Study on the pore structure characteristics of interbedded shale oil and formation mechanisms of high-quality shale oil reservoirs in the Chang 7 Member, Yanchang Formation, Ansai Oilfield
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摘要:
鄂尔多斯盆地安塞油田面临常规油气资源潜力匮乏、储量接替难度大的难题，目前增储的主要目标为页岩油储层。延长组7段（长7段）作为安塞油田的关键开发层系，其孔隙结构直接控制着储层质量，进而对页岩油开发产生显著影响。以安塞油田长7段夹层型页岩油储层为研究对象，通过开展扫描电镜、铸体薄片、低温氮气吸附、高压压汞和核磁共振等实验，明确不同尺寸孔隙对于储层品质的影响，并从沉积环境与成岩作用的角度揭示储层成因。研究结果表明，储层孔隙类型主要有长石溶孔、残余粒间孔、粒间溶孔、黏土晶间孔以及少量的微裂缝；其中长石溶孔主要发育微米级孔隙，而黏土晶间孔以纳米级孔隙为主。储层整体排驱压力较高，进汞饱和度较低，孔喉半径以纳米级为主，孔径在500 μm以下的孔隙多呈开放型的平行板状和狭缝形，同时发育少量的墨水瓶型孔隙。储层孔径主要小于300 μm，且随着物性变好，储层内较大尺寸的孔隙占比也在逐渐增大。优质储层的成因主要划分为2类：东北方向靠近物源，水动力强，岩石颗粒分选性好，储层内容易发育绿泥石膜，而绿泥石膜可以保护颗粒间的原生粒间孔，压实作用后可保留较多的残余粒间孔而形成优质储层；西南方向远离物源方向，水体深度增加，水动力变弱，因靠近烃源岩发育区，储层容易被烃源岩产生的有机酸溶蚀改造，发育较多溶蚀孔隙，从而形成优质储层。研究成果可为安塞油田页岩油藏的高效勘探开发提供支持。
Abstract:Objective As a key producing horizon of the Ansai Oilfield in the Ordos Basin , the pore structure of Chang 7 Member of the Yanchang Formation directly controls reservoir quality, and consequently influences shale oil productivity. The Ansai Oilfield is facing depleted conventional resource and difficult reserve replacement, making shale oil reservoirs the main target for reserve growth. Therefore, characterizing the pore structure and constraining the genesis of different reservoirs is of great significance for oilfield exploration and development.
Methods Targeting the interbedded shale oil reservoirs in the Chang 7 Member of the Ansai Oilfield, we carried out experiments including scanning electron microscopy, casting thin sections, low-temperature nitrogen adsorption, high-pressure mercury intrusion, and nuclear magnetic resonance, to identify the influence of pore size on the quality of the reservoirs, and to reveal the genesis of different reservoirs from the perspectives of depositional environment and diagenesis.
Results The reservoir pores are predominantly composed of feldspar pores, residual intergranular pores, intergranular pores, clay intergranular pores, and a small number of microcracks. The feldspar pores are mainly micrometer-sized, while clay intergranular pores are dominantly nanometer-sized. The reservoir exhibits relatively high discharge pressures and low mercury injection saturation, with pore-throat radii predominantly at the nanometer scale. Most pores with diameters below 500 μm are open-type parallel plate-shaped and slit-shaped, with a small number of ink-bottle-type pores also developed. The pore sizes in the reservoir are predominantly below 300 μm, and as physical properties improve, the proportion of larger pores gradually increases.
Conclusion The genesis of high-quality reservoirs can be categorized into two types. In the northeast, closer to the provenance area, strong hydrodynamic conditions lead to better sorting of rock particles, facilitating the development of chlorite coatings within the reservoir. These chlorite coatings can protect primary intergranular pores between particles, allowing more residual intergranular pores to be preserved after compaction, thus forming high-quality reservoirs. In contrast, the southwest area, being farther from the provenance, exhibits increasing water depth and weaker hydrodynamics. Due to its proximity to the source rock development zone, the reservoir is more susceptible to dissolution of organic acids from hydrocarbon source rocks, leading to the formation of numerous dissolution pores and the development of high-quality reservoirs. [Significance] The study can support the efficient exploration and development of shale reservoirs in the region.
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表 1 安塞油田长7段致密砂岩样品信息
Table 1. Information on tight sandstone samples from the Chang 7 Member in the Ansai Oilfield
序号 样品编号 井号 深度/m 层位 岩性 孔隙度/% 渗透率/mD 1 S1 D199 1324.95 长71 长石细砂岩 10.681 0.0498 2 S2 D199 1339.20 长72 长石中砂岩 10.428 0.0618 3 S3 D199 1357.33 长72 长石细—中砂岩 9.545 0.0653 4 S4 D199 1383.20 长73 长石细砂岩 5.599 0.0265 5 S5 D199 1399.25 长73 长石细砂岩 5.649 0.0284 6 S6 H15 1644.17 长73 长石细砂岩 3.662 0.0377 7 S7 H15 1654.05 长73 长石细—中砂岩 10.947 0.0457 8 S8 H15 1672.50 长73 长石细砂岩 7.629 0.0195 9 S9 H15 1685.49 长73 长石细—中砂岩 / / 10 S10 Q129 1622.46 长72 长石细—中砂岩 5.683 0.0844 11 S11 Q129 1666.88 长73 长石细砂岩 5.971 0.0995 12 S12 Q129 1667.94 长73 长石细砂岩 6.944 0.1015 13 S13 W538 1620.56 长72 长石细—中砂岩 12.165 0.1367 14 S14 W538 1623.66 长72 长石细砂岩 5.556 0.0469 15 S15 W538 1625.00 长72 长石细砂岩 4.804 0.0227 16 S16 W538 1625.75 长72 长石中砂岩 / / 17 S17 W538 1626.10 长72 长石中砂岩 6.760 0.0225 
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