The Geochemistry of Carbonate Rocks in Igwe Igarra, Southwestern Nigeria

Nigeria has an abundance of valuable solid minerals and rocks which can generate revenue for the government if they are fully exploited. Carbonate rock is one of such rocks that occur prominently in Igwe Igarra area of Edo State. Five (5) carbonate rock samples (Four marble and One Calcsilicate) were subjected to geochemical analyses. A petrographic study reveals that marble and calcsilicate contain calcite, quartz, microcline, plagioclase, biotite and opaque minerals. Geochemical data shows that carbonate rocks have variable geochemical characteristics: Igwe marble is highly Calcitic (96% - 98%) and very low in dolomite Mg(CO3). Calcsilicate rock is low in CaO (27.5%) and higher in marble. The Igwe marble is pure and contains fewer impurities compared to calcsilicate rock which makes it unideal for usage. Based on these characteristics the marble from Igwe is suitable as raw materials for the productions of glass, papers, beet sugar, aggregates, lubricants and fillers.

Carbon Fluxes and Sinks:the Consumption of Atmospheric and Soil CO_2 by Carbonate Rock Dissolution

Carbonate rock outcrops cover 9%-16% of the continental area and are the principal source of the dissolved inorganic carbon （DIC） transferred by rivers to the oceans, a consequence their dissolution. Current estimations suggest that the flux falls between 0.1-0.6 PgC/a. Taking the intermediate value （0.3 PgC/a）, it is equal to 18% of current estimates of the terrestrial vegetation net carbon sink and 38% of the soil carbon sink. In China, the carbon flux from carbonate rock dissolution is estimated to be 0.016 PgC/a, which accounts for 21%, 87.5%-150% and 2.3 times of the forest, shrub and grassland net carbon sinks respectively, as well as 23%-40% of the soil carbon sink flux. Carbonate dissolution is sensitive to environmental and climatic changes, the rate being closely correlated with precipitation, temperature, also with soil and vegetation cover. HCO3 in the water is affected by hydrophyte photosynthesis, resulting in part of the HCO~ being converted into DOC and POC, which may enhance the potential of carbon sequestration by carbonate rock dissolution. The possible turnover time of this carbon is roughly equal to that of the sea water cycle （2000a）. The uptake of atmospheric/soil CO2 by carbonate rock dissolution thus plays an important role in the global carbon cycle, being one of the most important sinks. A major research need is to better evaluate the net effect of this sink in comparison to an oceanic source from carbonate mineral precipitation.

Reform of Carbonate Rock Subsurface by Crustose Lichens and Its Environmental Significance

Crustose lichens are distributed extensively in karst areas in Southern China. They can be found on the surface of carbonate rocks. Through biophysical and biochemical processes, crustose lichens reform the subsurface of carbonate rocks and in the meanwhile change their physical and chemical properties: (1) the mechanical strength decreases by 17.04° on average (up to 33.2°); (2) the chemical solution surface area increases from 28.26% to 75.36% (lichen microholes considered only); and (3) the water-holding capacity is greatly improved. Comparative field experiments between biokarst samples underneath crustose lichens and fresh rock samples with the same composition and texture show that the corrosional rate of carbonate rocks of the former is 1.264–1.643 times higher than that of the latter. Crustose lichens are considered as an activator of the surface corrosion of carbonate rocks.

Role of Carbonic Anhydrase as an Activator in Carbonate Rock Dissolution and Its Implication for Atmospheric CO_(2)Sink

The conversion of CO2 into H+ and is a relatively slow reaction. Hence, its kinetics may be rate determining in carbonate rock dissolution. Carbonic anhydrase (CA), which is widespread in nature, was used to catalyze the CO2 conversion process in dissolution experiments of limestone and dolomite. It was found that the rate of dissolution increases by a factor of about 10 after the addition of CA at a high CO2 partial pressure (Pco2) for limestone and about 3 at low Pco2 for dolomite. This shows that reappraisal is necessary for the importance of chemical weathering (including carbonate rock dissolution and silicate weathering) in the atmospheric CO2 sink and the mysterious missing sink in carbon cycling. It is doubtless that previous studies of weathering underestimated weathering rates due to the ignorance of CA as an activator in weathering, thus the contribution of weathering to the atmospheric CO2 sink is also underestimated. This finding also shows the need to examine the situ distribution and activity of CA in different waters and to investigate the role of CA in weathering.

