New Kinetic Schema For Kerogen Cracking: Implication For Defining Biogenic and Thermogenic Windows For Hydrocarbon Generation
Understanding the transformation of organic matter in sedimentary basins, illustrates perfectly the complexity of developing robust analytical methods in oil exploration. From 1970s to the present day successive steps were necessary to propose a consistent kinetic schema using different experimental approaches have taken place, in both academia and the oil industry In the early 70s Bernard Tissot demonstrated that kerogen is converted to petroleum through two distinct reactions. 1. First, kerogen generates mainly polar compounds (kerogen 1 Polars + kerogen 2) such as resins and asphaltenes. 2. Then, the NSOs decomposed to generate hydrocarbons and a solid residue or char (NSOs HC + char). In parallel, during the last 20 years, different analytical workflows confirmed this kinetic schema: closed system pyrolysis under anhydrous (gold tube reactor) or hydrous conditions (Parr pressurized reactor). However, for decades and still today, bulk kinetics of HCs generation are derived using a simplified paradigm; kerogen cracking into HCs. This simplified schema directly derives from open system pyrolysis system which has been widely developed in the oil industry ‐ using either Rock Eval technique (IFP) or the Pyromat (LLNL). Consequently, when using open pyrolysis method, a bias occurs in the resulting kinetic parameters with a direct consequence for petroleum exploration. Indeed, depending of the initial maturity of the kerogen, bulk kinetics can be a mixture of the two reactions. When analyzing samples of maturity ranging from 0.3 to 0.6 % EqRo , the consequence can be either to underestimate or to overestimate the onset timing of HC generation. Correct kinetic parameters corresponding to the oil window must be those derived from the second equation to avoid this bias. In terms of minimum temperature for kerogen thermal cracking, very recent studies demonstrated that the onset of NSOs generation from kerogen starts at temperature as low as 30‐40°C under geological conditions. This means that during organic matter diagenesis, some polar compounds are released into the sedimentary medium and could be the source of biogenic gas during bacterial consumption (A. Kamga, PhD, 2016). All these results clearly show that new kinetic schema ‐ including the two distinct reactions ‐ must be implemented in basin simulators to better predict the real onset of the oil window. Additional equations for biogenic gas generation can be also proposed by deriving specific kinetic parameters for kerogen cracking in recent sediments. In conclusion, transferring research processes from the academic domain to industry, requires sometimes to lighten technical workflows. The simplified process based on open system pyrolysis was a real need for oil industry. However, resulting parameters need to be used with caution. Here the correct equations were written in the early 1970s and then overlooked. New research studies had to be restarted to understand the biases raised by the results of open system pyrolysis and to propose a new workflow. Consequently, kinetics of the equation including intermediate transformations of kerogen into NSOs was reviewed leading to a new definition for both biogenic and thermogenic windows.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019