鄂尔多斯盆地中央古隆起区中寒武统鲕粒碳酸盐岩

摘要

鄂尔多斯盆地中央古隆起区中寒武统鲕粒碳酸盐岩沉积于碳酸盐斜坡中的小规模浅滩，其主要矿物成分为白云石和方解石。根据全岩样品分析结果，鲕粒碳酸盐岩的δ18OV－PDB值在－11．3‰～－4．9‰之间，平均－6．5‰；δ13CV－PDB值在－1．7‰～0．3‰之间，平均－0．8‰。该碳酸盐岩沉积于浅水鲕粒滩，因受到淡水淋滤影响，之后又长期处于深埋环境，δ18 O值明显偏低。由于成岩作用过程中受有机质影响很小，δ13 C值没有表现出异常。白云岩的δ18 O值较石灰岩的高。显微激光采样分析结果表明，鲕粒较胶结物有较低的δ18 O和δ13 C值。前者形成于动荡的浅水环境，形成后常受淡水的淋滤。胶结物大多形成于埋藏期，孔隙水具有海水性质，盐度较高，胶结物沉淀后，受淡水影响较小。由于经受了强烈的埋藏作用，鲕粒碳酸盐岩表现出的特征为Na和Sr含量低、Fe含量高—特高及Mn含量较高。白云石大多数为埋藏作用阶段高Mg含量卤水条件下交代而成，整体上有较低的有序度，MgCO3含量高。在强烈的埋藏作用和胶结作用下，鲕粒碳酸盐岩现存孔隙度极低。%Middle Cambrian oolitic carbonate rocks were deposited in small shoals of the ramp in a central ancient upwelling area in the Ordos Basin. The oolitic carbonate rocks mainly consist of calcites and dolomites. The δ18 O and δ13 C values range from -11.3‰to -4.9‰and from -1.7‰to 0.3‰, averaging -6.5‰and -0.8‰V⁃PDB, respectively. Since they were deposited in oolitic shoals under meteoric leaching and experienced deep burial dia⁃genesis, the δ18 O values of the oolitic carbonate rocks are obviously low. During diagenesis, the influence of organic materials was slight, leading to normalδ13 C values for the oolitic carbonate rocks. Theδ18 O values of dolomites are higher than those of limestones. The results of analyses by laser⁃microsampling indicate that the δ18 O and δ13 C values of ooids are lower than those of the cements because the ooids formed in turbulent environments, and were leached by meteoric water after their formation. The cements precipitated in pore water with marine characteristics and high salinity, and were slightly influenced by meteoric water after their formation. The oolotic carbonate rocks are characterized with low Na and Sr, very high Fe, and medium Mn contents because of deep burial diagenesis. Most dolomites with low order degrees and high MgCO3 contents generally formed through replacement in brine during the burial stage. Deep burial diagenesis and good development of cements resulted in the development of ex⁃tremely low porosity in the oolitic carbonate rocks.