Shale Gouge Ratio
Encyclopedia
Shale Gouge Ratio is a mathematical algorithm
that aims to predict the fault rock types for simple fault zones developed in sedimentary sequences dominated by sandstone
s and shale
s.
The parameter is widely used in the oil and gas exploration
and production industries
to enable quantitative predictions to be made regarding the hydrodynamic behavior of faults.
content of the rocks that have slipped past that point.
The SGR algorithm assumes complete mixing of the wall-rock components in any particular 'throw interval'. The parameter is a measure of the 'upscaled' composition of the fault zone.
involves identifying and defining accumulations of hydrocarbon
s that are trapped in subsurface structures. These structures are often segmented by faults. For a thorough trap evaluation, it is necessary to predict whether the fault is sealing or leaking to hydrocarbons and also to provide an estimate of how 'strong' the fault seal might be. The 'strength' of a fault seal can be quantified in terms of subsurface pressure, arising from the buoyancy
forces within the hydrocarbon column, that the fault can support before it starts to leak. When acting on a fault zone this subsurface pressure is termed capillary threshold pressure.
For faults developed in sandstone and shale sequences, the first order control on capillary threshold pressure is likely to be the composition, in particular the shale or clay content, of the fault-zone material. SGR is used to estimate the shale content of the fault zone.
In general, fault zones with higher clay content, equivalent to higher SGR values, can support higher capillary threshold pressures. On a broader scale, other factors also exert a control on the threshold pressure, such as depth of the rock sequence at the time of faulting, and the maximum burial depth. As maximum burial depth exceeds 3 km, the effective strength of the fault seal will increase for all fault zone compositions.
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
that aims to predict the fault rock types for simple fault zones developed in sedimentary sequences dominated by sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s and shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s.
The parameter is widely used in the oil and gas exploration
Oil exploration
Hydrocarbon exploration is the search by petroleum geologists and geophysicists for hydrocarbon deposits beneath the Earth's surface, such as oil and natural gas...
and production industries
Petroleum industry
The petroleum industry includes the global processes of exploration, extraction, refining, transporting , and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline...
to enable quantitative predictions to be made regarding the hydrodynamic behavior of faults.
Definition
At any point on a fault surface, the Shale Gouge Ratio is equal to the net shale/clayClay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
content of the rocks that have slipped past that point.
The SGR algorithm assumes complete mixing of the wall-rock components in any particular 'throw interval'. The parameter is a measure of the 'upscaled' composition of the fault zone.
Application to hydrocarbon exploration
Hydrocarbon explorationOil exploration
Hydrocarbon exploration is the search by petroleum geologists and geophysicists for hydrocarbon deposits beneath the Earth's surface, such as oil and natural gas...
involves identifying and defining accumulations of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s that are trapped in subsurface structures. These structures are often segmented by faults. For a thorough trap evaluation, it is necessary to predict whether the fault is sealing or leaking to hydrocarbons and also to provide an estimate of how 'strong' the fault seal might be. The 'strength' of a fault seal can be quantified in terms of subsurface pressure, arising from the buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
forces within the hydrocarbon column, that the fault can support before it starts to leak. When acting on a fault zone this subsurface pressure is termed capillary threshold pressure.
For faults developed in sandstone and shale sequences, the first order control on capillary threshold pressure is likely to be the composition, in particular the shale or clay content, of the fault-zone material. SGR is used to estimate the shale content of the fault zone.
In general, fault zones with higher clay content, equivalent to higher SGR values, can support higher capillary threshold pressures. On a broader scale, other factors also exert a control on the threshold pressure, such as depth of the rock sequence at the time of faulting, and the maximum burial depth. As maximum burial depth exceeds 3 km, the effective strength of the fault seal will increase for all fault zone compositions.