Geological Carbon Dioxide Storage in Mexico: A First Approximation

problem Geology, Geochemistry, Seismology, Hydrology, Hydrogeology, Mineralogy, Soil, Remote Sensing and Environmental


Introduction
Carbon dioxide (CO 2 ) is one of the industrial gases that contribute to the greenhouse gas (GHG) effect. During the last decades, the emissions of CO 2 due to human activity have increased significantly all over the world. There are different and important efforts to reduce or stabilize the concentrations of greenhouse gases in the atmosphere, such as improvements in the efficiency of power plants and the development of renewable energies. However, those approaches cannot deliver the level of emissions reduction needed, especially against a growing demand for energy that promotes economic growth and prosperity. Carbon capture and storage (CCS) approach encompasses the processes of capture and storage of CO 2 that would otherwise reside in the atmosphere for long periods of time. Among the different carbon capture and storage options currently in progress all over the world, the geological storage option is defined as the placement of CO 2 into an underground repository in such a way that it will remain permanently stored. Mexico is one of the countries which are signatories of different international treaties which call for stabilization of atmospheric gases emissions at a level that prevent anthropogenic interference with the world's regional climates. In Mexico CO 2 represents almost 70% of the total greenhouse gases emissions where the primary sources of CO 2 are the burning of fossil fuels for power generation. CCS is a technological approach that holds great promise in reducing atmospheric CO 2 concentrations in Mexico. This is the first coordinated assessment of carbon storage potential across the country. and is not recommended for storage. Zone B encompasses also igneous rocks with less seismic and volcanic hazards than zone A, but not yet recommended for CO 2 storage. The zone G is a marine zone of exclusion comprising the ocean floor, deep marine sediments and high seismic and tectonic hazardous processes in the Pacific Ocean.  The inclusion zones are zones C, D, E and F. Zone C represents terrigenous geological formations and mainly carbonate sedimentary rocks cropping out in the area. Zone D includes terrigenous as well as carbonate sedimentary rocks sequences. Zone E is composed of evaporitic deposits and associated sedimentary rocks. And zone F reflects sediments deposited in the marine continental shelf, slope and deep waters beneath the Gulf of Mexico. All of these zones were outlined taking into account surficial lithological features, large geological subsurface structures and recent volcanic and tectonic activity in a country scale assessment. The exclusion zones were not recommended for geologic carbon storage due to its high seismic, geothermic and active volcanic hazardous potential. On the contrary, the inclusion zones yielded the best CO 2 storage potential and were recommended for further detailed studies in order to find geological provinces with a good CCS capacity.

Purpose and scope
The purpose of this chapter is to present the analysis of different geological provinces to address the possibility of storing anthropogenic CO 2 in deep underground geologic formations, particularly in eastern continental Mexico. Up to now, the assessment has been focused on five geological provinces in order to evaluate and quantify theoretically its CO 2 storage potential and to identify prospective regions and/or sectors that should form the object of further and detailed studies. The analysis has been considered in relation to a specific type of storage, that is, deep saline aquifers and to the location of the stationary CO 2 sources currently available for the whole nation. It must be noted though that an assessment of CO 2 storage potential is surrounded by large uncertainties, which increase in number with the lack of available data and detailed information. The proposed work in this chapter recognizes this uncertainty, and the envisaged output is an overview of possible scenarios rather than the quantification of specific areas or sites for CCS. The aim is to provide a high level summary of CO 2 geologic storage potential across Mexico where the capacity resource estimates presented are intended to be used as an initial assessment of potential geologic storage prior to a local area selection. It is expected that as new subsurface data and a more refined methodology are acquired, the CCS studies will be improved in the near future.

