1. A method for relating measurable or observable characteristics of hydrocarbon source rocks to estimated conditions at their time of formation, comprising: (a) selecting a first set of variables representing measurable or observable characteristics describing presence, quantity or quality of hydrocarbon source rocks; (b) selecting a second set of variables representing historical quantities that influence source rock formation, comprising at least one variable representing each of the following three types of influencing factors: (i) geologic age factors; (ii) paleogeographic factors; and (iii) paleoenvironmental conditions, said second set of variables being classifiable according to whether they affect rate of production, destruction or dilution of organic matter that forms hydrocarbon source rocks; (c) forming a network with nodes comprising both sets of variables, said network having directional links connecting causally-related nodes; (d) selecting one or more variables from either set to be unknowns and assigning at least one data value to each of the other variables along with associated probabilities of having the respective data values, said values and probabilities being estimated from measurement, observation or inferred indirectly; and (e) solving the network including the data and probability distributions for at least one of the one or more unknown variables using a Bayesian Network algorithm programmed on a computer and conservation of organic matter expressible as: 
 organic matter enrichment=production−(destruction+dilution) and downloading or saving the results to computer memory or storage.