Sedimentary characteristics and depositional evolution of carbonate platform during the Cambrian and Ordovician in eastern Tarim Basin,NW China

The eastern Tarim Basin (Tadong Area) has gained wide attentions on large-scale marine carbonate reservoirs in Cambrian-Ordovician due to significant hydrocarbon discoveries.A systematic analysis combining thin sections,cores,wireline logs,and seismic data is conducted on Cambrian-Ordovician carbonate platform in the whole eastern Tarim Basin,including Gucheng area,Majiaer area,and western Luobopo rise (Luoxi area).The results show that 8 sub-facies and more than 10 microfacies are developed including open platform,restricted/semi-restricted platform,reef-shoal around platform margin,drowned platform,foreslope,neritic platform,and deep-water basin.As both key areas for hosting petroleum reserves during the Cambrian and Ordovician,the Luoxi area is dominated by deep-water basin facies,while the Gucheng area is dominated by neritic platform facies and deep-water basin facies during the Lower Cambrian.The deposition evolution during the whole Cambrian is dominated by slope facies and deep-water facies,platform margin facies,and platform facies.In contrast,it is dominated by open platform facies during the whole Ordovician.The depositional evolution of carbonate platform is mainly controlled by paleo-geomorphology and sea-level changes.The distribution of paleo-geomorphologic units plays an important role in controlling types and distributions of carbonate platform facies.The transgression assists in growth of reef-shoal complex and lime mud mound in the Early Ordovician.However,with neritic platform and slope being to disappeared,in the Middle Ordovician,platform margin facies are well developed in Gucheng Area.Platform facies and deepwater basin facies are widely distributed.Finally,carbonate platform is drowned due to sea level rising in the Late Ordovician.The depositional evolution of carbonate platform coinciding falling and rising of sea-level changes can be beneficial for appropriate carbonate reservoirs identification and petroleum exploration.