四川盆地西北部中二叠统栖霞组白云岩特征及成因探讨

白云岩特征及其形成机制的研究对白云岩化理论与白云岩储集层勘探具有重要意义。本文以四川盆地西北部中二叠统栖霞组白云岩为研究对象,通过野外剖面及岩心观察并采用铸体薄片、阴极发光、地球化学分析等方法对该段白云岩特征及成因进行研究。结果表明川西北地区栖霞组白云岩主要表现为块状晶粒白云岩、角砾状白云岩及斑状白云岩3种类型;碳同位素为正低值,氧同位素为负值,反映出其成岩过程中伴有早期淡水参与。结合工区构造沉积背景,对比以往经典白云岩化模式,发现四川盆地西北部中二叠统栖霞组白云岩化成因模式具有一定自身特点。通过对Folk “镁笼效应”理论进行延伸,认为栖霞组白云岩化模式在纵向上分为淡水渗流带、混合渗流带、混合潜流带及海水潜流带4个区带,其中混合渗流带和混合潜流带是白云岩发育的主要位置。在混合渗流带中,淡水与海水交替进入,未完全成岩的海洋沉积物质在成岩过程中受到淡水冲洗导致大量镁离子随淡水流入下部地层,而后又有海水携带镁离子进行补给,这为白云石形成提供了有利条件;混合潜流带内不断进入的混合流体携大量镁离子使得该带具有较高Mg/Ca值;而地幔上涌造成的地温升高克服了白云石形成的动力学屏障,有效促进了白云岩的形成。

Characteristics and genesis of the Middle Permian Qixia Formation dolomite in the northwestern Sichuan Basin

The study of dolomite characteristics and its formation mechanism is of great significance for dolomite theory and dolomite reservoir exploration. In this paper, the dolomite of the Middle Permian Qixia Formation in the northwestern Sichuan Basin is studied. The characteristics and genesis of the dolomite are studied by field profile and core observation and using the cast thin section and cathode luminescence and geochemical analysis methods. The results show that the main types of dolomite in Qixia Formation are massive grain dolomite, karst brecciate dolomite and porphyry dolomite. The carbon isotope is positively low and the oxygen isotope is negative which reflects the participation characteristics of early fresh water in the process of rock formation. Based on the tectonic and sedimentary background of the work area and compared with the previous classical dolomitization model, we find that the dolomitization model of the Middle Permian Qixia Formation in the northwestern Sichuan Basin has its own characteristics. By extending the "magnesium cage effect" theory from Folk, the dolomitization model of Qixia Formation can be divided into four zones：fresh-water seepage zone, mixed seepage zone, mixed underflow zone and seawater underflow zone, in which the mixed seepage zone and mixed underflow zone are the main layer of dolomite. In the mixed seepage zone, freshwater and seawater enter alternately. During the diagenesis process, the sediment is flushed by fresh water, resulting in a large number of magnesium ions flowing into the lower stratum along with fresh water, and then the sea water carries magnesium ions for replenishment, which provides favorable conditions for the formation of dolomite. The mixed fluid constantly entering the mixed underflow zone carries a large number of magnesium ions, which makes the zone have a high Mg/Ca value. The earth temperature rise caused by mantle upwelling overcomes the dynamic barrier of dolomite formation and effectively promotes the formation of dolomite.