塔中地区与热液有关白云岩的形成机理探讨

A discussion on the formation mechanism of dolomite associated with hydrothermal solution in Tazhong area

与热液有关白云岩以晶体粗大为特征,较易形成物性良好的油气储集体。塔中地区寒武系-下奥陶统发育大量深层白云岩,以粉-细晶白云岩为主,局部可见中-粗晶白云岩。该中-粗晶白云岩流体包裹体均一温度经压力校正主要集中在173-199℃,比该层位最大埋藏温度（约160℃）高10-40℃,故认为其形成与热液活动有关。镜下观察两种类型的白云岩存在渐变过渡现象和碳氧同位素值的部分重叠,说明中-粗晶白云岩可能由粉-细晶白云岩在热液作用下重结晶而来。进一步结合其流体包裹体盐度（为正常海水盐度的3-5倍）和富^87Sr、贫^13C的特征认为,其成岩流体可能为岩浆热液经过下寒武统富^87Sr、贫^13C泥页岩质烃源岩的改造,并向上运移与中寒武统膏岩层孔隙间浓缩海水混合而形成的混合性流体。前者为其提供热量,后者可增加其盐度,遍布区内的深大断裂体系为其提供了运移通道。

The dolomite associated with hydrothermal solution is characterized by coarse crystallization and can form reservoirs with high porosity and permeability.There are a lot of deep buried dolomites in Cambrian and Lower Ordovician strata of Tazhong area,which mainly consist of powder-fine crystalline dolomite, and medium-coarse crystalline dolomite can be found in some parts of this area. A study of fluid inclusions shows that the medium-coarse crystalline dolomite has a fluid inclusion homogenization temperature of 173～199℃ corrected by pressure,which is 10～40℃ higher than its maximum burial temperature（about 160℃）,so its formation should be related to hydrothermal activity.The powder-fine crystalline dolomite has a δ13CPDB of-2.45‰～-1.08‰,-1.54‰ on average;its δ18OPDB is-6.90‰～-3.54‰,-5.35‰ on average;the medium-coarse crystalline dolomite has a δ^13CPDB of2.76‰～-1.12‰,-1.73‰ on average,which mostly overlap those from the powder-fine crystalline dolomite and is a little bit lower than the latter.Its δ^18OPDB is-10.91‰～-5.18‰,-7.44‰ on average,obviously lower than those from the powder-fine crystalline dolomite with partial overlap.The two types of dolomite show gradual change into each other under microscope.So the conclusion can be reached that the medium-coarse crystalline dolomite might have resulted from the recrystallization of powder-fine crystalline dolomite due to the hydrothermal activity.The powder-fine crystalline dolomite has ^87Sr/^86Sr ratios of 0.708 495～0.709 006,0.708 814 on average;the medium-coarse crystalline dolomite has ^87Sr/^86Sr ratios of 0.708 727～0.709 985,0.709 165 on average, suggesting the latter has more ^87Sr than the former.In addition,it has a fluid inclusion salinity of 10.73%～18.96%（NaCl equivalent）,which is 3～5 times that of normal seawater,so it can be concluded that the diagenetic fluid might have been the mixture of the magmatic hydrothermal solution and the concentrated pore seawater in the Middle Cambrian gypsolyte.The former brought the heat from the lower part of the bed and was transformed by the clay shale hydrocarbon source rocks in the Lower Cambrian which had more 87Sr and less 13C;the latter could increase its salinity,and the deep-large fault system spreaded all over the area might have been the pathway for fluid migration.