Methodology
The total CCS process is frequently analyzed from several viewpoints which include very wide technological, economic and environmental issues. Some of the issues are well constrained while others are poorly understood. In the particular case of CO 2 storage potential there are also various aspects involved, such as the separation and capture of CO 2 at the point of emission, the mass of CO 2 emitted by the point of emission, the infrastructure and transportation of CO 2 , and the storage of CO 2 in deep underground geologic formations [2]. However, here we are only concerned with the types of CO 2 emission sources, the searching of suitable geologic reservoir rock sequences and their location, and the quantification of the theoretical capacity of storing a given volume or mass of CO 2 in selected sectors across Mexico. This pragmatic methodology was based on the public domain accessible data and present-day geological knowledge, and it does not incorporate geological constraints in the theoretical capacity estimations, nor does it incorporate risk factors, environmental hazards, solubility and mineral trapping of CO 2 , or quantification of injectivity of the potential storage rock sequences. www.intechopen.com The first phase included a survey of CO 2 points of emission, production information, source category, emissions factors, and annual CO 2 emissions that were obtained from the mexican Pollutant Release and Transfer Inventory (RETC by its Spanish acronym) and the Ministry of the Environment and Natural Resources (SEMARNAT, by its Spanish acronym) databases [3,4]. These databases consider the stationary sources. A compilation for the United Nations Framework Convention on Climate Change (UNFCCC) [4] includes the stationary and the non-stationary source emissions. The non-stationary source emissions such as those that come from the transportation sector, the change of land use and forestry, and some others like landfills were excluded from the analysis. The CO 2 stationary sources included power plants, oil and natural gas processing facilities, cement plants, agricultural processing facilities, iron and steel production facilities, and other industry processing facilities. The spatial location of the stationary CO 2 emission sources were calculated and compiled through different mapping tools that contain latitude and longitude information for various Mexican locations. The analysis of CO 2 stationary sources was done to provide reliable emission estimations, identify major CO 2 emission sources within each region, and to asses the applicability of the data in subsequently infrastructure analyses. The second phase consisted of the identification of geological storage provinces through the careful analysis and screening of available geological data. In this regard, there are different proposed methodologies that are similar [5,6,7,8,9,27]. Only minor differences are evident depending upon the used weights that show the relative importance of the criteria. Therefore, our selection of candidate storage provinces was according to the basin level of the assessment scale [10] (Figure 3). This "basin scale" exploration assessment required a little more local data categories and a better level of detail than the "country scale". In this "basin scale" assessment, both terms, basin and province, are considered synonyms. The term basin has different meanings depending upon geologic features of the region, such as geothermal regime, size, age, boundaries, type and thickness of sedimentary fill, geologic deformation, tectonic context, and many others parameters that can change with time [11,12,13,14]. However, these variable geologic features are also possible to be applied to the meaning of the term province. The assessment was focused on the previously identified inclusion zone. Within the inclusion zone, twelve provinces were defined taking into consideration the types of geomorphological developments, stratigraphic successions, major structural deformation patterns, homogeneous tectonic history, and known subsurface geological boundaries www.intechopen.com between all of them ( Figure 4). Actually, their outlined boundaries are very similar with those of the petroleum basins previously named for those areas of Mexico [15,16,17,18,19]. From the twelve established provinces, at the moment, only five of them were considered to be studied in greater detail to estimate the geological resource for storing CO 2 . These provinces are: Burgos, Tampico-Misantla, Veracruz, Sureste and Yucatan, all of them located in the continental and marine platform areas along Gulf of Mexico. The screening and selection of the provinces was based on the published geologic maps from a scale of 1:250,000 to 1:4,000,000 and reports about surface geology, stratigraphic and structural features, regional geologic cross-sections (50-200 km in length and 500m to 3 km in thickness), geophysical information and available public oil well data within each province. Three main groups of sedimentary formations for underground geologic carbon storage were observed. These groups of sedimentary formations are referred to as carbonate, evaporite and terrigenous sequences depending upon the main, respectively, carbonated, evaporitic and clastic content of