鄂尔多斯盆地姬塬地区上三叠统坡折带、斜坡类型与油藏的关系

Relationship of Slope-Break,Slope Type with Hydrocarbon Pool-Forming of the Upper Triassic from Jiyuan Area,the Northwestern Ordos Basin

通过盆地边界性质、沉积相类型、地层厚度、沉积序列与砂体空间组合特征及砂岩的粒度变化与物性的研究,结合地震相对姬塬地区上三叠统延长组长4+5—长7油层组湖盆古地貌的恢复结果,认为鄂尔多斯盆地西北缘、西缘、西南缘、东南缘的沉积边界受构造控制,属于构造坡折带,形成以冲积扇、扇三角洲、辫状河三角洲共同发育的沉积格局;盆地东北缘和北缘为扰曲宽缓盆地边缘性质,属于沉积坡折带,发育缓坡曲流河三角洲沉积体系。姬塬地区三角洲前缘可划分为坡型前缘和台型前缘两种斜坡带类型。位于坡型前缘的砂体厚度大、物性好,是低渗背景上相对高渗带发育区,为油藏聚集的有利场所。该成果为在大型坳陷型盆地中寻找三角洲相隐蔽油气藏提供了新的研究思路与勘探方向。

The northwestern,western,southern and southeastern depositional margin of the Ordos basin is tectonic slope-break controlled by tectonic movement occurred at the end of the Triassic,where alluvial,fan-deltaic and braided-deltaic sedimentary suite developed.While the eastern and northeastern margin belong to depositional slope-break because of its gently uplifting during the late Triassic,which resulted in the meandering-deltaic sedimentary system.Two types of slopes including slope-shaped-front and platform-front can be recognized in the Jiyuan area,northwestern Ordos Basin,based upon analysis on feature of the basin margin,types of depositional facies and sediments,thickness of the strata,spatial assemblage and distribution of the sandstone bodies,and changes of the sandstone grain size,combined with seismic interpretation for topographic form of the Chang 4 ＋ 5-Chang 7 lake basin.Research result shows that sandstones at the slope-shaped-front are of greater thickness with relatively developed secondary dissolution pores and higher permeability values in delta front setting where sandstones tend to be of low porosity and very low permeability.Thus,sandstones at the slope-shaped-front with the better reservoir potential in Jiyuan area are optimistic regions for hydrocarbon accumulation.This study will provide a new idea and a fresh approach to explore and search for hidden oil pools at a delta depositional system in depression basins.