Gas Shales Characterization: Upper Mississippian Fayetteville Shale, Arkoma Basin, a Case Study

The Fayetteville Shale was analyzed and correlated between outcrops in north-central Arkoma Basin with subsurface data from the east Arkoma Basin and Mississippi Embayment area. Outcrop locations included Searcy and Stone counties in north Arkansas. Wells were located in east Arkoma Basin and Mississippi Embayment area.

The Fayetteville Shale in this area contains eight major lithofacies due to variations in depositional setting. These facies are laminated black shale, black shale with some concretionary elliptical septarian limestones, dark gray shale, phosphatic dark gray shale, gray shale interbedded with dark gray limestones, cherty limestones and chert nodules at the top. Parallel lamination, planar to slightly ondulatory bedding, cross bedded stratification, hummocky cross stratification, ripples and climbing ripples are the most common sedimentary structures; trace fossils include Teichichnus, Planolites and possibly Skolithos.

High amounts of pyrite as framboids or crystals indicate that the Fayetteville Shale was deposited under anoxic conditions and restricted water circulation as a Transgressive System Tract (TST). Organic matter sometimes is poorly preserved in flaky shapes, showing some partial or full chert replacement.

Organic matter (TOC) for outcrops samples is mature and kerogen is Type III (Gas Prone) and Type II/III (Oil and Gas Prone). Some samples correspond to Dry Gas. In the other hand, samples from wells show that dominant kerogen is Type III but with a low Oxygen Index perhaps due to diagenetic conditions from which all gas was generated or overcooked. Comparisons between facies and TOC distribution show that facies affect the oil or gas generation in north-central Arkoma Basin.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California