Origin of porphyry dolomite in the 5th sub-member of the 5th member of Ordovician Majiagou Formation (M55sub-member), Ordos Basin
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摘要: 鄂尔多斯盆地中东部奥陶系马家沟组五段5亚段发现大量不均一斑化白云岩,多口钻井油气显示良好,成为油气勘探关注的热点,但成因机理尚未明确。依据岩石学特征,辅以地球化学分析,研究了斑化白云岩成因。斑化白云岩可分为不均匀斑化白云岩、明显斑化含灰云岩和略微斑化灰质云岩3类,根据云斑的形态特征认为云斑构造受到生物扰动所致,白云岩斑点与基质的阴极发光相同;斑化白云岩总体具有较高的Fe,Mn,Na和较低的Sr含量,斑点与基质的Fe,Mn和Sr的含量无明显差别;斑化白云岩δ13C值与同期海水相同,δ18O值均偏负,三类白云岩无明显差别;87Sr/86Sr比值高于同期海水。岩石学和地球化学特征表明生物扰动使岩石不均一,扰动程度影响斑化程度,斑化白云岩系生物扰动灰岩经高盐度海水渗透回流白云石化作用而成,白云岩再埋藏后经历了重结晶作用改造。Abstract: Large number of heterogeneous porphyry dolomites have been found in the 5th sub-member of the 5th member of Ordovician Majiagou Formation (M55 sub-member) in the central and eastern parts of Ordos Basin, and many drilled wells have shown good oil and gas potential, which has become the focus for oil and gas exploration, but the genetic mechanism is still undetermined. Based on petrological characteristics and geochemical analysis, the origin of porphyry dolomite in this area is studied. Results show that porphyry dolomite can be classified into three types: unevenly, obviously and slightly porphyry dolomite. According to the morphology characteristics of cloud spots, the structure of cloud spots is supposed to be caused by biological disturbance, and the cathodoluminescence of spots and matrix is the same. Porphyry dolomite has relatively higher contents of Fe, Mn and Na and lower content of Sr, but there is no significant difference between the contents of Fe, Mn and Sr in spots and matrix. The δ13C values of porphyry dolomites are highly comparable to those of seawater at the same time, and the δ18O values are all negative with no significant difference among the three types of dolomites. The ratio of 87Sr/86Sr is higher than that of seawater in the same period. Petrological and geochemical characteristics show that biodisturbance causes the heterogeneity of rocks, and the degree of disturbance affects porphyry rate. The biodisturbed limestone of porphyry dolomite series is formed by the dolomitization of the osmotic reflux of high salinity seawater. The dolomite underwent recrystallization after re-burial.
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图 1 鄂尔多斯盆地中东部奥陶系马五5亚段岩相古地理(a)及地层柱状图(b)
据文献[23]修改
Figure 1. Lithofacies palaeogeographic map (a) and stratigraphic column map (b) of Ordovician M55 sub-member in central and eastern Ordos Basin
图 2 鄂尔多斯盆地中东部奥陶系马五5亚段斑化白云岩岩心、薄片特征及微区碳氧同位素分析点
