鄂尔多斯东南部延长组致密油储层微观特征及主控因素

通过岩心观察、薄片鉴定、测井、扫描电镜和恒速压汞实验等资料分析,利用储层地质学等理论技术和知识,对鄂尔多斯盆地东南部延长组致密油储层的岩石结构和组分、孔喉结构和类型、孔渗分布等微观特征及其相关性和主控因素进行研究。延长组致密油储层以细粒长石砂岩为主,岩石结构和成分成熟度较低;平均孔隙半径为10μm且孔隙连通性差,孔隙以粒间孔、长石溶孔和裂缝为主,平均喉道半径为0.41μm,喉道以微细-微喉道为主。孔喉结构的非均质性决定了致密油储层的储集和渗流能力,压汞曲线表现出高排驱压力、中值半径偏小、细歪度、分选中等、退汞效率低的特征显示研究区致密油储层储集能力较强,而渗流能力较差;孔隙度均值为8.5%,渗透率均值为0.69 m D,孔隙度和渗透率表现为线性正相关(R2=0.389),微裂缝的存在使部分样品表现出低孔高渗的特征,储层总体表现为特低孔、超低渗、非均质性强的致密性特点。通过相关性分析,岩石组分主要影响储层的孔隙度,岩石组分中(铁)方解石与孔隙度相关性最好;喉道主要影响储层的渗透率,压汞实验分析表明孔喉大小、分布和连通性特征参数中的孔隙体积、分选系数、排驱压力与渗透率的相关性最好,相关系数均大于0.9,沉积作用(埋深、沉积微相)、成岩作用(压实、胶结和溶蚀)、构造作用等是研究区储层特征的主要控制因素,其中水下分流河道的储层物性最好,是研究区"甜点"储层的指向区;压实作用对储层致密性的贡献大于胶结作用,酸性流体大量生成时储层已在压实和胶结作用下接近致密,因此无法与长石等易溶组分充分接触而发生大规模的次生溶蚀;此外,后期构造运动生成的裂缝由于缺乏酸性流体而无次生溶蚀孔隙生成,储层的致密性无法得到根本性的改善。

Microscopic characteristics and main controlling factors of tight oil reservoir of Yanchang Formation in southeastern Ordos Basin, China

We studied the microscopic characteristics and main controlling factors as well as their correlations in tight oil reservoirs of southeastern Ordos Basin, including structures and compositions of rocks, structure and types of pore throats, and porosity and permeability distributions. The methods include analysis of thin sections and core samples, logging well, X-ray diffractometry, scanning electron microscopy (SEM), constant rate mercury injection experiment. The main rock type in tight oil reservoirs of Yanchang Formation is fine arkose. Tight sandstone is found with low compositional maturity and textural maturity. Average pore radius is 10 um and the pore connectivity is poor. Main pore types are intergranular pores, feldspar dissolution pores and fractures. Average throat radius is 0.41 um, and main throat type is micro-throat. Heterogeneity of pore throat structure is the main factor determining the reservoir and flow capacity of tight oil reservoir of Yanchang Formation. Intrusive mercury curves indicate relatively high displacement pressure, low throat radius, moderate to good sorting, and low efficiency of mercury ejection. Which indicates the storage capability of reservoir is strong, while the seepage capability of reservoir is weak. The overall characteristics of extra-low porosity (8.5%), extra-low permeability (0.69 mD) and strong microscopic heterogeneity represent typical tight oil reservoir. There is a general trend (correlation coefficient: R2=0.389) of higher permeability with increased porosity. There are some samples characterized by low porosity but high permeability, possibly reflecting wide differences in dominant pore types and varying contributions of micro-fracture flow. Correlation analysis shows the porosity of tight oil reservoir is mainly affected by rock composition and pores and is most correlated with (iron) calcite. The permeability of tight oil reservoir is mainly affected by throats and most correlated with pore volume, sorting coefficient and displacement pressure. The main controlling factors of tight oil reservoir of Yanchang Formation include sedimentation (burial depth, sedimentary micro-facies), diagenesis (compaction, cementation, dissolution) and tectonization. The physical properties of underwater distributary are best and they are "dessert point" areas of the reservoir in the study area. The contribution of tight reservoir was greater due to compaction than to cementation. The reservoir had been became tight under the compaction and cementation when a large amount acid fluid entered the reservoir, so there are no mass dissolution in the reservoir. Furthermore, subsequent tectonization produced fractures, but the secondary pores could not be formed due to the lack of acid fluid, so the reservoir is still very tight in the southeastern Ordos Basin.