Core Description – Mineralogy and Petrology

Statoils Geological Laboratory performed whole rock X-ray Diffraction (XRD), Petrographic point count, and Scanning Electron Microscopy (SEM) analysis on cored intervals of the 15/9-19 SR Well. The analysis characterizes the mineralogy and petrography of these sandstone units and evaluates their controls on reservoir quality.

The data presented here are from the following cored intervals:

FormationInterval (mD MSL)Samples
Heimdal3643.00 - 3645.506
Hugin4328.85 - 4343.2516
Skagerrak4344.25 - 4354.0011

Mineral Characterizing techniques

All samples are examined by whole-rock XRD and selected samples by thin-section petrography/SEM analysis.

X-ray Diffraction (XRD) identifies the characteristic X-ray diffraction pattern of a mineral or compound. It can provide identification of interstitial clays that control porosity and permeability, and can help understand and evaluate well log data, stratigraphic logs and core logs.

Point counting is based on identifying the mineral and pores present in numerous points in a petrographic slide. This is used to determine potential reservoir problems (acid, fines migration and fresh water sensitivity) and perform depositional environmental interpretation (especially for carbonates).

Scanning Electron Microscope (SEM) is used to determine and identify the structure of substances like individual clay minerals and their physical locations in the pore system. This data helps us take engineering precautions to avoid adverse effects on the reservoir during the drilling, completion and production phases of reservoir development.

The Heimdal Formation

The 2.5 m Heimdal Fm. interval examined comprises of uniform, fine to medium-grained, sub-arkosic arenite sandstones. Whole rock XRD analyses (Image 1) show uniform petrologic composition of this interval, which is characteristic of the Heimdal Fm. in the area.

Image 1: XRD analysis for Heimdal formation

Petrographic analysis data (Image 2) is consistent with the XRD results and shows about 2 – 4 volume percent quartz cement.

The petrographic data also shows anomalous low intergranular macroporosity values of 5 – 11 volume percent. This intergranular macroporosity accounts for only about 34% of the total Helium porosity measured by conventional core analysis (average He porosity = 26%). The Substantial amounts of microporous dispersed clay and clay matrix (Image 2) are mainly responsible for this large discrepancy of the low intergranular macroporosity, and also probably for the anonymously low measured core permeabilities (12-40 mD)

Image 2: Petrographic Analysis of Heimdal formation

The Optical micrograph results show the medium to fine grained, moderately well sorted sandstone, with substantial amounts of dispersed clay (Image 3). It shows that all four samples consist predominantly of iron-rich chlorite, with minor amounts of illite/smectite (5 – 10% expandable layers). The results confirmed petrographic observations that dispersed clay is mainly derived from detrital clay matrix, clasts, and pellets.

Image 3: Optical micrograph from the Heimdal Formation, from 3644.50 m, plug-no. 7

SEM observations show that diagenetic recrystallization or alteration of the clay has also occurred (Image 4). It shows the substantial amounts of microporous clay (Fe-rich Chlorite) also formed on diagenetic quartz cement.

Image 4: Scanning Electron Micrographs, Heimdal Formation, from 3644.50 m, plug no. 7.

Sleipner Øst Petrologic Investigation

We integrate these results of Heimdal formation into the Sleipner Øst petrologic investigation of the resistivity transition zone. These petrologic features may represent a more extreme development related to resistivity transition zone problem of the Sleipner Øst Field.

Let’s have a look at the resistivity transition zone problem in Sleipner Øst.

Image 5: The Sleipner effect of low resistivity sands

Studies of the Palaeocene sands identified broad resistivity transition zones within the gas condensate reservoir (Image 6). We refer these as the High Resistivity Zone (HRZ), and Low Resistivity Zone (LRZ).

Image 6: The hypothesis explaining the low resistivity zone in the Sleipner area

High digenetic clay content shows changes in the resistivity as seen in the Sleipner field. An earlier oil charge with more extensive chlorite formation in the palaeo water zone may have affected the distribution of diagenetic clay.

Hugin Formation

The formation consists of highly variable, fine to coarse grained, well to poorly sorted, sub arkosic arenite sandstones with good to excellent reservoir properties. (Image 7)

Image 7: XRD analysis of Hugin formation samples

Permeability variation in three orders of magnitude (15300 vs. 115 mD) is exhibited by samples only 1 meter apart (4330.25 and 4331.25 m) with only 0.5% difference in porosity (25.2% vs. 24.7 %). The difference is primarily due to grain size sorting and clay content variation (Image 8).

Image 8: Petrographic analysis of Hugin formation samples

Variation in grain size, sorting, and reservoir quality are readily clear from the examination of three samples using photo micrographs from full diameter, blue epoxy impregnated thin sections (Image 9). As we go deeper, the reservoir quality diminishes.

Image 9: Optical micrograph from the Hugin Formation.

The image below shows that there is no correlation between porosity and permeability because of the influence of Grain size and Clay.

Image 10: Geologic controls on reservoir quality of the Hugin Formation, well 15/9-19 SR.

Skagerrak Formation

The Skagerrak interval examined consists of fine to very-fine grained sub-arkosic arenites, sandstones, siltstones, and shales with poor reservoir quality.

Only whole rock XRD data was acquired over this interval because of low economic interest of these lithologies (permeabilities are generally less than 1mD).

Image 11: XRD analysis of Skagerrak formation samples

The fine grain size and high clay content of the Skagerrak are mainly responsible for the poor reservoir quality of this formation (Image 11).

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1 thought on “Core Description – Mineralogy and Petrology”

  1. Hello, I am very interested in any petrographic thin sections that also have routine core analysis data too.

    I have created a github repository that uses python Altair and Panel that allows us to view the thin section photomicrographs for each poro-perm sample in the cross plot. Now I would like to add more data to this repository and this application.

    https://github.com/Philliec459/View-Thin-Section-Images-from-a-Porosity-Permeability-Cross-Plot-using-Python-Altair

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