OIL RESERVOIR TRACERS Field The present disclosure relates to compounds and composite materials, to their use as tracers that are suitable for inflow tracing of hydrocarbons, for example in a subterranean reservoir, formation or well structure, to methods of fabricating such compounds and composite materials, and to methods of monitoring hydrocarbon reservoirs using such compounds and/or composite materials. Background The use of tracers to monitor aspects of the performance of hydrocarbon wells is an established technique. The tracers may be water tracers, in that they are predominantly soluble or dispersible in water (e.g. produced water), oil tracers, in that they are soluble or dispersible in the hydrocarbons, or partitioning tracers, in that they are soluble or dispersible between both the water and hydrocarbon phases. Some tracing methods will employ more than one type of tracer and use the difference in behaviour to deduce properties of the hydrocarbon formation. For example, partitioning and water tracers may be injected into a production well along with injected water and then monitored as they are subsequently produced from the well. The time difference between the production of the water tracers, which are produced with the returning injected water, and the partitioning tracers, whose production is delayed by their interaction with the hydrocarbons, both in the formation and in the wellbore, can be used to deduce parameters relating to the local remaining hydrocarbon content of the formation. Alternatively, applications may use only water tracers. For example, water tracers may be introduced in an injection well and their presence monitored at adjacent production wells in order to obtain information about the movement and/or location of water from the injection well to the production well. In addition to injection techniques, it is also known to introduce tracers into a well by including them in articles placed in and around the wellbore or pipeline. Upon contact with the reservoir fluid of interest the tracer is released into the reservoir fluid. By detecting the concentration of tracer in samples of this reservoir fluid, information can be deduced about the performance of the hydrocarbon well. Tracers should be detectable in small to very small quantities, for example at levels below 100 parts per billion (ppb), preferably at levels of 50 ppb or lower, more preferably at levels of 10 ppb or lower, and most preferably in the parts per trillion (ppt) range (that is, at levels less than 1 ppb). The levels are typically determined on a mass/mass basis. The tracers should also be environmentally acceptable with low toxicity for insertion into the ground and usage, for example, in reservoir applications, but they must also be species that are not naturally present in the ground in such quantities as to contaminate the results of a tracer study. Typical detection methods include gas chromatography – mass spectrometry (GC-MS), gas chromatography – mass spectrometry – mass spectrometry (GC-MS-MS), liquid chromatography – mass spectrometry (LC-MS), liquid chromatography – mass spectrometry – mass spectrometry (LC-MS-MS), high-performance liquid chromatography (HPLC) and ultra- high-performance liquid chromatography (UHPLC), which can typically detect very low concentrations of the tracers in the produced fluids. It is desirable that tracers should be detectable in low quantities and also that they can be reliably distinguished from other tracers and species which are naturally present in reservoir fluids. Various families of chemicals can be used as tracer compounds. For example, GB2537530 (Rule et al) discloses the use of halogenated benzoic aldehydes or halogenated benzoic monoesters as oil tracers in hydraulic fracturing fluids to trace the production of crude oil during fracturing procedures. Whilst oil tracers are discussed in GB2537530 these are liquid tracers and are absorbed onto an insoluble medium. This limits their use to applications where a rapid desorption or dissolution of tracer from the insoluble medium material can be tolerated. Other tracer compounds exist that may be used to monitor the oil within a reservoir at a specific location. Tracers used to track the movement of oil soluble materials generally have low water solubility and high (>1000) octanol/water partition coefficients. Several families of such compounds have been used. Illustrative examples of suitable tracer compounds are organic compounds selected from halogenated hydrocarbons. Combinations of these compounds can also be used although single compounds are preferred. The tracer compound can preferably be a halogenated aromatic, polycyclic aromatic, heterocyclic aromatic, aromatic ketone, cycloalkane, or aliphatic compound, where the compound includes at least one halogen selected from the group consisting of Br, CI, F and I. Suitable tracers include, but are not limited to, 4-iodotoluene, 1,4-dibromobenzene, 1-chloro-4-iodobenzene, 5-iodo-m-xylene, 4-iodo-o- xylene, 3,5-dibromotoluene, 1,4-diiodobenzene, 1,2-diiodobenzene, 2,4-dibromomesitylene, 2,4,6-tribromotoluene, 1-iodonaphthalene, 2-iodobiphenyl, 9-bromophenanthrene, 2- bromonaphthalene, bromocyclohexane, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4- dichlorobenzene, 