report results of petrological and petrophysical investigation of

REPORT

Results of petrological and petrophysical

investigation of rock samples

from the Siljan impact crater

(Mora area)

Flotten AB

Stockholm, March 2015

This report is the result of the petrological and petrophysical analysis of core samples

from the Siljan impact crater area which was conducted by the researchers from the

Department of Lithology of the Gubkin Russian State University of Oil and Gas (Moscow).

The following researchers took part in this investigation:

Prof. Alexander Postnikov – the coordinator of research

Prof. Vladimir Kutcherov – the project

coordinator

Dr. Lyubov Popova – the senior researcher

Olga Sivalneva – PhD student

Alexander Buzilov – PhD student.

INTRODUCTION

One of the main prospective objects for oil and gas deposits accumulation are meteor

impact craters. The impact fractures are the result of impacts of asteroids, bolides, comets

on the Earth. Petroleum reserves were found in onshore and offshore meteor impact craters

carbonate, sandstone and granite rocks over the world [Donofrio, 1998]. The richest

petroleum meteor impact crater Cantarell is in Mexico. The current recoverable reserves are

equal to 1.6x109 m3 of oil and 146x109 m3 of gas in three productive zones.

In 1990-th two deep well (Gravberg-1 and Stenberg-1) to nearly 7 km depth were drilled in

the granitic Precambrian shield of the Siljan ring impact structure in the frame of Deep Gas

Drilling project. The main results of this project could be summarized as follows.

1. Gas samples collected from Gravberg-1 have a deep upper-mantle origin, or may be

product of abiogenic synthesis in crustal rocks.

2. Hydrocarbons found in the dolerite are similar, compositionally and isotopically, to

hydrocarbons in gases from East Pacific Rise (EPR) reported to be abiogenic in origin

[Welhan and Craig, 1983].

In 2009 Igrene AB has drilled five boreholes to 400-700 m depth. Methane gas was found in

several boreholes. The temperature of water (above 200C on the depth of 300-400 m),

amount of gas detected and the previous results from the Deep Gas Drilling project gave us

the possibility to suggest the present of shallow natural gas deposit with commercial

potential.

The main aim of the below-mentioned investigation is to get reliable information about

structure and properties of the rocks in and around the Siljan impact crater area in order to

evaluate the potential of possible hydrocarbon deposits in the area.

METHODS OF INVESTIGATION

The below-mentioned experimental investigations were conducted using core samples

received from different boreholes drilled by Igrene AB on the outer rim of the Siljan impact

crater (Mora area).

Petrological study of rock samples

The main objective of petrographic studies of the rock samples is the study of species

composition of the crust. Optical microscopy is the main method of research in this

science. With this technique accurate information about the chemical and physical

properties of minerals could be received.

Petrophysical investigation of rock samples

The main objective of the petrophysical investigations of the rock samples is to get data of

their reservoir properties - density, open porosity and permeability. These properties are

the basic parameters for evaluation of the geological reserves of natural gas in the

Siljan Ring impact crater area. The short description of the methods of investigation is

presented below.

A. Measurement of open porosity

To determine the open porosity of the sample the method of saturation of the sample by

formation water was used. According to this method, open porosity is calculated using

value of the weights for dry and saturated samples as well as samples weighted in the

solution with given mineralization.

NaCl solution with mineralization of 225 g/l (ρ = 1.146 g/cm3) was used as

saturating fluid.

Measurements were carried out at ambient conditions (pressure and temperature).

Open porosity was calculated as follows:

P=Vpor/Vsample, (1)

where Vpor – pore volume,

Vsample – sample volume.

B. Measurement of permeability

For measuring the absolute permeability the equipment of stationary filtration of gas

(nitrogen) with pressure on the outlet of the sample was used. The laminar flow of gas

through the sample realized in the equipment allowed use for permeability calculation the

Darcy law without additional amendments.

Preparation of samples: extracted from hydrocarbons and salts samples of right

cylindrical shape were dried to constant weight at a temperature of 1050C.