a.不均匀斑化白云岩,2 450.6 m,神29井;b.明显斑化含灰云岩,2 842.9 m,双4井;c.略微斑化灰质云岩,3 120.2 m,陕16井;d.不均匀斑化白云岩,2 450.6 m,神29井;e.明显斑化含灰云岩,2 842.9 m,双4井;f.略微斑化灰质云岩,3 120.2 m,陕16井;g.不均匀斑化白云岩,2 450.6 m,神29井;h.明显斑化含灰云岩,2 510.6 m,双108井;i.明显斑化含灰云岩,2 509.8 m,双108井;j.明显斑化含灰云岩,3 391.5 m,陕377井;k.明显斑化含灰云岩,2 462.3 m,米52井;l.略微斑化灰质云岩,3 120.6 m,陕16井;图中红色点位Sp为斑点(Spot),黄色点位Mat为基质(Matrix)
Figure 2. Cores, thin sections and carbon and oxygen isotopic analysis points in micro-area of porphyry dolomites in Ordovician M55 sub-member, central and eastern Ordos Basin
表 1 鄂尔多斯盆地中东部奥陶系马五5亚段斑化白云岩全岩地球化学特征
Table 1. Whole rock geochemical test results of porphyry dolomite in Ordovician M55 sub-member, central and eastern Ordos Basin
井号 井深/m 岩性 含量/10-6 87Sr/86Sr δ13CVPDB/‰ δ18OVPDB/‰ Na Mn Fe Sr 测试结果 误差(2σ) 神29 2 450.6 不均匀斑化白云岩 46.3 48.5 1 151.6 64.5 0.710 7 0.000 176 0 -0.64 -8.02 府4 2 531.0 59.1 38.8 846.3 86.2 0.709 8 0.000 095 8 1.29 -8.51 2 529.8 33.0 33.6 634.9 77.4 0.709 3 0.000 672 0 -3.31 -9.28 双4 2 841.8 明显斑化含灰云岩 66.5 46.7 1 420.2 97.1 0.709 8 0.000 052 6 -1.27 -7.99 2 832.8 72.4 49.6 1 482.7 80.9 0.712 9 0.000 041 0 -0.91 -7.62 双108 2 510.5 53.3 48.0 1 040.3 58.8 0.710 8 0.000 243 0 -0.65 -7.07 陕398 3 944.4 31.2 20.1 400.9 88.0 0.709 5 0.000 057 7 -1.46 -8.07 米52 2 462.3 34.0 986.3 64.6 0.710 3 0.000 018 8 -0.39 -7.08 陕16 3 120.2 略微斑化灰质云岩 62.8 40.7 859.1 102.0 0.709 8 0.000 023 7 -0.32 -7.37 陕399 3 860.7 泥晶灰岩 25.3 17.0 155.6 97.7 0.709 2 0.000 032 6 -1.81 -8.09 表 2 鄂尔多斯盆地中东部奥陶系马五5亚段斑化白云岩的微区地球化学特征
Table 2. Microgeochemical test results of porphyry dolomite in Ordovician M55 sub-member, central and eastern Ordos Basin
井号 井深/m 岩性 点位性质 含量/10-6 δ13CPDB/‰ δ18OPDB/‰ Na Mn Fe Sr 神29 2 450.6 不均匀斑化白云岩 1.基质 113.3 75.7 2 621.9 51.8 0.73 -7.03 2.斑点 101.1 43.2 2 050.4 37.7 -1.19 -10.41 府4 2 531.4 1.基质 238.3 49.0 3 033.2 41.7 2.斑点 110.8 48.1 1 692.8 39.8 双108 2 510.5 明显斑化含灰云岩 1.基质 139.1 41.8 2 161.4 40.0 -1.72 -6.88 2.斑点 113.9 44.6 2 086.9 47.3 0.90 -8.27 双108 2 509.8 1.基质 -1.64 -9.69 2.斑点 -1.78 -9.07 陕377 3 391.5 1.基质 137.0 48.4 1 468.0 45.3 -0.77 -8.52 2.基质 119.5 44.4 1 587.5 49.8 -0.77 -8.52 3.斑点 115.5 44.4 2 592.9 46.6 -0.41 -7.95 4.斑点 84.8 45.6 1 425.2 104.3 -0.41 -7.95 米52 2 462.3 1.基质 89.4 36.4 1 799.7 50.6 -1.18 -10.61 2.斑点 52.4 45.0 2 149.9 29.8 -1.81 -11.81 3.斑点 111.2 35.2 1 668.8 47.3 -1.54 -12.42 4.斑点 92.9 35.9 1 919.6 53.3 陕16 3 120.6 略微斑化灰质云岩 1.基质 58.2 45.9 1 824.7 33.7 -0.32 -8.59 2.斑点 74.2 44.0 1 819.3 39.7 0.15 -7.88 3.斑点 73.5 45.5 1 687.7 35.3 表 3 生物扰动指数及其特征描述 修改自文献[29]。
Table 3. Bioturbation index and its characteristics
生物扰动指数(BI) 生物扰动部分所占整块样品比值/% 描述 0 0 没有生物扰动,原生组构保存完好 1 1~4 少量生物扰动,有不连续的遗迹局部覆盖,沉积组构保存较好 2 5~30 中低等生物扰动,沉积构造保存仍然较好 3 31~60 中等生物扰动,有不连续遗迹化石的遗迹组构,仍然可以辨认出地层界面 4 61~90 强烈的生物扰动,遗迹化石密度高,原始沉积构造几乎全部被抹掉 5 91~99 强烈的生物扰动,层面沉积物被完全扰动 6 100 完全生物扰动和重新沉积,与遗迹化石重复覆盖有关 -
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