Abstract: Jubaila Formation is of the Upper Jurassic and covers the shallow shelf and the Central Arabian intrashelf basin. Unweathered limestone and soil samples were collected from the base of the roadcut (about 10-15 m below the surface of Jubaila Formation). XRD was applied for whole sample and clay fraction of limestone rock and soil samples. Moreover, Differential thermal and thermo gravimetric analysis (DTA, TGA) were carried out with whole sample fraction of limestone rocks and soils. Calcite was recognized as a major mineral of limestone rocks and other samples. Also, quartz and kaolinite were approved with all samples. Soil samples have low content of 2:1 clay minerals and more kaolinite. Clay fraction of samples has kaolinite, illite and illite/smectite. The content of illite and illite/smectite was more with soil samples comparison with other samples. The existence of calcite and absence of other calcium carbonates (such as aragonite, Mg-calcite and nesquehonite, etc.) indicates how old is study area (Jubaila Formation). Transformation of Mg-calcite to calcite, supply Mg ion which is necessary of smectite generated by transformation of illite. Because of, soil’s strap was stuck between limestone rocks made low leaching and drainage environment which supported generation of illite/smectite mineral.