A Study Of Deep Unconformities In Tarim Basin

The Tarim Basin is the most important oil-bearing basin in China.It is rich in oil and gas.Exploration studies in recent years show that large amounts of oil and gas reservoir exist in deep carbonate rock layers,which open up the exploration field of marine carbonate reservoirs in the basin.Unconformity interfaces of the deep strata have important controlling effects on the formation of oil and gas reservoirs.Based on the interpretation of seismic data and the study of outcrop,this paper studies the deep unconformities interface of the Tarim basin,especially the unconformities related to the Cambrian.The study includes dividing the types of unconformities,restoring their evolution history and analyzing the effect of unconformities on hydrocarbon accumulation.This study argues that the contact relationship between the upper and lower strata of the underlying interface of the Cambrian?T₉⁰?have 4 types,including conformity,disconformity,compressing fold unconformity and margin overlap unconformity.Upper and lower layers of the T₉⁰interface show the contact relationships of conformity in most areas of the Tarim basin.The unconformity only develop in local areas such as the central uplift zone and the southern basin.The T₇⁴ interface is the unconformity interface between the Upper Ordovician and the underlying strata,developing obviously in the central uplift zone.Above two interface?T₉⁰ and T₇⁴?are unconformity interfaces of the most importance between the Cambrian and middle and lower sections of the Ordovician,corresponding separately to the Keping movement and the first episode of the middle section of the Caledonian movement.The Keping movement has correlations with the regional mantle thermal uplifting and the first episode of the middle Caledonian movement is controlled by the NE-SW regional extrusion stresses.This study restores the process of morphological evolution of T₈¹ and T₉⁰ interfaces after their formation according to tectonic evolutionary histories of the T₇⁴ interface,the denudation thickness in the end of Middle Ordovician and ancient paleowater depth of the Cambrian.Through comparative analysis,the formation period of the uplift or depression of Manjiaer,Tabei,Awati,Bachu,Tazhong,Tangguzibasi,and Taxinan of the Tarim basin was preliminarily determined.Because the spatial location of T₉⁰ and T₇⁴ interfaces is different from source rocks and reservoirs,effects of these two interfaces on deep oil and gas reservoirs exist similarities and differences.Similarities lie in they can all become channels for oil and gas migration and provide reservoir spaces.The T₉⁰ interface mainly affects the formation of Cambrian source rocks and reservoirs,while the T₇⁴ interface mainly improves the property of Ordovician reservoirs.The further analysis believes that the southern slope of the Tabei area,the Shuntuoguole area,the northern slope of the Tazhong area and the Maigaiti area are all favorable blocks for developing deep oil and gas reservoirs,which can be regarded as key regions of the exploration in the future.