the rock units. It is worth to mention that the stratigraphic uncertainty is high since the specific subsurface geologic information is quantitatively scarce and sometimes restricted and/or no detailed. Otherwise, the disposal of CO 2 in geological formations, generally, includes unmineable coal seams, oil and gas reservoirs, and deep saline reservoirs. In Mexico unmineable coal areas are not considered as a CCS option because they are located inside the exclusion zone, that is, they are affected seismo-tectonically and located close to the surface. On the contrary, the oil and gas reservoirs are the best option, particularly the EOR (Enhanced Oil Recovery) technique in the exhausted oil fields. But, at the moment, this prospect is ruled out due to the inaccessibility to the public domain of the oil databases and information. Only PEMEX (Petróleos Mexicanos) the oil governmental industry could carry out such studies. So, based on the fact that subsurface layers of porous rocks are generally saturated www.intechopen.com with brine and that they form deep saline aquifers characterized by high concentrations of dissolved salts and unsuitable for agriculture or human consumption, they were envisaged as the favorable option for CO 2 storage in Mexico. The storing CO 2 in saline formations is achievable since there are examples from such projects [20,21]. The third phase dealt with the estimation of theoretical capacity within each identified geological province. At present, various calculation methods have been proposed to know the storage capacity of a rock formation [22,10,23,20,24,25,2]. They have been applied to different country projects within their respective areas and still there is uncertainty. The reasons for this uncertainty are diverse but they broadly comprise key aspects such as financial support, CCS technology research and development, and a real partnership between country organizations and academic teams [26,28]. The concept of storage capacity was referred to a completely free phase of the CO 2 , which means without taking into account the CO 2 reaction with the walls of the reservoirs or formations. It is considered only the volume of CO 2 that can be retained in the available porous space of the storage formation or reservoir at depths between 800 and 2500 meters. At such depths the CO 2 has some properties like a gas and some like a liquid due to the changes in temperature and pressure conditions [64]. These are known as the CO 2 supercritical conditions or the critical point of the CO 2 . The huge advantage of storing CO 2 in the supercritical condition is that the required storage volume is much less if the CO 2 were at standard pressure conditions. For the estimation of the theoretical capacity of storing CO 2 , it was used an approach here called "parameterization". The parameterization refers to observations, deductions, and calculations derived from the physical parameters obtained from geological maps, regional stratigraphic and structural cross-sections, and well data from the public petroleum industry. Different geological variables were taken into account since the estimation was done with respect to general storage capacity resources and following the standards used in the petroleum industry, that is, stratigraphic and structural traps, as well as seal (cap) rocks that play a decisive role within any geological province. One first step in the parameterization approach was the determination of important geological features that would fulfill the storage requirements such as structural or stratigraphic trap, seal formation, stratigraphic discontinuities, geological faults, depth conditions, appropriate porosity and thickness of the target sedimentary sequence. The critical features were: reservoir depth (more than 800 m and less than 2500 m), thickness, porosity, lithological composition (predominantly carbonates and clastic deposits) and, for effects of the volume calculation, the relationship between "net thickness" versus "total thickness". All of this, with the goal of having an expression figure of the fraction of the geological formation susceptible to become a reservoir. The previous information had to be homogeneously similar within the area with a radius between 10 and 20 kilometers around each oil well considered and the nature of trap boundaries. When the information was assumed to be minimally sufficient and it was valued as an attractive target from the point of view of the depth, thickness, porosity, and permeability, then it was selected to quantify its potential capacity to become a CO 2 storing sector. Otherwise, the portion of the regional section including the wells was discarded. One second step of the approach was the direct application of an equation whose variables were fulfilled with the information above mentioned for deep saline aquifers. Therefore, the critical parameters obtained in the previous step were substituted in the formula proposed by Bachu et al in 2007 [10]: Where A is the trap area, h is the average thickness, VCO 2 t is the theoretical volume available, φ is the effective porosity, V is the volume and S wirr is the irreducible water saturation. The solving of the equation yielded the theoretical storage capacity volume of the sector under consideration.

