Sedimentary Characteristics Of The Yanqi Prototype Basin In Mesozoic Era And The Determination Of The Basin Boundaries

The Yanqi basin has been developed since oil and gas was found in the stratum of Mesozoic erathem.The determination of semidentary characteristics of theYanqi basin in Mesozoic Era and the boundaries of paleobasin is a very important scientific research task for the exploration and development of oil and gas in this region.This article gives a detailed research of the sedimentary characteristics of the Yanqi PaleoBasin in Mesozoic Era through large amount of field study ,observation and analysis of boring well' s drilling core and an analysis of the given material which includes geology of boring well ,of logging ,and earthquake section .For the first time,mis article heuristic accept a series of new methods reconstruct the boundaries of paleobasin.Mesozoic Era strata of the Yanqi Basin 's comparative development is mainly located at the Bo Lake depression , which is consisted of Xiaoquangou Group of upper and middle Triassic Series, Badaowan formation of lower Jurassic Series,Sangonghe formation and Xishanyao Group middle Jurassic period and Toutunhe Group from bottom to top.Detrital rock is a major type of the sedimentary rock of the Yanqi Basin.The asseblage of the fragment heavy mineral of detrital rock ,of coarse fragment ,minerak>gic maturity is changed according to different transport distance,so they are essential foundations for analysis of sedimentary fades in this region and determination of deposit boundary of paleobasin. For this,we adopt three parameters,that is, stability coefficient of fragmental heavy mineral,percentage of pebble-bearing rudite fragmental rockrnineralogic maturity index.The stability coefficient of fragmental heavy mineraHhe relative content of stable fragmental heavy mineral(%ythe relative content of instable fragmental heavy mineral(%) = (zirconite+ tourmaline+ gamet+ magnetite+sphene+rutile+cassiterite+brockite+corundum+fluorite+anatase)/(mica+epidoze+zoisite+hornble nde+tremolite+riebeckite)The percentage of rudite fragmental rock=The thickness of conglomerate +mickness of grit rock +of psephitic sandstone +of pebbly sandstone/thickness of stratum 100%The fragmental mineralogic maturity index=content of quartz/content of (feldspar +debris).Based on the analysis of the changing of above three parameters in the basin,asseblage of fragmental heavy mineral ,of coarse fragment and mineralogic maturity,we can obtain the following information about The Yanqi Basin in Mesozoic Era;1) The partitioning of heavy mineral and debris reflect that ancestral basin has at least two big sources zones and two big corresponding ancient stream current systems.(1) There exists northern and northwestern source zones .The majority of the ancient water flows southeastward ,and the minority of the ancient water flows southward.(2) There exists southern and southwestern source zones,with the majority of the ancient water flowing northeastwanLand the minority of the ancient water flowing northward.2) In the northern sedimentary zone of the basin,from Badaowan period to Sangonghe period inJurassic period,Stability coefficient of the heavy mineral increases, while the percentage of pebble-bearingt clastic rock decreases. This shows its ancient sedimentary boundary becomes farther and farther with the development of times, the sedimentary area extends instead of shrinking from south to north, which implies that the main part of the ancestral basin expands from south to north at that time. In the Xishanyao period of Jurassic period, the stability coefficient of the fragmental heavy mineral in the southern sedimentary area of the basin decreases, which demonstrates the southern source zone had the tendency of moving northward in the Xishanyao period.3)The ancient water flowing mainly in the direction of east by south and the near extension of the basin from south to north shows that the main part of the ancient streams flew along the axis of the pulling-apart fault depression, moreover, the central and eastern part of the basin had the possibilit...