Post-oil solid bitumen network in the Woodford Shale, USA - A potential primary migration pathway

An important if not predominant component of porosity in many gas shales has been identified in organic matter. An organic network in shales has been described in the literature as organic matter (generalized term), kerogen (primary), or bitumen (secondary). Recognition of the type and origin of an organic network in shales has relevance in establishing the origin and timing of porosity and fracture development. The pervasive nature of the organic network adopting the shape of pores in Type II kerogen-rich Woodford Shale suggests it is the residue of primary oil migration. We use the term post-oil solid bitumen to distinguish this bitumen occurrence from pre-oil solid bitumen (defined as a precursor of oil). Three forms of this post-oil solid bitumen network are recognized in reflected white light at 500x magnification and confirmed in backscattered scanning electron microscope images at >2500x magnification, namely speckled (similar to 1-2 mu m), wispy (similar to 2-5 mu m), and connected (>5 mu m). The post-oil solid bitumen network demonstrates the prior occurrence of oil generation and migration within this hydrocarbon source rock, provides porosity for hydrocarbon storage sites, and forms hydrocarbon migration pathways. (C) 2014 Elsevier B.V. All rights reserved.