1-bromododecane, bromooctane, l-bromo-4-chlorobenzene, bromobenzene, 1,2,3-trichlorobenzene, 4-chlorobenzylchloride, 1-bromo-4-fluorobenzene, perfluoromethylcyclopentane (PMCP), perfluoromethylcyclohexane (PMCH), perfluorodimethylcyclobutane (PDMCB), m-perfluorodimethylcyclohexane (m-PDMCH), o- perfluorodimethylcyclohexane (o-PDMCH), p-perfluorodimethylcyclohexane (p-PDMCH), perfluorotrimethylcyclohexane (PTMCH), perfluoroethylcyclohexane (PECH), and perfluoroisopropylcyclohexane (IPPCH). The presence of these compounds can typically be detected in oil at concentrations down to about 5-10 ppb. Detection is limited to ppb levels partly by the fact that the molecules have to be a compromise between stability and functionality and by the fact that analysis is carried out in a medium with appreciable levels of background species, namely crude oil. By comparison, preferred tracer compounds for monitoring the flow of water within a reservoir may be detected at 50 ppt or less. Preferred tracer compounds for monitoring the flow of water are based on the alkali metal salts of halogen benzoic acids. The enhanced sensitivity of water tracers is partly a result of their analysis being carried out in an inherently cleaner medium, namely water, and partly because they can be derivatised prior to analysis. The derivatisation group added prior to analysis aids their detection by the preferred analytical technique. A typical derivatisation of fluorobenzoic acid water tracers in an aqueous medium is described in Fresenius' Journal of Analytical Chemistry volume 361, pages 797–802 (1998) by C. U. Galdiga & Tyge Greibrokk. The preferred analytical technique is GC-MS or GC-MS-MS. WO2016/174415, which is incorporated herein in its entirety, describes the application of oil field chemicals to a matrix and the release of such chemicals from the matrix. It exemplifies the composition of the matrix and discusses improvements that can be made to the composition of the matrix. It is an object of the present invention to address or mitigate one or more problems of the prior art. It is an object of the present invention to provide a tracer compound or a tracer composite material that may be suitable for use in solid materials intended to be deployed or provided at one or more strategic points or locations within a subterranean reservoir, formation or well structure, so as to monitor the inflow of hydrocarbons from that location. Summary The present inventors have found that commercially available tracers do not afford monitoring and tracing of the inflow of hydrocarbons from a subterranean reservoir, formation or well structure at sub-ppb levels. This is because currently available tracers either do not allow optimum sensitivity (i.e. in the parts per trillion (ppt) range) during subsequent detection techniques, or do not have the physical properties, such as high melting point, to allow them to be fabricated into composite materials that are to be deployed within a subterranean formation, reservoir or well structure. The present inventors have found a solution which advantageously combines the sensitivity of water tracers with the ease of deployment of oil tracers used in in- flow tracing applications. In particular, the present specification provides a tracer compound, wherein the tracer compound is a halogenated compound having a plurality of carboxylic ester functional groups, and wherein the tracer compound has a melting point equal to or greater than about 60°C. Advantageously, the use of tracer compounds having a melting point of at least or above 60°C allows such compounds to be fabricated, e.g. moulded or cast, into composite materials, for example polymeric resin composites, which can then be deployed at a predetermined location in a subterranean formation or well structure, typically in and/or around the wellbore or pipeline. Typically, the tracer compound may have a melting point equal to or greater than about 70°C, e.g. equal to or greater than about 80°C. Preferably, the tracer compound may have a melting point equal to or greater than about 90°C. By such provision, the melting point of the tracer may be sufficiently greater than the typical fabrication temperature of tracer resin composites, e.g. epoxy resin composites, to ensure that the tracer compound remains in solid form (i.e., does not melt) during the fabrication process. Without wishing to be bound by theory, it is believed that melting of the tracer compound within the polymer matrix during fabrication is undesirable as it may interfere with the curing process of the resin, and/or as it may affect the dispersion of the tracer compound within the resin. For example, melting of the tracer during manufacture of the composite may weaken the strength of the matrix. This may result in the matrix either having inferior physical and/or chemical properties. It may also cause the tracer not to be fully incorporated into the inner structure of the polymer matrix which may result in the tracer eluting too quickly from the exterior of the matrix. The present specification provides a family of oil tracers which are both very sensitive to detection in tracer applications and which have a sufficiently