Zebra carbonates are characterized by subparallel, rhythmic, mm-scale banding of host rock and vein. Their genesis has been interpreted by different authors as primary sedimentary structure, metasomatic infiltration or mechanical fragmentation followed by deposition of vein minerals. We studied zebra carbonates in the damage zones of normal faults formed during the drainage of an overpressure cell at about 7 km depth, in outcrops on Jebel Shams, Oman Mountains. They show a distinct pattern of mm-scale regularly spaced calcite veins in the dark grey, fine-grained carbonate host rocks, often connected laterally to a wide mode I fracture filled by a single calcite vein several cm thick. Veins in the zebra carbonates are filled with blocky crystals, indicating that the fractures remained open after their formation to allow crystal growth from a supersaturated fluid. Microstructures show no evidence of repeated crack-seal events and we conclude that all the veins in one zebra were formed simultaneously. The very high density of closely-spaced and simultaneously formed fractures indicates that they were formed very rapid loading, producing fracture densities much higher than expected during slow deformation. On the other hand, the highdensity fracture systems formed during explosive fracturing in dry rocks are much less regularly spaced. We hypothesize that the zebra carbonates were formed by rapid loading during faulting in highly overpressured carbonates, in places where coseismic rupture leads to a significant fluid pressure drop in dilatant jogs. The permeability of the matrix carbonate leads to drop in pore-fluid pressure close to the crack walls. This makes the host rock stronger close to the crack wall, so that the next fracture will preferentially propagate into the matrix away from the walls of the existing fracture. This process can lead to a more regularly spaced pattern of veins. Further work on zebra carbonates could provide a new tool to distinguish seismic from aseismic faults in carbonates.
The Zechstein-2-Carbonates represent one of the most prolific hydrocarbon systems of Central Europe. Carbonate reservoir quality is primarily controlled by mineralogy, with dolomite representing moderate-to-good porosities and calcite commonly representing low porosities. Current models suggest that this calcite is the result of a basin-wide phase of dedolomitization. The calcium (Ca) source for the dedolomites is thought to be derived from the fluids liberated during gypsum-to-anhydrite conversion. We present an easy-to-use and generally applicable template to estimate the dedolomitization potential of these fluids. Depending on reaction stoichiometry, salinity, and temperature, we estimate that between2.8⁎10 −3 m 3 and6.2⁎10 −3 m 3 of calcite may replace dolomite for each m 3 of anhydrite created. Within the constraints dictated by the environment of the late Permian Zechstein basin, we estimate that about5⁎10 −3 m 3 of dedolomite is created for each m 3 of anhydrite. Mass balance constraints indicate that fluids derived from gypsum-to-anhydrite conversion account for less than 1% of the observed dedolomite in most of the studied industry wells from northern Germany.
In the South Oman Salt Basin the Ara carbonates form an extensively cored, deeply buried intra-salt<br>hydrocarbon play. Six surface-piercing salt domes in the Ghaba Salt Basin (North Oman) provide the<br>only outcrop equivalents for carbonates and evaporites of the Ediacaran-Early Cambrian Ara Group<br>(uppermost Huqf Supergroup). Based on fieldwork, satellite imaging and isotope analysis it is<br>concluded that most of the carbonate bodies (so-called stringers) in the Ghaba salt domes are time<br>equivalent to the stratigraphically uppermost stringer intervals in the South Oman Salt Basin (A5-A6).<br>Maturity analyses demonstrate that the carbonate stringers in the salt domes were transported with<br>the rising Ara salt from burial depths of [|#24#|] 6 to 10 km to the surface. Petrographic and stable<br>isotope data show that their diagenetic evolution during shallow and deep burial was very similar to the<br>Ara carbonate stringer play in the SOSB. However, during the retrograde pathway of salt diapir<br>evolution, the carbonate stringers were exposed to strong deformation in the diapir stem and<br>diagenetic alterations related to dedolomitisation. As the salt domes contain facies that are in all<br>aspects identical to the deeply buried Ara play in the South Oman Salt Basin, this study provides<br>substantial additional information for hydrocarbon exploration in South Oman. In addition, our work<br>has implications for the hydrocarbon prospectivity of the Ghaba Salt Basin and possibly of other<br>Ediacaran-Early Cambrian evaporite basins in the Middle East such as for the time-equivalent ‘Hormuz’<br>salt basins.
The Northern Calcareous Alps are part of the Eastern Alps in Austria and Germany. The Mesozoic units of this fold-and-thrust belt were detached, thrusted and stacked along the evaporitic Haselgebirge Formation. Exposed in salt mines, rocksalt and mudrock form a two component tectonite: The rock type "haselgebirge" consists of 10-70 wt % halite with silt- to gravel- or block-sized components within a halite matrix, and the "kerngebirge" with >70 wt % halite. All rock types studied are fault rocks. By use of a temperature-independent subgrain size piezometer, the paleo-differential stress of halite was calculated at ca. 2.5 MPa in Altaussee and ca. 4.5 MPa in Berchtesgaden. Including data from a grain-size piezometer, temperatures were estimated at ca. 150 ± 20 °C and 110 ± 10 °C. This implies very high strain rates, which are about 10-10-10-9 s-1. During the tectonic movement, the halite deformed, recrystallized, and crystallized as veins in mudrock fractures. We interpret high overpressure of the pore fluid to have significantly contributed to fracturing of the mudrock.
In the South Oman salt basin (SOSB), diapirs of infra-Cambrian Ara Salt enclose isolated, commonly overpressured carbonate reservoirs. Hydrocarbon-impregnated black rock salt shows that it has repeatedly lost and then regained its sealing capacity. The black staining is caused by intragranular microcracks and grain boundaries filled with solid bitumen formed by the alteration of oil. The same samples show evidence for crystal plastic deformation and dynamic recrystallization. Subgrain-size piezometry indicates a maximum differential paleostress of less than 2 MPa. Under such low shear stress, laboratory-calibrated dilatancy criteria indicate that oil can only enter the rock salt at near-zero effective stresses, where fluid pressures are very close to lithostatic. In our model, the oil pressure in the carbonate reservoirs increases until it is equal to the fluid pressure in the low but interconnected porosity of the Ara Salt plus the capillary entry pressure. When this condition is met, oil is expelled into the rock salt, which dilates and increases its permeability by many orders of magnitude. Sealing capacity is lost, and fluid flow will continue until the fluid pressure drops below the minimal principal stress, at which point rock salt will reseal to maintain the fluid pressure at lithostatic values.
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