准噶尔盆地南缘中-下侏罗统浅水三角洲类型及其构造和气候指示意义

Lower to Middle Jurassic shallow-water delta types in the southern Junggar Basin and implications for the tectonic and climate

大型坳陷湖盆浅水三角洲砂体已成为我国陆相盆地岩性油气藏勘探的重要目标.中生代,准噶尔盆地开始全面进入陆相坳陷湖盆发展阶段,早-中侏罗世天山逐步的准平原化,为准噶尔盆地南缘(简称准南)浅水三角洲的发育提供了非常有利的地形条件.但目前准南中-下侏罗统沉积相研究仍缺乏系统性且存在较大的争议.本文对准南多条剖面中-下侏罗统地层进行了精细的沉积特征观察,并结合粒度分析,最终识别出3种不同的湖泊浅水三角洲相:(1)浅水辫状河平原三角洲,见于八道湾组,具有线状近物源,砂砾岩为主,发育广阔的水上和水下分支河道,河口坝和远砂坝欠发育;(2)浅水河口坝型曲流河三角洲,见于三工河组,河口坝非常发育,受湖浪影响横向延展性强;(3)极浅水曲流河三角洲,见于西山窑组,以发育广阔的三角洲平原为特征,前缘欠发育.中-下侏罗统三角洲相模式的转变体现了一个先湖侵后湖退的过程,与物源区持续后退、盆地持续扩张相矛盾,指示了中侏罗统西山窑组相较于下侏罗统八道湾组和三工河组相对干旱的气候或盆地基底的整体抬升.侏罗纪气候变化和盆地内部缓慢隆起共同控制了准噶尔盆地南缘可容纳空间的变化.

Shallow-water lacustrine delta sandstone in large continental sag basins have become the most important target for the stratigraphic reservoirs. The Junggar Basin was a continental sag basin in the Mesozoic. From the Early to Middle Jurassic, the Tian Shan experienced a regional peneplanation, providing a very favorable topographic condition for the developing of shallow-water deltas. However, studies on the sedimentary facies of the Lower to Middle Jurassic generally lack of systematicness and remain controversial. This article selected several sections in the southern Junggar Basin and carried out detailed sedimentary characteristic analysis and sedimentary facies identification in each section. Three shallow-water lacustrine deltas were identified in the southern Junggar Basin： 1） shallow-water braid plain delta, well developed in the Badaowan Formation and characterized by threadiness near-sources, abundant coarse-grained sediments, widespread distributary channels and undeveloped mouth bars; 2） shallow-water mouth-bar meandering-river delta, common in the Sangonghe Formation and showing well-developed mouth bars generally reformed by lake waves; 3） extremely shallow-water meandering-river delta, well developed in the Xishanyao Formation and characterized by widespread delta plain and undeveloped delta front. According to the lake delta evolution, a lake transgression to regression process happened in the southern Junggar Basin from the Early to Middle Jurassic, which is inconsistent with the continuous retrogradation of provenance and expansion of the Junggar Basin, indicating a relatively dry climate or basement uplift of the Junggar Basin during deposition of the Xishanyao Formation in contrast to the Early Jurassic Badaowan and Sangonghe Formations. Climate changes and basin basement slowly uplift collectively controlled the evolution of accommodation.