Abstract:
We use an evolutionary finite-element model to determine changes in stress and geomechanical features of sediments near a welding source layer of a salt wall. We show that a significant stress concentration develops at the weld tip, which is experienced by all sediments along the weld as the weld tip propagates along the weld. When the source layer welds, salt flow from diapir pedestal into diapir stops, resulting in a hydrostatic salt pressure. This pressure is less than the overburden stress in sediments over the pedestal. In fact, the salt in the pedestal supports only part of the weight of overlying sediments; the rest is transferred laterally to over-weld sediments through shear stresses above the weld tip. This transfer causes the stress concentration at the weld tip. We show that this concentration results in compression of sediments along the weld and hence enhancement of the weld sealing capacity. The resulting compaction can also explain the tightness of reservoirs near welds and the false pullup of their depths in seismic migration models. Our study is the first quantitative analysis of stress evolution near a welding salt layer and may help characterize and explore reservoirs near welds more accurately.
Introduction
Many petroleum prospects lie near salt diapirs that have a welded source layer, developed by complete or almost complete withdrawal of salt from the source layer (Jackson and Talbot, 1991) (Fig. 1). Several studies have addressed the important structural features of such welds for the exploration of these prospects, such as the permeability of adjacent sediments and presence of faults in them (Jackson et al., 2014; Peel, 2014; Rowan et al., 2012; Wagner, 2010). Previous studies, however, do not address the role of welding on the development of such features.
Field data indicate features that could be caused only by the welding process. For example, Hoetz et al. 2011 studied the data from 23 wells drilled through welds in the Zechstein salt basin, northern Netherlands, and found a consistent local decrease in the porosity and hence the permeability of reservoirs near the welds. They concluded that the salt layer welding is the cause of the decrease. Furthermore, Maione 2001 detected steeply dipping faults above welded source-layers of salt domes in the East Texas Basin. They also attributed the faults to the welding of the salt layers. Understanding and predicting the effects of welding on sediments is critical because these effects are often not detected in routine exploration (Hoetz et al. 2011; Maione 2001).
