鄂尔多斯盆地西北部上三叠统延长组长8油层组浅水三角洲沉积特征及湖岸线控砂

Sedimentary characteristics of shallow water delta and lake shoreline control on sandbodies of Chang 8 oil-bearing interval of the Upper Triassic Yanchang Formation in northwestern Ordos Basin

鄂尔多斯盆地西北部上三叠统延长组长8油层组主要受西北沉积体系控制,沉积期构造背景稳定,发育浅水三角洲沉积。大量的岩心观察表明,煤线、植物叶片、根系、虫孔、沉积构造等浅水环境标志广泛分布,反映了沉积时水体较浅(数米至十余米)。研究区长8油层组主要发育浅水三角洲前缘亚相沉积,水下分流河道是主要的沉积微相。与典型的河控三角洲不同,其沉积相带分布范围宽广,三角洲平原与三角洲前缘、前三角洲在平面上平缓相接,无明显的转折。其中,三角洲前缘可进一步细分为三角洲内前缘和三角洲外前缘。长8油层组水下分流河道砂体延伸较远,规模较大,具有顺河道方向局部厚层砂体呈坨状分布、垂直河道方向坨状砂体围绕湖盆呈环带状展布的特征,这种砂体展布特征主要受湖岸线位置控制,即湖岸线控砂,与拗陷湖盆坡折带控砂特征明显不同。

The Chang 8 oil-bearing interval of the Upper Triassic Yanchang Formation in the northwestern Ordos Basin was deposited in a stable tectonic setting and was mainly controlled by the northwestern sedimentary systems. During its deposition, shallow water delta was developed. The shallow water environment markers, such as coal seams, plant leaves and tributed in the cores indicating the relatively shallow water meters）. The Chang 8 oil-bearing interval was dominated foots, worm holes, and so on, were widely disduring its deposition （several meters to over ten by shallow water delta front subfacies and subaqueous distributary channel was the dominant microfacies. In comparison to the typical fluvial-dominated delta, the delta in the study area is characterized by its wide distribution, the gently connecting of delta plain with delta front and prodeha on the plain with no obvious transition. The delta front can be divided into the inner front and outer front deposits. The sandbodies of the Chang 8 oil-bearing interval were distributed to a long distance with larger scales and were characterized by pod-like distribution along the channel direction and in vertical direction. The pod-like sandbodies were distributed around the lacustrine basin. The distribution of the sandbodies is mainly controlled by the lake shoreline which is obviously different from the pattern of slope break belt controlling sandbodies in the depressional lacustrine basins.