Petroleum System

Source Rocks (SR) A source rock is a fine-grained, organic-rich sedimentary rock that has generated, or is capable of generating, movable amounts of hydrocarbon (oil or gas). The most important ✫ sedimentary rock to be SR is the: 1- Organic-rich Shale 2- Organic-rich mud 3- Organic-rich Limestone (less common) ✫ Why most major petroleum system in the world start with marine black shale? 1- Very fine-grained (deposited in calm environment) 2- Can accumulate large amount of Organic Matter 3- Often deposited in oxygen-poor (anoxic) environment 4- Early affected by increased of temperature and pressure ✫ Main types of Source Rocks (SR) 1- Potential SR: A sedimentary rock contains enough organic matter, but not mature (no enough temperature or pressure). Immature. 2- Effective SR: A sedimentary rock has reached maturity and generate hydrocarbon. Mature. 3- Relic Effective SR: A sedimentary rock previously generated hydrocarbon but stopped due to cooling or uplift, before using all organic matter. Postmature. 4- Spent SR: A sedimentary rock fully finished its ability to generate hydrocarbon, either because of spent all of OM, or become Overmature. ✫Evaluation Criteria of SR To determine if a SR is good or not, a Petroleum Geologist evaluate three main parameters: 1.Quantity (Amount of OM and Kerogen. 2.Quality (Type of OM and Kerogen). 3.Maturity (Temperature + Pressure). 1- Quantity (Total Organic Carbon TOC): Measured as a weight percentage (wt%) • TOC: 0.0 – 0.5% Poor • TOC: 0.5 – 1% Fair • TOC: 1 – 2% Good • TOC: > 2% Excellent 2- Quality (Kerogen Type): The type of Kerogen (OM) determines what the SR will produce Kerogen Type Source Material Environment Hydrocarbon Products Type I Algae Lacustrine Highly Waxy Oil Type II Maine Plankton Marine Oil and Gas Type III Plants Delta / Swamp Gas 3- Maturity: The thermal maturity is evaluated using Vitrinite Reflectance (Ro%). Vitrinite is a type of organic material derived from woody plants and is found in sedimentary rocks like shale and coal. The thermal maturity is determined by measuring the percentage of light reflected from the Vitrinite particles under a certain microscope. As burial depth and temperature increase, the reflectance of Vitrinite increased. Therefore, the Ro% value indicates if the SR within the: oil window, gas window, or overmature stage.

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