热液白云岩定年方法研究——以四川盆地二叠系热液白云岩为例

Study on Dating Method of Hydrothermal Dolomite—A Case Study of Hydrothermal Dolomite of Permian Formation in Sichuan Basin

摘要: 近期，四川盆地多口钻井在二叠系热液白云岩储层取得勘探突破，获得工业气流。然而热液白云岩化作用发生的时期尚存在较大争议，本次研究通过岩心观察、野外勘察、阴极发光和电子探针测试，得出以下认识：1) 四川盆地二叠系热液白云岩储层发育多种热液矿物，包括鞍形白云石、萤石、闪锌矿以及氟磷灰石等矿物；2) 经过电子探针对热液矿物元素含量的分析，方解石和白云石中U、Pb含量比其它热液矿物高，且方解石中U、Pb的含量高于白云石，斑马状裂缝中发育的巨晶鞍形白云石和方解石进行U-Pb定年的效果比较好；3) 通过对基质白云岩、鞍形白云石以及方解石的U-Pb定年测试得到，基质白云岩形成的时间介于230~251 Ma，鞍形白云石形成的时间介于130~184 Ma，方解石形成的年龄介于59~82 Ma，三者在岩相学上也是具有对应的先后关系。基于以上研究，本文建立了一套基于显微观察、阴极发光、电子探针、U-Pb测试的热液矿物定年方法。

Abstract: Recently, several wells in Sichuan Basin have made exploration breakthroughs in Permian hydrothermal dolomite reservoirs and obtained industrial gas flow. However, the period when hydrothermal dolomitization occurred is still highly controversial. In this study, core observations, field surveys, cathodoluminescence and electron probe tests led to the following understanding: 1) Permian hydrothermal dolomite reservoirs in the Sichuan Basin develop a variety of hydrothermal minerals, including saddle dolomite, fluorite, sphalerite, and fluorapatite. 2) According to the analysis of hydrothermal mineral element content by electron microprobe, U and Pb contents in calcite and dolomite are higher than those in other hydrothermal minerals, and U and Pb contents in calcite are higher than those in dolomite. The U-Pb dating effect of giant crystal saddle dolomite and calcite developed in zebra-like fractures is better. 3) U-Pb chronology analysis of matrix dolomite, saddle dolomite and calcite shows that the formation time of matrix dolomite is between 230 and 251 Ma, saddle dolomite 130~184 Ma, and calcite is 59~82 Ma. The three also have corresponding sequence relationship in petrography. Based on the above research, a set of hydrothermal mineral dating methods based on microscopic observation, cathodoluminescence, electron microprobe, and U-Pb testing are established.