北部湾盆地乌石凹陷低阻油层微观成因机理

北部湾盆地乌石凹陷古近系流沙港组砂砾岩油藏电阻率差异较大，高、低阻油层并存，流体性质难以识别，给油田开发带来一定难度。为此，利用测丼、录井、试井、岩芯分析化验等资料，对低阻油层成因机理开展系统深入的剖析。结果表明，泥浆侵入、导电矿物、地层水矿化度等对电阻率影响较小，高束缚水饱和度是油层电阻率偏低的主要影响因素。在此认识基础上，通过微观孔隙结构的精细评价，发现低阻油层储集空间为原生粒间孔、次生铸模孔混合孔隙组合，发育片状、弯片状喉道，排驱压力大，孔喉半径小，孔隙结构复杂是储层高束缚水饱和度的根本原因。

Microscopic Formation Mechanism of Low Resistivity Oil Layers in the Wushi Sag of the Beibu Gulf Basin

The conglomerate sandstone oil reservoir from the Paleogene Liushagang Formation in the Wushi Sag of the Beibu Gulf Basin exhibits significant resistivity differences. High resistivity and low resistivity oil layers co-exist; they can be hardly distinguished based on their fluid properties, leading to difficulties in oil field exploitation. Thus, we conducted in-depth investigation of the formation mechanism of low resistivity oil layers using data from well logging, mud logging, well testing, and core analyses. The results show that mud intrusion, conductive minerals, and mineralization of formation water slightly influence resistivity, while irreducible water saturation is the major factor leading to the formation of oil layers with relatively low electrical resistivity. Based on this knowledge and detailed assessments of the microscopic pore structures, we found that the enrichment and accumulation of low resistivity oil layers results from a combination of primary intergranular pores and secondary mold pores. The development of planar and curved throats, large displacement pressures, small pore throat radii, and complex pore structures are the fundamental causes of irreducible water saturation in the reservoir layers.