Characteristics and constrains of low-permeability reservoirs in the first member of Eocene Liushagang Formation, Weixinan Sag, Beibuwan Basin
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摘要: 为明确北部湾盆地涠西南凹陷始新统流沙港组一段储层特征及物性主控因素,综合应用壁心、薄片、物性、荧光、扫描电镜等储层分析资料,通过对储集特征、物性特征以及沉积作用、成岩作用、构造作用与油气成藏耦合等分析,表明涠西南凹陷流一段储层以长石岩屑石英砂岩和岩屑石英砂岩为主,总体成分成熟度和结构成熟度较低,储集空间主要为原生粒间残余孔和次生溶孔。涠西南凹陷北西源汇区和西部源汇区流一段多为中孔、中—高渗储层,东部源汇区主要为低孔低渗储层。沉积作用造成粒度、分选、泥质杂基等的差异,压实、碳酸盐胶结、有机酸溶蚀等成岩作用是低渗储层物性主控因素;而构造作用和油气成藏耦合所控制的早期烃类充注、异常高压可保护原生孔隙,加上断裂派生的裂缝可对低渗储层物性进行改善。涠西南凹陷北西源汇区和西部源汇区流一段储层由于埋深较浅,其水动力较强、粒度较粗、分选较好,在泥质杂基较少的区域储层物性较好;而东部源汇区在靠近生烃主洼的二号断裂附近,存在早期烃类充注、有机酸的强溶蚀以及部分高压保护,在2 500~3 500 m形成次生孔隙和高压孔隙发育带,亦发育深埋藏有利储层。Abstract: Reservoir analysis data such as core, thin-section, physical property and scanning electron microscope (SEM) were applied in this paper to study the reservoir characteristics and main constrains of the first member of Eocene Liushagang Formation in the Weixinan Sag, Beibuwan Basin. Reservoir features, physical properties, sedimentation, diagenesis, and the coupling of tectonics as well as hydrocarbon generation and accumulation were analyzed. Results showed that the reservoirs of the first member of Eocene Liushagang Formation in the area of this study were mainly feldspathic debris quartz sandstones and lithic quartz sandstones with low compositional and structural maturities. The reservoir space was mainly consisted of primary intergranular residual pores and secondary dissolved pores. In the northwestern and western source areas, the reservoirs mainly had moderate porosity and medium-high permeability, while those in the eastern source area had relative low porosity and low permeability. Sedimentation caused variations in grain size, sorting and argillaceous complexes. Diagenesis such as compaction, carbonate cementation and organic acid dissolution were the main controls for the physical properties of low-permeability reservoirs. The coupling of tectonics and hydrocarbon generation and accumulation controlled early hydrocarbon charging, while abnormal high pressure protected primary pores, which, combined with fractures derived from faults, improved the physical properties of low-permeability reservoirs. In the northwestern and western source areas, the shallow-buried reservoirs in the first member of Liushagang Formation had relatively strong hydrodynamic force, coarser grain size and better sorting, and showed better properties in the area with less mud. In the eastern source area close to the No. 2 fault around the main hydrocarbon-generating depression, there exist early hydrocarbon filling, strong dissolution of organic acids and partial high-pressure protection. Secondary pores and high-pressure pores were developed at 2 500-3 500 m deep, and favorable deep-burial reservoirs were formed.
