鄂尔多斯盆地漂白砒砂岩特征及成因

Characteristics and genesis of the bleached Pisha sandstone in Ordos Basin

漂白砒砂岩在鄂尔多斯盆地分布广泛,主要分布在中生界延安组中,以东胜地区煤层上部漂白砒砂岩为代表,是油气曾经存在及其运移的重要证据,对其研究具有重要意义。在野外地质调查及样品采集的基础上,运用偏光显微镜、扫描电镜、X-射线衍射、X-荧光光谱和电感耦合等离子体质谱进行了分析。结果表明,漂白砒砂岩的矿物组成主要为石英和碱性长石,其次为伊利石和高岭石,含有少量的伊蒙混层。SiO2含量为72.76%~74.96%,Al2O3含量为16.17%~17.48%,K2O含量为1.93%~2.65%,CaO含量为0.08%~0.57%,Fe2O3含量为0.01%~0.69%。Hf,Zr,Ss,Ba和U相对富集。∑REE介于67~296μg/g,LREE/HREE介于7.36~13.67;REE配分模式为轻稀土富集、重稀土亏损,Eu呈明显负异常,Ce表现出弱亏损;Ceanom介于-0.07^-0.17,平均-0.12。它们形成于缺氧、还原的古水体环境。上述研究结果表明,漂白砒砂岩是硫酸盐和天然气还原作用的结果,为研究区油气的形成和逸散作用提供地质依据。

Bleached Pisha sandstones are widely distributed in Ordos Basin and mainly occur in Yan＇an Formation of the Mesozoic,In Dongsheng area,the bleached Pisha sandstone overlying on the coal seam is an important evidence of the existence and migration of hydrocarbons. In this paper,the samples collected during field geological investigation were analyzed by using polarization microscope,scanning electron microscopy（ SEM）,X-ray diffraction and X-ray fluorescence spectrum and inductively coupled plasma mass spectrum. The results show that the main mineral composition of the bleached Pisha sandstone are quartz and alkali feldspar,followed by illite,kaolinite and a small amount of illite /smectite mixed layer. The content of SiO2 is 72. 76%-74. 96%,that of Al2O3 is 16. 17%-16. 17%,K2 O 1. 93%-1. 93%,CaO 0. 08%-0. 08% and Fe2O30. 01%-0. 69%. Hf,Zr,Ss,Ba and U are relatively enriched. The ∑REE and LREE /HREE was are in the ranges of 67-296 μg /g and 7. 36-13. 67,respectively. REE distribution pattern features in light REE enrichment,but heavy REE deficient,with significantly negative anomaly of Eu and weak deficient of Ce（ the content of Ceanom being in the range of-0. 07--0. 17,averaging at-0. 12）,suggesting an anoxic reducing environment. The studies show that the bleached Pisha sandstone is the result of sulfate and natural gas reduction,and thus can provide a geologic basis for the formation and dissipation of oil and gas in the study area.