Abstract

A study of the Upper Devonian–Lower Mississippian Woodford Shale was undertaken on samples at low thermal maturity from the Arbuckle Mountains, southern Oklahoma, to identify possible mechanisms by which natural gas might be stored in Woodford reservoirs in the adjacent Anadarko Basin. The two main lithologies in the Woodford, chert and mudstone, display different inorganic and organic characteristics. Chert beds contain >85 wt% quartz (early diagenetic from recrystallization of biogenic silica) and total organic carbon (TOC) contents of 2.8–6.4 wt%; the organic material is dispersed largely as small masses, although some is admixed with clays. In contrast, mudstone beds contain 26–77 wt% quartz, 10–40 wt% clays, and TOC contents from 4.1 to 22 wt%; the organic material is admixed dominantly with clay platelets, although Tasmanites microfossils are locally abundant. Cherts have (1) variable porosity (0.59–4.90%), (2) low calculated permeabilities (0.003–0.274 μD) and (3) small mean pore apertures (5.8–18.6 nm). Intercrystalline pores dominate in cherts. In contrast, mudstones generally have (1) porosities ranging from 1.97% to 6.31%, (2) low calculated permeabilities (0.011–0.089 μD), and (3) small mean pore apertures (6.2–17.8 nm). Interparticle, intraparticle, and moldic pores all are present in mudstones. Because of their high quartz content, cherts are brittle and commonly demonstrate microfracturing that is lithologically controlled and bedding perpendicular, whereas much less microfracturing exists in mudstones.

The early diagenetic intercrystalline porosity in cherts has likely been preserved since it formed because of the rigid, internal framework provided by the abundant authigenic quartz. Coupled with their relatively high TOC contents, cherts then may be important intervals of gas generation and storage in the Woodford. Where abundant, cherts may then play a significant role as source and reservoir intervals within the formation in the Anadarko Basin.