This is a summary of seven years of research conducted by the author at the Institute of Petroleum and Organic Geochemistry in Juelich, Germany. It contains a comparison of various depositional environments (lakes, deltas and seas) with respect to organic matter characteristics, a special chapter on the deposition of the Posidonia shale, a summary of organic matter maturation and related petroleum generation, and a chapter on the use of maturation parameters as calibration tools for numerical modelling of the temperature histories of sedimentary basins. Also, the microscopic effects of petroleum generation and oil to gas cracking are treated. The final chapters deal with coals as source rocks for oil and gas and with the effects of weathering on sediments which are rich in organic matter.