Estimated CO 2 emissions from stationary sources
The most recent update on the mexican national inventory (SEMARNAT) was compiled in 2006 (UNFCCC) [4]. This document shows that the total annual GHG in Mexico are above 709 million metric tons (Mt) of CO 2 equivalent. The carbon dioxide represents 69.5% out of a total of 492 Mt of emissions from stationary and non-stationary sources. There were estimated 285 Mt of CO 2 emissions from stationary sources ( Figure 5). In addition, RETC data shows approximately 216 Mt of CO 2 emitted from 1,860 stationary sources, according to the different industrial and economic activities in Mexico (Table 1). From the above data it is evident that the electricity supplier sector is the most important contributor to CO 2 emissions from stationary sources. It releases to the atmosphere 107 Mt of CO 2 , roughly 50% of the total. It includes emissions from the Federal Commission for Electricity (CFE, by its Spanish acronym) which is the national public service agency, as well as from private small electricity suppliers companies. The oil & petrochemicals facilities add another 22% and, therefore, the whole energy sector is responsible for 72% (154 Mt) of CO 2 emissions in the country. The cement, metallurgical, iron & steel industries are also major contributors to the overall CO 2 country emissions, though they are smaller in comparison to the energy industry. In fact, the electricity production industry is the largest contributor, and it does from a small number of stationary sources ( Figure 6). The industrial and chemical sectors show a much larger number of identified sources, but the relative share of their CO 2 emissions, compared to those of the energy sector, is lower.

Geologic CO 2 storage potential
In order to estimate the CO 2 storage potential and to identify different sectors that should be the object of detailed assessment five geological provinces were analyzed. From north to south the geological provinces are: Burgos, Tampico-Misantla, Veracruz, Sureste and Yucatan (Figure 7).

Burgos province
The Burgos province is located at the most northeastern portion of Mexico. This province is bordered to the north by the United States (sharing the Rio Bravo along the borderline), to the east by the Gulf of Mexico, to the south by Tampico-Misantla province, and to the west by the first exposures that form the contact between Cretaceous and Tertiary rocks [29]. The basement of the geologic province consists of metamorphic and intrusive igneous rocks [30,31]. However, the basement geometry and its age distribution have not been well established. On top of the basement, a sedimentary evaporitic and carbonated sequence was accumulated in Mesozoic times [50,62]. After a period of regional subsidence a thick sequence of mainly coarse to fine grained sediments was deposited starting in the Tertiary and continuing into the Quaternary. According to the geological analysis it is documented the existence of a thick terrigenous sequence composed by interbedded conglomerates, sandstones and shales of Cenozoic age [32]. These sequences have frequent lateral facies changes and abundant lenticular sand bodies which were deposited mainly in deltaic, shelf and deep marine environments. Exposures of these rock units extend from the Eocene to Quaternary (Figure 8). Regional geological sections B1, B2, B3 and B4 were studied to estimate the CO 2 storage capacity on the continental portion on the Burgos province. All of them document similar stratigraphic units and characteristic sets of faults as a result of both extensional tectonic and sedimentological events [36]. Section B4 has no public subsurface geological information available, consequently, it was not considered during the assessment process. After [29,33,34,35,46].
