Sedimentary Simulation of Xujiahe Fomation and Depositional Mechanism of Large Area Sandstone

Key words: 
• large area sandstone

Abstract: The widespread sandstone in Xujiahe Fomation of the Upper Triassic, Sichuan Foreland Basin, is one of the most important exploration strata. With the exploration  development of the gasfield, it needs to clarify the predominant factors that control the widespread sandstone developing to guide the further exploration and production.
  To elucidate  the depositional mechanism and spacial arrangement of the the widespread sandstone, the sedimentary physical simulation method was adopted to transform the original geological conditions into physical model, on base of investigating the evolution of tectonic, ancient landform and sedimentary system, and so on. And the simulation parameters were taken into consideration and designed to study the mechanism of widespread sandstone through the simulation of flume experiment technology. Three  provenances were designed in the experiment to incarnate the characteristics of sedimentary systems during different stages and at different locations under the multisource conditions. Six intervals were simulated to playback the sedimentary characteristics, by conducting the experiment which was designed referring to principle of simulation  and natrural model method. The main controlling factors of widespread sandstone were concluded, by designed such as simulation parameters,such as flow, sediment flux,gradient,subsidence and lake level et al. Some preliminary conclusions are obtained as follows:
  1.Under the gradual slope conditions, it is in the shallow areaat the depth of 2.3 cm, slope of 4 that distributary channels migrated frequently, where the area water passed became larger gradually. And so did the area where sandbody deposited and was retransformd, which was benefit for widespread sandstone developing. The sandbodies were universally thin and homogeneous. And the widththickness ratio of the deltas is grater than 98∶〖KG-*2〗1.
  2.In the experiment, when the flux was 4.35 l/s and sediment content was 9.5 g/s, the channels branched and sandbodies moved forward. While was 0.49 l/s and 0.4 g/s, single channel developed and sandbodies were eroded. So the flux and sediment content of different water consociation helped widespread sandstone development.
  3.With tectonic subsidence and accommodation extending, the sand sediment developed to sandbodies of certain scale.
  4. With frequent changes of the lake level, sandbodies of each stage expanded to different directions in the plane and overriding in vertical. Besides, the lobes combinated laterally, widespread sandstone was easily developed.
  5. In the experiment, convergence of 3 sources showed explain of the development of widespread sandstone. During 6th depositional stage, the lake level rised in ascending order: 10cm12cm14cm16 cm18cm,when the sandbodies deposited continuously towards the source regions.When the lake level dropped from 18cm to 12cm, the accommodation decreased,the sediment carried by rivers deposited in lowlying location and former sandbodies were reformed obviously,which was benefit to widespread sandstone.
  In the depositional process, the channels bifurcated into distributary channels and lateral migrated continuously. The sandbody prograted and retrogradated vertically by fluctuation of lake level and assembled in horizontal and vertical direction. Then widespread sandstone developed.