DIAGENETIC CONTROLS ON HYDROCARBON STORAGE IN CHERTS: EVIDENCE FROM THE WOODFORD SHALE, ARBUCKLE MOUNTAINS, OKLAHOMA

Diagenesis of mudstones is commonplace and can lead to significant modification of the original sediment, as evidenced in chert beds of the Devonian-Mississippian Woodford Shale, Arbuckle Mountains, OK. Recognition of and understanding the diagenesis, using petrography and SEM analyses, is critical insofar as the Woodford is both a world-class hydrocarbon source rock and unconventional reservoir. As such, identification of likely hydrocarbons storage intervals/compartments is important for evaluating effective Woodford target zones. Focus here is on the Woodford’s upper informal member, where cherts and phosphatic concretions are common.

Diagenesis in cherts is dominated by authigenic cements, largely apatite, quartz, and pyrite. Apatite, principally in concretions, occurs as single or twinned euhedral crystals (10-20 μm long), and textural relations indicate apatite is the paragentically earliest cement, but it’s largely restricted to concretions. The apatite 1) replaces uncompacted radiolarian skeletons, and 2) partly fills their interiors. Quartz (chalcedony and microcrystalline) precipitated episodically and 1) cements apatite, 2) infills remaining interiors of radiolarians, and 3) occurs as accumulations in the groundmass within and away from concretions. Pyrite occurs as 1) euhedral crystals (<30 μm across) co-genetic with chalcedony, and 2) euhedral-subhedral crystals (<100 μm) that post-date other cements. MICP analyses indicate cherts have both low porosities (<5%) and permeablities (<0.03 μD).

Because these cements dominate Woodford chert diagenesis, and because they all formed early (before radiolarian skeletons could be broken/compacted), the petrophysical character of cherts was largely fixed during early stages of diagenesis. Thus, hydrocarbon storage capacity was also largely controlled by early diagenetic events. Therefore, extensive early diagenesis in Woodford cherts rendered these beds as poor hydrocarbon storage intervals.