Abstract:

The Paleozoic slope zones in Tarim basin are divided into three types according to the characteristics of their formation and evolution:syndepositional paleo-slope,structural slope and superimposed slope.The slopes can also be classified into upper slope and lower slope in accordance with sediment compensation characteristics.The former is the extension of the platform and the latter is the stretch-out of the basin.Slope zones were developed in different evolution stages of the basin,mainly during the Paleozoic,the Cambrian and the Ordovician in particular.The high-energy phase belts of carbonates were developed in the upper slope of platform margin,while clastic stratigraphic-lithologic traps were formed in the lower slope of platform margin.Reservoir assemblage with deposits of reef-bank facies as the main reservoirs and tight carbonates or the Upper Ordovician mudstone as the caps was developed in the palaeo-slope zone.The structural slopes were resulted from regional tectonic movement or local uplift and are characterized by erosion in the upper part and deposition in the lower part.The slopes distribute around the margin of the palaeohighs,and the nearer of the slopes to the uplift,the stronger the erosion.For clastic sedimentary system,the transitional zone between the upper and lower slopes is a place where stratigraphy and lithology change and favorable for the formation of stratigraphic-lithology traps.As for the carbonate sedimentary system,the development of unconformity exerts a strong shaping effect upon the underlying layers,being favorable for the formation of karst and fractured-vuggy reservoirs.The karst reservoirs developed on the structural slope together with the mudstone caps form favorable reservoir assemblages.A comprehensive analysis of oil and gas fields in the basin indicates that multiphase concordant superimposed slopes and hinge zones are the potential areas for oil and gas accumulations.

Key words: slope zone, reservoir assemblage, oil and gas distribution, Tarim basin