塔里木盆地西北部下寒武统白云岩特征及多重白云石化模式

Characteristics and multiple dolomitization mode of the Lower Cambrian dolomite reservoir, northwestern Tarim Basin

塔里木盆地下寒武统白云岩储层研究尚处于起步阶段,开展白云石化模式系统研究对于明确白云岩储层特征具有重要意义。基于露头观察、岩石薄片鉴定、主量元素、微量元素、稀土元素和碳、氧、锶、镁同位素等资料,对塔里木盆地西北部下寒武统白云岩特征和成因模式进行了系统分析。塔里木盆地西北部阿克苏露头区下寒武统发育微生物白云岩、泥晶白云岩、粉晶—细晶白云岩和中—粗晶白云岩4类白云岩。微生物白云岩属于典型微生物白云石化成因模式,白云石化流体以海水为主。泥晶白云岩、部分粉晶—细晶白云岩和少量中—粗晶白云岩属于典型蒸发泵与回流渗透白云石化成因模式,白云石化流体以回流卤水和海水为主。绝大多数中—粗晶白云岩和部分粉晶—细晶白云岩属于埋藏热液白云石化成因模式,白云石化流体为与烃类充注相关的地层卤水及陆源水。早成岩阶段微生物白云石化作用和蒸发泵与回流渗透白云石化作用是塔里木盆地西北部阿克苏露头区下寒武统微生物白云岩和粉晶—细晶白云岩孔隙发育的基础。

Relative research about dolomite reservoirs in Lower Cambrian, Tarim Basin is still in its infancy, and carrying out systematic research on dolomitization model is of great significance to clarify the characteristics of dolomite reservoirs. Based on outcrop observation, thin section identification, geochemical analysis, and the data of major and trace elements, rare earth elements, carbon and oxygen, strontium and magnesium isotopes, we systematically analyzed the characteristics and genetic models of the Lower Cambrian dolomites in the northwestern Tarim Basin. Four kinds of dolomite were developed in the Lower Cambrian in the Aksu outcrop area in the northwest of Tarim Basin, including microbial dolomite, dolomicrite, powder-fine crystalline dolomite, and medium to coarse crystalline dolomite. Typically, microbial dolomite originated from microbial dolomitization, and the dolomitization fluid was mainly seawater. Dolomicrite and partial powder-fine crystalline dolomite resulted from evaporative pumping and reflux dolomitization, and its dolomitization fluids were dominated by brine reflux and seawater. The most of medium to coarse crystalline dolomite and some fine crystalline dolomites were formed through burial hydrothermal dolomitization, and the dolomitized fluids are formation brine and terrestrial water related to hydrocarbon charging. Both microbial dolomitization, evaporative pumping and seepage reflux dolomitization in early stage provide the basis for pore development in microbial dolomite and powder-fine crystalline dolomite.