The Preliminary Analysis Of Plasticity, Elastic And Rupture Of Cenozoic Sand And Mud Stone In The West Of Qaidam Basin

Generally, during the burial evolution period, loose sediment will experience the processes from compaction to consolidation diagenesis then to rupture in the oil and gas basin. However, sand and mud sediment are plastic when they deposit, sediment consolidate to rock and began the process of conversion from plastic to elastic during the basin evolution. Moreover, there is a great possibility of rock rupture after the process of sediment consolidating to rock. This process is bound to apply a dramatic impact on the formation and distribution of oil and gas reservoirs.In this paper, by core observing, thin section analysis, logging data analysis, simulation experiment, geological analysis and other methods are presented based on the observation, the plasticity, elasticity and fracture of Cenozoic sand and mudstone in the settlement burying process of Qaidam western basin are studied. Majorly achieved the following understandings:1. Base on the changing porosity with depth, compaction characteristics, core and thin section observation and description, rock compression, springback and burst experiment, according to the characteristics of rock mechanics properties change in the sediment burying process, the Cenozoic sand and mudstone in Qaidam western basin is divided into three stages in the settlement burying process: the plastic compaction stage, elastic-plastic compression stage and rupture stage.The plastic compaction stage is characterized by: in the sediments burial process, as the overburden pressure gradually increasing (or the increasing burial depth), the close degree of sandy or argillaceous clastic particles increases. However, the deformation is mostly not reversible, that’s to say, it’s mostly plastic deformation. Argillaceous sediments or mudstone buried depth of0-1000m commonly, sandy sediment or generally sandstone buried depth of0-1500m.Features of elastic-plastic compression stage are: after the plastic compaction, as the buried depth continues to increase, overburden pressure (or depth) increase further. Under compression stress condition, it reflects the formation elastoplastic characteristic, and porosity decreased slightly or increased a little. When the pressure decreases with the overlying pressure dropping, rocks may happen to rebound, but the magnitude is less than that of compression. It can’t return to the state before the compression, and the stress-strain curve of rock shows that the strain magnitude caused by stress increase is greater than that of stress decrease. Mudstone buried depth is usually1000-2000m and generally sandstone buried depth is1500-2000m.Characteristics of rupture stage are as follows: with the burial depth continually increasing (overburden pressure continues to increase), if it exceeds the compressive strength of rock, rock will convert into the rupture stage which is characterized by a large number of cracks in mudstone and sandstone, the fractures can parallel to the layer, as well as oblique and vertical to the layers. Mudstone rupture stage occurs generally greater than2000m depth, while sandstone rupture stage generally greater than2500m depth.2. The characteristics of the elastic-plastic compression stage, plastic compaction stage and rupture stage of Cenozoic sand and mud stone of western Qaidam Basin at different depths have different structural styles: they are relatively complete in a monoclinic structure, and the depth range of the various stages is similar to the general trend. The rupture stage is more obvious in fault zone. In anticline zone, the depth of elastic-plastic compression stage and plastic compaction stage will be smaller, ant the rupture stage will be more developed.