| Computer Code: |
IRAN |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Guadalupian Epoch (PUG)
— Lopingian Epoch (PUL) |
| Lithological Description: |
The Iris Anhydrite Member consists of white, or occasionally pale pink or cream anhydrite which, in cuttings samples, displays amorphous, cryptocrystalline, microcrystalline and sugary textures. In core from 29/27- 1, it occurs largely in millimetre-scale laminations, disrupted occasionally by irregular lenses of white displacive anhydrite, but there are also veins of anhydrite filling tension gashes and scattered crystals of replacive anhydrite (Taylor, 1993). Elsewhere, prominent wireline log spikes occur within the anhydrite locally, corresponding to thin beds of dolomite or mudstone. |
| Definition of Lower Boundary: |
The lower boundary of the Iris Anhydrite Member is defined by a downward change from dominantly anhydritic to dominantly dolomitic lithologies. This transition is commonly gradational, but a highly radioactive bed of mudstone or sapropelic dolomite occurs immediately below the boundary in many wells (e.g. 30/29a-l). |
| Definition of Upper Boundary: |
The top of the Iris Anhydrite Member is defined by a downward change from dolomites with thin mudstones (Innes Carbonate Member) to anhydrite. It is marked by a sharp downward decrease in gamma-ray values, commonly with an increase in velocity and resistivity. |
| Thickness: |
The Iris Anhydrite Member is generally less than 30 m thick, but reaches a maximum thickness of 118 m in well 27/3-1. |
| Geographical Limits: |
The Iris Anhydrite Member can be detected in most areas of the Northern Permian Basin. However, it is absent in much of the area between the Halibut Horst and Fladen Ground Spur, where the Argyll and Innes carbonate members amalgamate to form a composite carbonate unit. It is also absent in areas of partial Mesozoic erosion of the Permian section, and may have been dissolved during Mesozoic uplift in the Argyll Field (Bifani, 1985) and locally elsewhere. |
| Parent Unit: |
Halibut Carbonate Formation (HACA)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 30/29a- 1: 2562.5-2577 m (8407-8455 ft) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 20/03- 3: 3755-3819 m (12320-12530 ft) (Cameron, 1993). |
| Reference(s): |
| Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| Bifani, R. 1985. A Zechstein depositional model for the Argyll field. 11-118 in The role of evaporites in hydrocarbon exploration. Taylor, J C M et al. (editors). JAPEC Course Notes No. 39. |
| Taylor, J C M. 1990. Upper Permian-Zechstein. In: Glennie, K W (ed.) Introduction to the petroleum geology of the North Sea, 153-190. Blackwell Scientific Publications, Oxford. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
