Subsurface Structural Analysis Using Well Logs to Study Faults of the Lower Atokan Formation in the Arkoma Basin, Northcentral Arkansas

Abstract

Well logs observations have contributed to our understanding of the subsurface structures of the Arkoma Basin. These observations have led to a new explanation of the relationship between surface and subsurface structure of the basin. About 1500 well logs were used in this study to conduct a subsurface structural analysis of faults in the Arkoma basin of northcentral Arkansas. The Arkoma Basin is one of many petroleum basins in the United States which is structurally characterized by series of normal faults. In this study the faults were analyzed by building a structural contour map for Lower Atokan Formation (about 350 million years in age) and plotting nine cross sections using the well logs. The subsurface contour map shows very good match with the faults in the Arkoma basin. Two master fault zones were recognized in this study; the Mulberry and Clarksville fault zones, with displacement ranges from 1567 feet to 500 feet respectively. These fault zones trend northwest to southeast in the study area with an average strike azimuth of 285°. The principal stress axes that are responsible of forming the faults in the study area were calculated by measuring the attitudes of faults from the subsurface contour map and cross sections. The principal stress axes are represented by the plunge angle and plunge direction. The results show that the directions of the maximum, intermediate, and minimum stress axes of the Mulberry fault are 01°/286°, 69°/195°, 21°/016°, and of the Clarksville fault are 05°/287°, 66°/186°, and 24°/019° respectively. From these values, it can be seen that the two master faults are affected by the same tectonic stress regime. This study reveals a new finding of series of subsurface sub-basin structure in the analyzed area. One of the major sub-basins, named the Bullfrog Sub-basin, reveals tremendous extension about 4 miles to the surface and subsurface to the northcentral part of the area.