Influence of Texture and Diagenesis on Reservoir Quality in the Fayetteville Shale

Abstract

Southwestern Energy has drilled over 4,000 wells in the Mississippian Fayetteville Shale Formation across nearly one million acres in Arkansas, revealing that productivity and calculated GIP varies significantly throughout the lease area. Early hypotheses attributed these variations in production to mineralogical changes, which were subsequently not observed at the log scale. This study assesses if changes in productivity are controlled by fabric or textural properties of the rock. Petrographic analysis of 561 thin sections from 9 wells across the lease area was conducted and textural variability was documented both laterally and vertically through the stratigraphic intervals of the Fayetteville Shale. Additionally, XRD analysis was performed on 175 samples. Petrographic analysis shows that the Fayetteville Shale consists of five main facies based on texture, diagenesis, mineralogy, and organic content: siliceous organic-rich, mixed siliceous-argillaceous, carbonate-rich, argillaceous laminated, and silty argillaceous mudstones. Reservoir intervals in the Lower Fayetteville Shale are characterized by the siliceous organic-rich mudstone facies that are punctuated by zones of the mixed siliceous-argillaceous or carbonate-rich mudstone facies. Quartz, calcite and dolomite cement are the most common pore-reducing authigenic phases along with varying amounts of phosphate, pyrite, chlorite, kaolinite, and siderite. Silica is an important component of the Fayetteville Shale and it is primarily biogenic and not detrital in origin. The silica is sourced from radiolaria and sponge spicules (opal-A), which are highly unstable and undergo early diagenesis near the sediment-water interface. A more stable form of silica precipitated as chert in the matrix and filled Tasmanites algal cysts. Tasmanites are a common microfossil in the siliceous organic-rich mudstone facies. The abundance of phosphate, pyrite, and organisms such as radiolaria and Tasmanites in organic-rich sediment indicate that the Lower Fayetteville Shale reservoir intervals were deposited under high productivity and anoxic to suboxic conditions in a coastal upwelling system. A strong correlation between silica and TOC can be demonstrated and is related to high biogenic activity in algal blooms during upwelling. The distribution of silica both vertically and laterally is thus an essential control on reservoir quality.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015