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

• Primary oil migration leaves behind a post-oil solid bitumen network.
• Three post-oil solid bitumen network forms are speckled, wispy, and connected.
• Post-oil solid bitumen network demonstrates oil generation and migration.
• Post-oil solid bitumen network provides porosity for hydrocarbon storage sites.
• Post-oil solid bitumen network forms hydrocarbon migration pathways.

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 500× magnification and confirmed in backscattered scanning electron microscope images at > 2500 × magnification, namely speckled (~ 1 –2 μm), wispy (~ 2–5 μm), and connected (> 5 μm). 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.