柴达木盆地西部古近系储层裂缝有效性的影响因素

Influence Factors on Fracture Validity of the Paleogene Reservoir, Western Qaidam Basin

古近系盐湖相裂缝性泥灰岩储层是柴达木盆地西部地区的重要储层类型,储层中发育剪切裂缝、纵张裂缝、顺层滑动裂缝和成岩裂缝4种类型。裂缝普遍被方解石、石膏或钙芒硝等充填,裂缝的有效性对储层评价十分重要。有效裂缝不仅提供了储层的重要储集空间和主要渗流通道,还决定了油气的富集规律和储层的潜在生产能力。裂缝的有效性主要受裂缝形成时间、溶蚀作用、第四纪以来的构造抬升剥蚀作用、异常流体高压作用、膏盐层分布和断层的活动性等因素控制。晚期形成的裂缝有效性更好;溶蚀作用可明显地增加裂缝的有效性;第四纪以来的构造抬升剥蚀作用和异常流体高压作用可使裂缝重新裂开,使裂缝的有效性变好;离膏盐层越远,裂缝的有效性越好。裂缝的有效性与现今地应力方向关系不密切,主要受断层活动的影响,与主断层平行的裂缝有效性最好。

The Paleogene saline-facies fracture marlstone is one of the important reservoir types in the western Qaidam Basin. There are four kinds of fractures developed in the reservoir： shear fracture, tensional fracture, sliding fracture and diagenesis fracture. Fractures are generally filled with calcite, gypsum and glauberite. The fracture validity is very important to reservoir evaluation. Effective fractures not only provide storage spaces and channels for fluid flow, but also determine enrichment rule and output potential of oil and gas. The fracture validity is controlled mainly by formation time, dissolution, uplift and denudation since the Quaternary, abnormal fluid high pressure, distribution of gypsum and salt beds, and fault activity. Late fractures are more effective than early fractures. Dissolution can obviously increase the fracture validity. Since the Quaternary, tectonic uplift and denudation, and abnormal fluid high pressure make filled fractures reopen, improving the fracture validity. The farther fractures distribute from gypsum and salt beds, the better the validity. The fracture validity has no close relation with current Earth stress direction but is mainly influenced by fault activities, and fractures parallel to main faults are of the best validity.