The role of differential chemical compaction in the formation of the Gold Creek Collapse Structure, West Central Alberta

Abstract

Abstract The Gold Creek Gas Field has produced in excess of 2.5 billion cubic meters (>90 Bcf) of gas plus natural gas liquids from the Upper Devonian Wabamun Formation. The Wabamun is a 220 m thick limestone that is only porous and productive where fault-controlled dolomite is present. These faults are associated with a significant episode of structural collapse that has resulted in up to 160 m of negative relief on the Wabamun structure. Compensatory thickening of the overlying Mississippian Formations accompanied subsidence. The mechanism of collapse is interpreted to be differential compaction within the underlying Devonian Leduc reef. In response to overburden pressure, the muddy lagoonal sediments of the Leduc reef interior are interpreted to have undergone a higher degree of stylolitization than the coarser grained reef margin. Differential compaction commenced at the onset of Mississippian deposition and reached its zenith during Debolt Formation deposition. During the early stages of this process, the Leduc reef had been a closed diagenetic system that limited the magnitude of differential compaction. Debolt age rapid subsidence induced faulting that extended upwards from the Leduc through to the Debolt Formation. It is interpreted that CaCO3 saturated fluids were expelled from the Leduc reef and flowed upwards along the faults through the overlying Mississippian sediments and presumably to the surface. This permitted chemical compaction to proceed within the reef at a greatly accelerated rate leading to reef collapse. A second, more modest period of collapse occurred slightly later during Stoddart Group deposition. Seismic data along with isopach and structural mapping were used to quantify the amount of compensatory thickening that occurred and to establish the chronology of associated faulting. The Gold Creek Collapse Structure formed in three stages during the Mississippian Period. The first stage occurred during deposition of the Exshaw to Shunda Formation interval. The second stage occurred during Debolt Formation deposition and the remaining subsidence occurred during Stoddart Group deposition. Depth of burial of the Gold Creek Leduc Reef increased from 335 m to 743 m during stage one. During stages two and three the depth of burial increased to 992 m and 1035 m, respectively.