Origin Of Calcite Cement In The Sandstone Reservoirs Of The Upper Jurassic Qigu Formation In Fudong Slope,Junggar Basin

Based on the results of previous studies, this thesis takes stratigraphy,sedimentology, geochemistry and diagenesis theory as guide. Using test data of thin sections, cathodoluminescence, X-ray diffraction, vitrinite reflectance, electron probe analysis, heavy mineral analysis, scanning electron microscope analysis, major element, trace element, rare earth element, strontium isotope and carbon and oxygen isotope and physical property analysis etc. This paper carried out the study on the origin of calcite cement and effect on sandstone reservoir of the Upper Jurassic Qigu Formation. Made the following understanding:1. Research shows that sediments of Qigu Formation in Fudong slope were mainly from the northeast Shaqi-Beisantai Embossment, secondly from southern Bogda Mountains. The properties of parent rocks are magmatite with continental arc properties.2. The sandstone reservoirs of Qigu Formation in Fudong slope are mainly lithic sandstone, followed by feldspar lithic sandstone, which has low compositional maturity and high structure maturity.3. Calcite cements in Jurassic Qigu Formation sandstones could be identified two stages. Stage ? calcite cements with good crystalline were formed in A stage of eodiagenesis, the suspension-like clastic grains were mostly distributed in crystalline calcite cements. In this stage, the proportion of calcite cement was up to 30%. Stage?calcite cements filled in the residue primary intergranular pores and stored by authigenic clay mineral envelopes or replacement of clastic particles with orange-red cathodoluminescence. These properties indicate that stage ? calcite cements precipitate in B stage of eodiagenesis, which is uncommon in the Qigu Formation sandstones.4. Diagenetic dilution of calcite cements in Qigu Formation sandstones has reduced the content of Al₂O₃, TiO₂, MgO, Na₂O and K₂O, besides these samples were enriched in MnO.5. Origin of calcite cements is complex, there are three common types of origin. Calcite cements of Qigu Formation sandstones in Fudong slope are mainly origin from diagenetic calcite, followed by organic acid decarboxylation, a small amount of calcite cements may be derived from the biogas.6. Pore of Qigu Formation reservoir sandstones were filled with calcite cements with poor dissolution in the mesodiagenesis, so calcite cements had a significant destructive effect on reservoir quality. What's more, with the increase of content calcite cements, the damage to the reservoir is heavier. The calcareous cementation-weak compaction diagenetic phase identified not conducive to the development of reservoirs.