Characters and formation mechanism of unsteady reservoirs in Tarim Basin

The superimposed Tarim Basin has experienced a long evolutionary history,including multi-stage subsidence,uplift,hydrocarbon-accumulation and adjustment processes,thus,a variety of reservoir types with complex gas-water contact were formed.By systematically summarizing geological features of oil-gas reservoirs in the basin,we discovered that some of the oil reservoirs are characterized by imbalance of oil-water contact(OWC),gradational changes in the plane distribution of physicochemical properties and oil saturation,and discordant locations of the major reservoir body and trap center,indicating a dynamic aggregation or adjustment process that these reservoirs are currently experiencing,consequently a concept of 'unsteady reservoir' was proposed.An unsteady reservoir is formed under rapidly tectonic movement,during which the reservoir heterogeneity or other factors may result in the stabilization of the reservoir lagging behind the trap's adjustment and the uniform oil-water contact uncompleted.We classified unsteady reservoirs according to the formation mechanism,formation process,types of oil-water contact and accumulation patterns.It was proposed that the study of unsteady reservoirs should focus on integrating static and dynamic geological dissections,synthetically applying geological and geochemical methods,combining physical modeling with numerical simulation,quantitatively assessing paleo-and present-OWC migration and variation,and precisely restoring the evolutionary process of traps so as to reproduce the formation process of unsteady reservoirs and effectively predict their distribution.