throat for tight oil charging,and discussed the controls on hydrocarbon charging,seepage and enrichment in the mixed strata.Results show that the timing of hydrocarbon generation varies from different members in the formation:K₁g⁰ source rocks generated hydrocarbons early at the end of Early Cretaceous due to the high heat flow value in the early stage,while source rocks of K₁g² and K₁g³ started to generate hydrocarbons late in the Neogene,and K₁g¹ source rocks generated hydrocarbons in two periods(one early and one late period).Overpressure was believed to be attributed to hydrocarbon generation.Ever since 20 Ma,the remaining pressure on the boundary of source and reservoir rocks increased from 2 MPa to 32 MPa.The lower pore throat limit for oil charging on the boundary of source and reservoir rocks is 6 nm.Tight oil charging in the mixed strata of the Xiagou Formation and its short-distance migration accord with the invasion-percolation way,resulting in the pervasive accumulation of tight oil within and nearby the source rocks.The source-reservoir configuration and their internal architectures are the main controls on tight oil enrichment:the self-sourcing reservoirs and "sandwich-type"reservoirs are more oil-enriched than the lower-source upper-reservoir ones and thinly interlayered ones.