Weathering-pedogenesis of Carbonate Rocks and Its Environmental Effects in Subtropical Region

We investigated the weathering-pedogenesis of carbonate rocks and its environmental effects in subtropical regions of China. The investigation demonstrated that the weathering- pedogenesis of carbonate rocks is the process of a joint action of corrosion and illuviation and metasomatism in subtropical region. It is characterized by multi-stage, multi-path and multi-style. With the persisting development of weathering-pedogenesis of carbonate rocks, metasomatic pedogenesis progressively became the main process of the weathering-pedogenesis and the dominant style of formation of minerals. And it proceeds through the whole process of evolution of the weathering-pedogenesis of carbonate rocks. The stage evolution of weathering-pedogenesis of carbonate rocks and the fractionation evolution of newly produced minerals are characterized by obvious vertically zoning structures and the rules of gradation of elements geochemical characteristics in the carbonate rocks weathering profiles. The geochemical process of weathering-pedogenesis of carbonate rocks can be divided into three geochemical evolution stages, i.e., the Ca, Mg-depletion and Si, Al-enrichment stage; the Fe, Mn enrichment stage and the Si-depletion and Al-enrichment stage in the subtropical regions. Consistent with the three geochemical evolution stages, the sequence of formation and evolution of minerals can be divided into the clay mineral stage; the Fe, Mn oxide and the gibbsite stage. The influence of weathering-pedogenesis of carbonate rocks on the chemical forms of heavy elements is mainly affected via newly produced components and minerals in the process of weathering-pedogenesis, e.g., iron oxide minerals and organic matters. The important mechanism for the mobilization, transport and pollution of F and As is affected the selective adsorption and desorption of F and As on the surface of iron oxide minerals in the subtropical karst zones, i.e., the selective adsorption and desorption on mineral surfaces of newly produced minerals in the process of weathering-pedogenesis control the geochemical behavior of elements on the Earth＇s surface and environmental quality in subtropical regions.

Heterogeneity of Parent Rocks and Its Constraints on Geochemical Criteria in Weathering Crusts of Carbonate Rocks

Owing to the low contents of their acid-insoluble components, carbonate rocks tend to decrease sharply in volume in association with the formation of weathering crust. The formation of a 1 m-thick weathering crust would usually consume more than ten meters to several tens of meters of thickness of parent rocks. The knowledge of how to identify the homogeneity of parent rocks is essential to understand the formation mechanism of weathering crust in karst regions, especially that of thick-layered red weathering crust. In this work the grain-size analyses have demonstrated that the three profiles studied are the residual weathering crust of carbonate rocks and further showed that there objectively exists the heterogeneity of parent rocks in the three studied weathering crusts. The heterogeneity of parent rocks can also be reflected m geochemical parameters of major elements, just as the characteristics of frequency plot of grain-size distribution. Conservative trace element ratios Zr/Hf and Nb/Ta are proven to be unsuitable for tracing the heterogeneity of parent rocks of weathering crust, but its geochemical mechanism is unclear. The authors strongly suggest in this paper that the identification of the homogeneity of parent rocks of weathering crust in karst regions is of prime necessity.

A New Example of Retrograde Solubility Model for Carbonate Rocks

Objective The dissolution and precipitation of carbonate during burial diagenetic process controls the reservoir property in deep buried strata. The geological process related with it has become a research focus during recent years. The most important dissolution fluids to carbonates are probably H2S and CO2 as byproducts of sulfate reduction in deep-buried setting with sulfate minerals, but carbonates are more soluble in relatively low temperature, which is the so-called retrograde solubility. Several geological processes can result in the decrease of temperature, including the upward migration of thermal fluids and tectonic uplift.

