Dolomitization evolution and its effects on hydrocarbon reservoir formation from penecontemporaneous to deep burial environment

Aiming at the scientific problem that only part of dolomite acts as dolomite reservoir,this paper takes the multiple dolomite-bearing formations in the Tarim and Ordos basins,NW China and Sichuan Basin,SW China as the study object,by means of mineral petrological analysis and geochemical methods including carbonate clumped isotope,U-Pb isotopic dating,etc.,to rebuild the dolomitization pathway and evaluate its effects on reservoir formation.On the basis of detailed rock thin section observation,five dolomitic structural components are identified,including original fabric-retained dolomite(microbial and/or micrite structure),buried metasomatic dolomite I(subhedral-euhedral fine,medium and coarse crystalline structure),buried metasomatic dolomite II(allotriomorphic-subhedral fine,medium and coarse crystalline structure),buried precipitation dolomite and coarse crystalline saddle dolomite.Among them,the first three exist in the form of rocks,the latter two occur as dolomite minerals filling in pores and fractures.The corresponding petrological and geochemical identification templates for them are established.Based on the identification of the five dolomitic structural components,six dolomitization pathways for three types of reservoirs(preserved dolomite,reworked dolomite and limestone buried dolomitization)are distinguished.The initial porosity of the original rock before dolomitization and the dolomitization pathway are the main factors controlling the development of dolomite reservoirs.The preserved dolomite and reworked dolomite types have the most favorable dolomitization pathway for reservoir formation,and are large scale and controlled by sedimentary facies in development and distribution,making them the first choices for oil and gas exploration in deep carbonate formations.

Characteristics and Dolomitization of Upper Cambrian to Lower Ordovician Dolomite from Outcrop in Keping Uplift, Western Tarim Basin, Northwest China

Late Cambrian to Early Ordovician sedimentary rocks in the western Tarim Basin, Northwest China, are composed of shallow-marine platform carbonates. The Keping Uplift is located in the northwest region of this basin. On the basis of petrographic and geochemical features, four matrix replacement dolomites and one type of cement dolomite are identified. Matrix replacement dolomites include （1） micritic dolomites （MD1）; （2） fine-coarse euhedral floating dolomites （MD2）; （3） fine-coarse euhedral dolomites （MD3）; and （4） medium-very coarse anhedral mosaic dolomites （MD4）. Dolomite cement occurs in minor amounts as coarse saddle dolomite cement （CD1） that mostly fills vugs and fractures in the matrix dolomites. These matrix dolomites have δ18O values of ？9.7‰ to ？3.0‰ VPDB （Vienna Pee Dee Belemnite）; δ13C values of ？0.8‰ to 3.5‰ VPDB; 87Sr/86Sr ratios of 0.708516 to 0.709643; Sr concentrations of 50 to 257 ppm; Fe contents of 425 to 16878 ppm; and Mn contents of 28 to 144 ppm. Petrographic and geochemical data suggest that the matrix replacement dolomites were likely formed by normal and evaporative seawater in early stages prior to chemical compaction at shallow burial depths. Compared with matrix dolomites, dolomite cement yields lower δ18O values （？12.9‰ to ？9.1‰ VPDB）; slightly lower δ13C values （？1.6‰-0.6‰ VPDB）; higher 87Sr/86Sr ratios （0.709165-0.709764）; and high homogenization temperature （Th） values （98°C-225°C） and salinities （6 wt%-24 wt% NaCl equivalent）. Limited data from dolomite cement shows a low Sr concentration （58.6 ppm） and high Fe and Mn contents （1233 and 1250 ppm, respectively）. These data imply that the dolomite cement precipitated from higher temperature hydrothermal salinity fluids. These fluids could be related to widespread igneous activities in the Tarim Basin occurring during Permian time when the host dolostones were deeply buried. Faults likely acted as important conduits that channeled dolomitizing fluids from the underlying strata into the basal carbonates, leading to intense dolomitization. Therefore, dolomitization, in the Keping Uplift area is likely related to evaporated seawater via seepage reflux in addition to burial processes and hydrothermal fluids.

