Introduction
Sandstone is a clastic sedimentary rock comprised of sand sized detrital grains of predominantly quartz, feldspar and minor impurities, these are found ubiquitously throughout the World. Due to this sandstone reservoirs account for 60% of all petroleum reservoirs in the World (Br rlykke and Jahren, 2010) found in localities including; The North Sea, The Middle East and Australia to name but a few. Sedimentary rocks are the most common reservoir rocks as they have sufficient porosity and permeability to allow storage and transmission of petroleum (Glossary.oilfield.slb.com, 2017). Therefore, the utmost imperative controls on reservoirs quality are porosity and permeability. Porosity is the ratio of entire void space relative to bulk volume and permeability is the ability of a rock to allow fluid to pass through it (A ikalin et al., 2015).
Thus, it s important to comprehend the ways porosity and permeability can be effected. In an overall sense, both porosity and permeability in sandstones are effected by 2 factors; depositional and post depositional controls. Post depositional controls incorporate the realms of diagenesis, which in turn instigates chemical dissolution and quartz precipitation (Pye and Krinsley, 1985; Cao et al., 2016). Throughout diagenesis compaction reduces porosity and permeability to a greater degree than chemical dissolution and quartz precipitation (S. N. EHRENBERG(2), 1990). Having said this, this dissolution produces authigenic clays; Kaolinite, Illite, Smectite and Chlorite which occur as pore linings, pore fillings, pseudomorphous replacements, and fracture fillings (Michael D. Wilson, Edward D. Pittman, 1977) depending on the clay type fashioned. All have varying chemistries and ultimately varying properties. Nevertheless, authigenic clays have a major control on reservoir quality (Pittman, 1978). Therefore, knowing the type and its resultant controls is beneficial to oil and gas companies in determining if a sandstone is economically viable to extract oil and gas from. As well as this it s also applicable to groundwater reservoirs which are becoming increasingly valuable (Br rlykke and Jahren, 2010).
Aim To understand how the porosity and permeability of late carboniferous delta-front, valley confined sandstones from the Pennine Basin (UK) (Ventra et al., 2015) is effected by the amount, distribution and type of authigenic clays (Kaolinite, Illite, Smectite and Chlorite) present within the sandstones. The purpose of this research is to gain the information required to be able to apply this to any new sandstone hydrocarbon reservoir found throughout the World with comparable mineralogies. This can then be used to gage if the sandstone is economically viable for oil and gas extraction by a respective company.
Objectives and Hypotheses
1) Sandstones of the Pennine Basin will be examined to govern the mineralogy of the thin sections. This is a requirement to highlight the percentage of pore space and the generic clay minerals present, ultimately making a description of the thin section.
2) Once either clay minerals have been identified or minerals are unidentifiable, scanning electron microscope (SEM) and x-ray diffraction (XRD) will be used to answer the hypothesis If a greater amount of authigenic clay minerals are present is there less pore space under microscope examination? .
3) Moving on, rock samples analogous to the initial thin sections will be used on a Helium Porosimeter to answer one part of the project title through the hypothesis If more authigenic clay minerals are present is the porosity of sandstones reduced?
4) Finally, the Mercury Injection Method will be executed using the same analogous rock samples, only this time to establish permeability through devising pour thickness. Nevertheless, a comparable hypothesis will be answered If more authigenic clay minerals are present is the permeability of sandstones reduced?
Only once all 4 stages are completed in chronological order will the aim be satisfied, the hypotheses answered and the project title question be fulfilled.
Scope
To permit a manageable project certain aspects which could have been investigated have been required to be excluded. The main motives for their omission is time constraints and the depth of knowledge which is needed to solely answer the hypothesis and not get caught in a tangent. From the Pennine Basin, 5 logs were taken with sample numbers of each log ranging from a minimum of 10 to a maximum of 43. Due to the large number of samples attained many thin sections and ultimately hand samples will have to be excluded from the project. The reason for this being is to allow completion of the project within the specified time allocated. Nevertheless, a non-bias means of deciding which samples to use to be able to fulfil the aim as well as produce a final dissertation to a high standard will be chosen. With a maximum number of 30 being looked at. For example, including samples above and below the set boundary plus samples from logs across the logged cross-section.
