川西北下二叠统栖霞组斑马状白云岩地球化学特征及形成过程——以ST18井为例

四川盆地下二叠统栖霞组具有丰富的天然气资源。近年来，川西北地区在栖霞组取得了重大的油气勘探突破，其中优质储层主要位于白云岩层位。然而，有关该区栖霞组白云岩的成因至今仍有争议。ST18井钻遇栖霞组，岩心观察发现特殊的斑马状白云岩。针对斑马结构中的亮带和暗带，系统开展了岩石学和地球化学的对比研究。斑马结构中亮带由中粗晶—鞍形白云石组成，阴极发光呈亮红色；而暗带则由中—细晶白云石组成，阴极发光呈暗红色。亮带和暗带白云石的δ¹³C值均在同时期海水δ¹³C值范围内，而δ¹⁸O值较同时期海水偏负，表明白云石化流体来源于海水，且受到较高温度的影响，使得氧同位素发生分馏。亮、暗带白云石有序度均较高，推测是在后期埋藏作用下发生了调整，使得有序度增加到同一水平。此外，热液矿物黄铁矿的发现进一步证实了白云岩形成过程中热液流体的参与。研究表明，ST18井栖霞组斑马状白云岩的形成过程主要经历了以下三个阶段：(1)流体沿着构造裂缝溶蚀—交代原始灰岩，形成暗带白云石；(2)热流体沉淀形成亮带白云石；(3)深埋藏环境下亮暗带白云石均发生调整，形成有序度较高的斑马状白云岩。

Geochemical characteristics and formation process of zebra dolomites in Lower Permian Qixia Formation,northwestern Sichuan Basin: a case study of well ST 18

Abundant natural gas resources are stored in the Lower Permian Qixia Formation of the Sichuan Basin. In recent years, great breakthrough for oil and gas exploration has been achieved in the Qixia Formation in the NW Sichuan Basin, and high-quality reservoirs mainly locate in dolomite strata. However, the origin of dolomites in the Qixia Formation is still unclear. Special zebra-textured dolomites were observed in the Qixia Formation from the cores of well ST 18. In this paper, a comparative study of the light and dark belts in the zebra texture was systematically carried out with petrological and geochemical methods. Results show that the light belt is mainly composed of medium to coarse crystalline saddle dolomites and its cathodoluminescence is bright red, while the dark zone is composed of medium to fine crystalline dolomites and its cathodoluminescence is dark red. The δ13C values in light and dark zones are within the δ13C range of contemporary seawater, while the δ18O values are more negative than those of contemporary seawater. It is then indicated that the dolomitization fluid was derived from seawater and the oxygen isotopes were fractionated by the affection of high temperature. The order degrees of dolomites in the light and dark zones are consistently high. It was speculated that the order degrees of dolomites in the light and dark zones had been matured to the same degree during the later burial stage. In addition, the occurrence of pyrite confirmed the involvement of hydrothermal fluids in the formation of dolomites. Therefore, the formation of zebra dolomites in well ST 18 mainly went through the following three stages:(1) Fluids dissolved and dolomitized primary limestones along tectonic fractures to form medium to fine crystalline dolomites in the dark zone; (2) Hot fluids precipitated to form medium to coarse crystalline saddle dolomites in the light zone; (3) The order degrees of dolomites in the dark and light zones had adjusted to the same degree in the later deep burial stage.