鄂尔多斯盆地三叠系延长组长6—8油层组岩相古地理特征

Lithofacies Palaeogeographic Features of Chang6-8 Oil-Reservoir Group of Triassic Yanchang Formation in Ordos Basin

鄂尔多斯盆地延长组长6—8油层组是三叠系最重要的生油层和勘探目的层，其沉积演化及岩相古地理展布特征等问题的研究备受重视。本文综合运用露头、岩心、测井等资料，采用沉积演化和层序地层充填形式相结合的分析方法，指出长8沉积期盆地西部和西南地区地形相对较平坦，以辫状河、辫状河三角洲及滨浅湖沉积为主；长7沉积期．沉积格局发生重大变革，深湖范围明显扩大，湖盆中心向西南迁移了几十千米，坳陷幅度急剧增大，半深湖——深湖区范围可达1O×1O^4km^2以上，湖盆发育进入全盛期，在盆地西缘、西南缘地区发育冲积扇、扇三角洲沉积体系；长6沉积期，湖盆开始逐渐萎缩，三角洲沉积有所发育，但半深湖——深湖沉积仍较广泛，湖盆中心继承了长7沉积期的沉积格局。长6-8沉积期．鄂尔多斯盆地经历了从坳陷→，构造沉降→湖盆开始萎缩的一个完整的湖水进退旋回；沉积环境经历了由长8油层组时期三角洲→滨浅湖沉积→长7油层组时期半深湖——深湖沉积一长6油层组时期半深湖——深湖——三角洲沉积的演化过程。

In this paper, by the integrated use of outcrops, cores, well logging data, and combination analysis of sedimentary evolution and sequence stratigraphic filling forms, the authors pointed out that, at the Chang8 Sedimentation Age, in the west- ern and south-west region of the basin the terrain is relatively flat, with braided rivers, braided river delta and littoral and shallow lake deposits as the dominant; at the Chang7 Sedimentation Age, the sedimentation pattern had fatefully changed - the scope of deep lake expanded obviously, with the lake center migrated southwest dozens of kilometers; the depression range increases dramatically; with more than 10×10^4 km^2 range of semi-deep lakes - deep lakes, into the heyday of the lake basin development. An alluvial fan and fan-delta depositional system was developed in the western margin and southwest margin of the basin; at the Chang6 Sedimentation Age, the basin began to shrink gradually. The delta deposits have developed, but the semi-deep lake - deep lake deposition was still broad. The basin center continued the pattern of the Chang7 Sedimentation Age. At the Chang6-8 Sedimentation Ages, the Ordos Basin has experienced a complete cycle of lake water advance and re- treat of ＂depression → tectonic subsidence→the basin began to shrink＂. The depositional environment experienced an evo- lution process of the Chang8 oil reservoir group age （delta - littoral and shallow lake sediments）→ Chang7 oil reservoir group age （semi-deep lake - deep lake sedimentary）→ Chang6 oil-bearing group （the semi-deep lake - deep lake - delta sedi- mentation）.