re: Astrophysicist Dr. Willie Soon explains the abiotic origins of hydrocarbons.

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So my question: is the grade of crude based upon the type of decaying organism or unrelated and due to other sediments entirely?

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The grade of crude is based on its API number. A high API crude is thin and flows well. A low API is thick and can be thick as tar.

Whether a crude is sweet or sour depends on it's sulfur content. Generally 0.5% sulfur or above is considered sour. It really depends on whether or not a refinery can process the sulfur containing crude or not.