high melting point that they can be incorporated into a polymer matrix without melting during curing of the polymer matrix. This combination of features is advantageous in providing improved oil tracers for inflow oil tracer applications. Advantageously, the tracer compound may be soluble in oil or hydrocarbons. By such provision, in use, the tracer compound may be eluted from or released from its polymer matrix composite, thus being solubilised in the fluid, e.g. oil, to be traced. Advantageously also, the use of halogenated benzoic esters derivatives may allow such compounds to be monitored using high sensitivity detection techniques. Without wishing to be bound by theory, it is believed that such esters can be subjected to hydrolysis conditions (i.e. heating with an aqueous solution, e.g. an acid or alkali solution) so as to form the corresponding halogenated benzoic acids. The halogenated benzoic acids can then be extracted into an aqueous medium as a water-based tracer compound and then analysed accordingly. Such an approach may allow these compounds to be used as hydrocarbon tracers, but with the superior analytical performance of aqueous tracers. For example, acids are associated with better sensitivity as they can be removed from the reservoir fluid and concentrated, for example by 20 times or more, whereas oil tracers cannot. The tracer compound may have a generic formula (I): wherein Ar is an aromatic moiety, R1 is each independently selected from halogen or R2, l= 1, 2, 3, 4 or 5, R ₂ is a halogen-containing moiety, and m= 2, 3, 4, 5 or 6. The present specification also provides a composite material comprising: a polymer matrix; and an oil tracer compound (such as that described above), wherein the oil tracer compound has a melting point equal to or greater than the curing temperature of the polymer matrix. Brief Description of the Drawings For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows the reaction scheme for the preparation of the hydroquinone di-ester of 3-(difluoromethoxy)benzoic acid; Figure 2 shows the reaction scheme for the preparation of the hydroquinone di-ester of 2-chlorobenzoic acid; Figure 3 shows the reaction scheme for the preparation of the hydroquinone di-ester of 2,6-difluorobenzoic acid; and Figure 4 shows the reaction scheme for the preparation of the phloroglucinol tri-ester of 2-chlorobenzoic acid. Detailed Description As described in the summary section, the present specification provides a tracer compound or compounds having a melting point sufficiently high to allow such compound(s) to be fabricated, e.g. moulded or cast, into a composite material, for example a polymer resin composite, which can then be deployed at a predetermined location in a subterranean formation or well structure. Advantageously, the tracer compound has a melting point greater than the curing temperature of the polymeric resin in which it is mixed. Typically, the tracer compound has a melting point equal to or greater than about 60°C, e.g. equal to or greater than about 70°C, e.g. equal to or greater than about 80°C. Preferably, the tracer compound has a melting point equal to or greater than about 90°C. This may be particularly adapted to composites, e.g. using an epoxy resin matrix. Typically, such composite materials are fabricated by mixing a tracer compound within a polymer matrix. The matrix may be as disclosed in WO2016/174415, which is incorporated herein in its entirety. The matrix may preferably be a thermosetting polymer, a blend of a thermosetting polymer with one or more additional thermosetting polymers, a co-polymer, a thermoplastic polymer, a blend of one or more thermoplastic polymers with one or more thermosetting polymers, an elastomer, a wax, a binding agent, a rubber or a natural polymer. The thermosetting polymer can be an epoxy resin, a polyester resin, a cross-linkable polymer, a cross-linkable polyolefin, an amino resin, a phenolic resin, a polyurethane, a crosslink-able acrylic polymer, a phenol-formaldehyde resin, a melamine resin, a urea- formaldehyde resin, a melamine-formaldehyde resin, a polyimide, a silicone resin, a cyanate ester (a polycyanurate) or a diallyl-phthalate (DAP). The thermoplastic polymers can be a linear polyolefin (such as polypropylene (isotactic or syndotactic) or a polyethylene), a polyurethane, a polyester (polyethyleneterephthalate, polybutyleneterephthalate), a polyvinyldifluoroethylene, a polyamide, an acrylic polymer, a polyimide, a polystyrene, a polyvinyl chloride or a fluoropolymer. The elastomer may be degradable in oil or may be an oil-resistant rubber. In use, the matrix acts as a binder for the tracer compound. The tracer compound may be released via a number of mechanisms. For example, the matrix may slowly degrade or dissolve at the deployed location, thus slowly releasing the tracer compound. This may be particularly advantageous when the tracer compound is released in solid form and/or when the tracer compound does not dissolve at the deployed location, e.g. in oil. Alternatively, some tracer-matrix composites may release the tracer compound via a dissolution or diffusion mechanism with the matrix remaining intact or substantially intact. This may be particularly advantageous when the