- Source rocks are those sedimentary formations where hydrocarbons are generated. The sedimentary interval must have certain geologic criteria for it to have the ability to act as a source rock. First, it must contain a sufficient amount of organic material to generate these hydrocarbons. Organic content is measured as a percentage of the entire formation, or gross rock volume and referred to as total organic content, or TOC. We look for minimum TOC to be three percent or greater to act as the source rock. The majority of formations that act as source rocks are deposited in a marine environment. Hydrocarbons can be generated from land-based organics but these will generate primarily gas. The depth positional setting for source rocks must be anoxic, or severely oxygen depleted. The US Geologic Survey finds anoxic waters as those with dissolve oxygen in quantities of 0.5 milligrams per liter. These anoxate conditions take place in waters that had variations in density causing stratification of the water column. This stratification is due to primarily variations in salinity. This anoxic environment is necessary so that we won't have conditions that allow bacteria to be in the waters that would consume the organics. Thus, preserving those organics for later diagenesis into kerogen and then oil casts. The organic rich interval must reach depths where sufficient heat and pressure have been reached to allow the transformation of organic material into kerogen and then into oil and gas. Areas where sufficient temperature and pressure have been reached are referred to as 'the kitchen', as this is where the cooking takes place. This slide is simply a cartoon showing the process described in the previous slide. Note that the drawing is not to scale either in the horizontal or the vertical. The depth of the water in such an environment will be in the thousands of feet such that the pithemetry is an anoxic environment. Burial depth of the active source rock would be in the thousands of feet, such that we realize the proper temperatures and pressures required, and time required will be measured in millions of years. Here we have organic material in the form of algae, phytoplankton, zooplantkon, dying in the surface waters, and then being deposited as a pelagic rain in the anoxic bathymetric environment. Deposition continues for millions of years until such a time that the organic-rich material has reached sufficient pressure and temperature limits that it's transformed or undergone diagenesis and transformed into kerogen which is then transformed into oil, gas, or a combination. This slide is a pictorial of the basic stages for hydrocarbon generation. The generation of hydrocarbons from any source rock depends on four main factors. First, there must be an adequate amount of organic material present in the source rock to generate hydrocarbons. Then, there must be sufficient amount of time, pressure, and temperature in order to transform those organic materials from that stage to kerogen and then to oil and/or gas. The presence of bacteria and catalysts will also effect generation. The diagram on the left side of this slide shows transformation of kerogen first to oil, and then with additional time, heat, and pressure to gas. This diagram is a generalization, as each source rock will go through it's transformation based on it's particular circumstance. A type of organic material, either marine or land sourced, the temperature and pressure reached, and then the amount of time a source rock is in a particular temperature-pressure environment play critical roles in what type of hydrocarbons are generated. Note some specific scalers on this diagram, such as depth is measured in kilometers and hydrocarbon generation does not initiate until a temperature of at least 60 degrees centigrade or 140 degrees fahrenheit is reached.