Cemented fault zones

Fault properties and the development of cemented fault zones in sedimentary basins field examples and predictive models... 

Sverdrup, E. Bjhrlykke, K. 1997. Fault properties and development of cemented fault zones in sedimentary basins. Field examples and predictive models. In Mhller-Pedersen, P. Koestler, A. G. (eds) Hydrocarbon Seals. Importance for Exploration and Production. Norwegian Petroleum Society (NPF), Special Publication, 7, 91-106. 

In both cases, the CaCb-rich brine is thought to have evolved from the NaCI-rich brine after fluid-rock interactions in the basement. The fault zones and the breccia bodies at the base of the basins represent active drainage zones where different fluid reservoirs were connected, and thus a highly favourable location for fluid mixing. Temperature and pressure changes, combined with the effects of fluid mixing, appear to be key-factors in the main stages of quartz cementation and U deposition in both Australian and... 

Cemented faults and fractures The main porosity reduction mechanism is cementation (Fig. 2a). Cementation is taken here to cover situations where cements have developed from (i) fluids flowing along the fault zone, or (ii) preferential growth in cataclasites because of the high concentration of nucleation sites on the newly created fracture surfaces. 

Two variables are fundamental to assessing the flow across complex fault zones. The first variable is the cumulative fault-rock thickness across the fault zone, i.e., the total thickness of fault-rock from all faults along the flow path. This depends upon the fault frequency along the flow path and is not equivalent to the fault damage zone thickness (cf. Knott, 1993) unless the fault zone is invaded by cements. The second variable is the connectivity of the faults or deformation features with low permeabilities in the fault zone. In the case of a completely connected array with no windows of undeformed material along possible flow paths, the flow is controlled by the permeability of the fault rocks. Where a more open network of faults is present then the flow will depend upon the tortuosity associated with flow around the low permeability zones and the ratio of matrix to fault-rock permeability. The interaction of these two factors will control the effective transmissivity of the zone. We have constructed a database on... 

The exposed fault structures are very similar to the deformation bands as described by Antonellini and Aydin (1994) and Fowles and Burley (1994). Numerous, sub-parallel shear fractures up to ca. 10 mm thickness have developed fault zones up to 5 m wide. Displacements are in the millimetric scale for each individual band, adding up to a maximum of ca. 100 m for the fault zones. Based on isotope data of fault abundant quartz cement, the minimum depth at which faulting occurred has been estimated to ca. 3000 m (Fig. 8). 

Fig. 8. Isotope data from Brora fault zone quartz cement. Precipitation temperature is suggested to be in the range between 120 and 125°C based on a solution from Labyerie (1974).

Pittman (1981) reported permeability measurements down to <0.05 mD from sandstones collected near fault zones where granulation seams are developed in which porosity had been reduced by both quartz cement and cataclasis. Jamison and Steams (1982) reported reductions of permeability by microfaulting by as much as three orders of magnitude in sandstone they interpreted to have been deformed at a burial depth of 2 km. Harper and... 

In a similar way, calcite from the fault zone exhibits a unique REE pattern, with a marked positive Ce anomaly and a marked depletion in all heavy REEs (No. 7 in Fig. lOB), which does not resemble the patterns of other calcite samples. In agreement with O, C and Sr data, this confirms that the calcites in the fault and in the main blocks are not genetically linked. The REE patterns of calcite from the structural blocks are extremely variable, with any of the following features marked depletion in heavy REEs extreme depletion in light REEs enrichment in middle REEs marked positive Ce anomaly and small positive Gd anomaly (Fig. 10B,C). Different calcite-cemented intervals in the same well or in different wells generally... 

The timing and conditions of carbonate cementation in the Oseberg Formation in the Oseberg field have been discussed on the basis of petrological, fluid inclusion and geochemical evidence. Samples from structural blocks and from a major fault zone were investigated. 

Ankerite cement is very scarce in the Oseberg reservoir, except in one sample located at the boundary with the underlying shales. In this sample, fluid inclusion analysis indicates that ankerite formed at temperatures of 70-80 °C, i.e. during the latest Cretaceous or Early Tertiary. The Sr/ Sr ratios and REE patterns of ankerite are consistent with a significant influence of fluids and/or trace elements derived from shales. The 5 0 values indicate a marked contribution of meteoric water during ankerite formation. Ankerite from the fault zone and from the structural blocks have similar 5 0-5 C values, suggesting similar conditions of formation. 

This pattern of repeated filling events from a progressively maturing source rock basin, structural trap tilting, and migration up the fault zone, coupled with phase fractionation and separation of petroleum into oil and gas phases in the reservoir intervals (Fig. 32), which were subsequently progressively sealed off due to increased quartz cementation in response to increased burial, may explain the complex patterns of variable GOR, API and carbon isotope values observed in Smorbukk (cf. Fig. 14). 

Some would argue that such fault zones are unlikely conduits for fluid flow (petroleum migration) along the fault plane because the permeability along the fault plane is nearly always lower than in the matrix next to the fault. These faults may, however, not be impermeable for flow across the faults at moderate depth (<2 km) prior to cementation. 

Sverdrup Bjorlykke (1997) conclude that brittle deformation may occasionally generate faults which may remain open to fluid flow along the fault planes for some time, whilst ductile deformation and abundant clay smear as found in the fault-zone of clay-rich sandstones generally will have the opposite effect. Most of the observed carbonate cements in faults in the North Sea were found to have an in-situ rather than autochthonous origin by fault parallel fluid flow. 

Table 1. Examples of major carbonate-cemented zones as identified from sonic and bulk density logs and lithological descriptions of cuttings for a variety of structures in dilferent parts of the Eromanga basin (Fig. lA). Note that the carbonate-cemented zones in Jurassic sandstones vary in cumulative thickness, from a few metres (e.g. Strzelecki-10) to about 110 m (Spencer West-1), including over short distances (e.g. Strzelecki). All wells occur near the Cooper basin margin, or along major fault-bounded structural trends where the regional Nappamerri Group seal (Triassic) underlying the Jurassic sandstones is incompetent or missing...

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