The measurement process is performed as follows. The sample of cylindrical form is

placed in the rubber jacket of core holder. With the help of pneumatic pressure

crimping side pressure equal to 24 bars is creating a seal. This will not allow slippage of

gas between the sample and the jacket.

The permeability coefficient for stationary filtration with a linear gas flow is calculated

as follows:

2000 Pbar µQL Pe = ------------------- (2)

(P21-P2

2)F where Pe – permeability

- viscosity of gas (mPa·s);

Q - average gas flow rate, measured at atmospheric conditions at the outlet of

the sample (m3/s);

L – length of the sample (sm2);

P1 and P2 – input and output pressure (bar);

F - cross-sectional area of the sample (sm2).

Fig. 1. Experimental setup for study of rock permeability.

NMR spectroscopy

NMR techniques are typically used to predict permeability for fluid typing and to

obtain formation porosity, which is independent of mineralogy. The former application uses

a surface-relaxation mechanism to relate measured relaxation spectra with surface-to-

Input pressure

Output pressure Core

holder Reducer

Gas balloon Reducer

Throttle

Control panel

Flowmeter Throttle

volume ratios of pores, and the latter is used to estimate permeability. The common

approach is based on the model proposed by Brownstein and Tarr.

Brownstein, K.R.; Tarr, C.E. (1979), "Importance of classical diffusion in NMR

studies of water in biological cells", Physical Review A 19(6): 2446,

Bibcode:1979PhRvA..19.2446B, doi:10.1103/PhysRevA.19.2446

DESCRIPTION OF THE CORE SAMPLES SELECTED FOR INVESTIGATION

Petrological study

More than 600 core samples from two boreholes Vattumyren 1 (VM1) and Vattumyren 2

(VM2) drilled in Mora area were studied. Core recovery is nearly 100%. The study was

conducted on the full-size and sawn along the axis core material. Conclusion was made on

the basis of macro- and microscopic analyses, photograph in natural light.

Two large fragments - sedimentary rocks and basement rocks were recognized in both

borehole sections (fig. 2).

Fig. 2. Sedimentary rocks and basement rocks intervals for VM1 and VM2 boreholes.

In general, the sedimentary rocks are represented by Paleozoic sediments. They consist of

variety of rock types: limestone, sandstone, siltstone and mudstone.

Sedimentary rocks are relatively little disturbed by fractures. The thin chaotic fractures

which frequently change the orientation of the strike and sometimes fade even within the

same sample are the most widespread. It was found that in normal petrographic thin

sections their disclosure is not more than 0.01 mm. The vast majority of fractures found in

petrographic thin sections are filled with clay material.

The larger fractures are angled or sub-vertical orientated. The number of sub-vertical

fractures in the sedimentary section prevails. This indirectly indicates their relatively late

generation. The maximum extent of sub-vertical fractures is approximately 18 cm. The

value of the initial disclosure is not more than 0.6 cm. All the selected types of large

fractures are mineralized predominantly by calcite, or by clay material (fig. 3).

Carbonate and clay-carbonate rocks are present as reliable impermeable beds - caprocks.

Fig. 3. Algal limestone,VM1, depth 119.45 m. Fractures with confined clay substances. In general, the basement rocks are represented by acid vulcanogenic rocks in VM1 and by

metavulcanites - trachydacitic porphyrites with dolerite intrusions in VM2.

Basement rocks in contrast to the sedimentary rock intensively tectonically disturbed.

Numerous fractures of different orientation and initial disclosure, zones of intense crushing

rocks, cataclastic packs and mylonited zones were found in the basement section of both

boreholes.

Differently orientated cracks filled by different substances were observed in the most of

samples investigated. These cracks belong to the different generation – different tectonic

events (fig. 4).

Fig. 4. Systems of differently orientated fractures of different generations:

a) VM1, depth 471.47 m, b) VM2, depth 553.07 m.

