Cyclostratigraphy and Sequence Stratigraphy on Carboniferous Karashayi Formation of Tarim Basin,China

Carboniferous Karashayi Formation of Tarim basin formed in the continental shelf environment,is composed of several sedimentary assemblages of clastic rocks,carbonate rocks and gypsum rocks.It is one of the important reservoir combination in the Tarim basin.According to the research results of paleontology,the formation can be approximately correlated with the Visean and Serpukhovian Stage of the International Stratigraphic Chart,about ±27 Ma.Currently,the research of sequence stratigraphy on Karashayi Formation is relatively weak.Due to lack of obvious discontinuity surfaces,a limited number of studies have been divided the formation into 3-4 third-order sequences,which leads to a conflict as to the time limits ±27 Ma.In this paper,the cyclostratigraphy was carried on well logs GR(Natural Gamma-Ray Log),SP(Spontaneous Potential Log),U,Th,K,etc.of Karashayi Formation with spectral analysis and wavelet analysis methods.Integrated with core observation and sedimentary base level study,9-11 third-order sequences were identified in Karashayi Formation of Well Mai6,Zhong1,Zhong17 and Shun6.The average time duration of each cycle is about 2.40 Ma.This value corresponds to the intersection astronomical period of the three Earth orbital parameters.Not only do the results reflect the application value of the method,but also reflect that the climatic cycles are the controlling factors on the development of third-order sequences.