Dolomitization of the lower cretaceous carbonate reservoir in the Euphrates Graben,Syria

Results of this study are based on core materials description,thin sections,Cathodoluminescence(CL),and Scanning Electron Microscope(SEM)examinations.The Lower Cretaceous over the Euphrates Graben area was characterized by carbonate sedimentation in shallow marine environments.The low energy lagoonal to inner shelf sediments of the Judea Formation includes micritic mudstone to wackestone texture,dolomitic limestones and dolostones.Two types of dolomites recognized in the carbonates of the Judea Formation,the preserving microcrystalline dolomites which commonly founded in the partially dolomitized micritic limestones,and the destructive coarsely crystalline dolomites which commonly founded in the dolostones and dolomitic limestones.Petrographic examinations indicate that the preserving microcrystalline dolomites represent subtidal cycles developed in a shoal to open marine depositional environments,they probably formed under conditions of the shallow burial diagenesis.The destructive coarsely crystalline dolomites may develop in more basinward,open marine environments under conditions of the deep burial diagenesis that accompanied by rising in temperature,pressure,and burial depth.It is believed that evolution of the diagenetic history of the Judean Formation sediments occurred in two diagenetic stages;the shallow burial diagenesis,and the deep burial diagenesis.Compaction processes,early fracturing,micritization,early calcite,and the early phase of dolomitization were part of the multiple diagenetic alterations during the shallow burial diagenesis.The deep burial diagenesis was marked by dissolution,late stage of dolomitization and replacement,mechanical and chemical compaction,and the late calcite precipitation.It is believed that dolomitization of the Judea Formation carbonates in the Euphrates Graben has contributed to improving the reservoir properties by increasing the porosity and thus the permeability.

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.

Dolomitization of the Ordovician subsalt Majiagou Formation in the central Ordos Basin,China:fluid origins and dolomites evolution

The Middle Ordovician subsalt Majiagou Formation in the Ordos Basin comprises pervasively dolomitized shallow marine limestone and is a major reservoir rich in natural gas resources.Four types of dolomite matrix and cement were identified based on petrographic textures:(very)finely crystalline,non-planar to planar-s matrix dolomite(Md1);finely to medium crystalline,planar-s to planar-e matrix dolomite(Md2);microbialites comprising dolomite microcrystals(Md3);and finely to coarsely crystalline dolomite cement(Cd).The Md1 and Md2 dolomites were controlled by alternating lagoon-shoal facies and haveδ13C values(−1.89 to+1.45‰VPDB for Md1,−1.35 to+0.42‰VPDB for Md2)that fall within or are slightly higher than the coeval seawater,suggesting the dolomitizing fluid of evaporated seawater.Md2 dolomite was then subjected to penecontemporaneous karstification by meteoric water and burial recrystallization by sealed brines during diagenesis,as indicated by its relatively lowerδ18O values(−8.89 to−5.73‰VPDB)and higher 87Sr/86Sr ratios(0.708920–0.710199).Md3 dolomite comprises thrombolite and stromatolite and is interpreted to form by a combination of initial microbial mediation and later replacive dolomitization related to evaporated seawater.Cd dolomite was associated with early-formed karst system in the Md2 host dolomite.The lowestδ18O values(−11.78 to−10.18‰VPDB)and 87Sr/86Sr ratios(0.708688–0.708725)and fluid inclusion data(Th:123–175°C)indicate involvement of hydrothermal fluid from which the Cd dolomite precipitated during deep burial.These results reveal the multi-stage dolomitization history of the Majiagou Formation and provide new constraints on fluid origins and dolomites evolution during deep burial in old superimposed basins,such as the Ordos Basin and elsewhere.

Dolomitization in Campanian Oil Shale Rocks from South-Western Jordan

This work represents the first detailed study of an oil shale (OS) section from the Upper Campanian Amman Silicified Limestone (ASL) Formation in south-western part of Jordan. More than five meters of oil shale have been recorded. Using the petrography and geochemistry, this study aims to focus on such non-conventional types of oil shale rocks in Jordan and to shed some light on their composition and formational environment. Unlike oil shale from Maastrichtian to early Tertiary, this type of oil shale is highly dolomitized. The dolomite is diagenetic in origin. This oil shale type contains considerable amount of OM and remarkably enriched in some trace elements and shows quite low sulfur content. Results suggest that the formation of dolomite appears to be limited by the rate of organic matter oxidation.

