塔里木盆地深层寒武系地表岩溶型白云岩储层及后期流体改造作用

对塔里木盆地钻遇寒武系白云岩的塔深1等井的岩芯观察发现埋深超过8000m的深层寒武系白云岩中仍发育有丰富的几毫米至几厘米大小的地表岩溶作用形成的溶蚀孔洞。塔深1井测井曲线的突变反映出了多个寒武系沉积间断(暴露)面的存在;但塔深1井附近寒武系地震剖面同相轴连续性较好,没有明显的角度不整合接触。测井和地震特征表明了寒武系白云岩岩溶作用发生在同生或准同生期,是由于相对海平面下降造成的暴露作用,而没有发生强烈的构造抬升。塔深1等井寒武系白云岩溶蚀孔洞中常见有白云石、石英等矿物的充填。充填白云石具有弯曲晶面和波状消光的特征。白云石中流体包裹体均一温度主峰区间110～130℃,没有随深度增加而增加的特点。与基质白云岩相比,充填白云石具有相对较低的Na、K和Sr的含量以及相对较高的Fe、Mn和Ba含量。在同位素组成上,充填白云石具有相对较轻的氧同位素组成和较高的n(87Sr)/n(86Sr)比值;其氧同位素δ18OPDB的变化范围为-11.62‰～-5.13‰,平均为-8.00‰,锶同位素n(87Sr)/n(86Sr)比值变化范围为0.709361～0.709978,平均为0.709590。上述特征表明了白云石等的充填作用与深部热液活动有关。受深部热液作用强度的限制,寒武系白云岩中已有的岩溶型孔隙并没有被完全充填破坏,仍有较多的储集空间得以保存下来。

Deep Cambrian Surface karst Dolomite Reservoir and Its Alteration by Later Fluid in Tarim Basin

Observation of cores from wells such as TS1 and others which have drilled into the Cambrian dolomite indicates that there are plenty of pores which are some millimeters to some centimeters in size and were were though to be developed in surface-karst environment.Some of the Cambrian dolomite was buried to a depth more than 8000m.Abrupt variations of log curves of the TS1 well reflect several boundaries in the Cambrian dolomite,and the boundaries are related to deposition absence intervals or exposure to surface.However,seismic events within the Cambrian dolomite seismic profile near the TS1 well are well continuous indicating that there is not remarkable angular unconformity.Characteristics of well logs and seismics suggest that the karst dissolution of the Cambrian dolomite took place contemporaneously or penecontemporaneously during deposition due to exposure caused by fall of relative sea level and that there was not strong tectonic uplift.There are always some minerals such as dolomite and quartz filling in vugs of the Cambrian dolomites in the well TS1 and others.The filling dolomite shows curved crystal surface and wave extinction.The maximal frequency of the homogenization temperatures of the fluid inclusions in the filling dolomite is between 110~130℃ and the homogenization temperatures do not increase linearly with depth.Compared with matrix dolomite,the filling dolomite has relatively low Na,K and Sr but relatively high Fe,Mn and Ba contents.In isotope compositions,the filling dolomite is relatively light in oxygen isotope but has relatively high n(87Sr)/n(86Sr) values.The δ18OPDB values are between-11.62‰ and-5.13‰ and are-8.00‰ in average,and the n(87Sr)/n(86Sr) between 0.709361 and 0.709978 and 0.709590 in average.The above characteristics indicate that the infillings of the dolomite and other minerals are related to activities of deep hydrothermal fluids.Limited by strength of hydrothermal,the previously existed dissolution pores in dolomite were not completely destroyed by mineral filling,however,there are still some portion of the dissolution pores preserved.