Performance improvement of ionic surfactant flooding in carbonate rock samples by use of nanoparticles

Various surfactants have been used in upstream petroleum processes like chemical flooding. Ultimately, the performance of these surfactants depends on their ability to reduce the interfacial tension between oil and water. The surfactant concentration in the aqueous solution decreases owing to the loss of the surfactant on the rock surface in the injection process. The main objective of this paper is to inhibit the surfactant loss by means of adding nanoparticles. Sodium dodecyl sulfate and silica nanoparticles were used as ionic surfactant and nanoparticles in our experiments, respectively. AEROSIL~？ 816 and AEROSIL~？200 are hydrophobic and hydrophilic nanoparticles. To determine the adsorption loss of the surfactant onto rock samples, a conductivity approach was used. Real carbonate rock samples were used as the solid phase in adsorption experiments. It should be noted that the rock samples were water wet. This paper describes how equilibrium adsorption was investigated by examining adsorption behavior in a system of carbonate sample（solid phase） and surfactant solution（aqueous phase）. The initial surfactant and nanoparticle concentrations were 500–5000 and 500–2000 ppm, respectively. The rate of surfactant losses was extremely dependent on the concentration of the surfactant in the system, and the adsorption of the surfactant decreased with an increase in the nanoparticle concentration. Also, the hydrophilic nanoparticles are more effective than the hydrophobic nanoparticles.

Effect of overburden pressure on determination of reservoir rock types using RQI/FZI,FZI^* and Winland methods in carbonate rocks

Rock typing is an important tool in evaluation and performance prediction of reservoirs.Different techniques such as flow zone indicator(FZI),FZI~*and Winland methods are used to categorize reservoir rocks into distinct rock types.Generally,these methods are applied to petrophysical data that are measured at a pressure other than reservoir pressure.Since the pressure changes the pore structure of rock,the effect of overburden pressure on rock typing should be considered.In this study,porosity and permeability of 113 core samples were measured at five different pressures.To investigate the effect of pressure on determination of rock types,FZI,FZI~*and Winland methods were applied.Results indicated that although most of the samples remain in the same rock type when pressure changes,some of them show different trends.These are related to the mineralogy and changes in pore system of the samples due to pressure change.Additionally,the number of rock types increases with increasing pressure.Furthermore,the effect of overburden pressure on determination of rock types is more clearly observed in the Winland and FZI~*methods.Also,results revealed that a more precise reservoir dynamic simulation can be obtained by considering the reservoir rock typing process at reservoir conditions.

Carbon Isotope Excursions and Paleo-Oceanography of the Ordovician–Silurian Boundary Carbonate Rocks from the Xainza Area, Tibet

Objective The Ordovician–Silurian transitional period is a special time when the global paleo-environment changed greatly.It witnessed the first mass extinction as of Phanerozoic period and glaciations that occurred frequently at a large scale in a very short time,which has thus attracted much attention among geoscientists at home and abroad.

An Experimental Study of Mixture Corrosion Effects of Carbonate Rocks in the Transitional Zone of Littoral Karst Areas

The mechanism for development of littoral karst differs from that of inland karst, and the mixture corrosion effects are one of the most important factors that control the development of littoral karst. Through seven groups of static experiments carried out in a closed CO2-H2O system, basic conclusions can be drawn as follows: (1) the basic law of corrosion process in a transitional zone of seawater-freshwater in littoral karst areas is identical with that in the fresh water, i.e., the lithologic characteristics and rock structure are the main factors which control the development of littoral karst; (2) the mixture corrosion rate of the carbonate rock in the above transitional zone is faster than that in fresh water or seawater; (3) the mechanism for development of carbonate rocks differs at various pressures of CO2 in a transitional zone in littoral karst areas.