Moreover, only the 4 authigenic clays: Kaolinite, Illite, Smectite and Chlorite will be investigated to determine their effect on porosity and permeability. This again has comparable links with the prior reason for the number of samples due to time constraints. A supplementary reason is the sheer volume of varying chemistry such as mixed layer clay variations between clays that are even classed within the same group (Jeans, 1998). As not knowing the varying chemistry in such depths is required to answer my project title, sub-groups will be excluded and the focus will remain on the 4 groups aforementioned.
On the contra, diagenetic chemical reactions will have to be considered, even if not to the degree required for a project exclusively focus on diagenesis as different authigenic clays form under different diagenetic conditions (Curtis et al., 1985). This could aid in determining the specific clays present as certain depths and ultimately an estimation of porosity and permeability of that sandstone. Although this would be an idea for a sister project and will not be included in my own.
Initial Literature Review Bj rlykke (1998) illustrates in detail the main clay types which form during diagenesis and their accompanying reactions due to a thermodynamic drive. This information along with information from the book (Weaver and Pollard, 1975) sanctioned the development of the
aim to focus on 4 authigenic clay minerals present in sandstone. Moreover, evidence was also shown that at depths of 2-3km is mainly chemical through dissolution and precipitation of minerals, thus showing clays effect on porosity.
Several studies discuss the effect of individual clays on porosity and permeability. Storvoll et al., (2002) talks about how illite forms fibrous aggregates that have a greater detrimental effect on permeability compared to porosity. Anderson et al., (2010) examines the swelling effect of smectite and its challenge to the oil industry as it can swell to 20 times its original size, reducing both porosity and permeability significantly. Kaolinite forms aggregates in the form of booklets which plugs pores and pore throats causing macroporosity to degrade to microporisity (A ikalin et al., 2015). Not all clay aggregates have a negative impact on reservoir quality, (Pittman et al., 2012) states when illite and chlorite coat quartz grains they retard nucleation and so preserves both porosity and permeability.
Purcell (1949) examines the way in which Mercury can be forced into evacuated pour space of a porous rock. From here numerous equation are derived to relate the permeability of the porous material to its capillary curve, ultimately determining the permeability of the sample. This paper therefore provided a means of testing the effect or clay minerals on a rock permeability.
Nick Desiderio (2014) assesses 3 methods for calculating a rocks porosity, one of which is Helium Porosimetry. This is a modern experimental method relative to (Purcell, 1949). As helium is the smallest atom its able to penetrate even microscope pores which are impenetrable by normal fluids, which is highly beneficial with clay minerals. Using Boyles Law this provides the most accurate and efficient method of equating a rocks porosity and so illustrates an excellent method to use in this project.
Finally, physical properties of porosity and permeability of a rock are principal parameters to be considered by oil and gas companies. Baring this in mind and the fact that the recent depletion of abundant fossil fuels has resulted in a need for unorthodox locations to be examined, the effect of clay minerals on these reservoirs could have a profound effect on their economic extractability.
Proposed Methodology
1)
Using the Leica DM 2700P LED microscope to examine the thin sections from log 1.2 and 2.4 (Fig.1) of the sandstones from the Pennine Basin to determine the minerology present within. This will be done through the statistical technique of point counting in conjunction with NCH Suite, LAS V4.4 and Leica QWin V3 software to exploit; grain type, size, shape, overgrowths and dissolution. With the acquired information for each slide an accurate and sufficient petrographic description can be made of each locality within the log. Focus on pore space highlighted by blue resin and clay is prominent, although the latter may be tough to quantify. To device a systematic approach to the thin sections that will used to prevent bias, 5 random thin sections will be used from above and below the set boundary in log 1.2. The same random selection will be done for log 2.4 with 10 being chosen from above and below the set boundary (Fig.1).