Gabbroid intrusion was detected in both boreholes: in VM1 in the depth interval of 296.06-

369.76 m; in VM1 in the depth interval of 484.7-549 m. Gabbroid intrusion is a powerful

impermeable bed – caprocks.

a) b)

Basement rocks from VM1 are intensively tectonically disturbed. Numerous fractures, slit-

like and cavern-like voids confirm the presence of fractured volume (reservoir capacity) in

the basement rocks (fig. 5).

Fig. 5. VM1, depth 424.16 m. Slit-like and cavern-like voids in acid vulcanite cataclasite.

Basement rocks from VM2 tectonically disturbed much less. There are fractures, slit-like

and cavern-like voids but the fractured volume in the basement rocks is less compared

to the rocks from VM1 (fig. 6).

Fig. 6. VM2, depth 627.9 m. Fractures and voids in metavulcanite.

Basement complexes for both boreholes are presented on the fig. 7.

a) b)

Fig. 7. Basement complexes a) for VM1 and b) for VM2.

Petrophysical investigations

23 core samples from two boreholes Vattumyren 1 (VM1) and Vattumyren 2 (VM2) were

selected for petrophysical investigations. One sample represented sedimentary rocks,

22 samples were rocks from the crystalline basement.

Description of the samples and method used for investigation of each samples are

presented in Table 1.

Table 1. Description of samples investigated.

No Depth, m Description Method of investigation

Vattumyren 1

1 105.05 Fine-grained sandstones with spotty carbonate cement –sedimental rock.

Petrophysical, NMR

2 263.4 Acid vulcanites cataclasites. Petrophysical







9 409.96 Acid vulcanites cataclasites. Petrophysical, NMR

10 413,26 Acid vulcanites cataclasites. Petrophysical







17 467.04 Acid vulcanites cataclasites. Petrophysical, NMR

18 468.75 Trachydacitic porphyrite. Petrophysical

19 472.59 Trachydacitic porphyrite. Petrophysical, NMR

19 473,49 Trachydacitic porphyrite. Petrophysical, NMR

20 477,89 Trachydacitic porphyrite. Petrophysical

Vattumyren 2




To provide complex petrophysical measurement for all samples selected failed because

of the fragility of some highly fractured samples or because of their partial destruction

in previous studies.

Results of investigation for core samples are presented in Table 2.

Table 2. Results of the petrophysical and NMR investigations for core samples selected.

No Depth, m Density, kg/m3 P, % Pe, mD P, %, NMR Pe, mD, NMR

1 2 3 4 5 6 7

V attumyre n 1 1

105,05 2660 3,16

3,27 0,033

2 263,4 2720 0,50

0,77 0,001

3 288,35 2620 1,93

4 378,78 2630 5,24 0,2511

5 391,75 2640 6,12 0,6107

6a 392,25 2700 12,27

11,51 0,143

6b 392,25 2690 14,59 0,1157

7 392,49 2680 7,63

8,44 0,312

8 397,29 2690 5,38

5,74 0,093

9a 409,96 2660 10,23

11,64 0,813

9b 409,96 2670 8,88

9,19 0,05

10 413,26 2690 5,25

5,86 0,043

11 424,16 2680 1,96

12 444,08 2670 2,76 1,236

13 445,46 2700 4,55

14 448,04 2650 4,81

15 450,99 2690 4,45

16 452,78 2624 1,99 0,573

17 467,04 2740 4,51

4,5 0,824

18 468,75 2680 1,03 1,173

19 472,59 2667 1,44 0,319

20 473,49 2677 0,82 8,26

21 477,89 2767 1,31 1,22

Vattumyren 2

22 579,5 2767 0,53 0,87

23 655,7 2635 0,14 0,63

24 660,45 2628 0,43 0,417

As it is possible to see from Tables 2 the value for open porosity measured by

petrophysical method (column 4) and by NMR method (column 6) correspond each

other quite well.

Rocks from the basement.

In both boreholes VM1 and VM2 the gabbroid intrusion layer was found. In the VM1 the

gabbroid intrusion layer is located at a depth of 296 m below ground level. In the VM2 the

layer is located 485 m below ground level.