As all the sections depict similar stratigraphic and structural features, only Section B2 is presented ( Figure 9). The section B2 has approximately 150 km in length and show a basement covered by slightly deformed Jurassic and Cretaceous rocks sequences. On top of it, there is a thick tertiary sedimentary and faulted sequence of rocks. The sedimentary sequence and the fault system reveal a chronological pattern from older formations and faults on the west to younger ones on the east. Across the entire section are evident the Eocene and Oligocene rocks on the west, and Miocene formations on the east. According to the type of stratigraphical or structural trap and the lithological and petrophysical features obtained from the oil wells several extrapolations were performed along the regional geological sections in order to select the best potential sectors where saline formations could become CO 2 reservoirs. An example of detailed description of sector B2-4 of section B2 is presented ( Figure 10). The sector B2-4 displays an Eocene terrigenous sequence that is located at approximately 1500 meters depth and consists of thick bedded homogeneous sandstone layers with crossstratification and minor amounts of intercalated, laterally discontinuous, thin bedded shale. The thickness of the unit is 880 meters but the important fraction is 0.6, therefore the considered net thickness is about 528 meters. The unit is part of a structural trap in a "rollwww.intechopen.com over" anticline with a seal composed of shale from the upper limit of same sequence. The Oligocene sedimentary sequence overlies the Eocene sequence and consists of a siltstone and shale that are interpreted as a seal cap-rock.  The computed petrophysical parameters are porosity 0.1, irreducible water 0.6, permeability less than 10 milidarcies (mD), density of CO2 about 675 kg/m3. The respected volume of influence is assumed based on the lithological and petrophysical homogeneities of the rock unit supported by the extrapolation of features between oil wells, and the distances imposed by stratigraphical and structural elements. The use of these parameters in the theoretical calculation of the capacity results in 1.36 giga metric tons (Gt) of CO 2 for sector B2-4 (Table 3and 4). The same approach was used in all sections of Burgos province giving 31 potential sectors on terrigenous sequences. Sometimes several sectors are located at the same well area of influence but at different depths. The marine zone was not computerized although several projects at the shallow marine platform in the United States point out the great potential of that zone (Figure 11).

Tampico-Misantla province
The Tampico-Misantla province lies in the central-east portion of Mexico. It is bordered to the north by the Burgos province and the Sierra de Tamaulipas mountain range, to the south by the mountainous fronts of the Sierra Madre Oriental folded-thrust belt and the Trans-Mexican volcanic belt, and to the east by the Gulf of Mexico [29,37]. The deep basement of the Tampico Misantla province consists of Precambrian and Paleozoic metamorphic and granitic rocks, and faults zones caused by extensional tectonic events some of which dating back to the origin of the Gulf of Mexico [38,39]. Also, the basement pattern shows tectonic uplifts and through structures of different shapes and sizes.
Overlying the basement a thick succession of sedimentary materials have been deposited ranging from Jurassic red beds and evaporites to Cretaceous carbonate sequences originated in shelf, platform and abyssal marine facies. On top of this succession a number of terrigenous sedimentary sequences were deposited concurrently with contractional tectonic events of the Laramide orogeny, since the beginning of the Cenozoic [40].  www.intechopen.com Five regional geologic cross sections were analyzed to understand the Tampico-Misantla province. Due to the similar geologic patterns showed along all regional sections, only section TM4 is presented. Section TM4 represents approximately 130 km in length of the subsurface regional geological profile, where basement faults and, horst and graben structures of different sizes are clearly revealed (Figure 13). On the western portion of section TM4 are evident the folded and thrust faulted carbonate sequences of Cretaceous age, and on the eastern side is clear the minor tectonic deformation of the Cretaceous platform carbonates as well as the Cenozoic terrigenous sequences. In order to search sectors where saline aquifers could become potential CO 2 reservoirs the east sides of the regional sections were preferentially assessed because of their minor tectonic deformation. An example of the performed analysis is presented in sector TM4-6. Sector STM4-6 is located approximately at 2000 meters depth, and is part of carbonate reef platform sequence of Cretaceous age. The rock unit is a 635 meters package of medium to thick bedded light yellow gray fossiliferous limestone slightly deformed as an open anticline. This limestone is overlain by a sequence of thin bedded shale formed in deep basin conditions ( Figure 14). The shales is interpreted as a good seal cap rock. The petrophysical parameters from sector STM4-6 are porosity 9%, irreducible water less than 30%, net thickness 508 meters, and CO 2 density around 693.6 kg/m 3 . The use of these parameters in the theoretical calculation has resulted in 1.08 Gt (  After the analysis of the entire number of regional geological sections the Tampico-Misantla province yield 12 sectors. Four of them correspond to carbonate sequences and eight to terrigenous sequences. The total CO 2 capacity estimation corresponds to 9.75 Gt ( Figure 15 and Table 6).   Table 6. Theoretical storage capacity of the Tampico-Misantla province.

