Carbonate Cement Distribution and Carbon and Oxygen Isotope Enrichment Characteristics of Sandstone Reservoirs in M Oil Formation in Shiyougou Oilfield

Abstract

Carbonate cement is the most important authigenic mineral in M reservoir sandstone reservoir of Jianquanzi Formation in Jiuxi Basin, and its content and distribution form are the main factors affecting reservoir quality. Through mineralogy, petrology and geochemical analysis, it is concluded that the carbonate cements in this area are mainly composed of iron-free dolomite and calcite, and are often developed in the form of ring-side and speckle. The content of carbonate cement ranged from 2.11% to 56.39%, with an average of 12.93%. Microzone observation shows that the iron-free microcrystalline dolomite cements are distributed outside the particles and cover the whole particles, and they are mainly formed from cogenesis to the early stage of premorphic rock. The iron-free dolomite cements formed in the early stage increase the mechanical strength and compact-resistance of the rocks, which is beneficial to the development of the reservoirs in the study area. Speckle like sparry calcite cement mainly grows along the pore space and finally occupies the entire pore space. It is the chemical precipitation product in the late diagenesis, which is unfavorable to the development of reservoir. The value of δ18O (PDB) is −11.14‰ ~ −3.25‰, and the value of δ13C (PDB) is −7.32‰ ~ −0.42‰, indicating that the carbonate cement in this area comes from the brachywater environment and is related to the dissolvation-deposition process and is of chemical origin. The δ13C isotope is generally negative in the speck distribution of carbonate cements, which may be due to the addition of light organic carbon in the M oil layer, and the calcium ion produced by the organic acid produced by decarboxylation of organic matter under high temperature and pressure and the reaction with clay minerals provides the Source of Ca2+ for diagenetic diagnosis of carbonate cements. The study also shows that carbonate cementation may be one of the important factors that make the sandstone reservoir physical property of M oil group worse, and the porosity loss rate of M oil group is up to 16% due to carbonate cementation. The research results on the characteristics and formation mechanism of carbonate cements provide key parameters for quantitative evaluation of reservoirs and experimental support for acidification and stimulation measures.