古潜山周缘滩坝沉积模式与岩性油藏勘探实践——以南苏丹Melut盆地Ruman地区Galhak组为例

Depositional models and lithologic reservoir exploration of sandy beach-bar around buried-hill:a case from Galhak Formation in Ruman region of Melut Basin,South Sudan

Melut盆地历经了近20年的构造圈闭勘探,目前可钻的剩余构造圈闭已十分有限,为应对储量和产量递减,亟待开展岩性油藏等新领域的探索。基于古地貌恢复、物源体系、单井微相、砂体展布等分析,综合青海湖现代沉积调查结果,提出Ruman潜山周缘白垩系Galhak组发育大型滩坝沉积,建立了潜山隆起周缘型和水下潜山台地型2类沉积模式,认为长轴三角洲前端侧翼的古潜山地貌所造就的高能滨浅湖环境是滩坝沉积的关键控制因素。Galhak组沉积初期,因强烈断层掀斜作用,Ruman潜山短暂露出水面,基岩-长轴三角洲复合供源,发育大型坝砂,单层厚度10～20 m,面积约10 km^2;随后,因相对湖平面上升,Ruman潜山没入水下,发育水下潜山台地型滩坝,长轴三角洲单一供源,以大面积薄层滩砂为主,局部发育厚层坝砂。滩坝砂体与上倾尖灭线、下伏Renk组烃源岩、顶板洪泛泥岩、侧向湖相泥岩封隔等配套,形成上倾尖灭、孤立透镜体等类型的岩性圈闭。滩坝沉积的提出有效指导了岩性油藏勘探,部署的多口探井获勘探发现,提升了Ruman地区的储量规模。

After almost 20 years’exploration,only a few structural traps were left for drilling in Melut Basin,South Sudan.Exploration for new areas such as lithologic reservoirs is needed to tackle the reserve and production decline.Based on analysis of palaeogeomorphic reconstruction,provenance system,mico-facies in single well,sandbodies distribution and investigation of modern deposition in Qinghai Lake,large-scale sandy beach-bars were discovered in Cretaceous Galhak Formation around Ruman buried-hill in Melut Basin.Two depositional models of buried-hill periphery and underwater buried-hill platform were built,and high energy shore and shallow lake environment caused by buried-hill topography located beside the long axis delta was considered as the main controlling factor of sandy beach-bar deposition.In the initial stage of Galhak Formation deposition,Ruman buried hill exposed for a short duration because of intense tilting of faults,where large scale of sandy bar was developed with an area of 10 km2 and a thickness of 10-20 m,the sediment source was provided by basement and long-axis delta.Ruman burial hill was sinking into the water as the relative lake level rising,sandy beach-bars of underwater buried hill platform(dominated by large area of thin layer beach-sand with thick bar-sand in local)were developed,the sediments source was supplied by long-axis delta.Beach-bar sandbodies,up-dipping pinched-out boundary,underlying source rock of Renk Formation,flooding mudstone in the top and lateral lacustrine mudstone seal were matched to form lithological traps.The discovery of sandy beach-bar guided the lithologic reservoir exploration effectively,several exploratory wells that were deployed on the basis of“beach bar”were succeeded,promoting the reserve scale of Ruman area.