榆林地区奥陶系马五5亚段白云岩成因研究

Study of genesis of Ordovician Ma55 sub-Member dolomitite in Yulin region

鄂尔多斯盆地奥陶系中组合马五；亚段白云岩是继靖边奥陶系风化壳气藏成功勘探以来的又一重要碳酸盐岩勘探领域，目前靖边西侧南北向厚层、带状展布的马五。亚段白云岩勘探效果良好。已经形成近千亿立方米的储量规模，而盆地东部榆林地区马五，亚段白云岩呈孤立岛状分布，其形成机理及分布规律制约着对该区白云岩体的钻探及综合研究。通过岩心和薄片观察，发现孤立岛状白云岩体与其之间的石灰岩沉积组构明显不同，前者的形成更多反映了有生物活动的参与。形成一些生物丘。后者更多反映了一种干旱气候条件下的机械或化学沉积。参考现今新近系生物岛礁石灰岩能大规模云化的事实，将研究区马五。亚段碳酸盐岩与中国南海西沙群岛西琛l井新近系碳酸盐岩的地球化学特征进行了对比．发现两类白云岩较石灰岩均具有碳氧同位素正偏、锶含量偏低及海水盐度较大的特征，从而认为其云化环境可能具有相似性。都是在沉积期后高盐度的海水环境下发生的白云石化．

Ma55 sub-Member dolomite reserw^irs in the Middle Combination of Ordoviciau of Ordos Basin are another important domain for carbonate rock exploration since the successfill exploration of the Ordovician weathered crust gas reservoirs in Jingbian, northern Shaanxi. At present, the north-south zonally-distributed thick Ma55 sub-Member dolomite reservoirs in the west side of Jingbian are with good results of exploration, in which the reserves of nearly 100 billion cubic meters have been found. The Ma55 sub- Member dolomite reservoirs in Yulin area of the eastern part of the basin assume an isolated island-like distribution. Its formation mechanism and distribution law restrict the drilling and comprehensive research of dolomite in this area. Through the core and cast thin section observation, it is found that the isolated island-like dolomite is obviously different from the limestone sedimentary structure among the dolomite. The formation of the former （ the isolated island-like dolomite） reflects more of the participation of biological activities, forming some biological mound; the latter （the limestone sedimentary structure） more reflects mechanical or chemical deposition under dry climate conditions. In this paper, the geochemical characteristics of carbonate rock of the Ma55 sub-Member in the study area are compared with those of the Neogene System in the well Xiehen- 1 of the Xisha Islands in the South China Sea, taking the fact that today＇ s Neogene bio-reef limestone can be large-scale dolomilized as a reference. It is found that both types of the dolomite and limestone have the characteristics of positive bias of carbon and oxygen isotopes, low strontium content, and high salinity of seawater. Thereby the dolomitization environment may be similar. They all occurred in the high salinity seawater environment after the deposition period.