tracer compound is soluble in the fluid to be traced, e.g. in hydrocarbons or oil. When tracers are used for in-flow applications, a desirable parameter is a sustained but suitably slow release of tracer into the reservoir fluid. Additionally, the release rate should be such that the concentration of the tracer in the fluid is above the quantitation limit of the tracer. Typically, the matrix may not dissolve in the reservoir fluid but may remain intact or substantially intact. Without wishing to be bound by theory, once the tracer or other oil field chemical has left the matrix, it is thought that the latter may define a porous structure , with small pores existing where the tracer or other oil field chemicals had previously been. The mechanism of release of the oil field chemical from the matrix may be either by dissolution or by diffusion depending on the location of the oil field chemical within the matrix. Where the oil field chemical is at the surface of the matrix then the oil field chemical is believed to leave the matrix mainly by a dissolution process with a minimal release attributed to a diffusion process. However, where the tracer or oil field chemical is not close to the edge of the matrix, or is contained in a coated particle within the matrix, then migration of the oilfield chemical through the matrix to the reservoir fluid is believed to occur mainly by diffusion or Brownian motion. The tracer or other oilfield chemical, once sufficiently close to the outer edge of the matrix, may be carried away by the reservoir fluid. Typically, the oilfield chemical will dissolve in the reservoir fluid and be carried away from the polymer matrix. In the examples in this application the tracer compound would dissolve freely in the reservoir hydrocarbons. The tracer can be in a physical state of a solid, liquid or gas at the temperature of which the compounds are to be released. In a tracer/matrix composition without a coating, the liquid oil field chemicals will have a much higher release rate from the matrix and their usage would be less favourable than an oil field chemical in the solid state in the matrix as it will have a shorter lifetime. The fabrication process of the composite material typically involves moulding, casting or otherwise fabricating the composite material. This process typically involves curing the polymeric (matrix) resin, for example an epoxy resin, at temperatures above ambient temperature, and typically in the region of 50-70°C, e.g. approximately 60°C. As a result, the use of straight chain alkyl or single ring aromatic mono-esters of halogenated benzoic monoesters is frequently not possible as such compounds typically have a melting point lower than the temperature of fabrication or preparation of conventional composite materials, e.g. epoxy- based materials. A survey of the literature, in particular 2,3,4,5-tetrachlorobenzoic acid shows that the progression in melting point of aliphatic esters of halogen benzoic acids is not straightforward and certainly not as simple as equating increasing ester chain length with increasing melting point. In accordance with the present specification, the tracer compound is a halogenated compound having a plurality of carboxylic ester functional groups, and wherein the tracer compound has a melting point equal to or greater than 60°C, 70°C, 80°C, 90°C, or 100 ^o C. The tracer compound may be an ester of a (polyhydroxy)aromatic compound and a halogenated benzoic acid derivative. The tracer compound may be an ester of a (polyhydroxy)benzene and a halogenated benzoic acid derivative. The tracer compound may be a di-ester or a tri-ester of a (polyhydroxy)aromatic compound and a halogenated benzoic acid derivative. The tracer compound may be an di-ester or a tri-ester of a (polyhydroxy)benzene and a halogenated benzoic acid derivative. The tracer compound may be a di-ester of a dihydroxybenzene and a halogenated benzoic acid derivative. The tracer compound may be a tri-ester of a trihydroxybenzene and a halogenated benzoic acid derivative. The tracer compound may have the generic formula (I): wherein Ar is an aromatic moiety, R1 is each independently selected from halogen or R2, l= 1, 2, 3, 4 or 5, R ₂ is a halogen-containing moiety, and m= 2, 3, 4, 5 or 6. Preferably, R2 may be a haloalkoxy group, e.g. R2 may be –O-CHnX3-n, wherein X is a halogen or trifluoromethyl, and n is 0, 1 or 2; or R2 may be CF3. Typically, m may be 2 or 3. That is, the tracer compound may be a diester of a di(hydroxy)aromatic compound or may be a triester of a tri(hydroxy)aromatic compound. Advantageously, such compounds may be easier to synthesize than esters of polyhydroxy aromatic compounds having four or five hydroxyl groups, whilst possessing a melting point sufficiently high to avoid melting during the fabrication of a resin composite material, e.g. an epoxy-based composite. Ar may be a substituted or unsubstituted aromatic moiety, e.g. a substituted or unsubstituted monoaromatic (e.g. benzene), diaromatic (e.g. naphthalene), polyaromatic (e.g. anthracene), biphenyl-based, or bisphenyl-based, moiety. Advantageously, Ar may be an unsubstituted monoaromatic moiety, e.g. an unsubstituted benzene ring. By such provision, the polyol residue