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鄂尔多斯盆地西南缘浅埋藏储层致密化成因
张英男1,白青林2,束青林3,路智勇4,张戈2,窦祥骥5,李真2,何之明2
(1.青岛理工大学土木工程学院,山东青岛 266520;2.中国石化胜利油田分公司现河采油厂,山东东营 257068;3.中国石化胜利油田分公司,山东东营 257061;4.中国石化江汉油田分公司,湖北潜江 433100;5.常州大学石油与天然气工程学院,江苏常州 213164)
摘要:
鄂尔多斯盆地西南缘镇原地区三叠系延长组8段细粒砂岩在较浅的埋藏条件下便发生了致密化。综合利用岩心分析、铸体薄片、扫描电镜与图像分析等技术,对其孔喉及成岩特征进行分析。结果表明:储层的成分及结构成熟度较低,孔隙结构以细孔—微细喉型为主,压实作用强烈,并且具有多期胶结与多期溶解的特征。以储层的“埋藏史-油气充注史-成岩演化序列-孔隙结构”为约束条件对储层的致密化定量表征发现,长8段在稳定的构造背景与辫状河三角洲环境的控制下,形成了细粒、富含塑性颗粒且高杂基含量的沉积,配合其后的快速埋深造成了强烈压实,其对储层致密化的贡献值为85%。成岩环境的多期变化造成了富集不同盐类的孔隙流体对储层的强烈胶结作用,对储层致密化贡献值为40.8%。早期绿泥石膜对颗粒的保护、稳定的构造背景及顶底板围岩处的硅质与钙质胶结层对孔隙流体活动的限制、过早的低渗和远离盆地生烃中心造成的缺乏酸性流体持续性注入等导致了溶蚀作用较弱,对储层致密化的抑制值仅为26.1%,且形成的次生孔喉易被后期胶结物充填。构造、沉积、成岩作用及其与油气充注在时空的耦合关系造成了长8段储层在浅埋藏条件下的致密化。
关键词:  镇原地区  延长组8段  致密砂岩  致密化  定量表征  成岩环境
DOI:10.3969/j.issn.1673-5005.2024.02.003
分类号::P 62
文献标识码:A
基金项目:江苏省基础研究计划自然科学基金项目(BK20220623);江苏省高等学校自然科学研究项目(20KJB440003)
Mechanism of tight sandstone reservoirs with shallow burial in southwest of Ordos Basin
ZHANG Yingnan1, BAI Qinglin2, SHU Qinglin3, LU Zhiyong4, ZHANG Ge2, DOU Xiangji5, LI Zhen2, HE Zhiming2
(1.School of Civil Engineering in Qingdao University of Technology, Qingdao 266520, China;2.Xianhe Oil Production Plant, Shengli Oilfield Company, SINOPEC, Dongying 257068, China;3.Shengli Oilfield Company Limited, SINOPEC, Dongying 257061, China;4.Jianghan Oilfield Company Limited, SINOPEC, Qianjiang 433100, China;5.School of Petroleum and Natural Gas Engineering in Changzhou University, Changzhou 213164, China)
Abstract:
The densification of fine sandstone occurred under relatively shallow burial conditions in the 8th member of the TrassicYanchang Formation, deposited in the Zhenyuan area of the southwest Ordos Basin. The pore-throat structure and diagenesis of Chang 8 member tight reservoir were analyzed through core analysis, casting thin sections, scanning electron microscope, fluorescence thin section with picture analysis, and other methods. The results indicate that both compositional and structural maturity are relatively low,with the dominant pore-throat structure being fine pores with minimal throats. Diagenesis is characterized by strong compaction, multi-period cementation, and dissolution accompanied by variations in the diagenetic environment. The quantitative characterization of densification process was carried out under the comprehensive control of "burial history-oil and gas filling history-diagenetic evolution sequence-pore-throat structure". It reveals that Chang 8 member contains a large amount of fine grains and plastic fragments with a high matrix content, leading to significant reservoir space loss under rapid burial after deposition. This was influenced by a stable tectonic setting combined with a braided river delta sedimentary system, resulting in strong compaction contributing to 85% of the densification. Additionally, pore fluids saturated with various salts led to intense cementation, affected seriously by multiple changes in the diagenetic environment, contributing 40.8% to the densification. The early-formed chlorite film protected clastic particles from being dissolved. Cement layers limited the exchange of pore fluid with surrounding rock under the stable tectonic background, which were formed at the roof and floor of Chang 8 member by strong siliceous cementation and carbonate cementation. Early entry into a low-permeability stage, coupled with distance from the hydrocarbon-generating center of the basin, hindered continuous acid fluid injection, reducing the dissolution intensity. Consequently the inhibiting effect of dissolution on densification was only 26.1%. Besides, the secondary pores generated by dissolution were easily filled by later-period cements. In summary, the tight sandstone reservoirs in the Chang 8 member were buried at shallow depths, controlled by multiple factors, including tectonic setting, sedimentary environment, diagenesis, and their coupled relationship with periods of hydrocarbon generation and expulsion.
Key words:  Zhenyuan area  8th member of Yanchang formation  tight sandstone  densification  quantitative characterization  diagenetic environment
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