Veracruz province
Veracruz province lies to the east of Mexico, sitting in the central part of the state of Veracruz. This province is bounded to the north by the Trans-Mexican volcanic belt, to the southeast by Los Tuxtlas volcanic field complex, to the west by Sierra Madre Oriental folded-thrust belt (known in this area as Sierra de Zongolica), and to the east-northeast by the Gulf of Mexico [42,43]. The current geological context suggests a quick subsidence process along with several tectonic deformational events since Mesozoic times. The surficial geology suggests a faster subsidence process at the north of the province (Figure 16). Six geologic sections were analyzed in order to estimate theoretical CO 2 potential capacity for this province. From the subsurface point of view, the Veracruz province can be clearly divided into two geologic subprovinces. The first subprovince is the Sierra Madre Oriental folded-thrust belt and its continuation at depth known as the "Frente Tectonico Sepultado" (Buried Tectonic Front). It is characterized by folded calcareous rocks deformed by reverse faulting. The second subprovince is known as "Cuenca Terciaria de Veracruz" (Veracruz Tertiary Basin) composed by a thick succession of interbedded shale, siltstone, sandstone and conglomerate [40,42,47]. This terrigenous sequence has been, in turn, affected tectonically in distinctive styles and at different depths. Fig. 16. Simplified geologic map of the Veracruz province, and location of regional geologic sections and wells. After [43,33,34,35,46].
For reference, figure 17 shows one of the regional sections that display structural features customarily found in the area. Section V3, about 180 km in length, lies in the middle of Veracruz province. The western half of the section displays calcareous sequences highly deformed by reverse faulting [42]. These sequences reveal Cretaceous facies from platform to basin environments. The eastern half of the section reflects terrigenous sequences wherein Paleocene and Eocene units expose reverse faulting folds.  Based on the regional geological sections and available oil well data, potential CO 2 storage sectors were searched in the Veracruz province. One of them is sector V2-5 in section V2. Sector V2-5 is characterized at 2450 meters depth by a lower Miocene terrigenous sequence that consists of interbedded green to gray bentonitic shale, layers of bentonite, coarse grained to conglomeratic sandstone, and conglomerate composed by fragments of gray to dark grayish brown clayey limestone and light brown bioclastic limestone [40,43]. The conglomerate and the sandstone horizons were interpreted as potential formations to store CO 2 . So, at the top of the lower Miocene sequence is a 50 meters thick horizon that is part of an anticline. It is overlain by homogeneous greenish gray shale interpreted as a good seal cap rock ( Figure 18).   According to the theoretical calculations carried out in the Veracruz province resulted 21 sectors with CO 2 capacity potential ( Figure 19). Five of the sectors correspond to carbonate sequences, and the remaining 16 are terrigenous sequences. The estimated capacity targets reach 15.23 Gt (Table 8). Fig. 19. Sectors with CO 2 storage potential in saline aquifers at the Veracruz province.

