Quantitative modeling of organic matter connectivity in source rocks using fractal geostatistical analysis

Numerical modeling of hydrocarbon expulsion requires accurate assessment of the 3-D geometry and connectivity of hydrocarbon pathways (such as pore systems, organic matter networks, fractures, and stylolites) in source rocks. In this paper, a method combining scanning electron microscopy (SEM), digital image analysis, fractal geostatistical analysis, and a renormalization algorithm is developed to determine the 3-D geometrical properties, defined as organic matter connectivity factor (OCF), of these pathways in source rocks. The method is applied to derive fractal geostatistical models and the OCFs of organic matter networks (OMN) of fourteen source rocks with wide variation in total organic carbon (TOC) content and texture. The results show that the connectivity of the OMNs is affected by both the amount and distribution of organic matter in source rocks. Uniformly distributed OMNs lose 3-D connectivity when the TOC content is below approx. 6%, whereas OMNs with preferred orientation may maintain lateral connectivity at 4% and perhaps lower TOC content. Thus, in shales such as the Bakken in which vertical OMN connectivity may not exist, the fracture systems may provide not only primary migration pathway, but hydrocarbon reservoirs that are recharged through the laterally connected OMNs. The method developed in this paper can be used to quantify geological models and average geological properties over a wide range of scales and thereby provide reliable geometric parameters for basin modeling using a numerical technique.