川西北地区中二叠统栖霞组白云岩形成机理——以车家坝剖面为例

Origin of dolomite in Middle Permian Qixia Formation in northwestern Sichuan Basin: a case study of Chejiaba section

曾杰 董少峰

ZENG Jie DONG Shaofeng

西南石油大学 地球科学与技术学院, 成都 610500

School of Geoscience and Technology, Southwest Petroleum University, Chengdu, Sichuan 610500, China

以岩石学观察为基础，结合稳定同位素与团簇同位素分析，探讨了川西北地区中二叠统栖霞组白云岩形成机理。川西北车家坝剖面白云岩集中发育于栖霞组上段，可识别出基质白云石与白云石胶结物2种不同类型的白云石。基质白云岩呈灰白色，与灰岩接触界线截然，呈指状交错接触；白云石胶结物呈乳白色，局限分布于溶蚀孔洞的边缘。镜下可进一步划分为3种类型基质白云石，即漂浮状细晶直面自形—半自形基质白云石(Md1)、细—中晶直面自形—半自形基质白云石(Md2)、中—粗晶它形曲面基质白云石(Md3)，以及一种鞍形白云石胶结物(Sd)。Md1沿缝合线呈漂浮状分布于灰泥基质中，可能为浅埋藏环境下压溶作用的产物。Md2与Md3具有与宿主灰岩相似的δ13C、δ18O和87Sr/86Sr同位素特征，指示白云石化流体主要来源于二叠纪同期海水，而相对较高的成岩温度说明受二叠纪末大规模火山活动的影响显著。鞍形白云石及其后的方解石胶结物具有明显偏负的δ18O值和放射性87Sr/86Sr值，说明它们形成于高温外源富放射性锶的白云石化流体中，可能为来自或穿过碎屑岩地层的深部热流体。川西北地区栖霞组白云岩的形成受原始沉积相与断裂活动的双重控制，热流体的注入使原始滩相地层发生白云石化作用，形成大量溶洞与晶间溶孔，对储层的改造起到了积极的建设性作用。

This study focuses on the origin of dolomite within the Middle Permian Qixia Formation in the northwestern Sichuan Basin by the approaches of petrographic examination and stable isotopic as well as clumped isotopic analysis. Two types of dolomite (matrix dolomite and dolomite cement) in the upper part of the Qixia Formation at the Chejiaba (CJB) section were classified. The gray matrix dolomite shows a clear inter-finger contact with limestone, while the milky white dolomite cement lines the edges of dissolved pores. Microscopic examination revealed three types of matrix dolomite including floating fine crystalline, planar-e(s) matrix dolomite (Md1); fine to medium crystalline, planar-e(s) matrix dolomite (Md2); and medium to coarse crystalline, planar-a matrix dolomite (Md3), along with saddle dolomite cement (Sd). The presence of Md1 dolomites along stylolites suggests they were formed due to pressure dissolution during shallow burial. The similar isotopic signatures of Md2 and Md3 with the host limestone indicate that the dolomitizing fluids likely originated from Permian seawater. However, elevated diagenetic temperatures suggest an influence from the Late Permian volcanic activity. The Sd dolomite and subsequent calcite cementation with negative δ18O values and radioactive 87Sr/86Sr ratios point to formation in high-temperature, externally-derived, radio-genic strontium-rich dolomitizing fluids, possibly from deep-seated thermal fluids within siliciclastic strata. The formation of dolomite in the Qixia Formation of the northwestern Sichuan Basin was influenced by both original sedimentary facies and faulting activities. The influx of hydrothermal fluids triggered widespread dolomitization, resulting in the creation of numerous dissolved and inter-crystalline pores, which played a beneficial role in reservoir development.

国家自然科学基金面上基金(42072139)资助。

https://doi.org/10.11781/sysydz202402288