What are two factors causing decrease in the supply of petrol?What two factors have lead to the decrease in the supply of petrol?

There are many factors. Some of these are:
1.There are many concerns about the Middle East: is it stable enough politically to assume that petro will be supplied regularly.

2.The Russians are cutting the supply of oil, to hurt the US economy, and at the same time helping their economy by keeping their own oil

3.There is competition from the Chinese, Indian, Japanese, Korean, and Brazilians since they are needing more petro.  They're making big deals with Venezuela and Sudan to get a bigger piece of the petro for themsleves.

4.The big oil companies are having problems uncovering more fields.

5. Companies are having problems getting money to fund looking for more oil. For example Cheetah Oil recently stated to Reuters that their funding sources are making it difficult for them to go ahead with their search for more oil reserves.