鄂尔多斯盆地三叠系延长组砂岩钠长石化与热液成岩作用研究

鄂尔多斯盆地延长组储层砂岩发育大量自生钠长石矿物,其成因和产出与浊沸石有关,含量和分布比浊沸石多而广泛,但没有引起人们的注意。大量详细岩相学观察发现延长组储层砂岩发育丰富的自生钠长石,其主要为由斜长石碎屑蚀变形成或呈胶结物充填分布在砂岩碎屑颗粒之间。钠长石中含有大量原生的发亮黄色荧光的油气包裹体,表明其形成与油气注入同时。电子探针成分分析表明,钠长石Na2O含量较高,几乎为斜长石的纯钠长石端员(NaAlSi3O8),没有钙长石端员(CaAl2Si2O8)。激光拉曼光谱特征反映出钠长石为沉积成岩期形成的低温钠长石,而非来源于岩浆岩或者变质岩区的碎屑钠长石。应用LA-MC-ICP/MS原位微分析技术对钠长石进行的稀土元素分析表明,延长组砂岩钠长石具有热水成岩作用地球化学特征,属于热液成岩作用产物。认为大量钠长石形成与石油充注的同时进行导致了储层致密过程中岩性油藏的形成。延长组储层砂岩中热液成岩作用对油藏形成和分布意义重大,值得重视和研究。

Albitization and hydrothermal diagenesis of Yanchang oil sandstone reservoir, Ordos Basin

The Triassic low permeability reservoir sandstone of Yanchang Formation in Ordos Basin is composed of fine-grained arkose and feldspathic litharenite. Lots of research work has been devoted to the diagenesis of reservoir sandstone of Yanchang Formation.It is considered that the formation of secondary pore through dissolution of laumontite improved the reservoir property of sandstone,and the dissolution of laumontite was closely related to alteration of plagioclase.According to the equilibrium theory of mineral reaction,the laumontite formed by the alteration of plagioclase accompanied by the formation of albite is two times as many as the laumontite in quantity.Therefore,if laumontite of sandstone in Yanchang Formation was dissolved to form dissolution pores,the content of albite with more stability should be higher in theory.The sandstone of Yanchang Formation actually contains plentiful albite which was altered from plagioclase fragments and cements of authigenic minerals that filled the fragments.Microscopic observation indicates that plagioclase fragments were altered to form the assemblage of albite,laumontite and kaolinite.The formation of authigenic minerals of albite and kaolinite caused sandstone compaction and led to the formation of low permeability and low porosity reservoirs.Many primary hydrocarbon inclusions with bright yellow fluorescence contained in cleavages of albite show that albite was formed synchronously with reservoir compaction and oil accumulation,indicating that the formation and distribution of albite were closely related to the formation and distribution of reservoirs.The existence of albite and its significance were ignored because previous researchers focused their attention on laumontite dissolution.Composition analysis by means of electronic microprobe shows that chemical composition of albite in sandstone of Yanchang Formation reservoir is similar to that of standard albite mineral,in which the SiO2 is from 67.07% to 68.51% and Al2O3 from 19.59% to 20.49%.Na2O content of albite is high while K2O and CaO content is very low.The albite is almost pure albite end member of plagioclase(NaAlSi3O8) without anorthite member of plagioclase(CaAl2Si2O8).Laser Raman spectrum analysis shows that albite in sandstone of Yanchang Formation is characterized by four obvious peaks at 293 cm-1,329 cm-1,407 cm-1 and 458 cm-1 respectively,implying that the albite is a sedimentary mineral formed during the diagenetic stage at the low temperature rather than a detrital mineral of magmatic or metamorphic rocks.The in-situ micro-analysis technology of LA-MC-ICP/MS was employed to analyze REE of albite.The results show that REE distribution curves are gentle or somewhat rightly-oblique.The average ΣREE of albite is 138.74×10-6,which is close to that of the upper crust.The average δEu is 1.62,assuming strong positive Eu anomaly.The average δCe is 0.74,assuming weak negative Ce anomaly.Rare earth fractionation is weak with average(La/Yb)n of 4.95 and average(La/Sm)n of 1.88.Characteristics of REE suggest that albite is a product of the hydrothermal diagenesis.Combined with regional geological evolution history of Ordos Basin,the authors hold that albitization of sandstone reservoirs in Yanchang Formation of Ordos Basin was related to hydrothermal activity of Yanshanian period.It is suggested that the hydrothermal diagenesis of sandstone was closely related to the formation and distribution of oil reservoirs and therefore deserves attention and research.