塔河地区下奥陶统蓬莱坝组白云岩特征与成因分析

Characteristics and genesis analysis of dolomite in Lower Ordovician Penglaiba Formation in Tahe area

徐勤琪 彭军 何成其 夏金刚 刘瑞 黄苇

XU Qinqi PENG Jun HE Chengqi XIA Jingang LIU Rui HUANG Wei

中国石化 西北油田分公司 勘探开发研究院, 乌鲁木齐 830011 西南石油大学 地球科学与技术学院, 成都 610500 天然气地质四川省重点实验室, 成都 610500

Exploration and Production Research Institute, Northwest Oilfield Company, SINOPEC, Urumqi, Xinjiang 830011, China School of Geoscience and Technology, Southwest Petroleum University, Chengdu, Sichuan 610500, China Sichuan Provincial Key Laboratory of Natural Gas Geology, Chengdu, Sichuan 610500, China

下奥陶统蓬莱坝组白云岩是塔河油田油气勘探的重要接替领域，明确白云岩的沉积特征、成因机理对塔河油田的油气勘探与开发具有重要意义。综合岩心描述、薄片鉴定、扫描电镜、阴极发光及地球化学等分析手段，系统研究了塔河油田下奥陶统蓬莱坝组白云岩的地质地球化学特征及成因机制。结果表明：(1)研究区以晶粒白云岩为主，残余颗粒白云岩次之。泥—微晶白云石以他形为主，粉—中晶多呈半自形—自形，中—粗晶以异形为主；残余颗粒白云岩以粉—细晶为主，可见残余内碎屑结构。(2)阴极发光显示：泥—微晶呈暗淡蓝紫或暗褐色光；粉晶为暗淡紫红色；细—中晶具紫色核心、橙红色亮边环带结构；中—粗晶呈紫红与明亮棕红色环带发光。(3)泥—微晶白云岩主要为重稀土富集，δ18O值波动大，87Sr/86Sr值接近早奥陶世海水值，指示高盐度蒸发海水的准同生白云石化作用；残余颗粒白云岩主要为残余内碎屑结构，属埋藏云化成因；粉晶白云岩、细—中晶白云岩稀土元素(REE)配分模式与同期泥晶灰岩类似，重稀土富集，δ18O值跨度较大，87Sr/86Sr值部分高于同期海水锶同位素值，为浅—中埋藏云化叠加热液改造的产物；中—粗晶白云岩轻稀土相对富集，δ18O值偏负，87Sr/86Sr值高于同期海水值，属碎屑岩层热液交代成因。

The dolomite of the Lower Ordovician Penglaiba Formation is an important successor field for oil and gas exploration in the Tahe oilfield. Clarifying the sedimentary characteristics and genetic mechanisms of the dolomite is of great significance for oil and gas exploration and development in the Tahe oilfield. Based on this, the geological and geochemical characteristics and genetic mechanisms of the dolomite in the Lower Ordovician Penglaiba Formation of the Tahe Oilfield were systematically studied through core description, thin section identification, scanning electron microscopy, cathodoluminescence, and geochemical analyses. The results showed that: (1) The study area was dominated by crystalline dolomite, followed by residual granular dolomite. Mud-microcrystalline dolomite was mainly anhedral. Silty-medium crystalline dolomite was mostly subhedral to euhedral. Medium-coarse crystalline dolomite was predominantly xenomorphic. Residual granular dolomite was mainly composed of silty-fine crystalline dolomite, showing residual intraclastic texture. (2) Cathodoluminescence showed that the mud- microcrystalline dolomite emitted dull blue-purple or dark brown luminescence. Silty dolomite emitted dull purple-red luminescence. Fine-medium crystalline dolomite had a purple core with a bright orange-red rim-shaped band. Medium-coarse crystalline dolomite exhibited purple-red and a bright brown-red rim-shaped luminescent band. (3) Mud-microcrystalline dolomite was mainly enriched in heavy rare earth elements (REE), with large δ18O fluctuations and 87Sr/86Sr ratios close to those in the Early Ordovician seawater, indicating quasi-syngenetic dolomitization by high-salinity evaporative seawater. Residual granular dolomite was mainly characterized by residual intraclastic texture and its genetic mechanism was burial dolomitization. The REE distribution patterns of silty crystalline dolomite and fine-medium crystalline dolomite were similar to those of contemporaneous micrites, which were enriched in heavy REEs with large δ18O fluctuations. The 87Sr/86Sr values were partially higher compared with the strontium isotope values in contemporaneous seawater, indicating products of shallow-medium burial dolomitization superimposed with hydrothermal fluids. Medium-coarse crystalline dolomite was relatively enriched in light rare earth, with negative δ18O values and 87Sr/86Sr values higher than those in contemporaneous seawater, indicating formation by hydrothermal metasomatism related to clastic rock layers.

中国石化西北油田分公司勘探开发研究院测试分析项目“塔里木盆地碳酸盐岩储层发育机理与重点区带评价方法” 34400008-22-ZC0613-0064

https://doi.org/10.11781/sysydz2025040805