塔中地区热液改造型白云岩储层

Hydrothermal alteration dolomite reservoir in Tazhong area

塔里木盆地寒武系—下奥陶统以深灰色的粉—细晶白云岩发育为特征,在塔中等地区局部可见浅灰色—白色的中—粗晶白云岩。中—粗晶白云岩中的流体包裹体均一温度主峰区间为160~200℃,盐度为6.88%~14.15%;氧同位素δ18OPDB值为-10.91‰^-7.65‰,平均为-8.98‰,与粉—细晶白云岩δ18OPDB值相比,具有明显偏轻的特征。根据白云石与水之间的氧同位素分馏系数计算出形成中—粗晶白云岩流体的δ18OSMOW值为+4.2‰^+10.2‰,据此判断该流体为岩浆热液流体。中—粗晶白云岩的87Sr/86Sr值为0.709 098~0.709 236,平均为0.709 171,高于粉—细晶白云岩和下古生界正常海相碳酸盐岩的87Sr/86Sr值。锶同位素组成特征也表明了中—粗晶白云岩的形成与热液流体作用有关。再结合其产状特征,可以推测塔中地区中—粗晶白云岩是粉—细晶白云岩在热液作用下发生重结晶作用的结果。重结晶形成的中—粗晶白云岩不但有丰富的晶间孔隙,而且还发育有大量的因热液溶蚀作用而形成的溶蚀孔隙,是良好的油气储集层,在塔里木盆地下古生界深层油气勘探中应该给予足够的重视。

Dolomite accounts for a significant proportion of the Cambrian-the Lower Ordovician strata of the Tarim Basin,which mainly consists of dark gray powder-fine crystalline dolomite.A lot of shallow gray-white medium-coarse crystalline dolomite is locally developed in the Tazhong area.The maximum frequency of homogenization temperature of the fluid inclusions in the medium-coarse crystalline dolomite ranges from 160 ℃ to 200℃,and the salinity is from 6.88% to 14.15%.The δ^18OPDB values of the medium-coarse crystalline dolomite ranges from 160℃ to 200℃, and the salinity is from 6.88% to 14. 15%. The δ^18OPDB values of the medium-coarse crystalline dolomite are from 10.91‰ to 7.65‰, and the average value is - 8.98‰, which is lighter than that of the powder-fine crystalline dolomite. The δ^18OSMOW values of the fluid are from ＋ 4.2‰ to ＋ 10.2‰ according to the fractionation factor between dolomite and water, such fluid is considered as the hydrothermal fluids. The ^87 Sr to-^86 Sr ratios of the medium-coarse crystalline dolomite range from 0. 709 098 to 0. 709 236, and the average ratio is 0, 709 171, which is higher than those of the powder-fine crystalline dolomite and the Paleozoic normal marine carbonate. The characteristics of the strontium isotope composition demonstrate that the formation of the medium coarse crystalline dolomite is relative to hydrothermal fluid. By combining with the occurrence, the medium-coarse crystalline dolomite is predicted to be the recrystallization of the powder-fine dolomite under the effect of hydrothermal fluid. The altered dolomite is good reservoir because it contains abundant intererystalline pores and dissolution pores. The hydrothermal alteration dolomite reservoir is extensively developed in the Lower Paleozoic of Tarim Basin and should be paid more attention during petroleum exploration of deep hydrocarbon reservoir in the Paleozoic.