The Sachakou Deposit in West Kunlun of Xinjiang: A Pb-Zn Polymetallic Deposit Associated with Magmatic Metasomatism of Carbonate Rock

Objective The Sachakou Pb-Zn polymetallic deposit is located in Hetian County, Xinjiang （geographical coordinates of E78° 57＇ 54.30＂-78°59＇ 53.63＂, N34° 39＇ 27.50＂-34° 40＇ 57.21＂）. It belongs to the West Kunlun orogenic belt on the northwest edge of the Qinghai-Tibet Plateau and is connected to the Sanjiang orogenic belt to the south （Spurlin et al., 2005）. In recent years, a series of Pb-Zn mineralized spots and deposits have been discovered in this area one after another, which is called the Huoshaoyun ore concentration area. Among them, the Sachakou Pb-Zn deposit has reserves up to140 Mt, which has reached a large scale. However, the study on the genesis of deposits in this area has only just begun. This work studied the genesis ofthis Pb-Zn deposit in order to provide new ideas for the genesis of regional deposits and regional prospecting.

Mixed carbonate rocks lithofacies features and reservoirs controlling mechanisms in a saline lacustrine basin in Yingxi area,Qaidam Basin,NW China

Taking the mixed pre-salt carbonate rocks in the upper member of Eocene Xiaganchaigou Formation(E_(3)^(2))of Yingxi area in the Qaidam Basin as an example,the lithofacies and controlling mechanisms of reservoir formation are analyzed based on a large dataset of cores,thin sections and geochemical analysis.The reservoirs in E_(3)^(2)pre-salt layers have five types of lithofacies,of them,mixed granular calcareous dolostone,massive calcareous dolostone,plaque calcareous dolostone,and laminated dolomtic limestone are of sedimentary origin,and breccia calcareous dolostone is of tectonic origin.The four types of sedimentary lithofacies are divided into two types of saline sedimentary sequence lithofacies combinations,low-energy type in the sag area and low to high-energy type in the slope and paleo-uplift zone in the depression.Affected by high-frequency supply of continental clastic material,the two types of salty sedimentary sequences are mostly incomplete subtypes of lithofacies.Lithofacies have strong impacts on pre-salt reservoirs in E_(3)^(2):(1)Lithofacies type and sedimentary sequence controlled the formation and distribution of dolomite intercrystalline pores and dissolved pores during the pene-sedimentary period.(2)The structure of laminated dolomitic limestone controlled the formation of large-scale laminated fractures and high permeability channels during the diagenetic period.(3)Granular,massive,plaque calcareous dolostones have low mud content and strong brittleness,in the late tectonic reactivation period,the distribution of the three types of lithofacies,together with their distance from the top large slip faults and secondary faults,controlled the formation and distribution of high-efficiency fracture-cave brecciaed calcareous dolostone reservoirs.The above research led to the composite lithofacies-tectonic formation model of pre-salt reservoir in E_(3)^(2)of Yingxi area.The tempo-spatial distribution of tectonic breccia calcareous dolostone reservoirs,laminated dolomitic limestone shale oil reservoirs and granular,massive calcareous dolostone dissolved-intercrystalline pore tight reservoirs in various structural belts of the studied area have been figured out.These findings gave new insights into tight-shale oil accumulation theory in mixed carbonate successions from saline lacustrine basins,aiding in high efficient exploration and development of petroleum in the studied area.

The First Report of Thermochemical Sulfate Reduction Reaction in the Upper Paleozoic Carbonate Rocks of Southeastern Ordos Basin

Thermochemical sulfate reduction （TSR） is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce H2S and CO2. TSR has been studied in many sedimentary basins such as China＇s Sichuan and Tarim basins because it has a profound impact on the commercial viability of petroleum resources, with HzS typically being undesirable.

Combination and distribution of reservoir space in complex carbonate rocks

This paper discusses the reservoir space in carbonate rocks in terms of types,combination features,distribution regularity,and controlling factors,based on core observations and tests of the North Truva Oilfield,Caspian Basin.According to the reservoir space combinations,carbonate reservoirs can be divided into four types,i.e.,pore,fracture-pore,pore-cavity-fracture,and pore-cavity.Formation and distribution of these reservoirs is strongly controlled by deposition,diagenesis,and tectonism.In evaporated platform and restricted platform facies,the reservoirs are predominately affected by meteoric fresh water leaching in the supergene-para-syngenetic period and by uplifting and erosion in the late stage,making both platform facies contain all the above-mentioned four types of reservoirs,with various pores,such as dissolved cavities and dissolved fractures,or structural fractures occasionally in favorable structural locations.In open platform facies,the reservoirs deposited continuously in deeper water,in an environment of alternative high-energy shoals（where pore-fracture-type reservoirs are dominant） and low-energy shoals（where pore reservoirs are dominant）.

