Fracture patterns in synclinal folds, Miaofengshan, Beijing

Abstract:

The anticlinal bends are of interest for the oil/gas exploration and drilling designs as they are structural traps associated with high intensity of natural fractures due to bending curvature extension. However, some petroliferous areas with proven oil reserves were identified in synclinal structures, e.g. Songliao, Ordos and Bohai Bay Basins, northeast China, Bonaparte Basin, Australia, and Santa Maria Valley field, California. We analyze the fractures in synclines that are expected to carry curvature related fractures similarly to anticlines. The analysis is conducted on a 500m long and ~300 tall exposure of a folded sequence of dolomite and limestone layers at Miaofengshan, Beijing. Two general fracture groups are recognized: (1) layer crossing joints that are sub-parallel to the syncline axial surface; and (2) a distinct system of extension veins, which are joints filled with secondary calcite, that was found only in two layers of 0.8 and 2.2 m thick. These veins are layer-bound, they are up to 5 cm wide, and their width tapers toward the top and bottom of the host layers. Most of them are oriented normal to the bedding surfaces and radially with respect to the syncline shape. We recognized two phases of secondary mineralization that indicate layer-parallel extension of 5% or more. Apparently, these veins developed by bending extension of the most brittle layers whereas the more ductile layers above and below extended quasi-continuously. The analysis suggests that synclinal fracturing should be considered as possible mechanism for exploration of unconventional.