Sedimentary Features And Sequence Stratigraphic Analysis Of The Triassic Baikouquan Formation In The Southern Portion Of The Western Slope Of The Mahu Depression In The Junggar Basin

Located in the northwestern part of the Mahu depression,the western slope of the Mahu depression is bordered by the Dabasong and Xiayan uplifts on the east,the Wuxia fault zone on the north,the Zhongguai uplift on the south,and the Kebai fault zone on the west.Overall,this slope is a SE-dipping single secondary structure to the central depression.This study focused on the Triassic Baikouquan Formation in this area.Based on lithology,grain sizes,well logs,and sedimentary characteristics,the sediments in the study area were classified into fan-delta facies and lacustrine facies.The fan-delta facies can be subdivided into two subfacies:the delta-plain subfacies and delta-front subfacies.The delta-plain subfacies is composed primarily of braided channel deposit and the delta-front subfacies consists largely of subaqueous distributary channel deposit.The lacustrine deposits were subdivided into marginal lacustrine and shallow lacustrine subfacies.As it is difficult to distinguish between the two subfacies and between the marginal lacustrine mudstone and shallow lacustrine mudstone within them,the marginal and shallow lacustrine subfacies were treated as a whole in the subsequent analysis and discussion,so were the marginal and shallow lacustrine mudstone microfacies.This study analyzed the sequence stratigraphy of the study area using the principles of sedimentology,well logging,and seismic stratigraphy.It was based on the seismic reflection data,well logs,and records of denudation,onlap,and abrupt changes in sedimentary facies preserved in rock cores and outcrops.The results reveal that there are a total of five 3rd order sedimentary cyclic successions in the study area.The entirety of the Baikouquan Formation constitutes a 3rd order cycle and it can be divided into three 4th order cycles based on vertical variations in lithology and electrical characteristics:the first member,second member,and third member of the Baikouquan Formation.A map was created to show the horizontal distribution of the sedimentary facies within the three cyclic successions,with the corresponding 4th order cycles being the stratigraphic units.The patterns of evolution and spatial distribution of the facies in each succession were analyzed to provide a basis for predicting the horizontal distribution of favorable reservoirs in the study area.The reservoirs in the target formation are characterized by overall extra-low porosity and permeability and distributed primarily in the delta-front subfacies in the first and second members of the formation.They consist mainly of lithic glutenite and feldspathic glutenite.There are 5 types of reservoir spaces:residual intergranular pores,intragranular dissolution pores,shrinkage cavities,and micro-fractures,with the first two types being the main reservoir spaces.Residual intergranular pores exist in large numbers because the rigid particles present in the reservoir rocks helped preserve some primary intergranular pores due to their resistance to compaction.The intragranular dissolution pores in the interiors of feldspar particles,which are widespread in the reservoir rocks,were created by the oil and gas injection in the early period and the abnormally high formation pressure.The reservoirs with extra-low porosity and permeability in the study area can be divided into good reservoirs and poor reservoirs,based on shale content,physical properties,displacement pressure,pore throat radii,and grain size characteristics.The good reservoirs mainly occur in the first member of the Baikouquan Formation.They are comprised of sandy fine-grained conglomerate,conglomerate,and coarse-grained sandstone containing pebbles and have favorable physical properties and pore throat structures.The poor reservoirs,largely in the second member of the target formation,are characterized by sandy fine-grained conglomerate,medium physical properties and relatively poor pore throat structures.The third member of this formation is made up predominantly of dense mudstone and thus unfavorable for formation of reservoirs.But this member can act as a good cap rock to trap oil and gas in the underlying reservoirs.A comprehensive analysis of sedimentological,sequence stratigraphic and reservoir characteristics suggests that,in the southern portion of the western slope of the Mahu depression,favorable reservoirs occur in the first and second members of the Baikouquan Formation.In the first member,favorable reservoirs are distributed across the study area;they are found around wells Maxi-1,Aihu-2,Bai-64,Ma-18,and Aihu-1,especially in the delta front subfacies.In the second member,favorable reservoirs are distributed around wells Aican-1,Bai-65,and Aihu-2 in the northern part of the study area,especially concentrated in the northern delta-front subfacies and the northwestern delta-plain subfacies.