$1.$ Petroleum is formed from the decomposition of ancient organic materials under high pressure and $\_\_\_\_\_\_\_\_\_\_$ over millions of years.

$2.$ The main locations where petroleum reserves are found include $\_\_\_\_\_\_\_\_,\_\_\_\_\_\_,$ and $\_\_\_\_\_\_\_\_.$

$3.$ The $\_\_\_\_\_\_\_\_\_\_$ of petroleum can vary significantly, affecting its color and viscosity.

$4.$ Crude oil classifications are primarily based on sulfur content and API gravity, categorizing crudes as either $\_\_\_\_\_\_\_\_,\_\_\_\_\_\_,$ or $\_\_\_\_\_\_\_\_.$