Diagenesis And Effects On Porosity In Triassic Sandstones Of Yanchang Formation,north Of Shaanxi Province,Ordos Basin

Diagenesis and effects on porosity in Triassic sandstones ofYanchang formation, north of Shaanxi province, Ordos BasinLiu Jie(Chengdu University of Technology)AbstractThe theme of this thesis is the clastic diagenesis Yanchang formation, Triassic,north of Shaanxi province, Ordos Basin, and control factors on the porosity of thefCsefVloLWe study the petrologic characters of Yanchang sandstone using the microscope,the scanning electronic microscope, energy spectrum analysis, cathodoluminescenceanalysis, the electron-probe analysis, X-ray diffraction, and the isotope analysis ect.Based on thermodynamic theory We calculate the Gibbs free energy of two reactionsfor laumonite's formation. We get results when the pressure change from 5 to 40 Mpa,the minimtun temPeratUr for the reedions is 30 to 40oC, therefore, the laumonite inthe Yanchang formations should have been precipitated in the early diagenesis.Considering the geological setting and the burial history, we also studied the diagenesisand established the mode of the diagenesis. We get more knowledge of theprecipitation mechanism of the chlorite, carbonate and laumonite cements. Combiningthe data of carbon, oxygen, stfolltium isotopes and trace elemeflts, we illterpret theformation mechanism of them.According to the characters of the porosity and diagenesis of the sandstone, wediscussed the diagenesis' effects or condolling factors on the porosity We suggest thatthe chlorite coats are positive to preserve the original porosity and secondary porosity.Although the cemeniation of the carbonate resist the mechanical comPaction, its rarelydissolution result in blocking the pore in sandstone. The laumonite cements occuPiedthe most porosity, bat its dissolution provides more secondry porosity it is postive tothe reservoir.Base on all the study, we conclude that: besides sedimental facies, the diagenesiscontrols the reservoir quality Because of less comPactiing, more chlorite coating, lesscementation and dissolution, the sandstone in Chang 2 section has more porosity andbetter quality. Whereas the dissolation of early cements, together with the chloritecoating generated abundam secondary porosity in the sandstone of Chang 6 section.In the end, we use multiple stepwise regression analysis to evaluate the main factors, which dominate the reservoir quality of Chang 6 section in YangMiJian area, Ordos basin, using the results of the regression analysis and diagenesis. According to the facts that the chlorite coat, the feldspar and laumonite dissolution drop off from the south to north, at the same time, laumonite and other cements increase gradually, we proposal that the reservoir quality in the south is better than that of the north.