Organic Mater (OM) preserved in the flagstones provide a relatively rich source of Carbon to be eventually converted by burial in the earth to Hydrocarbons.
The picture shows one of the initial Lake Orcadie 100,000year cycles. Approx. 5m thick
Blue grey deep permanent lake laminite, organic rich, at the base is 75cm thick (Top showing). The lake shallows through light grey muds and sands (1m) with stromatolites. This is followed by a section (3m) of alternating lake sediments and long periods of drying out forming mud-cracks of differing lengths and width indicative of the drought severity.
The abundance of organic mater (OM) varies throughout the cycle being richest in the permanent lake facies and associated with the stromatolites. Fish and Plant remains make a significant contribution to the OM.
The organic mater swept into the lake is preserved on the anoxic (without oxygen) bottom to be eventually covered and buried by sediments.
Progressive burial increases temperature and pressure transforming the OM into Kerogen and eventually Oil and Gas.
The kerogen is the part of the organic matter contained in the sediment that produces hydrocarbon molecules when heated over a period of time. This process is called maturation. It is a chemical process governed by heat and time which can be described by a collection of first order parallel partial reactions. If the maturation is performed to its term, each partial reaction will be responsible for the transformation of a part of the kerogen called the partial potential. The sum of all the partial potentials is referred to as the Hydrogen Index (HI). It is given in mg HC / g Organic Carbon.
This description is not sufficient to describe the nature of generated chemical products. These products are mostly hydrocarbon molecules. Their number and complexity is such that they cannot be simulated individually. For the sake of simplicity (and practicality), the hydrocarbon molecules are grouped into classes according to their physical behaviour to enable prediction of their potential for Hydrocarbon Generation, often termed source rock quality.
Generally Type I is strongly oil prone, Type II is oil and condensate prone while Type III is gas prone. Type IV is generally non productive.
The Wikipedia free encyclopaedia has an excellent article on the types of KEROGEN and how it is transformed into Hydrocarbons.
The kerogen is related to the type of organic matter contained in the source rock. The kerogen Type indicates hydrogen richness and a kinetic scheme governing transformation into hydrocarbon.