The gabbroid intrusion layer is presented in other wells drilled in the area (fig. 10).

According to our consideration the reservoir rocks represent by acid vulcanites cataclasites

in VM1 and by metavulcanites - trachydacitic porphyrites with dolerite intrusions in VM2.

The reservoir rocks are located under the gabbro layer. Acid vulcanites cataclasites from

VM1 has open porosity value from 0.82 to 14.6% and permeability from 0.1 to 8.26 mD.

Trachydacitic porphyrites from VM2 has much lower value of open porosity - from 0.14 to

0.53% and permeability from 0.4 to 0.87 mD. Only three core samples from VM2 were

investigated. The result received cannot be considered as representative. We have selected

7 samples that will be investigated in March 2015.

Our preliminary suggestion about existence of the thrust structure made on the basis of

investigation of core samples from VM1 was not confirmed by the result of investigation of

the core samples from VM2 and visual analysis of core samples from other wells.

The experiments where vulcanite was saturated by liquid markers confirm our suggestion

that fluid and/or gas could move in the basement rocks along the fractures (fig. 8).

Fig. 8. Liquid marker (blue color) in the fracture of the metavulcanites from VM2.

The sections for both boreholes VM1 and VM2 are presented on the fig. 9.

a)

b) Fig. 9. Sections for a) VM1 and b) for VM2.

Fig. 10. Section, Mora area

Suggestion about possible reservoir volume.

According to our consideration the potential reservoir in the Siljan impact crater area and

traps correspondently could be located in the basement rocks. In general the trap structure

could be recognized by seismic survey. But in the case of the Siljan impact crater where the

suggested trap is located in the basement rocks under comparably thin sedimentary layer

seismic method does not allow to determine the structure.

In this case our suggestion is the following. The trap(s) could be located under the gabbroid

intrusion layer down to 500-700 m. We suggest that the trap in the Siljan crater area is

limited by a network of lineaments location of which was recognized in our previous report

(the color area between two violet lines on fig. 11). The uplifted ring area is the potential

zone for shallow natural gas deposit location.

Fig. 11. Block structure of the Siljan impact crater area.

Considering the average thickness of potential reservoir below the gabbroid intrusion of

about 200 meters (and maybe more) each sq. km of the area corresponds to about 0.2 cub.

km of reservoir rocks. Assuming an average porosity in the range of 2 to 3%, the available

pore volume for gas or water is 4-6 mln m3 per sq km. Only the Mora area might be as large

as 100 sg km. Hence the total pore volume available for water or gas may be as high as 600

mln m3 for Mora area only. These rocks are likely saturated with water and possibly some

with free gas. Some water contains methane in solution (e.g. Mora area). At present we

cannot predict what part of the above-mentioned pore volume is occupied by water with and

without gas or free gas (if any). Nor is the reservoir capacity known. To better understand the

gas source and reservoir capacity it is recommended to conduct a production test.

Conclusion

1. The results of this study confirm the presence of cap rocks being a) the layer of

sediments (about 250 m) and b) the layer of gabbro (about 70 m) in Mora area. Both layers

are characterized by almost zero permeability.

2. The value of open porosity in the basement rocks investigated varies widely from 0.14

to 14.6%. The permeability varies from 0.1 to 8.26 mD. These differences indicate the

presence of different beds in the basement some of them (the vulcanites located under

the gabbroid intrusion layer) could be taken into a consideration as potential reservoir rocks.

3. Experiments with meta-vulcanites saturating by markers confirm our suggestion that fluid

and/or gas could move in the basement rocks along the fractures.

4. Each sq. km of the Mora area corresponds to at least 0.2 cub. km of reservoir rocks.

Assuming an average porosity of 2-3 % the available pore volume for water or gas is

expected to be in the order of 4-6 mln m3 per sq km. These rocks are likely saturated with

water and possibly some with free gas. Some water contains methane in solution.

5. To better understand the gas volumes in place and the reservoir capacity it is

recommended to conduct a production test.

report results of petrological and petrophysical investigation of

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