须家河组沉积模拟实验及大面积砂岩成因机理分析

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

四川前陆盆地上三叠统须家河组砂岩大面积分布,是整个盆地现今天然气勘探的重点层系之一,随着气田的勘探和开发,亟需弄清形成大片砂岩的主控因素来指导勘探和生产。为研究其成因机理,在对须家河组区域构造演化、古地貌、沉积体系等地质条件调查研究的基础上,运用沉积物理模拟的方法,将原型地质条件转化为物理模型,并设计模拟实验参数,通过水槽模拟实验技术进行研究。实验主要设计三个物源,通过控制流量、加沙量、地形坡降、活动底板沉降量及湖水位等参数,在水槽装置再现形成大面积砂岩的演变过程,系统研究大面积砂岩形成的主控因素。模拟结果表明,地形坡降小、不同来水特征组合、湖水频繁进退、各沉积体系间交汇及盆地沉降与沉积速率的匹配等,影响着大面积砂岩的形成。实验发现多期河道不断分汊改道,在侧向上不断迁移,纵向上受湖水位的变化表现为前积和退积,致使砂体在横向和纵向上相互拼接,导致砂岩大面积分布。

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∶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 6th 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.