柴西缘阿尔金山前下侏罗统层序地层与岩相古地理研究

柴西缘阿尔金山前侏罗系与柴北缘侏罗系具有相似的沉积背景,发育较好的泥页岩层段,具有一定的页岩气资源潜力。通过对野外露头与钻孔岩芯沉积特征的研究,建立了柴西缘阿尔金山前下侏罗统小煤沟组层序地层格架,恢复了基于三级层序的岩相古地理,并对沉积演化特征进行分析。小煤沟组岩性主要为泥岩、页岩、粉砂岩、砂岩、砾岩,发育湖泊、扇三角洲和辫状河沉积体系,包含半深湖、滨浅湖、湖湾、扇三角洲平原、扇三角洲前缘、河床、泛滥平原等沉积相。根据区域不整合面、河流下切谷冲刷面等层序界面,将小煤沟组划分为3个三级层序,每个层序内均发育一定厚度的泥页岩段。在单剖面和对比剖面沉积相分析的基础上,以三级层序为作图单元,利用单因素分析多因素综合作图法恢复了研究区古地理面貌。小煤沟组整体呈现北部半深湖、滨浅湖,南部以及东北部扇三角洲、河流的古地理格局,地势具有东南高西北低的特点,此时阿尔金山尚未隆升,物源主要来自于南部的柴达木盆地腹部隆起以及东北部的古阿拉巴什套山,盆地沉积中心主要为清水沟以及小西沟东北地区,发育厚度巨大的泥页岩段,为侏罗系页岩气勘探的有利地区。

Sequence Stratigraphy and Lithofacies Paleogeography of the Lower Jurassic in Southern Altyn Tagh,Western Qaidam Basin

The Jurassic strata of southern Altyn tagh in the western Qaidam Basin have a similar sedimentary background as that in the northern Qaidam Basin, with well-developed shale beds and promising resource prospects. Based on the sedimentary characteristics of outcrop and borehole cores, sequence stratigraphic framework of the Lower Jurassic Xiaomeigou Formation of southern Altyn tagh was established, the lithofacies paleogeography of third-order sequence was reconstructed, and the sedimentary evolution characteristics were analyzed. The lithology of the Xiaomeigou Formation mainly consists of mudstone, shale, siltstone, sandstone, and conglomerate. Three depositional systems were classified, including braided fluvial, fan delta, and lake.The braided fluvial system is composed of river bed and floodplain facies; the fan delta system consists of fan deltaic plain and fan deltaic front facies; and the lacustrine system includes shore-shallow lacustrine, lake-bay, and semi-deep lake facies. The sequence boundaries, including the regional unconformities, fluvial incision surfaces, and abrupt change surfaces of both lithology and color were identified. According to these sequence boundaries, the Xiaomeigou Formation was subdivided into three third-order sequences. Based on the analysis of the sedimentary facies in both single and comparative profiles, the single factor analysis and multifactor comprehensive mapping method were used to reconstruct the lithofacies palaeogeography of each third-order sequence. The semi-deep lake and shore-shallow lacustrine facies mainly developed in the north, while the fan delta and the river developed in the south and northeast of the research area. From southeast to northwest, the terrain gradually decreases, and Altyn tagh had not yet risen in this period. The uplift of the Qaidam Basin in the south and the ancient Alabashitao mountain in the northeast are mainly provenance. The northeast region of Qingshuigou and Xiaoxigou is the depocenter in the prototype basin, which developed shale with large thicknesses, and is the favorable area for exploration of Jurassic shale gas.