resulting from hydrolysis of the ester tracer compound should be easily separable as it will be organic soluble. By contrast, the acid species formed in the hydrolysis, can easily be rendered water soluble by making aqueous phase alkaline. In an embodiment, Ar may be a benzene ring. In such instance, the tracer ester compound may derive from a di- or tri-ol, e.g. from a dihydroxybenzene such as catechol, resorcinol or hydroquinone, or from a trihydroxybenzene such as pyrogallol, hydroxyquinol, or phloroglucinol. In one example, the tracer compound may have the generic formula (Ia): R1 is each independently selected from halogen or R2, l= 1, 2, 3, 4 or 5, R ₂ is a halogen-containing moiety, and m= 2, 3, 4, 5 or 6. Preferably, R2 may be a haloalkoxy group, e.g. R2 may be –O-CHnX3-n, wherein X is a halogen or trifluoromethyl, and n is 0, 1 or 2; or R2 may be -CF3. Typically, m may be 2 or 3. The tracer compound may have the generic formula (II): wherein R ₁ is each independently selected from halogen or R ₂ , l= 1, 2, 3, 4 or 5, and R2 is a halogen-containing moiety. Thus, the tracer ester compound of formula (II) may derive from a dihydroxybenzene diol compound such as catechol, resorcinol or hydroquinone. Preferably, R ₂ may be a haloalkoxy group, e.g. R ₂ may be –O-CH _n X _3-n, wherein X is a halogen or trifluoromethyl, and n is 0, 1 or 2, or R2 may be -CF3. The tracer compound may have the generic formula (IIa): Thus, the tracer compound may be or may comprise an ester of hydroquinone and 3- (difluoromethoxy)benzoic acid. The tracer compound may have a melting point of at least 100°C, e.g. of about 101°C to 106°C. The tracer compound may have the generic formula (IIb): Thus, the tracer compound may be or may comprise an ester of hydroquinone and 2- chlorobenzoic acid. The tracer compound may have a melting point of at least 100°C, e.g. at least 105°C, e.g. of about 107°C to 113°C. The tracer compound may have the generic formula (IIc): Thus, the tracer compound may be or may comprise an ester of hydroquinone and 2,6- difluorobenzoic acid. The tracer compound may have a melting point of at least 120°C, e.g. at least 140°C, e.g. of about 143°C to 150°C. The tracer compound may have the generic formula (III): wherein R1 is each independently selected from halogen or R2, l= 1, 2, 3, 4 or 5, and R ₂ is a halogen-containing moiety. Thus, the tracer ester compound of formula (III) may derive from a trihydroxybenzene triol compound such as pyrogallol, hydroxyquinol, or phloroglucinol. Preferably, R2 may be a haloalkoxy group, e.g. R2 may be –O-CHnX3-n, wherein X is a halogen or trifluoromethyl, and n is 0, 1 or 2; or R2 may be CF3. The tracer compound may have the generic formula (IIIa): Thus, the tracer compound may be or may comprise an ester of phloroglucinol and 3- (difluoromethoxy)benzoic acid. The tracer compound may have a melting point of at least 60°C. The tracer compound may have the generic formula (IIIb): Thus, the tracer compound may be or may comprise an ester of phloroglucinol and 2- chlorobenzoic acid. The tracer compound may have a melting point of at least 80°C, e.g. about 89°C to 94 °C. A composite material is also provided comprising: a polymer matrix; and an oil tracer compound (e.g. as previously defined), wherein the oil tracer compound has a melting point equal to or greater than the curing temperature of the polymer matrix. Advantageously, the composite material may be suitable for inflow tracing of hydrocarbons. Preferably, the tracer compound may have a melting point greater than the curing temperature of the polymer matrix. The tracer compound may comprise or may be a halogenated compound having a plurality of carboxylic ester functional groups. The tracer compound may have a melting point equal to or greater than about 60°C. Typically, the tracer compound may have a melting point equal to or greater than about 70°C, e.g. equal to or greater than about 80°C. Preferably, the tracer compound may have a melting point equal to or greater than about 90°C. By such provision, the melting point of the tracer may be sufficiently greater than the typical fabrication temperature, e.g. curing temperature, of tracer resin composites, e.g. epoxy resin composites, to ensure that the tracer compound remains in solid form (i.e., does not melt) during the fabrication process, e.g. during the curing process. Without wishing to be bound by theory, it is believed that melting of the tracer compound within the polymer matrix during fabrication is undesirable as it may interfere with the curing process of the resin, and/or as it may affect the dispersion of the tracer compound within the resin. Advantageously, the tracer compound may be soluble in oil or hydrocarbons. By such provision, in use, the tracer compound may be eluted from or released from the polymer matrix, thus being solubilised in the fluid, e.g. oil, to be traced. Advantageously, the composite material may be provided in solid form, and/or may be suitable for use in monitoring the inflow of hydrocarbons from a location within a subterranean reservoir, formation or well structure. The polymer matrix may comprise a resin. The resin may comprise one or more resins selected from the group consisting of epoxy