--> The Maness Shale: A Comparison of the Geomechanical and Mineralogic Properties within the Lower Eagle Ford Near the San Marcos Arch

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The Maness Shale: A Comparison of the Geomechanical and Mineralogic Properties within the Lower Eagle Ford Near the San Marcos Arch

Abstract

The Lower Cenomanian Maness Shale is a clay-rich mudrock originally identified in the East Texas field lying between the Woodbine and Buda Limestone that has been correlated to the basal Lower Eagle Ford in the vicinity of the San Marcos Arch. Where present, the Maness has been known to pose instability problems for horizontal wells that have encountered it. However, presence of the Maness may prove beneficial if it acts as a fracture barrier between hydraulically fractured Eagle Ford wellbores and underlying aquifers. Petrographic, x-ray diffraction (XRD), and geomechanical (point load penetrometer and micro-rebound hammer) analyses were performed on two industry cores taken in the vicinity of the San Marcos Arch that sampled the section from the lower Austin Chalk, Eagle Ford, Maness, and the uppermost Buda. The geomechanical studies demonstrated that the Maness is significantly weaker than the other formations; after converting the geomechanical raw data to unconfined compressive strength (UCS), average UCS values derived from the penetrometer for the Maness were 32% less than those for the Eagle Ford and 75% less than the Buda. Similarly, average micro-rebound hammer UCS values for the Maness were 36% less than the Eagle Ford and 77% less than the Buda. XRD analyses found that the shale samples from the Maness contained an average of 50% clay, whereas the overlying Eagle Ford marls contained an average of 40% clay.

Four horizons (top of overlying phosphate lag, Maness top, intra-Maness limestone, Buda top) were correlated in 345 wells within a six-county region (Karnes to Fayette counties) near the arch. Thicknesses of the Maness were found to trend northeast-southwest, in alignment with the Karnes-Gonzales troughs and the Sligo-Stuart City reef trends. The thickest intervals (>25 ft) occurred within the Gonzales trough, whereas the Maness was found to pinch-out south of southern Karnes County. Regression analysis found a 91.7% correlation between Maness thickness and oil/water ratios, which were based on cumulative first year oil and water production data from over 2000 horizontal wells in the study area, indicating that the Maness may be acting as a fracture barrier in the region. This analysis also found a 50% decrease in oil/water ratios between Maness thicknesses of 5 to 10 ft, suggesting that a minimum of 10 ft is needed for the Maness to effectively act as a fracture barrier.