Xorg.conf is the configuration file of Xorg ,the location of this file is /etc/X11 IF have.X can automatically configures itself with reasonable defaults,so there is no xorg.conf file in many unix/linux systems

Actually we can configure X in Ubuntu use xorg.conf.d which is a directory locates under

/usr/lib/X11/xorg.conf.d (Ubuntu 10.04)

/usr/share/X11/xorg.conf.d(Ubuntu 10.10 and newer)

You can refer https://wiki.ubuntu.com/X/Config/ to learn how to configure X using xorg.conf.d

If you prefer to configure your X using xorg.conf,you can generate it using below command in Ubuntu

sudo  X  :1 -configure

(There will be an error after generating xorg.conf.new file ,just ignore it)

Above command will create a xorg.conf.new file under your home directory ,then you can copy and rename it to xorg.conf to directory /etc/X11

sudo cp ~/xorg.conf.new /etc/X11/xorg.conf

Now you can configure X using xorg.conf and configurations of xorg.conf will overwrite all settings of xorg.conf.d since it has higher priority