The Study On Depositional Systems And Spreading Of Sandbody Of Triassic Yangchang Formation Chang 6 At Middle-West Area, Ordos Basin

With the exploration to be deepened, there exists the phenomenon that the Ordos basin was full of sands in Triassic Yanchang Formation Chang 6 in middle-western area. In order to deeply understand sedimentary types of depositional systems and lithofacies palaeogeographic pattern and sand spreading in different stage of the Chang 6 sedimentary period further, then we can cognitive the main reservoir sands from the macroeconomic angle, offering important basic data to point out further gas exploration and development in the study area.In this paper, the author synthetically applied sedimentation, well logging, petroleum geology, mathematics geology, ect, on the basis of studying on the core of well drilling in the district, fully utilize logging well, well drilling core and testing and analyzing materials to study sedimentary facies,sedimentary microfacies profoundly and carry on sedimentary facies contrast to study chang6 sector lithofacies palaeogeography evolution character and regular. Then we can study the sandstone spreading rules and detailed analysis the sand of various genetic type, acquiring some cognition as follows:There are Anbian delta depositional system in northeast, Yanding delta depositional system in northwest, Huanxian and Yanwu delta depositional system in west, Jinchuan-Zhenyuan delta depositional system in northwest, Gucheng-heshui turbidite fans depositional system in south by means of provenance analytical method consisting of sandstone types, the heavy minerals character, cathode luminescence feature of quartz, ect. Depositional system that have various inner constitutions and combination controlled by different provenance will control the spreading character of main reservoir sandstone body.According to the rock types, sedimentary structure, section architecture, grain distribution, well logging and geochemistry indicators, alluvial fan sedimentary system, fluvial sedimentary system, delta sedimentary system, lacustrine sedimentary system and turbidite fans sedimentary system can be identified in the study area. It particularly discussed the character of sedimentary facies and sedimentary microfacies, as well as establishing corresponding sedimentary patterns. On the basis of the above mentioned, it studied the character of lithofacies palaeogeography of Chang6 sector to discover the evolution, and pointed out that the evolution of lithofacies palaeogeography controlled the development degree and spreading of sandstone in the great extent.According to the analyzing longitudinal evolution and flat spreading of sandstone body, it found the spreading of Chang6 oil-bearing formation exiting strong regularity. The development of sandstone gradually developed better from Chang6~3 to 6~1 mainly because of that with the uplifting of lake basin in chang6 oil formation and the descending of lake level, it is benefit to form overriding progradation complex delta sandbodies, it analysis various genetic type of the main reservoir sands in detailed and sandbodies were divided into five types including underwater distributary channel sandbodies in front of the delta, overriding underwater distributary channel sandbodies, mouth bars sandbodies, combination of underwater distributary channel and mouth bars or distal bars sandbodies, the fluxoturbidite in front of the delta. Finally, reservoir characteristic of various genetic types of the sandbodies and the rate it account are discussed in detailed, and to provide referential important schedule oil-gas exploration further.