Study on the Carboniferous Washover Fan Behind Barrier lsland In Donghetang of Tarim Basin

Key words: 
• washover fan /
• depositional characteristics depositional model /
• oil and gas exploration /

Abstract: In recent 20 years, while having payed attention to studying the modern barrier island sedimentarysystems, reople have made some researches on sedimentary characteristics of washover fans associatedwith barrier island and pointed out that there are many washover fans in ancient barrier island sedimen-tart' sequences, which opens up a new field for oil- gas exploration. Directed by modern sedimentary the-ory, authors studied the Carborriferous washover fans ind barrier island in the Donghetang area of theTarim basin by full uses of geological, logging and seismic data. The washover fan can be divided intosandstone type and limestone type. The former consists of siltstone,fine sandstone and brown mudstonewith graded and parrallel beddings. In the sandstone, quartz and chert content is 8070一9070，there aresome normal and multicrystal oolites in it. The latter is made up of brecciated tearing debris, fine silt andquartz, chert, coarse silt, which are associated with grey brown siltstone and grey green arenaceous lime-stone vertically. Grain probability curves of the washover fan almost are characterized by two segmentswith high content of suspension substance which is up to 30%.The thickness of sandstone typewashover fan vertical sedimentary structure sequence, which can be describedas“Bouma sequence AE,AAA and ABE'，i s 40-60 cm. The washover fan of limestone type shows massive structure as a whole,and there developed limestone brecciated with various sizeand shape, suspending among lime mud andsilty sand, and sedimentary thickness is about 1 m, which ismud. Affected by storm factor, tempestuous current carryingassociated with normal tidal flat lagoona great deal of basinal debris passed overbarrier island and formed washover fans extended toward lagoon about 1 kilometer in length and about10 square kilometerin area in atidal flat lagoon environment at the back of barrier island. The washoverhas good reservoir potential and pinches out quickly laterally, therefore, it‘s favorable for formidip pinch一outoil- gas po of w hi ch has been found in the Carboniferous stfata in the Lunnan area of the Tarim basin.