Dolomitization Mechanism Based on Petrography and Geochemistry in the Shotori Formation (Middle Triassic), Central Iran

Middle Triassic carbonate sequences of Shotori Formation have a thickness of 70 m and are deposited Robat-e-Kalmard region of Tabas city in Central Iran basin. Gradationally and conformably overlying Sorkh shale Formation, Shotori Formation, mostly composed of medium to thick dolomites (50 m), interbeded with thin lime and sandstones, is disconformable by a laterite horizon at its upper boundary. This Formation mainly consists of fine-to-coarsely crystalline dolomites. According to petrographic (fabric and grain size) and geochemical (elemental analysis of Ca, Mg, Na, Sr, Fe, Mn) evidence, five various types of dolomites were recognized in Shotori Formation. This variety results from early and late diagenetic processes, triggering a change in dolomitizing fluids and thereby forming various dolomites. Geochemical studies have revealed that the dolomites of Shotori Formation have formed under meteoric diagenesis and reducing conditions. Various dolomitization mechanisms are proposed for various types of dolomites;that is to say, Sabkha model is considered for type 1 dolomite, mixing zone model for type 2 and 3 dolomites and burial model for type 4 and 5 dolomites.

Episodic dolomitization of Paleozoic limestones in the Kinta Valley,Malaysia:Implications on porosity evolution and reservoir properties

Carbonates undergo a variety of physical,mineralogical,chemical changes during depositional and postdepositional stages that are of interest to exploration geologists and sedimentologists.World over,about 60%of hydrocarbon reserves occur in carbonates with the dominant proportion associated with dolomitic reservoirs,in particular fault-associated dolomites which appear to be proven hydrocarbon reservoirs.Dolomitization as the critical diagenetic process thus plays an important role in determining reservoir characteristics and in designing exploration strategies.The buried Paleozoic dolomites exhibit relatively higher porosity than their counterparts,Paleozoic limestones.Fault-associated dolomites are usually targets of hydrocarbon exploration due to their good reservoir quality.High heterogeneity in carbonate reservoirs due to varied porosity types and distribution makes reservoir characterization a daunting task.However,the key elements that control porosity development and evolution in these dolomites remain debatable.Studying the influence of multiphase dolomitization,associated diagenetic events,hydrothermal alteration and their causative thermo-tectonic events on porosity evolution and reservoir quality is essential to understanding the critical processes and controlling factors that result in diverse impacts on reservoir quality.In this paper,an attempt is made to characterize dolomitization in homogeneous carbonate mud and its relationship with porosity evolution and reservoir property.In the field,six facies types excluding host limestone are recognized,which are in turn characterized into seven petrographically recognizable phases,namely the host limestone,early replacement dolomite,sucrosic dolomite,metamorphosed dolomite,late replacement dolomite,brecciated limestone and dolomite and late stage calcite.Mineralogical analyses reveal the dominance of stoichiometric dolomites,followed by calcareous dolomites and magnesium calcites.Among all the field-based facies and petrographic phases,the sucrosic dolomites show appreciable porosity within a range from 4%to 8%,whereas the other phases show either nil or insignificant porosity under megascopic and petrographic observations.Our study confirms that in the process of dolomitization that substantially modifies porosity,dolomitic crystal morphologies play a significant role in enhancing or reducing the porosity.

Origin of Dolomitization in Demirkazik Formation(Adana-Turkey)

Demirkaz&#305k formation consisting of the limestones and dolomites is common in the Adana, Turkey. In this study, it is aimed to find the origin of dolomitization by preparing a detailed geological map. For this purpose, firstly, thin section/polished section and, chemical analysis of the twenty-one samples characterizing dolomitization of the field were prepared and examined. In the chemical analyzes, 21 samples were determined MgO-CaO ratio. Geological map was prepared by field study and MgO contents of the samples that are known coordinates in the chemical data, and the regions where dolomitization was effective were determined. Dolomitization in these carbonates occurs at early stages and late stages. Two dolomite types are petrographically determined;(1) fine crystalline dolomite (TD-1) with 10 - 25 μm crystal size and (2) coarse crystalline dolomite (TD-2) with 50 - 120 μm crystal size. TD-1 is formed by synsedimentary replacement and TD-2 derived from TD-1. The evidence of dolomitization due to pressure changes in the TD-2 dolomites is evidence of late diagenetic. There are three types of carbonate by chemical data;limestone, dolomitic limestone, calcitic dolomite and MgCO3 ratio of these is increased respectively. The rate of dolomitization is very variable and at the fracture regions, the Mg ratio increases, indicating that tectonism is the cause of dolomitization.