Autochthonous Dolomitization and Dissolution in the Microbial Carbonate Rocks of the Fengjiawan Formation in the Ordos Basin

Many euhedral dolomite crystals and related pores are found in the microbial siliceous stromatolite dolomite and siliceous oolitic dolomite in the Fengjiawan Formation of the Mesoproterozoic Jixian System in the southern Ordos Basin.With the application of the microscope,scanning electron microscope,cathodoluminescence and in-situ trace element imaging,it can be seen that different from the phase I dolomite that was damaged by silicification,the intact euhedral phase II dolomite occurred through dolomitization after silicification,concentrated mainly in the organic-rich dark laminae of the stromatolite and the dark spheres and cores of the ooids.A considerable number of phase II dolomite crystals were dissolved,giving rise to mold pores and vugs which constituted the matrix pores and also the major pore space of the Fengjiawan Formation.The formation and dissolution of the dolomite were controlled by the microenvironment favorable,respectively,for carbonate precipitation and dissolution under the influence of microbial biological activities and related biochemical reactions.The driving force and material supply of dolomitization and dissolution were confined to the fabrics enriched with microorganisms,which are highly autochthonous.This mechanism may be a key factor for the development of Precambrian dolomite and related reservoirs,in the context of the domination of microbial rocks.

Transformation mechanism of muddy carbonate rock by the coupling of bioturbation and diagenesis:A case study of the Cretaceous of the Mesopotamia Basin in the Middle East

The transformation mechanism of muddy carbonate rock by the coupling of bioturbation and diagenesis was stud-ied based on core,cast thin section and physical property data of Cretaceous strata in the Mesopotamia Basin,the Middle East.There are 3 ways of biological transformation of rocks:(1)The living creatures transformed formations mechanically to make the rocks looser and intergranular pores increase.(2)After formation,burrows were backfilled with coarse-grained debris,and then unsaturated fluid infiltrated into the burrows during the penecontemporaneous period,resulting in dissolution.(3)Chemical alteration occurred in abandoned burrows and dolomitization produced a large number of intercrystalline pores.The·coupling of bioturbation and dissolution occurred mainly in the penecontemporaneous phase,and was controlled by rock type,sea level decline,burrow abundance,infillings,and water environment etc.As the burrows had better physical properties than the matrix,unsaturated fluid preferentially migrated along the burrows,leading to dissolution and expansion of the burrows first and then dissolution of the matrix.The coupling of bioturbation and dolomitization occurred mainly in the burial phase.The rich organic matter and reducing bacteria in the burrow provided material basis,reducing conditions and alkaline envi-ronment for dolomitization.The metasomatism in the penecontemporaneous period had little effect on the physical properties of the burrows.When the burrows were se parated from the deposition interface,equimolar metasomatism occurred in the burrows in a closed environment,forming euhedral fine-crystalline dolomite with intercrystalline pores.The transformation degree of bioturbation to muddy carbonate reservoir depends on rock type,density,connectivity,infillings and structure of the burrows.With the increase of the carbonate mud content,the im provement to rock physical properties by bioturbation be-comes more prominent.When the burrows are filled with coarse-grained debris or fine-crystalline dolomite,the greater the density,the higher the connectivity,and the lower the tortuosity of burrows,the better the physical properties of the muddy carbonate rocks are.