resin, acrylic resin, polyurethanes, polyesters, vinyl esters, and phenol formaldehyde resin. Typically, the resin may comprise or may consist of an epoxy resin. According to another aspect, there is provided a composite material comprising: a polymer matrix; and a tracer compound, wherein the tracer compound has the generic formula (I): wherein Ar is an aromatic moiety, R1 is each independently selected from halogen or R2, l= 1, 2, 3, 4 or 5, R2 is a halogen-containing moiety, and m= 2, 3, 4, 5 or 6. Preferably, R ₂ may be a haloalkoxy group, e.g. R ₂ may be –O-CH _n X _3-n, wherein X is a halogen or trifluoromethyl, and n is 0, 1 or 2; or R2 may be CF3. Typically, m may be 2 or 3. That is, the tracer compound may be a diester of a di(hydroxy)aromatic compound or may be a triester of a tri(hydroxy)aromatic compound. Advantageously, such compounds may be easier to synthesize than esters of polyhydroxy aromatic compounds having four or five hydroxyl groups, whilst possessing a melting point sufficiently high to avoid melting during the fabrication of a resin composite material, e.g. an epoxy-based composite. Advantageously, the composite material may be suitable for inflow tracing of hydrocarbons. Advantageously, the tracer compound may be soluble in oil or hydrocarbons. By such provision, in use, the tracer compound may be eluted from or released from the polymer matrix, thus being solubilised in the fluid, e.g. oil, to be traced. Advantageously, the composite material may be provided in solid form, and/or may be suitable for use in monitoring the inflow of hydrocarbons from a location within a subterranean reservoir, formation or well structure. The polymer matrix may comprise a resin. The resin may comprise one or more resins selected from the group consisting of epoxy resin, acrylic resin, polyurethanes, polyesters, vinyl esters, and phenol formaldehyde resin. Typically, the resin may comprise or may consist of an epoxy resin. The features described above in relation to the tracer compound are equally applicable in relation to the composite material and, merely for brevity, are not all repeated here. According to another aspect, there is provided the use of a tracer compound or a composite material as described above in monitoring the inflow of hydrocarbons from a location within a subterranean reservoir, formation or well structure. According to another aspect, there is provided a method of preparing a composite material, the method comprising mixing a polymer matrix with a tracer compound as described above. The method may comprise curing the polymer matrix. The method may comprise fabricating the composite material into a solid composite. The method may comprise casting the composite material. The method may comprise moulding the composite material. The method may comprise preparing the composite material by immobilising the tracer in the polymer matrix, as described in WO2016/174415, which is incorporated herein by reference in its entirety. The method may comprise preparing the composite material by microencapsulating the tracer in the polymer matrix, as described in WO2016/174414, which is incorporated herein by reference in its entirety. Advantageously, the use of a tracer compound having a melt temperature above the processing temperature applied during manufacture of the composite material, e.g. above the curing temperature of the resin, allows the provision of a solid composite tracing material suitable for use within a subterranean formation, reservoir or well structure. According to another aspect, there is provided a method of monitoring an inflow of hydrocarbons from a subterranean reservoir, formation or well structure, the method comprising: providing a composite material as described previously at one or more locations within the reservoir, formation or well structure; producing hydrocarbons from the reservoir, formation or well structure; and analysing the produced hydrocarbons. The method may comprise producing hydrocarbons at or near the one or more locations within the reservoir, formation or well structure. The method may comprise detecting and/or measuring the tracer compound within the produced hydrocarbons. The method may comprise processing an amount of the produced hydrocarbons prior to analysis or detection. The method may comprise subjecting an amount of the produced hydrocarbons to hydrolysis. By such provision, the tracer compounds may be hydrolysed, allowing the ester groups to be split and the acid derivatives, e.g. the halogenated benzoic acids, to be extracted into an aqueous medium and analysed as a water-based tracer compound. Advantageously, this may increase the performance and/or sensitivity of the analysis, e.g. of the detection and/or measure of the tracer compound. According to another aspect, there is provided a method of preparing a tracer compound as described previously, the method comprising reacting a (polyhydroxy)aromatic compound with a halogenated benzoic acid derivative. The method may comprise reacting a (polyhydroxy)aromatic compound and a halogenated benzoic acid derivative. The method may comprise reacting a (polyhydroxy)benzene and a halogenated benzoic acid derivative. The method may comprise reacting a dihydroxybenzene