Multiple Dolomitization and Fluid Flow Events in the Precambrian Dengying Formation of Sichuan Basin, Southwestern China

The Precambrian Dengying Formation is a set of large-scale, extensively dolomitized, carbonate reservoirs occurring within the Sichuan Basin. Petrographic and geochemical studies reveal dolomitization was a direct result of precipitation by chemically distinct fluids occurring at different times and at different intensities. Based on this evidence, dolomitization and multiple fluid flow events are analyzed, and three types of fluid evolution models are proposed. Results of analysis show that Precambrian Dengying Formation carbonates were deposited in a restricted peritidal environment （630- 542 Ma）. A high temperature and high Mg2＋ concentration seawater was a direct result of dolomitization for the micrite matrix, and for fibrous aragonite in primary pores. Geochemical evidence shows low δ18O values of micritic dolomite varying from -1.29%o to -4.52%o PDB, abundant light rare earth elements （REEs）, and low dolomite order degrees. Microbes and meteoric water significantly altered dolomite original chemical signatures, resulting in algal micritic dolomite and the fine-grained, granular, dolosparite dolomite having very negative δ18O values. Finely crystalline cement dolomite （536.3-280 Ma） and coarsely crystalline cement dolomite have a higher crystallization degree and higher order degree. The diagenetic sequence and fluid inclusion evidence imply a linear correlation between their burial depth and homogenization temperatures, which closely resemble the temperature of generated hydrocarbon. Compared with finely crystalline dolomite, precipitation of coarsely crystalline dolomite was more affected by restricted basinal fluids. In addition, there is a trend toward a more negative δ18O value, higher salinity, higher Fe and Mn concentrations, REE-rich. Two periods of hydrothermal fluids are identified, as the exceptionally high temperatures as opposed to the temperatures of burial history, in addition to the presence of high salinity fluid inclusions. The early hydrothermal fluid flow event was characterized by hot magnesium- and silicon-rich fluids, as demonstrated by the recrystallized matrix dolomite that is intimately associated with flint, opal, and microcrystalline quartz in intergranular or intercrystalline pores. This event was likely the result of a seafloor hydrothermal chimney eruption during Episode I of the Tongwan Movement （536.3-5.5 Ma）. In contrast, later hydrothermal fluids, which caused precipitation of saddle dolomite, were characterized by high salinity （15-16.05wt% NaCI equivalent） and homogenization temperatures （250 to 265℃）, δ18O values that were more enriched, and REE signatures. Geochemical data and the paragenetic sequence indicate that this hydrothermal fluid was related to extensive Permian large igneous province activity （360-280 Ma）. This study demonstrates the presence of complicated dolomitization processes occurring during various paleoclimates, tectonic cycles, and basinal fluids flow; results are a useful reference for these dolomitized Precambrian carbonates reservoirs.

Characteristics and Dolomitization of Dolostones in Nenjiang Formation Unit 1,Northern Songliao Basin

Songliao Basin is a large-scale Cretaceous continental sedimentary basin in northeastern China. In the northern Songliao Basin Nenjiang Formation Unit 1 are mainly dark mudstones with dolostone interbeds.Understanding the sedimentary characteristics and dolomitization of the dolostones is of great significance to the paleoenvironment and paleoc-

Application of Stable Strontium Isotope Geochemistry and Fluid Inclusion Microthermometry to Studies of Dolomitization of the Deeply Buried Cambrian Carbonate Successions in West-Central Tarim Basin, NW China