Formation of fractures in carbonate rocks by pad acid fracturing with different states of carbon dioxide

Carbonate outcrops were taken from Ma 51 sub-member in the Lower Paleozoic in the Yan’an gas field to conduct true tri-axial hydraulic fracturing experiments with water, liquid CO_(2) and supercritical CO_(2). CT scan was applied to analyze initiation and propagation laws of hydraulic fractures in carbonate rocks. The experiments show that supercritical CO_(2) has low viscosity, strong diffusivity and large filtration during fracturing, which is more liable to increase pore pressure of rocks around wellbore and decrease breakdown pressure of carbonate rocks. However, it would cost much more volume of supercritical CO_(2) than water to fracture rocks since the former increases the wellbore pressure more slowly during fracturing. For carbonate rocks with few natural fractures, tensional fractures are generated by fracturing with water and liquid CO_(2), and these fractures propagate along the maximum horizontal principal stress direction;while fracturing with supercritical CO_(2) can form shear fractures, whose morphology is rarely influenced by horizontal stress difference. Besides, the angle between propagation direction of these shear fractures near the wellbore and the maximum horizontal principal stress is 45°, and the fractures would gradually turn to propagate along the maximum horizontal principal stress when they extend to a certain distance from the wellbore, leading to an increase of fracture tortuosity compared with the former. For carbonate rocks with well-developed natural fractures, fracturing with fresh water is conducive to connect natural fractures with low approaching angle and form stepped fractures with simple morphology. The key to forming complex fractures after fracturing carbonate rocks is to connect the natural fractures with high approaching angle. It is easier for liquid CO_(2) with low viscosity to realize such connection. Multi-directional fractures with relatively complex morphology would be formed after fracturing with liquid CO_(2).

Geochemical characteristics and sedimentary environment significance of carbonate rocks in Nanfen Formation of Qingbaikou System in Tonghua,Jilin Province

In order to reveal the sedimentary environment of carbonate rocks in Nanfen Formation of Qingbaikou System in Tonghua,Jilin Province,the mineral composition and petrochemistry of carbonate rocks in Nanfen Formation were analyzed.The mineral compositions of five carbonate rock samples in Nanfen Formation mainly consist of calcite,with minor clay minerals and quartz,and the rock type is siliceous marlite.The Mn/Sr va-lues range from 1.52 to 4.08,with an average of 2.64,indicating that the carbonate rocks experienced weak diagenesis;the Sr/Ba values range from 1.26 to 2.51,with an average of 1.93,indicating marine environment;the ratio of Mg/Al ranges from 35.33 to 86.34,with an average of 62.95,indicating the seawater environment,which is consistent with the result from Sr/Ba;the MgO/CaO values range from 0.01 to 0.04,with an average of 0.02,indicating humid environment;the values of V/(V+Ni)range from 0.63 to 0.73,with an average of 0.70,indicating anoxic environment.In summary,geochemical analyses show that the Nanfen Formation carbonate rocks are marine deposits,in a warm,humid,anoxic environment with poor flow of seawater,and subsequently underwent weak diagenetic alteration.

Impact of effective stress on permeability for carbonate fractured-vuggy rocks

To gain insight into the flow mechanisms and stress sensitivity for fractured-vuggy reservoirs,several core models with different structural characteristics were designed and fabricated to investigate the impact of effective stress on permeability for carbonate fractured-vuggy rocks(CFVR).It shows that the permeability performance curves under different pore and confining pressures(i.e.altered stress conditions)for the fractured core models and the vuggy core models have similar change patterns.The ranges of permeability variation are significantly wider at high pore pressures,indicating that permeability reduction is the most significant during the early stage of development for fractured-vuggy reservoirs.Since each obtained effective stress coefficient for permeability(ESCP)varies with the changes in confining pressure and pore pressure,the effective stresses for permeability of four representative CFVR show obvious nonlinear characteristics,and the variation ranges of ESCP are all between 0 and 1.Meanwhile,a comprehensive ESCP mathematical model considering triple media,including matrix pores,fractures,and dissolved vugs,was proposed.It is proved theoretically that the ESCP of CFVR generally varies between 0 and 1.Additionally,the regression results showed that the power model ranked highest among the four empirical models mainly applied in stress sensitivity characterization,followed by the logarithmic model,exponential model,and binomial model.The concept of“permeability decline rate”was introduced to better evaluate the stress sensitivity performance for CFVR,in which the one-fracture rock is the strongest,followed by the fracture-vug rock and two-horizontalfracture rock;the through-hole rock is the weakest.In general,this study provides a theoretical basis to guide the design of development and adjustment programs for carbonate fractured-vuggy reservoirs.