鄂尔多斯盆地延长组次生矿物与裂缝形成时限分析

The analysis of secondary minerals and fracture formation time in Yanchang Formation of Ordos Basin

摘要鄂尔多斯盆地延长组发育大量构造裂缝，对于裂缝的形成机理、成因动力学以及与油气成藏关系都已得到了比较系统的研究，但对于裂缝中充填的次生矿物，以及次生矿物与裂缝形成时限之间的关系，尚鲜有研究。本文在次生矿物的矿化作用和成岩作用分析基础之上，应用碳氧同位素和流体包裹体分析方法，结合区域构造热事件以及盆地热演化史，对次生矿物与裂缝形成时限的约束关系进行研究。首先，通过次生矿物的镜下观察分析，获知充填次生矿物的裂缝为张性裂缝，且可判断裂缝扩展方向；根据成岩作用分析，裂缝在形成时间上早于或同期于次生矿物形成时间。其次，根据延长组储层砂岩和裂缝中碳酸盐胶结物的碳氧同位素分析，得知露头中大量被碳酸盐胶结物充填的E—W和N—S向裂缝，在形成时间上早于或与碳酸盐脉体同期形成。最后，结合流体包裹体和盆地埋藏热演化史分析，得知盆内地下大量的热液活动主要发生在140～100Ma之间（即早白垩世），也是盆地大规模成岩作用时期。基于上述分析，充填碳酸盐胶结物的E—W向和N—S向裂缝组，在形成时间上，应早于或同期于充填次生矿物的形成时间，即在早白垩世或者早白垩世之前形成。

There are a large number of structural fractures in Yangchang Formation of Ordos Basin; the crack formation mechanism, formation dynamics, relation with hydrocarbon accumulation all have get the systematic research. Secondary minerals filled in fractures, as well as relationship between secondary minerals and fracture forming time is still unrevealed. Based on the analysis of mineralization and diagenesis, the analyzing of carbon-oxygen isotopes and fluid inclusions, combining with regional tectonic thermal event and thermal evolution history of basin, is employed to research how the secondary minerals constrain the fracture forming time. First of all, through thin section observation of secondary minerals, we know that the mineral- filled fracture is tensile fracture, and can infer the fracture propagation direction. According to the diagenesis analysis, the forming time of fracture is earlier than or consistent with forming time of secondary minerals. Secondly, according to analysis of carbon-oxygenisotopes from carbonate cements in Yanchang Formation sandstone and fractures, we get the fact that forming time of E-W and N-S trending fractures filled carbonate cements in the outcrop is earlier than or the same period with carbonate veins formed. Finally, combining with the analysis of fluid inclusions and basin thermal evolution history, we know basin range hydrothermal activity mainly occur in 140 - 100 Ma （i.e., the Early Cretaceous）, is also the basin diagenesis period. Based on the above analysis, the forming time of EW and NS trending fracture filled carbonate cements should be earlier than or the same period in the formation of secondary minerals, namely forming time is the Early Cretaceous or before the Early Cretaceous.