| Recently, It has been a hot point to study the Petrological characteristics of sandstone in Yanchang 9 formation of Upper Triassic Jiyuan region of the Ordos Basin. It has been not quite ready yet now, but It become inevitably an important aspect to study the characteristics of the sandstone in diagenesis.In addition, there has been a lot of self-generated laumontite cements in sandstone of Chang 9 formation, which fill in Intergranular pore, solution pore. Laumontite exist not only preserved sandstone section intergranular pores during the diagenesis process, but it is more important to explain the properties related pore fluids, as well as other causes of authigenic cements and formation time in Chang 9 layer.The dissolved pore from authigenic laumontite cements become a major oil and gas reservoir space in some layer in northern Shanxi Yanchang. Therefore, It is very important to study authigenic laumontite cements existing in sandstone layers in Yanchang Ordos Basin with low porosity and permeability characteristics.Using core observation, the polarizing microscope, and the methods of particle size test and analysis, Through the crystal characteristics of Laumontite mineral in combination with the occurrence of other the authigenic cements between the particles,We can explain the process of sandstones changes and the formation of laumontite in Yanchang 9 during diagenesis and further analysis the formation mechanism of the laumontite in Chang 9 formation of Upper Triassic of the Ordos Basin in Jiyuan region.Through the discussion, the main knowledge we get is:1. Feldspar sandstone and lithic feldspar sandstone are mainly two kinds of sandstone in Jiyuan region Chang 9 formation of Upper Triassic of the Ordos Basin, and their proportions in the total samples are 23.8% and 57.2%.The components of Feldspathic and lithic in sandstone framework grains are generally higher levels With an average of 39.30% and 39.58%, and the lithic sandstone content showed a modest growth trend from north to south in this Area.2. The sediments of the provenance from Yinshan the northern of this region have a greater impact in Chang 9 formation in Jiyuan region. The sandstone framework grains have a poor roundness, subangular, and medium separation generally, and mid-fine grained sandstone is the main type of sandstone.3. The dissolution of feldspar framework grains is more common in Chang 9 layer in Jiyuan region. And the solution residue's shape like a harbor, but the solution pore is often filled by some self-generated cements likcalcite in the latter part of the cementation process.4. Although the sandstones suffered intense dissolution in this area,but they are rich in abundant authigenic cements. Authigenic chlorite occurs mainly in capsule form. Filling pores is the main form for the laumontite and calcite cement. And it is more common to the quartz and stone cements by overgrowth.5. Diagenetic environment for the sandstone in Yanchang formation was in favor of the formation of authigenic cement, especial the authigenic laumontite cements during some times, where it was a suitable temperature, a high Ca2+? Na+ concentration in the fluid, and a higher activity of SiO2.6. The alkaline diagenetic environment was formed by hydrolysis or alteration for some kinds of dark minerals like biotite, which would provide a stable alkaline fluid environment for some authigenic minerals.7. The Ca2+ ions concentration elevated in the pore fluid on the albitization of plagioclase, and the high Na+ ions concentration in the pore fluid also could help autogenous laumontite cements to form preferentially8. The dissolution phenomena about the feldspar grains and the authigenic laumontite cements is extremely common, but there are few authigenic kaolinite and a lot of organic residues, So according to previous studies, we can guess the dissolution fluid at that time is rich in organic matter.9.Comparing with the carbonate cements, authigenic laumontite cements need higher pore connectivity for cementing requirements. Meanwhile Laumontitization growth pattern and the presence are related with he formation temperature. |
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