Distributional and Sedimentary Characteristics of Sandstones in Jurassic Zhiluo Formation, Ordos Basin

Key words: 
• Ordos Basin /
• Jurassic /
• Zhiluo Formation /
• sandstone

Abstract: According to the cycle architecture of lithology, the Zhiluo Formation is divided into two members (named upper and lower ones) in the scale of whole basin. The figures about member thickness, sandstone thickness and layer numbers of sandstone indicate that: the thickness distribution patterns of sandstone about both upper and lower members are consistent with that of the whole formation, all of them show it is thicker in the west while thinner in the east; the thickness of upper member are thicker than that of lower member, generally. The lower member has less layer numbers of sandstone and bigger thickness of single sandstone layer, while the upper member holds the opposite characteristics. The analysis of detrital components, detritus type,supporting style and regional mapping indicate that: the sandstone of Zhiluo Formation has the characteristics of multiprovenances and proximal deposition. The analysis of grain size shows that the sandstone has the characteristic of fluvial channel deposition. Several types of surface and bedding structures existed in the sandstone of Zhiluo Formation, which reflect the fluvial deposit environment. The vertical sequence of lower member has the typical characteristic of braided stream system, while the upper member reflects the combination characteristic of superimposition of multiple phase meanderingbraided stream system. Via comprehensive analysis, it is considered that the main sedimentary facies of lower member of Zhiluo Formation is braided stream system, and transited to meandering river at latter in the residual basin. While that of the upper member are mainly braidedmeandering stream system, delta and lake. The sandstone deposited in braided channel of lower member of Zhiluo Formation has excellent permeability, connectedness and well bedding, so it is the well reservoir of sandstonetype uranium deposits and hydrocarbon.