Study On Sedimentary System Of The Benxi Formation In The Southeastern Ordos Basin

There are abundant natural gas resources in the Benxi Formation of Southeastern Ordos basin,which is a very prospecting region.With many kinds of lithology,the sediment facies of Benxi Formation are complex and variable.Previous researchers taken the total formation as one unit when they studied the sediment character of whole basin,as a result it's not systematic and not precise for the Benxi Formation of our study area.So it's significant to learn the deposit character and sediment system of Benxi Formation in the Southeastern Ordos basin.Based on the former research,we use core data,well log data,microscope data and many other analysis data to decide the sediment character and deposit sequence of each face under the principle of sedimentology and sequence stratigraphy,then corresponding core and well log to build the logging facies template.After that,we divide the sediment facies through log data under the guidance of logging facies template and sediment model to discuss the distribution regulation of facies and sandbody.At last,combining efficient sand contour,sand contour,limestone contour and coal contour,this paper will explain the sediment distribution of Benxi Formation in plane.The main achievements are as follows: 1 Considering the factors such as marked bed,sediment environment,sequence and source-reservoir combination,we defines the Benxi Formation as a bed from the top of Majiagou Limestone to the bottom of Miaogou Limestone,than divide the formation into two Members by Jinci Sand.2 This conclusion is than applied to stratigraphic correlation and we found that with a thickness of 10~50m the Benxi Formation presents a regular of northern and eastern are more thicker than southern and western.Furthermore,the 2th Member is thicker than 1th Member.3 We recognize 5 types facies from Benxi Formation according to core data,tidal flat,barrier,lagoon,tidal delta and washover fan,and tidal flat can be divided into subtidal zone,intertidal zone and supratidal zone.Further cross bedded conglomerate and coarse sand are developed very well in subtidal zone,above which is the tidal bedding of intertidal zone.While the supratidal is characterized by mud flat and swamp.Barrier is constitutes of mid-coarse sand with planar cross bedding,and lagoon is full of mud as well as very-thin siltstone layers formed in quiet water.We can recognize some tidal delta in the formation,the deposit sequence of which presents as the stack of channels.In addition,washover fans are defined by mid-fine grained sand and low angle bedding.4 Sediment facies and deposits are different in different base level stage,so the base level change can be judged by the variety of facies and grain size in tidal flat and barrier-lagoon deposit.The 2th Member has two cyclic stacking pattern forms,descending half cycle to ascending half cycle than to descending half cycle,and ascending half cycle to descending half cycle,while the 1th Member is made up of ascending half cycle and descending half cycle.5 With the guidance of sediment model,we detailed analysis efficient sand contour,sand contour,limestone contour and coal contour to discuss the sediment distribution of Benxi Formation in plane.Tidal flat located in the west of study area,and the east is occupied by barrier-lagoon which parallel to coast in 3 arranges.Tidal delta can be found between barriers while washover fans often site on the barrier.