准噶尔盆地南缘多口探井在白垩系清水河组底部储层获得高产油气流,证明清水河组底部发育优质储层,但不同钻井揭示储层特征差异较大.在清水河组沉积前古地貌分析的基础上,结合砂体结构特征、岩矿特征、重矿物组合特征及古水流方向特征分析了清水河组底部砂体沉积相的差异,明确储层砂体的主控因素和分布规律.研究认为清水河组沉积前,准噶尔盆地南缘古地貌呈现南陡北缓的特征,南部为一陡坡区,北部为一个大型的缓坡区,且在缓坡区发育两级坡折.清水河组沉积时期南缘处于湖侵沉积,清水河组沉积初期形成了受古地貌控制的退积型辫状河(扇)三角洲沉积:南部陡坡带发育扇三角洲沉积体系,所形成的砂砾岩储层厚10～30 m,北部缓坡带发育受两级坡折控制的3期退积型辫状河三角洲沉积体系,所形成的砂岩储层厚20～50 m,砂岩储层在南缘中部和东北部大面积分布.北部和东北部物源形成的辫状河三角洲前缘砂体物性相对较优,是清水河组有利的储层发育区.

Many exploration wells in the southern margin of Junggar Basin have obtained high-production oil and gas flow from the Lower Cretaceous Qingshuihe Formation in wells,which proves that high-quality reservoirs is developed in Qingshuihe Formation.Based on the paleo-geomorphologic data,this paper analyzes the differences in sedimentary facies of the bottom sand body of Qingshuihe Formation,the reasons for the stable distribution of sand bodies,and the distribution patterns of different rock types,by combining analysis of the sand body structures,petrologic characteristics,heavy mineral assemblages and paleocurrent characteristics of the reservoir.The study suggests that before the deposition of Qingshuihe Formation,the paleo-geomorphology of the southern margin in Junggar Basin was characterized by the development of a steep-slope area in the south and a large gentle-slope area in the north,whereas two slope breaks were globally developed in the gentle-slope area.During the depositional period of Qingshuihe Formation,the study area was in the process of lake transgression,and retrogradational braided-river deltas and fan deltas were formed under the control of paleo-geomorphology.In the southern steep-slope area,fan deltas depositional system was developed,and the thickness of sandstone and conglomerate reservoirs is at the range of 10-30 m.In the northern gentle-slope area,three stages of regressive braided-river deltas depositional system were developed under the control of two slope breaks,resulting in the widely developed sandstone reservoirs in the middle and northeastern parts of the southern margin in Junggar Basin.The sandstone reservoir formed by northern and northeastern source system with a thickness of 20-50 m and relatively good physical properties,making it a favorable reservoir development area for the Qingshuihe Formation.