MULTIPHASE DEFORMATION OF THE PALEOZOIC AND MESOZOIC UNITS WITHIN THE PANHANDLE OF NEBRASKA

Recently, there has been renewed interest in potential hydrocarbon system within the panhandle of Nebraska, due to the increasing economic viability of unconventional plays. This timely study presents preliminary results from a detailed investigation of a seismic dataset from the panhandle region of Nebraska.

The study area is located on the margins of the Denver-Julesburg basin. Structurally, features such as the Denver and Alliance sub-basins, and Morrill County and Wattenberg Highs affect the northern reaches of the D-J basin. Stratigraphically, the region consists of cyclical sandstone-limestone-dolomite units of Pennsylvanian-Permian age, overlain by the shale, sandstone and limestone-rich Cretaceous succession. Hydrocarbon plays in the region have traditionally targeted Cretaceous units (both clastic and carbonate) but the deeper carbonate and shale sequences are additional modern targets.

The present study analyses a series of 2D seismic lines, dominantly oriented NE-SW, with sufficient crossing lines to build a 3D representation of the structure in the survey area. Formation tops were picked from key local wells and associated velocity profiles, and mapped across the remaining lines. The Mesozoic and late Paleozoic units are clearly delineated on the seismic lines, and the base Cenozoic unconformity is clearly visible, marking the onset of a flux of clastic sediments from the growing Rocky Mountains. The clearly imaged sections display a regional dip to the SW, consistent with the position on the margins of the D-J basin. The late Paleozoic section is subtly folded, with the folds dying out upwards. Faults observed in the Cretaceous section show both normal and reverse offset, suggesting that there have been multiple phases of deformation or fault reactivation affecting this sequence.

The subtle folds within the late Paleozoic section may be zones of enhanced porosity, due to fold-related fracturing, suitable as targets for unconventional exploration. In addition, these folded and fractured zones may have been further deformed in subsequent tectonic events.