Paleobiological Characteristics and Its Reservoir Significance of Bioclastic Migmatite in First Member of Shahejie Formation in Bohai Sea
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摘要: 渤海海域古近系沙一段广泛发育湖相混积岩,其中生物碎屑混积岩物性好、产能高,是混积岩中的优质储层.对生物碎屑混积岩中的生物碎屑成分展开系统的古生物研究,明确腹足类是渤海海域沙一段生物碎屑混积岩中最主要的生物碎屑类型,共鉴定出腹足类21属27种,介形类19属74种,并且均以适宜一定盐度浅水环境生活的小型化类型为主.结合古环境研究表明,高能环境腹足类易富集,相对低能环境介形类含量上升.通过铸体薄片、扫描电镜、电子探针等分析发现,生物碎屑对混积岩储层演化具有重要影响.生物碎屑体腔孔、壳体铸模孔、遮蔽孔、差异收缩缝直接提供了储集空间;生物碎屑早期溶蚀能有效改善储层渗透能力,促进生物碎屑富集段的白云岩化作用,形成生物碎屑白云岩优质储层;生物碎屑在微生物作用下形成性质稳定的泥晶套,起到了孔隙格架支撑作用.受壳体成分及各门类生物沉积环境差异影响,腹足类为主的生物碎屑混积岩物性更好.Abstract: Lacustrine migmatite is widely distributed in the first member of Shahejie Formation of Paleogene in Bohai Sea. The bioclastic migmatite is the high-quality reservoir with good physical properties and high productivity. In this paper, a systematic paleontological study of bioclastic components in bioclastic migmatites is carried out. It is confirmed that gastropods are the most important bioclastic types in the first member of the Shahejie Formation in Bohai Sea. 21 genus and 27 species of gastropods and 19 genus and 74 species of ostracoda are identified. They are mainly miniaturized types suitable for living in a certain salinity and shallow water environment. It is found that the gastropods are easily to accumulate in high energy environment, while the content of ostracods is relatively high in low energy environment combined with paleoenvironmental studies. The analyses of casting thin section, SEM and EPMA show that bioclasts play an important role in the evolution of migmatite reservoir. The reservoir space is directly provided by the cavity hole of the bioclastic body, the mold hole of the shell, the shielding hole and the differential contraction joint. Early dissolution of bioclasts can effectively improve the permeability of the reservoir, promote dolomitization of the bioclastic enrichment section, and form a high-quality reservoir of bioclastic dolomite. Bioclasts form a stable mud crystal set with the action of microorganism, which plays a supporting role of pore lattice. The physical properties of the bioclastic migmatite mainly composed of gastropods are better than the others, as influenced by the differences of shell composition and various kinds of biological sedimentary environment.
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Key words:
- Bohai Sea area /
- Paleogene /
- bioclastic migmatite /
- palaeontology /
- reservoir /
- sedimentation
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图 2 渤海海域古近系湖相生物碎屑混积岩中常见化石照片
