浅水三角洲沉积体系与储层岩石学特征

浅水三角洲已经成为当前沉积学研究和油气勘探的重点领域，浅水三角洲常规和非常规砂岩储层中发现了丰富的油气资源。鄂尔多斯盆地中—古生界发育大型浅水三角洲沉积，结合岩芯、测井、分析化验等资料，对盆地西部地区山1—盒8段浅水三角洲的母岩特征、沉积充填、砂体展布与储层岩石学特征进行研究，为研究区常规砂岩储层和致密砂岩油气勘探提供地质依据。分析表明，研究区南部和北部砂岩重矿物组合相似，以锆石和白钛矿为主。综合La/Yb-REE与Dickinson三角图解、稀土元素配分模式和碎屑锆石定年分析认为，北部物源主要来自盆地西北部阿拉善古陆太古界和元古界花岗岩、片麻岩、石英砂岩，向南影响到陇东地区北部；南部物源主要来自盆地南部北秦岭地区太古界、元古界花岗岩、片麻岩和片岩，向北影响陇东地区中南部。山1段发育曲流河三角洲，盒8段发育辫状河三角洲，孤立型与垂向叠置型水下分支河道砂体是主要的砂体类型。盒8段沉积时期，古湖泊可能萎缩；南北物源形成的三角洲在研究区中部环县东南一带交汇混合。砂岩类型主要为岩屑质石英砂岩、石英砂岩和岩屑砂岩，结构成熟度中等—较差。砂岩成岩改造强烈，水—岩反应复杂，压实作用是储层致密化的主因，多期胶结作用使砂岩的孔喉结构变得十分复杂。孔隙类型以微米—纳米级的岩屑溶蚀孔、剩余粒间孔和高岭石晶间孔为主，整体上属于非常规致密砂岩储层。砂体分布与“甜点”预测是研究区致密油气勘探的关键。

Sedimentary system and sandstone reservoir petrology of a shallow water delta: Case study of the Shan-1 and He-8 members in the western Ordos Basin

The shallow water delta has become the focus of sedimentology research and oil and gas exploration.Abundant oil and gas resources have been found in conventional and unconventional sandstone reservoirs within shallow water deltas.Large scale shallow water delta deposits were developed during the Mesozoic to Paleozoic in the Ordos Basin.Based on the core,well logging,and geochemical data,the parent rock,sedimentary filling,sand body distribution,and reservoir characteristics of the shallow water deltas from the Shan-1 and He-8 Members in the western part of the basin are studied in this paper,which provides a geological basis for conventional sandstone reservoirs and tight oil-gas exploration in the study area.The results show that the heavy mineral assemblages of sandstone in the south and north regions of the study area are similar,mainly composed of zircon and white titanium.Using LA/Yb rare earth elements(REEs),Dickinson triangle diagrams,REE distribution models,and the analysis of detrital zircon dating,we find that the provenance in the north is mainly from Archean and Proterozoic granites,gneisses,and quartz sandstone of the Alashan ancient land in the northwest part of the basin and affects the northern part of the Longdong area southward.The provenance in the south part of the basin mainly comes from Archean and Proterozoic granites,gneisses,and schists in the North Qinling area and affects the central and southern Longdong area northward.Meandering river delta was developed in the Shan-1 Member,braided river delta was developed in the He-8 Member.Isolated,and vertical superimposed underwater branch channel sand bodies are the main sand body types.During the depositional period of the He-8 Member,the ancient lakes have shrunk;the delta lobes formed by the North-South provenance converged and mixed in the southeast part of Huanxian County in the central part of the study area.Sandstone types are mainly lithic quartz sandstone,quartz sandstone,and lithic sandstone,with medium to poor textural maturity.Sandstone has undergone a heavy diagenetic transformation and complex water-rock reaction.Compaction is the main cause of reservoir densification,and multi-stage cementation makes the pore throat structure of the sandstone very complex.The pores are micro-nano scale,and the main types are dissolution pores of rock debris and residual intergranular pores,as well as kaolinite intercrystal pores.The studied sandstone belongs to the unconventional tight sandstone reservoir as a whole.Sand body distribution and"sweet spot"prediction is the key to tight oil and gas exploration in the study area.