The Analysis Of The Sediment Source And Reconstrucation Of The Framework Of The Depositional Environment In The Mesozoic Ordos Basin

The exploration of oil and gas has been going on for nearly one century in the Ordos basin. The research on basin's basic theory and productive experience indicates that, the sandstone reservoir with good permeability is the base, and the various traps are the conditional factor, among many geological factors in the Mesozoic oil accumulation. In the present research, therefore, identification of the shape and distribution of the permeable sands seems very important. In a word, it's of great significance in the theory and in practice to find out the location,shape and extending direction of lacustrine delta and subaquatic fan delta in the Upper Triassic Yanchang Formation, and the channel sandstone in the Middle Jurassic Yan'an Formation in the Ordos basin.The surface feature , extending direction of lakes and rivers, the location of the lake shoreline and basin prototype were mainly studied in this paper through the analysis of plaeocurrent and sediment source with other methods for basin analysis,such as the correlation of strata thickness. It shows the framework of depositional environment and the relation between the depocenter and the center of subsidence in different sedimentary times, which helped for determining the shape and distribution of the permeable sands.It suggested that there were multiple sediment source areas in the Mesozoic Ordos basin. Sediment mainly came from the north,the west,the south margin of the basin in different times. At the same time, the sediment might origin from the folded belts, the orogenic belts and the pre-existed sedimentary rocks within the basin. The sedimentary context in the Mesozoic includes lake facies,lake delta facies, fluvial facies and fluvial delta facies. The Mesozoic Ordos basin had undergone several cycles. The facies belt distributes in EW direction and the major source rock and reservoir rock in Mesozoic oil accumulation had been formed. The depocenter had been moved from east to west, while the center of subsidence had been migrated from west to east as time gone. The time-space distribution and matching relation of the source and reservoir rock in Upper Triassic and Lower-Middle Jurassic were controlled by the migrating laws, the laws of the distribution and spreading of the oil and gas accumulation was also determined. The source rock had begun to produce petroleum after the structure-heat event in the Late-Jurassic epoch, and the source sag laid in the southeast of the basin. While the present favorable region for Triassic reservoir is located in the middle to north and southwest of the basin, and the prospecting region for Jurassic reservoir is located in the middle-west of the basin.