砾质辫状河型冲积扇岩相类型、成因及分布规律 ——以准噶尔盆地西北缘现代白杨河冲积扇为例

冲积扇砂砾岩储层是准噶尔盆地一类重要的油气储层类型,由于其具岩相类型多、连续性差等特点,对冲积扇内部岩相成因解释一直是冲积扇相带认知的基础和难点。以准噶尔盆地西北缘现代白杨河冲积扇为例,在大量的野外露头资料和粒度分析数据的基础上,结合冲积扇源区母岩类型、水文资料以及冲积扇文献资料,对现代白杨河冲积扇岩相的类型、成因及分布规律进行探讨。按沉积机制,白杨河冲积扇属于辫状河型冲积扇,具有规模大(扇体总面积约327.6 km^2),坡度平缓(约1‰～7‰),沉积粒度粗等特征。在白杨河冲积扇内共可识别出16种岩相类型,并根据岩相形成的流体动力差异划归为5类成因,即重力流成因、高流态牵引流成因、低流态牵引流成因、静水沉积成因以及风成沉积成因。重力流以洪流沉积为主;高流态牵引流主要包括片流沉积和湍流沉积;低流态牵引流以砂(砾)质河道沉积为主;静水沉积以蓄水细粒沉积为主;风成沉积以风携细粒沉积为主。根据各岩相沉积构造、粒度特征及展布规模,可将岩相划分为四类:Ⅰ类岩相沉积构造特征明显并具有较大展布规模;Ⅱ类岩相沉积构造特征明显但展布规模局限;Ⅲ类岩相为不具层理构造但具有较大展布规模的岩相;Ⅳ类岩相不具层理构造并且展布规模局限。其中Ⅰ类和Ⅱ类岩相多为牵引流成因,多发育于洪水期扇体扇中、扇缘区域以及间洪期扇体的扇中区域,并可在地下继承性发育为较好的储集相带。

Research on Lithofacies Types,Cause Mechanisms and Distribution of a Gravel Braided-River Alluvial Fan: A case study of the modern Poplar River alluvial fan,northwestern Junggar Basin

Conglomerate reservoirs in alluvial fans are an important oil and gas reservoir type in the Junggar Basin. However, alluvial fan glutinite reservoirs consist of several lithofacies and poor continuity, which causes the lithofacies interpretation of the alluvial fan facies to be a fundamental and difficult research area. As an example, the lithofacies mechanism and distribution of the modern Poplar River alluvial fan at the northwestern margin of the Junggar Basin is discussed on the basis of a large amount of outcrop data, source rocks, hydrological data and alluvial fan studies reported in the literature. According to the deposition mechanism, the Poplar River alluvial fan is a braided-river alluvial fan with features typical of its large scale (fan total area is 327.6 km2), gentle slope (about 4‰-7‰), and rough sediments. Sixteen lithofacies were recognized; their hydrodynamic differences indicate that they may be classified into five causes:debris flow, high-flow traction current, low-flow traction current, hydrostatic deposition, and aeolian deposition. Debris flow is mainly composed of flood deposits; high-flow traction current mainly includes sheet-flood deposits and turbulent deposits; low-flow traction current is mainly composed of (gravelly) sand channel sediments; hydrostatic deposits are dominated by fine material, and aeolian deposits mainly consist of wind-transported sediments. The lithofacies are divided into four types according to sedimentary structure, granularity and distribution scale. The first type has an obvious sedimentary structure and wide distribution. The second type has obvious sedimentary structural characteristics but limited distribution. The third type, which is present on a large scale, has characteristics in which the sedimentary structures are not apparent. The sedimentary structure and physical properties of the fourth type are also not apparent, and occur only on a limited scale. The first and second types of lithofacies are mainly caused by traction currents that mainly form in the middle and distal parts of the flood period, and in the middle part of the inter-flood stage; those lithofacies form higher-quality reservoirs underground in due course.