Sureste province
The Sureste province is situated in the southeastern region of Mexico on the southern edge of the Gulf of Mexico. This province is bordered to the south by the Sierra de Chiapas mountainous range, to the east by the Yucatan Peninsula, to the west by the Veracruz province, and to the north and northeast by the Gulf of Mexico. The Sureste province comprises both mainland and offshore areas. In mainland the extensive geological exposures show evidence of the last episode of sedimentary infilling, therefore, most of the area is covered mainly by late Cenozoic sedimentary deposits ( Figure 20). The internal subsurface configuration of the province is characterized by very deep and fragmented basement affected by different tectonic deformational events. At depth the Sureste province is divided into four subprovinces: Salina del Istmo, Comalcalco, Reforma-Akal and Macuspana [40,44,45]. The basement of the province consists of crystalline rocks of Precambrian and Paleozoic age [30,49] most of which are covered by Mesozoic rock units composed of red beds, marine evaporites and carbonates of basin and platform marine facies [53]. Overlying the Mesozoic rocks are Paleogene terrigenous deposits of deep and shallow marine, deltaic, lagoonal and even alluvial facies [51,52]. In addition, there are terrigenous sequences belonging to deltaic, lagoonal and shallow marine sedimentary facies that cover all the earlier deposits [40,52,54]. Six regional geologic cross sections (SE1, SE2, SE3, SE4, SE5 and SE6) were analyzed in order to estimate theoretical CO 2 potential capacity in the province. The regional cross sections show that the sedimentary sequences from Jurassic to Oligocene-Lower Miocene were folded and reversely faulted. Also, it is evident that the younger late Cenozoic terrigenous sequences were faulted, but this time, under an extensional tectonic regime. The entire province was first under contractional tectoni c r e g i m e s , a n d t h e n i t w a s a f f e c t e d b y extensional tectonic events during erosion-sedimentation stages.The position of the Sureste province could be viewed in terms of the jointly evolution of a passive continental margin associated to a strike-slip and a subduction margins both related to the plate tectonic interaction at the pacific region of Mexico. However, the complete and detailed tectonic history of the province is not yet well known. The subsurface stratigraphical and structural complexity is shown in Section SE2 which is approximately 135 kilometers long, is located in the middle of the province, and is running along a northwest-southeast line ( Figure 21). synchronous erosion and sedimentation processes. At the Comalcalco basin the Pliocene and Plesitocene sediments can reach up to five kilometers in thickness, and the regularly spaced faults do not meet at the surface. All along the cross section is evident that the development of the basins is linked to the widespread fault systems and to subsidence mechanisms.
During the screening and selection of the sectors to estimate the CO 2 capacity, several stratigraphic and anticline traps structures were found. One of them is presented in figure  22 to illustrate the procedure. The sector SE2-4 consists of an anticline structure verging in northeast direction with an average axis orientation of N 300°. The anticline includes rock units from Jurassic to Oligocene times that are marked first by reverse faulting episode, and then by a regional unconformity. The unconformity is overlain by Miocene and Pliocene rock units. The CO 2 storage target is in a wedge of late Miocene well-bedded sequence about 280 meters thick and located 1550 meters deep. The storage sequence consists of a light gray, medium to coarse-grained, medium-bedded sandstone interbedded with occasional graygreenish shale containing mollusks and lignite fragments. The sandstone is overlain by a wide package of greenish gray shale of Pliocene age and interpreted as the seal layer. The petrophysical parameters of the sandstone target sequence are net thickness about 240 meters, clay content less than 4 %, porosity (Ф e ) about 30%, irreducible water saturation (S wirr ) less than 20% and permeability about 60 miliDarcys (mD)(  On the basis of the estimations conducted in the Sureste province resulted 17 sectors with CO 2 capacity potential ( Figure 23). Six of them are within offshore subsurface lands. The total capacity estimate is around 24.10 Gt on terrigenous sedimentary sequences (Table 10).   Table 10. Theoretical storage capacity of the Sureste province.

Yucatan province
The Yucatan province is bounded to the northeast by the Campeche Escarpment (which is formed on the edge of the marine continental shelf), to the east by the Caribbean Sea (where the marine platform is quite narrow), to the west by the Sonda de Campeche and to the south and southeast by the Sierra de Chiapas mountain ranges, Los Chuchumatanes Dome in Guatemala, and the Maya Mountains of Belize [43,16,55]. The area of study comprises the onshore portion known as Yucatan Peninsula and some offshore submerged areas in the Sonda de Campeche and the Yucatan marine platform regions ( Figure 24). The geology of the province can be characterized in subsurface terms by a huge basement block composed of Paleozoic rocks [43]. This crustal tectonic element has been present since the origin of the Gulf of Mexico [56]. On top of the basement, Jurassic evaporites, Cretaceous carbonates, as well as both Tertiary carbonates and terrigenous sedimentary sequences were deposited [57,38,58]. The sedimentary sequences were not under intense tectonic stress since they show a nearly horizontal depositional pattern and some minor faults. However, at the surface level, the central part of the huge province presents normal faults of considerable length that could bear testimony of extensional tectonic events which affected Mesozoic and lower Tertiary rocks. Under this g e o l o g i c a l c o n t e x t , f o u r l o n g r e g i o n a l geologic cross sections were analyzed to estimate the CO 2 storing capacity in the Yucatan Province. Fig. 24. Simplified geology map of Yucatan province showing regional geologic sections and wells. After [40,43,34,35,33,55,63].
