塔深1井寒武系白云岩储层同位素流体地球化学示踪

GEOCHEMICAL TRACING OF ISOTOPIC FLUID OF THE CAMBRIAN DOLOMITE RESERVOIR IN WELL TASHEN 1

通过对塔里木盆地沙雅隆起阿克库勒凸起东部塔深1井寒武系白云岩岩石学特征及成岩成因分析,影响塔深1井寒武系地层流体改变主要成岩有准同生期、埋藏期和后期热液改造期等。塔深1井寒武系白云岩及充填孔、洞、缝内方解石的氧、碳、锶同位素地球化学特征表明:准生期白云岩δ13CPDB值(0.9‰～1.8‰)偏正、δ18OPDB值(-10.1‰～-4.2‰)偏负反映准同生期泥微晶白云石成因属于高盐度的海水使得碳酸盐泥发生白云石化;埋藏期白云岩碳、氧随重结晶作用加强,白云岩晶粒由细向粗变化值随埋深增加,由于同位素分馏作用而偏负,δ18OPDB值(-10.02‰～-5.7‰)呈明显的下降,但δ13CPDB值(-1.4‰～0‰)组成变化不大;后期热液白云岩在热液作用下δ18OPDB值普遍低于-10‰(δ18OPDB/‰-13.1～-9.4,δ13CPDB/‰-2～-0.647);基质方解石δ18OPDB值为-10.1‰～-10.13‰,δ13CPDB值为-1.48‰～-1.62‰;充填孔洞缝粗-巨晶方解石δ18OPDB值为-10.89‰～-14.28‰,δ13CPDB值为-2‰～-3.09‰,反映准同生期→埋藏期→后期热液晶粒大小由泥微晶→细晶→中晶→粗晶氧碳同位素值逐渐变小偏负。据87Sr/86Sr(0.707284～0.746888)值均远高于现今海洋中海水的锶同位素组成(0.708)及围岩的锶同位素(0.707284),说明鞍形白云石以及方解石结晶时的孔隙流体不是残余在岩石孔隙中的同生期海水,而是外来的富含锶的流体,也就是深部热液流体。渗透回流白云石化、埋藏白云石化和高温热液白云石化等特征表明白云岩形成于超盐度、埋藏和高温热液等3种不同的环境,因此影响储层形成与分布,从而影响对白云岩的勘探。

The analysis of petrological characteristics and diagenesis of Cambrian dolomite from well Tashen 1 in eastern Akekule Salient of Shaya Uplift,Tarim Basin, revealed that. The variation of Cambrian-strata-fluid in well Tashen 1 was mainly affected by pencontemporaneous period,burial period,late hydrothermal-transformation period and so on. Based on the study of isotopes of oxygen, carbon and stontium and geochemical characteristic showed following features. The δ^13OPDB/‰ in pencontemporaneous period dolomite varied from -10.1 to -4.2,and the δ^13CPDB/‰ varied from 0. 9 to 1.8. The δ^13CPDB was partial positive and the δ^18OPDB partial negative,indicating that the formation of pencontemporaneous period microcrystalline dolomite is duo to the dolomitization of carbonate mud,and the dolomitization was a result of high-salinity seawater. In burial environment, the δ^18OPDB/‰ in burial-period dolomite varied from -10.02 to -5.7,and the δ^13CPDB/‰ from -1. 4 to 0. The stable isotopes of carbon and oxygen enhanced with recrystallization, and the dolomite grains changed from fine to coarse grained with increasing depth. Due to isotope fractionation, the δ^18OPDB showed a significant decrease, and therefore ,the value was partial negative. But the composition of δ^13CPDB showed a little change. The δ^18OPDB/‰ in late hydrothermal-transformation-period dolomite varied from - 13.1 to - 9.4. and the δ^13CPDB/‰ from -2 to -0. 647. Under hydrothermal condition,the δ^18OPDB/‰ values were generally lower than -10‰. The δ^18OPDB/‰ in matrix calcite varied from -10.1 to -10. 13, and the δ^13CPDB/‰ varied from -1.48 to -1.62. The δ^18OPDB/‰ filled in the coarse-grained calcite and megacrysts calcite varied from - 10.89 to - 14.28, and δ^13CPDB/‰ varied from - 2 to - 3.09. It was indicated that, with the time from pencontemporaneous period to burial period and to late hydrothermal-transformation period,the grain size varied from mud microcrystalline to fine grain to medium grain to coarse grain. Oxygen and carbon isotope value were decreased to partial negative. The values 0.707 284-0.746 888 of ^87Sr/^86Sr were higher than that of present ocean water （^87Sr/^86Sr=0. 708） and surrounding rock（^87Sr/^86Sr=0. 707 284）. The pore fluid in saddle-shaped dolomite and calcite crystallization,was not the pencontemporaneous period seawater remained in the pores of rock, but the outside strontium-rich fluid, that is, the deep hydrothermal fluids. In the environment of super salinity, burial and hydrotherm with high temperature, the dolostone formed finally through three types of dolomitization,including seepage reflux dolomitization, burial dolomitization and high-temperature hydrothermal dolomitization,which influenced the formation and distribution of reservoirs, and consequently influenced the future exploration of oil and gas.