FRACTURE ARCHITECTURE IN THE MIOCENE-PLIOCENE OGALLALA FORMATION AND QUATERNARY STRATA, NORTHEASTERN TEXAS PANHANDLE: IMPLICATIONS FOR GEOLOGIC STORAGE OF CARBON DIOXIDE

Aquifer protection is a central imperative of underground injection control in the United States, and so understanding aquifer architecture is an important part of environmental protection in areas favorable for subsurface storage of carbon dioxide. The High Plains Aquifer of the northeastern Texas Panhandle includes loess-like sandstone of the Ogallala Formation and Quaternary strata that include sand, clay, chert, and caliche. Target formations for CO₂ storage and enhanced oil recovery are in Pennsylvanian sandstone deeper than 2,000 m in this area, and shale forms the primary reservoir seal; Ogallala strata rest disconformably on Permian strata, which contain widespread salt layers that secondary seals that further help protect the aquifer. Above the aquifer is a thick section of caliche and chert that would further buffer any fugitive carbon dioxide in the unlikely event that seepage would occur. Field study reveals that the Miocene-Quaternary formations contain numerous joints that provide insight into aquifer architecture and subsurface flow pathways.

Length, orientation, spacing, and cross-cutting relationships of more than 1,000 joints were measured in the field and in high-resolution satellite imagery. The fracture networks consist of well-developed systematic joints and cross-joints. Systematic joints are strongly aligned and have length that commonly exceeds 60 m. Cross-joints tend to terminate at systematic joints, and so joint length is typically equal to systematic joint spacing (~2 m). In vertical section, the joints are typically curvilinear and strata-bound, cutting indurated sandstone and chert and terminating within friable sandstone. Analysis of joint indicates that that strike of the systematic joints varies among beds and regionally. Some of the joints appear to be the product of tensional stress associated with hill slopes and thus have little relevance to aquifer architecture beyond the flanks of stream valleys. However, a large part of the fracture population maintains a west-northwest orientation over large areas. These fractures may be the product of subtle regional tectonic stresses and may have a significant effect on flow in the High Plains aquifer system.