and a halogenated benzoic acid derivative. The method may comprise reacting a trihydroxybenzene and a halogenated benzoic acid derivative. The method may comprise reacting a dihydroxybenzene diol selected from the list consisting of catechol, resorcinol or hydroquinone, with a halogenated benzoic acid derivative selected from the list consisting of 3-(difluoromethoxy)benzoic acid or derivative thereof, 2- chlorobenzoic acid or derivative thereof, or 2,6-difluorobenzoic acid or derivative thereof. It will be understood that features of the different aspects of the present specification as described above can be combined. As described above, the tracer compound is typically an ester of a (polyhydroxy)aromatic compound and a halogenated benzoic acid derivative. Typically, the tracer compound is the tri-ester of a trihydroxy aromatic compound and a halogenated benzoic acid derivative, or the di-ester of a dihydroxy aromatic compound and a halogenated benzoic acid derivative. The trihydroxy aromatic compound is typically a trihydroxybenzene triol compound such as pyrogallol, hydroxyquinol, or phloroglucinol. The dihydroxy aromatic compound is typically a dihydroxybenzene diol compound such as catechol, resorcinol or hydroquinone. A generic method of preparing such a diester tracer compound is as follows: A halogenated benzoic acid (1.0 eq), a dihydroxy aromatic compound (such as hydroquinone) (0.5 eq), and 4-dimethylaminopyridine (DMAP) (0.05 eq) are combined and dissolved in anhydrous dichloromethane at room temperature. The mixture is cooled to 0°C for five minutes before N,N'-dicyclohexylcarbodiimide (DCC) (1.0 eq) is added in a single portion. After stirring the resulting mixture for five minutes at 0°C, the mixture is allowed to warm to room temperature and stirred for a further three hours. Subsequently, the mixture is diluted with additional dichloromethane and chilled to 0°C for several minutes. The mixture is filtered, washed twice with hydrochloric acid (2 M), washed with brine, and dried via the addition of anhydrous magnesium sulfate. The slurry is then filtered and the solvent removed under reduced pressure and chromatographically purified to yield the hydroxy aromatic di-ester of the halogenated benzoic acid derivative. A generic method of preparing such a triester tracer compound is as follows: A halogenated benzoic acid (1.0 eq), a trihydroxy aromatic compound (such as phloroglucinol) (0.33 eq), and 4-dimethylaminopyridine (DMAP) (0.05 eq) are combined and dissolved in anhydrous dichloromethane at room temperature. The mixture is cooled to 0°C for five minutes before N,N'-dicyclohexylcarbodiimide (DCC) (1.0 eq) is added in a single portion. After stirring the resulting mixture for five minutes at 0°C, the mixture is allowed to warm to room temperature and stirred for a further three hours. Subsequently, the mixture is diluted with additional dichloromethane and chilled to 0°C for several minutes. The mixture is filtered, washed twice with hydrochloric acid (2 M), washed with brine, and dried via the addition of anhydrous magnesium sulfate. The slurry is then filtered and the solvent removed under reduced pressure to yield the hydroxy aromatic tri-ester of the halogenated benzoic acid derivative. Analysis was carried out by GC-MS with the following parameters Column: Agilent 19091S-433I HP-5ms Inert 30 m x 250 μm x 0.25 μm Temperature profile: 80 °C, 3 minute hold, rising at 50 °C/min to 300 °C, 10 minute hold. Pressure: 11.7 psi Flow rate: 1.2 ml/ min Average velocity: 40.5 cm/sec Injection volume: 1 microlitre Mass spectrometer: electron impact, scan mode, 100-600 a.m.u. A generic method of preparing a tracer composite including a tracer immobilised into a polymer matrix is as follows: A tracer (100 g) is sieved through a 1 μm sieve. 10 g of this material is mixed with 10 g of an epoxide component (e.g. bisphenol A diglycidyl ether) in a plastic container and mixed with a wooden spatula. Triethylenetetramine (2 g) is added to the tracer/epoxide mixture. The mixture is then poured into a silicon mould. The top of the mould is levelled using the wooden spatula so that the level of the resin is flush with the mould. This is then cured in an oven at a temperature of at least 60 °C. After 1 h the mould is removed, allowed to cool to room temperature and the polymer is released from the mould. A number of more detailed examples are set out below to illustrate different embodiments of the present invention. Examples Example 1 – Preparation of the diester reaction product of hydroquinone and 3- (difluoromethoxy)benzoic acid Synthesis Purified 3-(difluoromethoxy)benzoic acid (from Sigma Aldrich) (1.4309 g, 5.52 mmol), hydroquinone (0.3109 g, 2.82 mmol), 4-dimethylaminopyridine (DMAP) (0.0372 g, 0.30 mmol), and dichloromethane (~10 mL) were combined in a flask in an ice bath and stirred for 10 minutes. A solution of N,N'-dicyclohexylcarbodiimide (DCC) (6.44 mmol in 14 mL) was added with continued stirring at 0 °C. After 5 minutes, the ice-bath was removed and stirring continued for three hours. The reaction mixture was diluted with more dichloromethane (10 mL), chilled in an ice- bath for 10 minutes. The solid was then filtered and washed with hydrochloric acid (2×25 