Thin section petrography

We offer three options, depending on the degree of information required:

Quicklook petrography: involves a brief description of the major detrital minerals and diagenetic phases. This is the least detailed option but facilitates rapid scanning of a number of thin sections to assess trends and variations. It is also applicable to poor quality thin sections, for instance from cuttings or sidewall core samples.

Detailed description: includes a description of framework grains, ductile phases, authigenic clays and cements with a visual abundance estimate. Pore type and distribution are assessed, and the main factors controlling reservoir quality are identified.

Detailed description with point count: This is the most detailed option. Detrital and authigenic phases, pore types etc are described as for the previous option, but in addition a 200 grain point count is performed to generate more reliable quantitative compositional data. This is the most common option for routine petrographic studies.

All thin sections are impregnated with blue-dyed araldite to identify porosity and are fully stained for K-feldspar and carbonate cements to aid mineral identification.

Examples of a petrographic data sheet and photomicrographs are shown below.

Petrographicservices

WELL: DEPTH: 3750.34m PLUG NO.: 14

DESCRIPTION: A general view of a well sorted, fine (lower) Facies 1 sandstone. There is a significant amount of open, moderately connected remnant primary porosity. Note also the relatively common, orange stained, uncorroded K-feldspar grains.

He Ø = 19.5% Kh = 273mD FIELD OF VIEW: 3.6mm

WELL: DEPTH: 3737.94m PLUG No.: 33 DESCRIPTION: This photo shows a mica-rich lamina within a very fine (upper) Facies 1 sandstone. The lamina is significantly more compacted than adjacent mica-poor areas and contains little or no remnant porosity. Fluid flow within the sand is therefore likely to be anisotropic. Note also the presence of a siltstone clast.

He Ø = 21.5% Kh = 88mD FIELD OF VIEW: 3.6mm

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WELL: PLUG NO: 33 He POROSITY: 30.4%

FORMATION: DEPTH: 6611.70ft PERMEABILITY: 2455mD

GRAIN SIZE: Fine (upper) COMPACTION: Mod low TOTAL FRAMEWORK: 92.5%

RANGE: StM - ML SHAPE: Subangular to subrounded TOTAL DUCTILE: 2.0%

SORTING: WS STRUCTURE: Grain size lamination TOTAL AUTHIGENIC: 5.5%

TOTAL MACROPOROSITY: 22.5%

FRAMEWORK GRAINS: 92.5%

Monocrystalline qtz 77.5% Dominantly strained metamorphic grains

Polycrystalline qtz 6.0% Strained metamorphic grains

Microcrystalline qtz 0.5% Chert and ?igneous grains

K-feldspar 4.5% Untwinned to locally lamellar twinned grains, little or no dissolution

Plagioclase 1.5% Scattered lamellar twinned grains showing minor dissolution

Lithics 1.0% Mainly ?phyllitic grains showing some dissoluion

Bioclasts 0.0%

Heavy minerals 1.5% Mainly sphene, with scattered opaque oxides, zircon, garnet and amphibole/pyroxene

Volcaniclasts 0.0%

Carbonate grains 0.0%

DUCTILE COMPONENTS 2.0%

Muscovite 0.5% Rare laths, slightly altered to kaolin

Biotite Tr% Rare relatively fresh laths

Chlorite 0.0%

Organic material 0.0%

Mudclasts 0.0%

Detrital clays 1.5% Minor poorly formed, rarely pore-filling clays

AUTHIGENIC CEMENTS 0.5%

Quartz overgrowth 0.5% Minor euhedral terminations

Microcrystalline quartz 0.0%

Calcite Tr% Minor slightly ferroan rhombic cement is probable calcite

Dolomite 0.0%

Siderite 0.0%

Haematite 0.0%

Barite 0.0%

Pyrite 0.0%

Hydrocarbon 0.0%

AUTHIGENIC CLAYS 5.0%

Illite/smectite 0.0%

Chlorite 0.0%

Kaolin 5.0% Interstitial, microporous pore-filling masses, rarely demonstrably formed from

muscovite

MACROPOROSITY 22.5%

Predominantly open, well connected primary porosity (22.5%) with trace amounts of secondary porosity in partly corroded feldspars and lithics

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