鄂尔多斯盆地神木地区下二叠统太原组浅水三角洲沉积特征

Sedimentary characteristics of shallow water delta of the Lower Permian Taiyuan Formation in Shenmu area,Ordos Basin

鄂尔多斯盆地神木地区太原组是在北隆南倾的古地形背景下形成的以浅水三角洲为主的充填沉积。携带沉积物的河流进入海水后,由于河水与海水之间存在着较大的密度差异、侧向扩散较少,其三角洲前缘沿着海底继续向前快速推进,使水下分流河道延伸较远。研究区地形坡度平缓、水体浅,三角洲平原向前推进并进一步降低了地形坡度,从而减弱了携带沉积物的流体的动能,使得大部分沉积物在三角洲平原的分流河道中沉积下来。同时因水体浅,河口坝、席状砂等前缘沉积物常遭受进积的水下分流河道的冲刷和侵蚀而难以保存。研究区三角洲平原分流河道沉积极为发育,前三角洲相对不发育,三角洲前缘也以水下分流河道沉积为主。分流河道、水下分流河道常对下伏沉积物强烈冲刷,切割先期的沉积物乃至包括海相沉积物在内的深水沉积物。在三角洲废弃期,三角洲前缘沉积物常被潮汐作用改造。三角洲平原分流河道及三角洲前缘水下分流河道砂体呈带状分布,是天然气勘探的有利目标。

The Taiyuan Formation is dominated by shallow water delta deposits in a north uplifting and south inclining tectonic setting. Its characters are as follows： when the sediments-carrying river flew into the seawater, due to the different density between river and seawater, and a little latteral diffusion, the delta front would move along the seabed rapidly, and the subaqueous distributary channel extended a long distance. The study area is characterized by a gentle slope and shallow water setting, thus the delta plain extended forward and decreased the topographic slope furthermore which led to the decrease of energy of fluids carrying the sediments and most of the sediments were deposited in the distributary channel in the delta plain. Otherwise, due to the shallow water setting, mouth bar and sheet-like sandbodies of delta front were easily exposed by the transgressive subaqueous distributary channel and were difficult to be preserved. Distributary channel was well developed, prodelta was relatively not well developed and delta front was dominated by subaqueous distributary channel deposits. Distributary channel and subaqueous distributary channel usually strongly eroded the underlying sediments, including the sediments deposited during early periods even the deep water sediments including marine sediments. During the abandonment period of delta, sediments of delta front were usually rebuilt by tidal action. Distributary channel sandbodies of delta plain and subaqueous distributary channel of delta front are distributed in belt which are the favorable targets for natural gas exploration.