mL, 2 M). The dichloromethane layer was then dried with magnesium sulfate, chilled in an ice-bath for 10 minutes, then filtered again to remove the magnesium sulfate . Upon removal of solvent, a fine white powder was obtained (0.7914 g, 1.76 mmol, 64% yield). The reaction scheme for the preparation of the compound of example 1 is shown in Figure 1. Analysis A few specks of the product were dissolved in toluene and analysed by GC-MS. A peak with the mass of the molecular ion, m/e = 450.2 was observed at 13.18 min. A major fragment was also observed at m/e = 171.0 corresponding to cleavage of a 3-difluoromethoxy- phenylcarbonyl group. The melting point of the product was measured using a Stuart Scientific SMP3 melting point apparatus, and found to be 100.9 -105.4 °C (Δ= 4.5 °C). Example 2 – Preparation of the diester reaction product of hydroquinone and 2-chlorobenzoic acid Synthesis 2-chlorobenzoic acid (p/no 135577 from Sigma Aldrich) (0.4818 g, 3.08 mmol), hydroquinone (0.1651 g, 1.50 mmol), DMAP (0.0233 g, 0.19 mmol), and dichloromethane (10 mL) were combined in a flask. The mixture was stirred over an ice-bath for 10 minutes and then a solution of DCC in dichloromethane (3.10 mmol in 7.2 mL) was added. After stirring for 5 minutes, the ice-bath was removed and stirring continued for three hours. The mixture was diluted with dichloromethane (10 mL), chilled in an ice-bath, and filtered. The filtrate was washed twice with hydrochloric acid, dried with anhydrous magnesium sulfate, filtered, and the solvent removed. A small quantity (0.2553 g, 0.66 mmol) of off-white solid was obtained. All material adhered to the sides of the flask. The reaction scheme for the preparation of the compound of example 2 is shown in Figure 2. Analysis The crude yield of the product was 21%. A few specks of the product were dissolved in toluene and analysed by GC-MS. A peak with the mass of the molecular ion, m/e = 387.6 was observed at 22.52 min. A major fragment was also observed at m/e = 139.0 corresponding to cleavage of a 2-chloromethoxy- phenylcarbonyl group. The melting point of the product was measured using a Stuart Scientific SMP3 melting point apparatus, and found to be 106.7 -112.6 °C (Δ= 5.9 °C). Example 3 – Preparation of the diester reaction product of hydroquinone and 2,6- difluorobenzoic acid Synthesis 2,6-difluorobenzoic acid (p/no 190039 from Sigma Aldrich) (0.5016 g, 3.17 mmol), hydroquinone (0.1818 g, 1.65 mmol), DMAP (0.0228 g, 0.19 mmol), and dichloromethane (10 mL) were combined in a flask. The mixture was stirred over an ice-bath for 10 minutes and then added to a solution of DCC in dichloromethane (3.17 mmol in 7.4 mL). After a couple of minutes, the solution turned from white to bright yellow. After 5 minutes, the ice-bath was removed and stirred for another three hours. After this time, the solution's intensity of colour had halved. The mixture was diluted with dichloromethane (10 mL), chilled in an ice-bath, and filtered. The filtrate was washed twice with hydrochloric acid, dried with anhydrous magnesium sulfate, filtered, and the solvent removed. A small quantity (0.1066 g, 0.27 mmol) of off-white solid was obtained. All material was adhered to the sides of the flask. The reaction scheme for the preparation of the compound of example 3 is shown in Figure 3. Analysis The crude yield of the product was 8.6%. A few specks of the product were dissolved in toluene and analysed by GC-MS. A peak with the mass of the molecular ion, m/e = 390.0 was observed at 11.09 min. A major fragment was also observed at m/e = 141.0 corresponding to cleavage of a 2,6-difluoro-phenylcarbonyl group. The melting point of the product was measured using a Stuart Scientific SMP3 melting point apparatus, and found to be 142.5 -150.0 °C (Δ= 7.5 °C). Example 4 – Preparation of the triester reaction product of phloroglucinol and 2-chlorobenzoic acid Synthesis 2-chlorobenzoic acid (p/no 135577 from Sigma Aldrich) (1.2517 g, 7.99 mmol, 1.0 eq), phloroglucinol (0.3473 g, 2.75 mmol, 0.34 eq), DMAP (0.0506 g, 0.41 mmol, 0.05 eq), and dichloromethane (25mL) were combined in a flask. The mixture was stirred for a couple of minutes at room temperature and then cooled in an ice-bath for several minutes. A DCC solution (12.5 ml, 0.66M in dichloromethane, 1.0 eq.)) was then added and stirred for a further 5 minutes at which point the ice-bath was removed. After another three hours of stirring, the mixture was diluted with more dichloromethane (25 mL) and cooled in an ice-bath for several minutes. The precipitate on the surface was filtered off and the filtrate washed twice with hydrochloric acid (1 M). Anhydrous magnesium sulfate was used to dry the mixture which was chilled again prior to filtering. The solvent was removed under reduced pressure to yield a white solid (0.7110 g, 0.97 mmol, 16%). The reaction scheme for the preparation of the compound of example 4 is shown in Figure 4. Analysis The crude yield of the product was 16%. The melting point of the product was measured using a Stuart Scientific SMP3 melting point apparatus, and found to be 89 -94 °C (Δ= 5 °C). While this invention has been described with reference to certain examples and embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.