Predicting and assessing the amounts and types of a petroleum in a petroleum system requires obtaining thermally immature samples of the suspected source rocks in the generative part of a petroleum system. Marginal outcrops are typically thermally immature and provide petroleum geologists and geochemist with samples that help predict the potential of a petroleum system. Outcrop weathering can influence source rock evaluations resulting in erroneous predictions. Thousands of dollars may be spent sampling outcrops with subsequent tens of thousands of dollars spent on geochemical laboratory analyses to evaluate their potential. Integration of this geochemical data with geophysical data and geology is used in deciding where the greatest potential of petroleum reserves reside with subsequent millions of dollars being spent in subsurface drilling. This book provides field and geochemical laboratory criteria for collecting and evaluating unweathered outcrop samples to ensure accurate data describing the potential of a petroleum system. Bio: Mike was raised in Chicago, obtained bachelor of science degree under Sam Goldich in the department of geology at Northern Illinois University, completed Master of Science at Michigan Technological University, Worked as an offshore geologist and geochemist for Shell Oil Company in New Orleans, and completed PH.D. in the Geology Department at the University of Cincinnati. Worked at the Amoco Production Company Research center in Tulsa, Oklahoma. Afterwards, he worked for the USGS as a research geochemist in the energy program of the United States Geological Survey in Denver, Colorado. After retiring from the USGS, he formed a consulting s-corp entitled Lewan GeoConsuting Corporation in Golden, Colorado where he now resides.