REPORT. Results of petrological and petrophysical investigation of rock samples from the Siljan impact crater (Mora area)

Transcription

1 REPORT Results of petrological and petrophysical investigation of rock samples from the Siljan impact crater (Mora area) Flotten AB Stockholm, March 2015

2 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.

3 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 1.6x10 9 m 3 of oil and 146x10 9 m 3 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 m depth. Methane gas was found in several boreholes. The temperature of water (above 20 0 C on the depth of 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.

4 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 = g/cm 3 ) was used as saturating fluid. Measurements were carried out at ambient conditions (pressure and temperature). Open porosity was calculated as follows: P=V por /V sample, (1) where V por pore volume, V sample 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.

5 Preparation of samples: extracted from hydrocarbons and salts samples cylindrical shape were dried to constant weight at a temperature of C. of right 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: where Pe permeability - viscosity of gas (mpa s); 2000 P bar µql Pe = (2) (P 2 1-P 2 2 )F Q - average gas flow rate, measured at atmospheric conditions at the outlet of the sample (m 3 /s); L length of the sample (sm 2 ); P1 and P2 input and output pressure (bar); F - cross-sectional area of the sample (sm 2 ). Reducer Input pressure Core holder Output pressure Gas balloon Reducer Throttle Control panel Flowmeter Throttle 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-

6 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 B, doi: /physreva

7 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.

8 Fig. 3. Algal limestone,vm1, depth 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). a) b) Fig. 4. Systems of differently orientated fractures of different generations: a) VM1, depth m, b) VM2, depth m. Gabbroid intrusion was detected in both boreholes: in VM1 in the depth interval of m; in VM1 in the depth interval of m. Gabbroid intrusion is a powerful impermeable bed caprocks.

9 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 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 m. Fractures and voids in metavulcanite. Basement complexes for both boreholes are presented on the fig. 7.

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

11 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 Fine-grained sandstones with spotty Petrophysical, NMR carbonate cement sedimental rock Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical, NMR ,26 Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical Acid vulcanites cataclasites. Petrophysical, NMR Trachydacitic porphyrite. Petrophysical Trachydacitic porphyrite. Petrophysical, NMR ,49 Trachydacitic porphyrite. Petrophysical, NMR ,89 Trachydacitic porphyrite. Petrophysical Vattumyren Trachydacitic porphyrite. Petrophysical Trachydacitic porphyrite. Petrophysical Trachydacitic porphyrite. Petrophysical

12 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/m 3 P, % Pe, md P, %, NMR Pe, md, NMR V attumyre n 1 105, ,16 3,27 0, , ,50 0,77 0, , , , ,24 0, , ,12 0,6107 6a 392, ,27 11,51 0,143 6b 392, ,59 0, , ,63 8,44 0, , ,38 5,74 0,093 9a 409, ,23 11,64 0,813 9b 409, ,88 9,19 0, , ,25 5,86 0, , , , ,76 1, , , , , , , , ,99 0, , ,51 4,5 0, , ,03 1, , ,44 0, , ,82 8, , ,31 1,22

13 Vattumyren , ,53 0, , ,14 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 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.

14 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.

15 Fig. 10. Section, Mora area

16 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 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.

17 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 m 3 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.