Abstract: Deep to ultra-deep hydrocarbon resources constitute a strategic exploration frontier in central and western China. Carbonate reservoirs discovered in Tarim Basin and Sichuan Basin currently hold the record for the greatest burial depths exceeding 8 000 meters, and current exploration efforts are progressively extending toward the targets at 10 000-meter depths. Compared with sandstone reservoirs, carbonate reservoirs exhibit a deeper hydrocarbon accumulation window, as well as substantially lower porosity and permeability thresholds for hydrocarbon enrichment, suggesting a broader exploration potential. However, the dynamic mechanisms controlling the formation and distribution of hydrocarbons in carbonate reservoirs remain poorly understood. Based on independently designed physical simulation experiments, the grain-size ratio (R/r) between reservoir particles (R) and source-rock particles (r) was employed to qualify the relative magnitude of capillary pressure differentials. Under identical experimental conditions, simulations were performed on the variations in oil content when oil-saturated source-rock layers contact water-saturated carbonate and clastic reservoirs. From the perspective of capillary pressure differentials, the differences and interrelationships in the dynamic mechanisms of hydrocarbon accumulation between the two reservoir types were elucidated, thereby further clarifying the distribution patterns of carbonate and clastic hydrocarbon reservoirs within hydrocarbon-bearing basins. The results show as follows. (1) Capillary pressure differential serves as a key driving force for hydrocarbon accumulation. In both carbonate and clastic reservoirs, hydrocarbon accumulation occurs only when the grain-size ratio between the reservoir and source rock particles exceeds a critical threshold (R/r≥2), and oil saturation increases progressively with increasing R/r. (2) At a constant grain-size ratio, oil saturation in the reservoir increases with decreasing reservoir grain size. Owing to their larger contact angles, carbonate rocks exhibit lower capillary resistance and a larger capillary pressure differential between the source rock and the reservoir, thereby resulting in higher oil saturation. (3) Carbonate rocks exhibit lower capillary resistance and a larger capillary pressure differential driving hydrocarbon accumulation, as compared to clastic systems at equivalent grain sizes. This indicates both deeper buoyancy-driven entrapment thresholds and lower hydrocarbon accumulation limits. Consequently, in carbonate-dominated basins, conventional hydrocarbon resources account for the majority of total recoverable reserves, exhibiting a broader hydrocarbon distribution.

Key words: sandstone, carbonate rock, physical simulation, hydrocarbon accumulation dynamics, dynamic mechanism, deep to ultra-deep layers, exploration field

摘要： 深层—超深层油气资源是中国中西部油气勘探的重要领域,塔里木盆地和四川盆地已发现的碳酸盐岩油气藏的最大埋深超过8 000 m,目前正在向万米领域拓展。碳酸盐岩储层的油气成藏底限较砂岩储层更深,富集油气的孔隙度和渗透率下限远较砂岩储层低,展示出更加广阔的勘探前景,但其形成分布的动力机制尚不清晰。基于自主设计的物理模拟实验,使用储集体颗粒粒径(R)与烃源岩颗粒粒径(r)的粒径比(R/r)反映毛细管压力差的相对大小,模拟了相同条件下饱含油的烃源岩层在与饱含水的碳酸盐岩和碎屑岩接触过程中的含油性变化,从毛细管压力差的角度揭示了2类储层在油气成藏动力机理上的差异性和关联性,进而阐述了碳酸盐岩油气藏和碎屑岩油气藏在含油气盆地中的分布规律。研究结果表明:①毛细管压力差是一种重要的驱动力,对于碳酸盐岩储层和碎屑岩储层而言,均表现为当储层与源岩间的粒径比超过临界条件(R/r≥2)时才能聚集油气,且粒径比越大含油饱和度越大;②粒径比相同时,储集体中的含油饱和度随储集体颗粒粒径的减小而增大,其中,碳酸盐岩因其颗粒的润湿角更大而具有更小的毛细管阻力,更大的源-储间毛细管压力差,因而含油饱和度更高;③碳酸盐岩在相同颗粒粒径下的毛细管阻力更小且聚油气的毛细管压力差动力更强,分别指示其具有更深的浮力成藏下限和油气成藏底限,这意味着在以碳酸盐岩地层为主的盆地中,常规油气资源的占比相对更高且整体的油气分布范围更广阔。

关键词: 砂岩, 碳酸盐岩, 物理模拟, 成藏动力, 动力机制, 深层—超深层, 勘探领域