The nature of authigenic clay minerals can mean that individual clay minerals that haven t flocculated can be at their largest only 0.004mm (Ucl.ac.uk, 2017). This meaning they could be easily missed under the Leica DM 2700P LED microscope or the type of clay be unknown. In order to answer this question both XRD and a SEM will be used to identify areas of uncertainty. XRD will be used on a sub set of samples to clarify the main clay types within the system. As XRD only provides information on the crystallographic clay groups and so type, SEM will be used to highlight the clays distribution (pore-fill, grain coating excreta), in the same sub set of samples. Once the previous 2 methods are complete the analysis of thin sections is concluded for this study.
Hand samples collected at the same site will be used to classify quantitatively true density and true porosity using a Helium Porosimeter (Desiderio, 2014). The hand samples used will be the same rock used to create the thin sections used prior. With helium being the smallest atom its able to penetrate even the smallest pore spaces and so therefore provides the highest accuracy measurement of porosity for each hand sample (Glover, 2014). The data received can then be linked with the quantitative point count measurements made earlier to see if there is a relationship between clay type and porosity.
4)
Finally, as permeability is also a parameter that needs to be addressed to conclude how the reservoir quality of a hydrocarbon reservoir is effected by clay minerals a further test needs to be performed. Once again, the complementary hand samples to the thin sections are used with the mercury injection method. Mercury is injected at different pressures to cause mercury to replace expelled air even in the smallest of pore spaces. From this a capillary curve is created and using Washburn equation permeability is derived. Once again, the data received can then be linked with the quantitative point count measurements made earlier to see if there is a relationship between clay type and permeability, but also a relationship between clay type and porosity and permeability.
Skill Requirements Initially petrology skills are required to allow the recognition of minerals within each thin section. Factors that permit the proper identification of minerals using the Leica DM 2700P LED microscope require the knowledge of knowing if
they re present or not and if so what s the answer to the respective question.
Training will be needed to use the software NCH Suite, LAS V4.4 and Leica QWin V3 linked to the microscope to full effect to get the maximum information accessible. Further training will also be vital at this stage of proceedings to use point counting to describe the thin sections in an unbiased and quantitative way. Weather clay minerals have been identified or minerals are unidentifiable, an SEM microscope is more than likely going to be essential to allow the specific mineral to be identified. As this is the case tuition will be required either prior or at the required time on how to use a SEM microscope.
Finally, to establish the porosity and permeability characteristics of the rock samples, laboratory equipment including a Helium Porosimeter and Mercury Injection Method will need to be performed. Despite a qualified laboratory technician performing these procedures, the output data will need to be explained along with the theory involved with each respective machine.
Resource Requirements Various resources are a mandatory requirement needed to allow the completion of this project, resources vary from University to industry supplied.
Data Sets
A complete data set of thin sections have been supplied that are dyed with a blue resin to highlight porosity. The complete data set matches the samples taken from each log as shown in (Fig.1). Furthermore, hand samples that also conform to the log codes shown in (Fig.1) are provided. This means all relevant data sets required to answer the project title are supplied. As well as this, additional information is supplied in (Ventra et al., 2015) the study in which all prior data mentioned was collected for, thus acting as supplementary information and or data to aid in the intended research.
By the end of the project when all formalities are complete, from previous reading of literature a generalised conclusion at this point can be roughly predicted. As the percentage of clay minerals increase in the sample both the porosity and permeability will decrease, underlining poor hydrocarbon reservoir quality. Furthermore, one clay type in particular, illite, will have the paramount effect on porosity and permeability reduction due to fibrous pour filling which strongly reduces permeability in sandstones (Giles et al., 1992; Bj rlykke et al., 1986).