柴北缘冷湖七号地区碳酸盐胶结物特征及其意义

Characteristics and Significance of Carbonate Cement in the No.7 Area of Lenghu,Northern Margin of Qaidam Basin

碳酸盐胶结物是碎屑岩储层中最常见的胶结物类型,其形成环境、物质来源、反映的成岩温度对储层研究具有重要意义。通过对柴北缘冷湖七号下干柴沟组砂岩碳酸盐胶结物进行岩石学、矿物学和地球化学分析,研究区下干柴沟组砂岩碳酸盐胶结物包含方解石、含铁方解石、铁方解石和白云石等4种类型。碳同位素(δ13CPDB)和氧同位素(δ18OPDB)值分别为-8.81‰^-3.51‰和-8.77‰^-17.36‰,平均值分别为-5.33‰及-13.50‰;利用碳同位素和氧同位素计算出古盐度值和古温度值分别为113.61~102.85和63.03~123.06℃,平均值分别为109.66及94.47℃。反映出研究区下干柴沟组所处成岩阶段为早成岩阶段B期——中成岩阶段A期,形成过程受到有机质脱羧基作用影响,成岩环境偏酸性,成岩流体来自碎屑的析出水和大气淋滤淡水。储层物性除了受成岩环境和温度的影响外,与碳酸盐胶结物含量具有明显的负相关关系,随着碳酸盐胶结物含量的增加,储层的孔隙度和渗透率降低。

Carbonate cement is the most common type of cement in clastic reservoirs,and its formation environment,material source and diagenetic temperature are of great significance to the reservoir research.Through the analysis of petrology,mineralogy and geochemical of the sandstone carbonate cementation in the Xiaganchaigou Formation of Lenghu No.7 in the northern margin of Qaidam Basin,this paper shows that there are four types of sandstone carbonate cements in the Xiaganchaigou Formation in the study area which are calcite,iron-bearing calcite,iron calcite and dolomite respectively.The content of carbon isotopes(δ13CPDB)and oxygen isotopes(δ18OPDB)are 8.81‰∼3.51‰and 8.77‰∼17.36‰respectively,with averages value of 5.33‰and 13.50‰,respectively.Paleosalinity and temperature are 113.61∼102.85 and 63.03∼123.06◦C according to carbon isotopes and oxygen isotopes calculation,the average value is 109.66 and 94.47◦C,respectively.It reflects that the diagenetic stage of the Xiaganchaigou Formation is from early diagenetic stage B to middle diagenetic stage A,whose formation process is influenced by the decarboxylation of organic matter.The diagenetic environment is acidic,and the diagenetic fluids mainly come from the precipitated water of the clastic and the leaching fresh water in the atmosphere.Besides the influence of diagenetic environment and temperature,the physical property of reservoir has a significant negative correlation with the carbonate cement content.And with the increase of carbonate cement content,the porosity and permeability of the reservoir decrease.