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Modified Athy-Law Compaction to Account for Porosity Generation and Preservation From Kerogen Conversion in Terzaghi-Like Models of Petroleum Source Rocks

Abstract

A new algorithm is proposed and calibrated for assessing the effect of organic matter on compaction, porosity generation, and porosity preservation in organic-rich fine-grained sediments at various maturities. The algorithm involves the addition of simple terms to the Athy-law exponent relating porosity to effective stress in Terzaghi-like compaction models, which are often used in basin and petroleum systems models to calculate expulsion of water and petroleum from source rocks. The central concept in these models is that porosity is related to the difference between vertical lithostatic pressure and pore pressure, and pore pressure is calculated from a simple permeability model, either 0D or 1D. The new model presented here is empirical and requires calibration for the source rock of interest. It considers that because kerogen is softer than most inorganic grains, when in high concentration, it can lead to lower rock porosity prior to catagenesis. This part of the model was calibrated for the Green River Formation using log data at 600-700 m that shows porosity decreasing from 15-25% to about 7% as the kerogen volume fraction increases from negligible to 50 vol%. In addition, the new model was designed to consider that preservation of porosity created from kerogen conversion can be related to its geometric shape and the ductility of the surrounding mineral grains. Model results are shown for the ranges of residual kerogen porosities observed in source rocks. The model has been incorporated into TRESORS, a 0D simulator of source rock maturation and expulsion at both laboratory and geological conditions.