Relationship Between Lineaments and Shale-Filled Fractures

Shale-filled fractures have been interpreted at the top of the Mississippian in a horizontal well from central Ness County, Kansas, approximately 8 miles from the Dickman Field study area (Carr et al., 2000). In this well, numerous near-vertical shales were identified at intervals of 10-100 ft along the lateral length of the well. These intervals were interpreted as solution-enhanced fractures extending down from the karst surface at the top of Mississippian that have been filled by Pennsylvanian shale of the Cherokee Formation (Carr et al., 2000). The shale-filled fractures provide a barrier to fluid flow in the reservoir.

Map of Ness County, Kansas, showing the relative locations of the Dickman 3-D seismic survey and the Ummel #4H horizontal well. Gravity (red), magnetic (dark gray), and Mississippian structural (blue) lineaments are also depicted.

Evidence for shale-filled fractures in Dickman Field

Cores from several wells in Dickman Field contain fractures and green shale in the Mississippian subjacent to the pre-Pennsylvanian unconformity surface, and Well 6 (located directly on a volumetric curvature lineament) shows a zone of reworked Mississippian dolomite above the unconformity surface. In general, fractures that were solution-enhanced by post-Mississippian karst and remained open during the Pennsylvanian are likely to have been filled by both Pennsylvanian shale and weathered Mississippian debris. Therefore, it is assumed that locations with evidence of a thicker section of weathered Mississippian material at the base of the Pennsylvanian (subsequently referred to as the “karst zone”) will have a higher incidence of shale-filled fractures. For the purposes of this study, the karst zone in a well was defined as the interval between the highest occurrence of the basal chert conglomerate in the Pennsylvanian section (chert weathered from the Mississippian) and the top of the un-weathered Mississippian, as identified from cuttings and core.

 

Lithologic (top) and seismic (bottom) cross section through Well 6, which is located directly on a volumetric curvature lineament. The synthetic seismogram for Well 2 is displayed in blue on the seismic section.

Well 6 Core Description

 

In order to determine whether either of the two dominant lineament trends seen in the volumetric curvature data relate to filled solution-enhanced fractures, crossplots were created of the thickness of the karst zone versus the distance to the nearest NE- and NW-trending lineaments. These crossplots show that there is no apparent relationship between the thickness of the karst zone and the NW-trending lineaments; however, there is an increase in the thickness of the karst zone closer to the NE-trending lineaments. The relationship between the thickness of the karst zone and distance to the NE-trending lineaments suggests that these lineaments are likely to represent fractures preferentially solution-enhanced during karst formation and subsequently filled with shale and debris. Since some of the NE-trending lineaments have interpreted lengths in excess of 0.5 mile, they may provide significant barriers to fluid flow in the northwest-southeast direction.

Thickness of Karst Zone vs. Distance to Lineaments