The field of geochemistry involves study of the chemical composition of the Earth and other planets, the composition of rocks and soils, the cycles that involve the earth's chemical components, and the interaction of those cycles with land and water.
Some questions geochemistry tries to answer include what elements and chemicals are present in various soils and rocks in different locations? What can we learn from those differences? How are these soils and rocks changing, and how have they changed through the centuries? How do once-living things like plants and animals decompose after their deaths and what sorts of new things do they form as they interact with the environment (such as fossils or hydrocarbons, a.k.a, oil)? How do these various processes affect the environment, and how does the environment, weather, and other influences affect them?
Organic Geochemistry at answers.com