Characteristics of Carbonate Cements and Reservoir Quality of the Lower Cretaceous Sandstone in Jiudong Sag

Key words: 
• Jiuquan Basin /
• Ying'er Depression /
• carbonate cements /
• carbon and oxygen isotopes /
• material source

Abstract:  The deep burial sandstone reservoir quality is seriously affected by the carbonate cements which is an essential autogenic mineral in the Lower Crataceous formation in Jiudong sag. The thin sections and scanning electron microscope are analyzed, the energy spectroscopy ,carbon and oxygen isotopes of carbonate cements are tested. The study results indicate that the carbonate cements of Lower Crataceous sandstone from the Jiudong sag are mainly ferrous dolomite, ferrous calcite and dolomite with heterogeneous distribution and apparent zonation. They were formed in the late stage of diagenesis and the source are mainly derived from clay minerals transformation, thermal evolution of organic matter in the mudstone and the part of the calcium from the alteration of plagioclase. The key reason for development of the highquality reservoir in the deep burial lowpermeability reservoirs is the dissolution of carbonate cements. In addition, the content of carbonate cement also have an impact on reservoir quality, if the content is greater than 5%,  with the increase  of  the content of carbonate cements the porosity decreases, while it's less than 5%,  there  is  no  obvious  correlation between porosity and carbonate cement content.