What kind of wood block is at block 0 0 0?

Solution 1:

It seems there's a bug in world generation where sometimes the block at 0 0 0 is sideways log(Data value of 4 or 8) instead of bedrock. It is very inconsistent and glitchy and even sometimes block's data value is changed in-game naturally. It is somehow related to the seed of the world and tree generation. When tested with superflat world, this does not happen when tree generation is turned off.

See this reddit thread for more information.

Solution 2:

Look like you've found one of those rare holes-in-the-void. Must be a glitch in tree generation. Also, since 1.6(?) - Layer 1 is always solid bedrock.

Here's what happened:

1. Your terrain is generated. (The Terrain; stone, hills, ores etc.)
2. Then, the world generates structures (trees, dungeons, caves, villages, etc.). Sometime at this point the game placed a tree there during world generation.
   This is the point during world generation which the bug occurred.
3. Because the way tree generation works, wood will override any block, while leaves will only appear if there is space for it.

As for what kind of wood block?
It's should be one of the possible Minecraft logs. So, since you're in a desert (though it's weird for a tree to spawn there), it would most likely be Oak.