准噶尔盆地盆1井西凹陷八道湾组低位体系域砂体沉积模式及油气勘探意义

Sedimentary model and significance for oil and gas exploration of the LST sand body of Badaowan Formation in Western Well Pen-1 Sag, Junggar Basin

准噶尔盆地腹部八道湾组是侏罗系潜在的勘探接替领域。通过分析盆1井西凹陷八道湾组低位体系域砂体形成条件、沉积特征,结合成藏条件研究,提出了坳陷湖盆坡折之下发育低位体系域砂体,低位体系域砂体成藏条件优越:(1)盆1井西凹陷八道湾组发育1个完整的三级层序,凹陷周缘发育坡折,来自东部物源体系的辫状河三角洲向凹陷方向进积,在坡折之下的凹陷区卸载沉积物,为凹陷区低位体系域砂体的发育创造了条件。(2)盆1井西凹陷八道湾组发育低位体系域砂体,低位体系域砂体呈砂泥互层,侧向与三角洲平原河漫滩泥岩接触,顶底板和侧向遮挡条件较好,岩性圈闭条件优越,具有形成规模岩性圈闭群的条件。(3)盆1井西凹陷发育低孔特低渗低位体系域砂岩储层,局部发育的微裂缝改善了储层物性;凹陷发育二叠系风城组和下乌尔禾组2套烃源岩,烃源条件较好。(4)盆1井西凹陷周缘发育深浅2组断裂,在垂向上沟通八道湾组低位体系域砂体与二叠系烃源岩,有利于油气聚集成藏,勘探潜力大,为准噶尔盆地下一步天然气规模发现的有利勘探领域。

Badaowan Formation in the hinterland of Junggar Basin is a potential exploration replacement field of Jurassic. By analyzing the formation conditions and sedimentary characteristics of the lowstand system tract(LST) sand body of Badaowan Formation in Western Well Pen-1 Sag, combined with the study of reservoir forming conditions, this paper proposes that LST sand bodies are developed below the slope break of depressional lake basin, and the reservoir forming conditions of LST sand body are superior. The results show that:(1)A complete third-order sequence is developed in the Badaowan Formation of the Western Well Pen-1 Sag in the Junggar Basin, and during the Badaowan sedimentation period, slope breaks are developed around the Western Well Pen-1 Sag. Braided river delta from the eastern provenance system prograde towards the sag, and unload sediment in the sag area below the slope breaks, creating conditions for the development of LST sand body in the sag area.(2)LST sand bodies of braided river delta are developed in Badaowan Formation of Western Well Pen-1 Sag, which is formed through the continuous unloading of sediment below the slope break after long distance transportation. The LST sand body is sand-shale interbeded, and laterally contacts with the flood plain mudstone of the delta plain, so the roof and floor and shielding conditions are good.Therefore, the lithologic trap conditions are superior, and the LST sand bodies have the conditions to form a large-scale lithologic trap group.(3)The low porosity and ultra-low permeability reservoir of LST sandstone is developed in the Western Well Pen-1 Sag, and the local microfractures improve the physical properties of the reservoir. Two sets of source rocks of the Permian Fengcheng Formation and Lower Wuerhe Formation are developed in the sag, so the source conditions are good.(4)In addition, two groups of deep and shallow faults are developed around the Western Well Pen-1Sag, vertically connecting the LST sand bodies of Badaowan Formation with Permian source rocks, which is conducive to the accumulation oil and gas, so the LST sand bodies have great exploration potential and are favorable exploration fields for the next step of oil and gas discovery in the Junggar Basin. This understanding provides a geological basis for the next step of oil and gas exploration in Junggar Basin, and also provides a reference for oil and gas exploration in depressional lake basins with similar geological backgrounds at home and abroad.