Organic matter input and depositional conditions of the sediments that become source rocks exert primary controlon the biomarker fingerprints of source–rock extracts and crude oils. An example of organic matter input is the distribution of C27, C28, and C29 sterols from eukaryotic organisms that reaches the sediment from an overlying water column. This initial distribution of sterols might be altered by many diagenetic factors during and after sedimentation, but ternary plots of the relative amounts of C27, C28, and C29 steranes largely reflect original source input. However, the extent of conversion of C27 sterols to C27 diasterenes reflects the acid catalytic or oxidative potential of minerals in the water column and sediment, which is an effect of the depositional environment. Thus, the diasterane/sterane ratio of crude oils can be used to indicate relative amounts of clays in the related source rock.