膏盐岩盖层封闭性动态演化特征与油气成藏——以库车前陆盆地冲断带为例

受岩石力学特征及区域构造应力的影响,膏盐岩层也会产生断裂和裂缝,大规模油气可穿盐盖层运移。膏盐岩三轴加温加压应力应变实验表明,膏盐岩具有"浅埋低温脆变、深埋高温塑变、强挤压破坏形变"的岩石力学特征,膏盐岩脆-塑转换的关键地层温度约为100℃。结合库车前陆冲断带成藏期古地温参数,确定3 000 m是库车坳陷前陆冲断带膏盐岩脆-塑转换的关键埋深,塑性膏盐岩盖层挤压流动不易破裂,已有断层因岩盐的流动而弥合、焊接,野外盐构造特征及盐岩流体包裹体显微特征提供了有力的佐证。库车前陆冲断带膏盐岩盖层封闭性地质历史时期是演化的、且空间上具有一定分布规律,从而控制着冲断带油气成藏过程及差异。

Dynamic sealing evolution and hydrocarbon accumulation of evaporite cap rocks: an example from Kuqa foreland basin thrust belt

Due to the effects of mechanical factors and regional tectonic stress, evaporites can form faults and fractures, leading to large-scale oil and gas migration through the cap rocks. The results of triaxial stress-strain experiment with heating and pressurizing show that evaporites feature embrittlement with shallow depth and low temperature, ductile deformation with deep depth and high temperature, and failure deformation with strong extrusion. The crucial formation temperature for brittle-ductile transition of evaporites is approximately 100℃. Taking into consideration the paleotemperature parameters during hydrocarbon accumulation, it is determined the crucial burial depth for brittle-ductile transition of evaporites reaches 3 000 m in Kuqa foreland thrust belt. The ductile evaporite cap rocks are hardly broken by flow during compression, and the existing faults are closed or welded due to salt rock flow. The above findings can be supported by strong evidence including field characteristics of salt structures and microscopic characteristics of fluid inclusions contained in evaporites. Therefore, the sealing of evaporite cap rocks is evolutive during the geological history in Kuqa foreland thrust belt and features specific spatial pattern, thereby controlling the process of hydrocarbon accumulation and associated difference in the thrust belt.