a. BZ262A井, 3 264.51 m,小恒河螺Gangetia minys;b. BZ262A井,3 250.20 m,小恒河螺Gangetia minys;c. BZ262A井, 3 264.51 m,细弱恒河螺Gangetia exilis;d. BZ262A井, 3 264.51 m,短圆恒河螺Gangetia brevirota;e. BZ262A井, 3 264.51 m,均匀狭口螺Stenothyra paritis;f. BZ262A井, 3 264.51 m,优美狭口螺Stenothyra elegans;g. BZ262A井, 3 264.51 m,狭口螺属未定种Stenothyra sp.;h. BZ262A井,3 250.20 m,热河台高盘螺Valvata rehetaiensis;i. BZ262A井, 3 264.51 m,下辽副贝加尔螺?Parabaicalia xialiaoensis?;j. BZ262A井,3 264.51 m,乐陵真星介Eucypris lelingensis;k. BZ262A井,3 264.51 m,玻璃介未定种Candona sp.. 图 2a~2h线段比例尺=1 mm
Fig. 2. Photos of fossils in the lacustrine bioclastic migmatite of Paleogene, Bohai Sea area
图 6 生物碎屑对湖相混积岩储层的改善作用
a. QHD292EB井,3 353.50 m,生物体腔孔;b. B362Q井,2 390.25 m,介形虫体腔孔发育,部分被粉晶白云石胶结充填;c. QHD292ED井,3 433.00 m,生物碎屑遮蔽孔发育;d. QHD292E井,3 370.36 m,生物碎屑遮蔽孔发育;e. QHD292E井,3 382.14 m,遮蔽孔;f. BZ262A井,3 257.10 m,铸模孔大量发育,仅剩泥晶套;g. BZ262A井, 3 252.06 m, 生物碎屑铸模孔;h. BZ262A井,3 263.96 m,铸模孔发育,部分生物碎屑体腔被充填;i. B362Q井,2 381.60 m,微裂缝沿介形壳体裂开、扩散
Fig. 6. Improvement of lacustrine migmatite reservoir by bioclastic
表 1 QHD292E井与BZ262A井元素分析
Table 1. Analysis data of typical elements of Well QHD292E and Well BZ262A
井号 深度(m) Ni(g/g) Co(g/g) MnO(%) Fe2O3(%) Ni/Co MnO/Fe2O3 Ni/Co平均值 MnO/Fe2O3平均值 QHD292E 3 255 46.70 19.40 0.08 3.70 2.41 0.02 2.15 0.03 QHD292E 3 320 25.60 12.70 0.07 1.82 2.02 0.04 QHD292E 3 335 23.30 12.60 0.06 1.69 1.85 0.04 QHD292E 3 390 15.50 7.43 0.06 1.19 2.09 0.05 QHD292E 3 430 9.75 4.36 0.03 1.24 2.24 0.02 QHD292E 3 475 14.50 6.26 0.11 2.78 2.32 0.04 BZ262A 3 195 36.93 15.68 0.30 2.49 2.36 0.12 2.51 0.14 BZ262A 3 215 34.48 13.34 0.33 2.26 2.59 0.15 BZ262A 3 225 33.68 13.98 0.31 2.17 2.41 0.14 BZ262A 3 235 33.68 14.31 0.32 2.16 2.35 0.15 BZ262A 3 246 20.97 8.99 0.67 4.19 2.33 0.16 BZ262A 3 278 44.60 16.72 0.42 4.57 2.67 0.09 BZ262A 3 290 32.31 14.45 0.34 3.28 2.24 0.10 BZ262A 3 300 50.72 17.12 0.42 2.95 2.96 0.14 BZ262A 3 310 38.85 15.54 0.42 2.87 2.50 0.15 BZ262A 3 320 33.39 12.45 0.38 2.45 2.68 0.15 表 2 B362Q井生物碎屑混积岩井段碳、氧同位素值
Table 2. Analysis data of carbon and oxygen isotopes in bioclastic migmatite of Well B362Q
样号 深度(m) 层位 岩石类型 δ13C(‰,PDB) δ18O(‰,PDB) 古盐度S(‰) 古盐度指数Z 古温度(℃) B362Q 2 386.42 沙一段 生物碎屑 2.60 -7.22 27.53 129.03 48.71 B362Q 2 386.95 沙一段 鲕粒 1.97 -9.40 25.35 126.65 60.06 B362Q 2 391.00 沙一段 生物碎屑 0.15 -14.85 19.90 120.21 90.09 B362Q 2 392.45 沙一段 泥晶套 1.75 -11.30 23.45 125.26 70.26 B362Q 2 394.20 沙一段 砂屑 2.55 -5.57 29.18 129.75 40.38 B362Q 2 395.00 沙一段 砂屑 2.76 -6.00 28.75 129.96 42.53 B362Q 2 395.00 沙一段 洞充填物 1.49 -6.70 28.05 127.01 46.06 B362Q 2 395.70 沙一段 泥晶云岩 -0.50 -6.53 28.22 123.02 45.20 B362Q 2 396.10 沙一段 泥晶云岩 0.04 -2.62 32.13 126.08 26.02 B362Q 2 396.10 沙一段 泥晶云岩 0.40 -3.70 31.05 126.28 31.20 B362Q 2 396.50 沙一段 泥晶云岩 0.74 -2.19 32.56 127.72 23.98 注:古盐度S=δ18OPDB +21.2/0.61;古盐度指数Z=2.048×(δ13CPDB+50)+0.498×(δ18OPDB+50);古温度T=13.85-4.54×δ18OPDB +0.04×(δ18OPDB)2.据Keith and Weber(1964). 表 3 生物碎屑壳体电子探针数值
Table 3. EPMA data of bioclastic shell
井号 井深(m) 点号 MgO(%) QHD292E 3 375.65 1 45.05 QHD292E 3 375.65 2 40.18 QHD292E 3 375.65 3 41.49 QHD292E 3 375.65 4 39.41 -
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