苏北盆地溱潼凹陷阜宁组二段页岩油储层裂缝形成机制及对烃类赋存的影响

Fracture formation mechanism in shale oil reservoirs,Qintong Depression,North Jiangsu Basin and its influence on hydrocarbon occurrence

苏北盆地溱潼凹陷阜二段是页岩油勘探开发的有利层段,页岩天然裂缝发育,是页岩油的重要储集空间和渗流通道,对页岩油的开采与增产有重要影响。综合利用岩心观察、成像测井、薄片观察和实验分析测试等资料,应用先进环境扫描电镜、激光扫描共聚焦显微镜等仪器,对溱潼凹陷阜二段页岩天然裂缝的成因类型、发育特征、形成机制、赋存状态和对油气赋存的影响开展了研究。溱潼凹陷阜二段天然裂缝以构造裂缝和沉积成岩裂缝为主,沉积成岩裂缝主要以层理缝为主,发育少量缝合线;构造裂缝包括穿层剪切裂缝、顺层剪切裂缝和层内张裂缝。在纵向上阜二段各小层裂缝发育特征差异较大,从Ⅰ亚段到Ⅴ亚段,裂缝发育类型逐渐单一,裂缝密度逐渐减小,裂缝的延伸长度逐渐增大,并且裂缝的填充性逐渐减弱。阜二段页岩天然裂缝受沉积成岩、构造和异常高压等多种因素作用形成复杂的缝网,沟通储层基质孔隙形成了立体的孔缝系统,为页岩油的储集提供了良好的空间,也极大地提升了储层的渗流能力。环境扫描电镜和激光扫描共聚焦显微图像显示阜二段页岩油主要以游离态和吸附态两种赋存形式存在,游离油主要以液态油滴形式赋存于孔隙和微裂缝中,吸附油以油膜形式包裹于矿物颗粒表面。裂缝中原油主要以轻质组分为主,孔隙中既有轻质组分又包含重质组分。天然裂缝发育,明显提高了页岩的储集物性,有利于后期压裂改造,是溱潼凹陷阜二段页岩油得以富集并实现高产的重要保障。

The second member of the Funing Formation in Qitong Depression,Subei Basin is a favorable section for shale oil exploration and development,where natural fractures provide important reservoir spaces and seepage channels for shale oil and have an important impact on shale oil exploitation and production increase.Based on core observation,imaging logging,thin section observation and experimental analysis,combined with advanced environmental scanning electron microscopy(SEM)and laser scanning confocal microscopy(LSCM),this paper investigates into the genetic type,development characteristics,formation mechanism and distribution and occurrence state of natural fractures and their influence on oil and gas occurrence in the study area.The natural fractures are mainly tectonic fractures-including cross-bedding shear fractures,in-bedding shear fractures and in-bedding tensile fractures-and sedimentary rock fractures-mainly bedding fractures with a few sutures.The characteristics of fracture development in the sublayers of the second member vary greatly in the vertical direction.From sublayersⅠtoⅤthe fracture development intensity decreases and fracture types reduce to a single type;the fracture density is gradually reduced;the fracture extension length is gradually increased;and the fracture filling is gradually weakened.A complex fracture network is formed under various influencing factors including sedimentary rocks,structural setting and abnormal high pressure.The fracture network communicates with the reservoir matrix pores to form a three-dimensional pore-fracture system,providing a good space for shale oil storage and greatly improves reservoir permeability.As SEM/LSCM images show shale oil in the study area occur mainly as free or adsorbed oils in the forms of liquid oil droplets in pores and microfractures or oil films wrapping around mineral particles.The crude oil in fractures is mainly composed of light components,and the pores contain both light and heavy components.The development of natural fractures obviously improves the physical properties of shale reservoir rocks and is conducive to the later fracture network reconstructure—an important guarantee of shale oil enrichment and high-production in the study area.