志丹地区长82砂层组缓坡浅水三角洲前缘砂体发育模式及成因

Development pattern and genesis of gentle slope shallow water delta front sand bodies of Chang 82 sand set in Zhidan area,Ordos Basin

浅水三角洲前缘地区的油气成藏条件优越,其内部砂体历来是常规油气勘探的重点目标,弄清其沉积规律与沉积模式,是勘探取得成功的关键。利用鄂尔多斯盆地志丹地区长82砂层组岩心、薄片、测井和录井资料等,对该区长82砂体的岩石学特征、矿物学特征、测井响应特征、空间展布规律及成因类型进行研究,在考虑沉积期湖盆古地貌、古水深、古气候等的基础上,建立了缓坡浅水三角洲前缘砂体的沉积模式,并分析了其成因。研究表明:志丹地区发育2类沉积特征明显不同的浅水三角洲前缘砂体,分别是水下分支河道砂体和河口坝-滩坝复合砂体;2类砂体的岩石类型均以细粒长石砂岩和岩屑长石砂岩为主,其中水下分支河道砂体单层厚度大、砂质成熟度低,垂向上多期砂体连续叠置,平面上连片性差,而河口坝—滩坝复合砂体单层厚度小、砂质成熟度较高,频繁地夹泥—粉砂薄层,平面上连片性好;志丹地区处于湖盆的浅水缓坡上,区内砂体发育的类型与湖平面高低密切相关,水下分支河道砂体沉积主要受控于低湖平面时期的河流进积作用,而河口坝—滩坝复合砂体则主要与高湖平面期湖浪作用有关。该研究成果可为湖相缓坡浅水三角洲前缘砂体体系的研究提供借鉴。

The oil and gas accumulation conditions in the front area of the shallow water delta are superior,and the internal sand bodies are always the crucial target of conventional oil and gas exploration.The key to the suc‐cessful exploration is to find out their sedimentary law and model.Based on the core,thin section and logging data of Chang 82 sandstone set in Zhidan area of Ordos Basin,the petrological characteristics,mineralogical charac‐teristics,logging response characteristics,spatial distribution and genetic types of Chang 82 sand bodies in this area were studied.On the basis of considering the palaeogeomorphology,palaeowater depth and palaeoclimate of the lake basin during the sedimentary period,the sedimentary model of the sand bodies was established,and the genesis was analyzed.The results show that there are two types of shallow water delta front sand bodies with distinct sedimentary characteristics in Zhidan area,which are underwater distributary channel sand bodies and mouth bar-beach bar composite sand bodies.The rock types of the two types of sand bodies are mainly fine-grained feldspathic sandstone and lithic feldspathic sandstone.The single-layer thickness of the underwater dis‐tributary channel sand body is larger,the sand maturity is lower,the vertical multi-stage sand bodies are continuously stacked,and the planar connectivity is poor,while the mouth bar-beach bar composite sand bodies are characterized by smaller single layer thickness,higher sand maturity,frequent mud-silt thin layer and good planar connectivity.Zhidan area is located on the shallow gentle slope of the lake basin,and the type of sand body development is closely related to the lake level.The sedimentation of underwater distributary channel sand bodies is mainly con‐trolled by the fluvial progradation during the low lake level period,while the mouth bar-beach bar composite sand bodies are mainly related to the Lake wave transformation during the high lake level period.The research results can be used for reference in the study of the sand body system in gentle slope shallow water delta front of lake facies.