FORMATION LEVEL CHARACTERIZATION OF THE ARBUCKLE GROUP IN CENTRAL OKLAHOMA: A KEY TO UNDERSTANDING INDUCED SEISMICITY

Recent seismicity rates in Oklahoma far exceed historic or background seismicity, so research examines a multitude of factors related to induced seismicity. Geologic investigations aim to understand the basal sedimentary unit, the Arbuckle Group (Arbuckle), and the connections to basement rock where earthquakes are potentially triggered. Where the Arbuckle is not an oil or gas producer, it is the predominant wastewater disposal zone, and correlations have been shown between increased Arbuckle disposal rates and seismicity in OK and KS. The Arbuckle is comprised of carbonate formations deposited in the late- Cambrian to early- Ordovician, when a warm shallow sea covered much of North America. Type logs of the Arbuckle are available in regions where it has been a producer of oil and gas; however, limited information is available for the rest of the state and a type log is needed for central and northwestern Oklahoma.

Our research objectives are to examine stratigraphy, lithology, fracture patterns, and hydraulic characteristics of the Arbuckle in seismically active regions. Type logs that we are developing indicate that the Arbuckle may be from 200-900 feet thick in the Anadarko Shelf and Cherokee Platform geologic provinces, as compared to other parts of Oklahoma where the Arbuckle sedimentary sequence is up to 3000 feet thick. The type log represents Cleveland, Lincoln, Logan, Payne, and Oklahoma Counties.

Observations were made on the Texas Union I-Idema core (Idema core) in Cleveland County to validate the central Oklahoma Arbuckle type log. Five Arbuckle formations were featured in the core, but the Signal Mountain was not readily identified. The carbonates are tidally influenced and have sharp boundaries between formations. Small-scale measurements were taken horizontally, vertically, and along fractures, on the Idema core using a hand held air permeameter. Horizontal (along bedding surfaces) permeability measurements had a median value of 2.6 mD, while vertical permeability had a median of 37.2 mD. Thirty percent of the core had vertical or sub-vertical fractures, with a median permeability of 37.7 mD. A continuation of this research will provide detailed geologic characteristics at the formation level and can be used to refine fluid flow, geomechanical, and seismological models of the region.