玛湖凹陷二叠系夏子街组限制性湖盆扇三角洲沉积特征

根据岩心观察、测井和地震资料解释成果等，利用古地貌、岩相特征、单井相及连井沉积相对比，对玛湖凹陷二叠系夏子街组的沉积背景和沉积体系进行了系统分析和研究。研究结果表明：①受可容空间控制，玛湖凹陷二叠系夏子街组岩性多为砂砾岩，发育扇三角洲沉积，构造运动形成的古地貌使得玛湖凹陷夏子街组的沉积空间受到限制，最终表现为“北断、西剥、东南超”的构造格局；②玛湖凹陷夏子街组发育四大物源沉积体系：北部地区为长轴远物源，砂地比低，储层较薄；西部、南部、东部等地区主要为短轴近物源，快速沉积，垂向厚度大，相带平面展布较窄，但在南部地区存在1套近南北向的长轴远物源；③玛湖凹陷夏子街组主要为粗砾岩和中砾岩，分选性较差，磨圆度中等，杂基含量较高，主要分布于坡折带附近，并发育于辫状河道和水下分流河道内；④夏子街组沉积时期，玛湖凹陷靠山一侧可容空间大，发育巨厚的扇三角洲砂砾体且延伸较局限，玛南地区的狭长沟谷地貌为沉积物搬运至玛湖凹陷中部提供了通道，这种长轴远物源沉积体系可形成规模较大的有效储层，砂地比可超过50%，沉积物分选性、磨圆度均相对较好，是有利储层发育带。

Sedimentary characteristics of fan delta in restricted lacustrine basin of Permian Xiazijie Formation in Mahu Sag

Based on core observation,well logging and seismic interpretation,the sedimentary settings and sedimentary system of Permian Xiazijie Formation in Mahu Sag were systematically analyzed by using paleogeomorphology,lithofacies characteristics,single well facies and well-tie sedimentary facies contrast. The results show that:(1)Controlled by the accommodation space,the lithologies of Xiazijie Formation in Mahu Sag are mostly sandy conglomerate,and fan delta deposits are developed. The paleogeomorphology formed by tectonic movement controls the sedimentary space. The tectonic pattern shows "fault in the north,denudation in the west,and overlap in the southeast".(2)There are four major provenance systems in Xiazijie Formation in Mahu Sag. The northern region is a long axis far-source area with low sand-stratum ratio and thin reservoirs. The western,southern and eastern regions are mainly short-axis near-source areas with rapid deposition,large vertical thickness and narrow plane distribution of facies zone. However,a long-axis far-source is developed in the southern region.(3)The Xiazijie Formation in Mahu Sag is mainly composed of coarse and medium conglomerates with poor sorting,medium roundness and high content of matrix. The reservoirs are mainly distributed near the slope break zone and developed in braided channels and underwater distributary channels.(4)During the sedimentary period of Xiazijie Formation,the accommodation space is large at the mountain side of Mahu Sag where huge thick fan delta sand gravel body developed and extended limitedly. The narrow and long valley landform in Manan area provides a channel for sediment transportation to the middle of Mahu Sag. This long axis far-source sedimentary system can form large-scale effective reservoirs,the sand-stratum ratio is greater than 50%,and the sediment sorting and roundness are relatively good,which is a favorable reservoir development zone.