The Yucatan province exposes a very wide and nearly horizontal sedimentary Mesozoic and Cenozoic rock sequences, where the topographic elevations rarely exceeds 200 meters above sea level. Because of this quite regular geologic homogeneity it is believed that the Yucatan peninsula remained stable throughout its geologic history. In contrast, at the edge of the basement block in the Sonda de Campeche, the offshore submerged area display Miocene contractional and extensional tectonic deformations linked to the geologic evolution of the Sureste province [59,60]. The regional cross section Y2, approximately 400 km in length, depicts geological features frequently found in the entire province. At the offshore area within the Sonda de Campeche region gently folds structures in Mesozoic and early Cenozoic strata indicate a tectonic regime not so intense. Later, Cenozoic sequences of rocks denote normal faults systems that affected almost the complete stratigraphic column ( Figure 25). Sector PY2-1 illustrates one of the selected potential sectors where saline aquifers could eventually become CO 2 reservoirs. The Miocene terrigenous sequence is characterized by a thick succession of light colored sandstone interbedded with calcareous breccias and some layers of shale that alternate with calcareous arkoses lenses ( Figure 26). Within the Miocene  The net thickness of the target sequence is about 353 meters with porosity (Ф e ) about 10% and irreducible water saturation (S wirr ) 30%. Based on these parameters the theoretical capacity is 3.25 Gt of CO 2 in sector PY2-1(  The analyses of the Yucatan province yield seven sectors capable of storing CO 2 with a total theoretical capacity estimate of 14.44 Gt. Most of them are located in the offshore submerged lands of the Sonda de Campeche (Figure 27). The sectors are divided in terrigenous rock sequences with 10.46 Gt and carbonate sequences with 3.98 Gt (Table 12).  In summary, the theoretical CO 2 capacity estimates in Mexico stands currently at 81.59 Gt on terrigenous and calcareous sequences located within the outlined inclusion zones. The total assessed sectors are 88 with possibilities of CO 2 storage in potential saline aquifers (

Conclusions
In Mexico the energy sector is responsible of more than 70% of the carbon dioxide emissions. In order to address the possibility of storing such anthropogenic CO 2 in deep underground geologic formations three lines of analysis were performed. First, the type, location and magnitude of CO 2 sources indicate approximately 216 Gt of CO 2 emissions coming from 1860 point sources. Second, five out of twelve geological provinces were analyzed. The assessed provinces are Burgos, Tampico-Misantla, Veracruz, Sureste and Yucatan which have the best favorable conditions for underground CO 2 storage in sedimentary rock successions of Mesozoic and Tertiary age. They are geologically well defined and located within the coastal plain region around the western portion of Gulf of Mexico. Third, theoretical storage capacities in potential saline aquifers sectors were estimated for each geological province. The theoretical CO 2 storage estimates and the number of assessed sectors are: Burgos province 17.81 Gt in 31 sectors, Tampico-Misantla province 10.01 Gt in 12 sectors, Veracruz province 15.23 Gt 21 sector, Sureste 24.10 Gt in 17 sectors and Yucatan province 14.44 Gt in 7 sectors. The total theoretical CO 2 storage potential currently stands at 81.59 Gt within 88 assessed sectors for the entire nation. During the CO 2 storage capacity estimations, it became clear that some areas yielded more and better quality data than others. Therefore, it is acknowledged that these data sets are not complete. However, it is anticipated that CO 2 storage capacity estimates, geological formation maps as well as regional geological cross sections will be updated as new information, particularly oil wells data, are acquired and methodologies for CO 2 storage capacity estimates are improved in Mexico.