鄂尔多斯盆地环县地区长8油层组浅水三角洲内前缘沉积特征

Sedimentary Characters of Shallow-Water Delta Front in Chang 8 Oil Layers in Huanxian Area

对鄂尔多斯盆地上三叠统长8油层组进行了深入的沉积环境研究,通过岩心观察、测井曲线识别,发现研究区属于西南三角洲沉积体系,具备形成浅水三角洲的地质条件,研究区内主要发育特殊的浅水三角洲内前缘沉积。浅水三角洲发育时期,地貌平缓,构造稳定,三角洲平原、前缘、前三角洲各微相之间无大的间断面。水下分流河道微相是主要的沉积微相,河口坝、水下分流间湾、前缘席状砂等沉积微相在这一时期均有发育;砂体以浅灰色及灰色中—细粒砂岩及粉细砂岩为主,垂向上发育多套正韵律砂体;底部发育冲刷面,可见到泥砾,层理类型以平行层理和块状层理为主。水下分流河道电测曲线呈钟形、箱形。长8油层组沉积时期研究区整体以建设型三角洲为主要特点,水下分流河道因河流的频繁改道而分布广泛,河口坝不发育或难以保存,砂体规模大,与湖相泥岩交错接触,具有优良的成藏条件,利于岩性油气藏的形成。

There are more and more researchers being interested in the sedimentary characteristics of shallow-water delta rich in hydrocarbon. Chang 8 oil-bearing formation of Triassic series in Ordos Basin has been further studied for the de- ployment of oil and gas exploration. By means of core observation, logging identification and granularity analysis, it is found that northwest sources and sedimentary system of the study area rendered the geological conditions for shallow-water delta. It mainly develops shallow-water delta front sedimentary in the area. Due to the gentle landform, stable structures and minor differences of height, it seems that In plane view, there are no discontinuous surfaces among the delta plain, delta front and prodelta at the development period of shallow-water delta, progradation was the major sedimentation. The major skeleton sedimentary micro-phase is underwater distributary channel, and the other micro-phases, such as mouth bar, distributary clearance, frontal sheet sand, are formed in this period. The sand body is mainly composed of light gray and gray middle-fine sandstone and siltstone. Mult-set positive rhythm sand bodies are formed in vertical direction. At the bottom, scour surface is formed, and scour surface is also developed, Parallel bedding and massive bedding are the main bedding types. The SP curve of underwater distfibutary channel presents bell-shaped, thin- walled and the compound of them. It can be determined that the main characteristic of the study area is constructional delta deposit, and underwater distributary channel caused by river diversion is widely distributed. The large sand bodies in touch with lacustrine mudstone was favourable for occurrence of lithologic reservoir.