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Key words:
- organic acid /
- oil and gas charging /
- diagenesis /
- reservoir characteristics /
- Liushagang Formation /
- Eocene /
- Weixinan Sag /
- Beibuwan Basin
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图 2 北部湾盆地涠西南凹陷始新统流一段重矿物特征及源汇分区
平面位置见图 1。
Figure 2. Heavy ore characteristics and its source and sink zone in the first member of Liushagang Formation, Weixinan Sag, Beibuwan Basin
图 3 北部湾盆地涠西南凹陷始新统流一段储层孔隙类型与成岩作用典型特征
a.颗粒点接触为主,部分线接触,原生粒间孔发育,早期方解石胶结物大量溶蚀,形成次生溶孔和混合孔隙,WZ5井,2 391.8 m;b.方解石胶结充填粒间孔,颗粒悬浮状或点接触,WZ25井,2 391.5 m;c.颗粒线接触为主,部分点接触,发育原生粒间孔和次生溶孔,WZ16井,2 515.0 m;d.颗粒悬浮状或点接触为主,次生溶孔发育,WX6井,2 390.0 m;e.颗粒呈线接触和凹凸接触,粒间溶孔发育,塑性矿物云母呈弯片状,LS5井,3 575.0 m;f.颗粒溶孔、粒间溶孔和铸模孔发育,LS13井,3 249.2 m;g.构造作用派生裂缝,LS9井,3 217.5 m;h.构造作用派生裂缝,WZ22井,3 249.9 m;i.方解石胶结充填粒间孔,颗粒多为点接触,WX75井,2 474.5 m;j.晚期含铁方解石胶结物充填孔隙或交代早期方解石胶结物和长石等,LS21井,3 224.0 m;k.石英次生加大,LS5井,3 561.1 m;l.石英加大边内侧油气荧光,存在早期烃类充注,荧光薄片,LS5井,3 561.1 m;m.方解石(Cc)等碳酸盐矿物与假六边形片状高岭石(K)、片状绿泥石(Ch)等黏土矿物充填孔隙,LS15井,2 488.6 m;n.铁白云石(Ak)等碳酸盐矿物与假六边形片状高岭石(K)、片状伊蒙混层(I/S)等黏土矿物充填孔隙,LS15井,2 810.0 m;o.假六边形片状高岭石(K)、片状绿泥石(Ch)、片状伊利石(I)等黏土矿物及石英(Q)充填孔隙,LS15井,2 548.0 m
Figure 3. Pore types and typical characteristics of diagenesis of the first member of Liushagang Formation, Weixinan Sag, Beibuwan Basin
表 1 北部湾盆地涠西南凹陷始新统流一段各源汇区碎屑组分
Table 1. Clastic composition of source areas in the first member of Liushagang Formation, Weixinan Sag, Beibuwan Basin
源汇区 井名 样类 碎屑组分百分比/% 成分成熟度 单晶石英 多晶石英 长石 岩屑 其他 北西源汇区 WZ5 岩心 31.50 27.03 9.21 7.94 0.44 0.71 WZ7 岩心 25.47 26.25 12.93 12.84 0.00 0.49 WZ9 岩心 34.16 19.54 12.43 9.43 0.00 0.83 西部源汇区 WZ17 岩屑 37.06 27.63 4.94 9.25 0.56 0.89 WZ10 壁心 40.21 16.74 11.37 7.00 0.00 1.15 WZ6 岩屑 44.00 9.75 6.65 5.35 0.00 2.02 WZ8 岩心 42.21 13.87 10.50 5.82 0.00 1.40 WZ16 壁心 43.80 20.40 4.80 0.50 0.00 1.70 东部源汇区 LS8 壁心 54.85 8.30 4.00 6.80 0.00 2.87 LS3 岩心 45.31 7.44 4.78 5.97 8.69 2.49 LS9 壁心 51.62 11.54 5.00 10.00 0.00 1.94 LS11 岩屑 69.00 15.00 5.67 7.67 0.00 2.44 LS13 壁心 55.75 12.13 6.03 11.03 2.13 1.91 LS5 壁心 55.10 8.20 10.35 13.30 0.00 1.73 LS21 壁心 47.88 12.92 7.25 11.88 6.50 1.49 LS12 岩屑 57.00 2.33 2.17 2.67 0.00 7.95 西南源汇区 WX5 岩心 41.21 9.50 9.77 7.90 0.00 1.52 WX3 岩心 37.33 6.67 3.67 3.33 0.00 2.73 WX8 壁心 46.00 18.00 5.00 24.00 0.00 0.98 WX2 壁心 33.50 1.00 1.00 2.00 0.00 8.38 WX11 岩心 40.98 14.37 6.02 9.88 0.00 1.35 WX6 岩屑 50.00 10.17 3.67 6.75 0.00 2.43 -
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