优质烃源岩在鄂尔多斯低渗透富油盆地形成中的关键作用

鄂尔多斯盆地中生界低渗透油藏分布广泛,主要分布于延长组各油层组,其中长6、长8、长4＋5为主力油层组。储集岩热解资料显示,湖盆中部长7、长6油层组广泛分布的致密深水砂岩普遍具有良好的含油性,普遍成藏特征明显,呈现出“满凹含油”的低渗透富油盆地特征。恒速压汞、启动压力梯度和石油充注成藏试验结果显示,延长组低渗透—致密砂岩储层的喉道细小,毛细管阻力大,加之地层平缓,使得储层中油水重力分异困难。石油运聚表现为“被动”的动力学属性,石油充注与大规模运聚成藏不仅需要较强的动力,而且富烃流体要更有利于成藏富集。大范围分布的长7优质烃源岩具有生排烃规模大、生烃增压作用十分强烈（提供石油运聚的主要动力）、高效排烃（提供大规模的富烃流体）等特征,因而可满足低渗透—致密砂岩大规模成藏富集所需的动力和油源条件,在中生界低渗透富油储层形成中起着关键作用。

Key Role of High-quality Source Rocks on the Formation of Low-permeability Oil-rich Reservoirs in Ordos Basin

Mesozoic low-permeability oil reservoirs are extensively distributed in Ordos Basin, mainly in all members of Yanchang Formation, and Chang-6, Chang-8 and Chang-4＋ 5 are the dominating reservoirs. The data from pyrolysis of reservoir rocks show that the oil-rieh features of deep water facies compact sandstone bodies of Chang-7 and Chang-6 in the central part of lake basin are favorable, oil reservoirs are universal in these members, which enable the lake basin to be oil-rich and low-permeability at full basin scale. Rate-controlled mercury, startup pressure gradient and oil infilling tests for reservoir show that gravitational differentiation of oil and water in reservoirs is very difficult, because the sandstone reservoirs of Yanchang Formation are lowpermeability and tight, and consist of tiny migration channels which show high capillary resistance, and the stratum is only relatively slowly inclined. Oil migration and accumulation appear to be passive in terms of dynamic attribute, since oil infilling and large-scale migration, accumulation and reservoir formation require strong dynamics as well as hydrocarbon-bearing fluid. Chang-7 high-quality source rock, which is distributed extensively on large scale in the basin, is capable of large-scale hydrocarbon generation and expulsion, strong hydrocarbon- generating pressurization and efficient hydrocarbon expulsion. Hydrocarbon-generating pressurization provides the power of oil migration and accumulation, and hydrocarbon expulsion provides large-scale hydrocarbon-bearing fluid. So, it can satisfy the dynamics and oil-source conditions as required for formation of large-scale low-permeability and tight sandstone reservoirs, and play a key role in formation of Mesozoic low-permeability and oil-rich reservoirs in Ordos Basin.