Detailed petrographic,geochemical(O-C-Sr isotopes)and fluid inclusion studies of the deeply buried Cambrian carbonates in the West-central Tarim Basin revealed three types of crystalline dolomites(fine-crystalline,nonplanar-a(s),dolomite(RD1),fine-to medium-crystalline,planar-e(s)dolomite(RD2),and medium-to coarse-crystalline,nonplanar-a dolomite(RD3)),medium-to coarsecrystalline,nonplanar-a saddle dolomite cement(CD)and early and later-stage calcite cement.The occurrence of RD1 along low-amplitude stylolites points to link with pressure dissolution by which minor Mg ions were likely released for replacive dolomitization during early-to intermediate-burial seawater dolomitization.The increasing crystal sizes of RD2 and RD3 with irregular overgrowth rims suggests intense recrystallization and replacement upon the RD1 or remaining precursor limestones by dolomitizing fluids during late intermediate burial dolomitization.The overlap ofδ^(18)O,δ^(13)C and ^(87)Sr/^(86)Sr values of RD1-RD3 and CD dolomite with coeval seawater values,suggests that the principal dolomitizing fluids that precipitated these dolomites was connate(Cambrian)seawater preserved in the host limestones/dolomites.Their high ^(87)Sr/^(86)Sr ratios suggest influx of radiogenic strontium into the Cambrian seawater.Two regimes of fluid flow are recognized in the study area:firstly,influx of magnesium-rich higher-temperature basinal brines along deep-seated faults/fractures,resulting in cementation by CD dolomite.Secondly,the incursion of meteoric waters,mixing with ascending highertemperature basinal brines,and an increase in Ca^(2+)/Mg^(2+)ratio in the fluids probably results in the precipitation of calcite cement in vugs and fractures.

Dolomitization by Penesaline Sea Water in Early Cambrian Longwangmiao Formation, Central Sichuan Basin, China

The Lower Cambrian Longwangmiao Formation shoal dolostone reservoir in Sichuan Basin is currently an exploration and research highlight in China. Reservoir rocks mainly consist of crystalline dolomite with residual grain texture, and dolarenite of which the arene is mainly composed of muddy to micritic dolomite with some crystalline grain directionally aligned. The trace element indicates that the dolomites of Longwangmiao Formation may be related to the high salinity of sea water. The oxygen isotope values of crystalline dolomite and dolarenite are both similar to that of the Early Cambrian marine dolomites, and the carbon isotope values of every kind of dolomites are completely overlapped with that of the seawater in Early Cambrian, indicating the dolomitization fluid is originated from the Early Cambrian sea water. The restricted marine biological communities and a small amount of gypsum pseudonodule seen in muddy to micritic dolomite indicate that the sea water in Early Cambrian was restricted and evaporated. However, the general lack of massive evaporite mineral and gypsum karst breccia indicates that the salinity of sea water during dolomitization was lower than the value of gypsum＇s precipitation. The Longwangmiao Formation consists of several high-frequency sedimentary cycles, indicating frequent sea level changes. This study indicates that massive dolomitization may also occur in underwater palaeohigh in carbonate platform through the reflux of penesaline sea water driven by a combination of high-and low-frequency sea-level changes. This kind of dolomitization can explain the generation of massive dolomites in the absence of evaporite precipitation, and further indicates that replacement dolomites can be produced by sea water with a wide range of salinity（normal, penesaline to hypersaline）.

Dolomite geochemistry of the Cambrian Longwangmiao Formation,eastern Sichuan Basin:Implication for dolomitization and reservoir prediction

Based on petrology and mineralogy,the geochemical characteristics of dolomite in the Cambrian Longwangmiao Formation in the eastern Sichuan Basin,were carried out.Results showed that dolomites mainly occurred in the middle and upper parts of Longwangmiao Formation,with most commonly as a crystalline dolomite and less commonly as a gain dolomite;the percentage content of dolostone in dolomite ranged from 48.6%to 75%,ratios of Mg/Ca were generally small,and positive and negative relationships between content of MgO and content of CaO were identified,respectively;the dolostones were generally characterized by high contents of Fe and Na while low contents of Sr and Mn,and only a part of dolostones had high content of Mn;compared with composition of contemporaneous seawater,most of dolostones were characterized by positiveδ^(13)C values and negativeδ^(18)O values.Based on the analyses of regional geological setting and geochemical characteristics,the dolomite in Longwangmiao Formation was mainly from the seepage-refluxion dolomitization,while evaporative-concentration dolomitization and burial dolomitization were not developed.In general,the dolomitization of Longwangmiao Formation was not complete.Furthermore,through the dolomitization model of Longwangmiao Formation,the distribution of sedimentary facies as well as the controlling effect of dolomitization on reservoir,the favorable reservoir was predicted,and was mainly developed around Qiyueshan fault zone.

The relationship between dolomitization and organic matter occurrence in Lower Paleozoic carbonate in the Ordos Basin

Based on observations on the core and surface sections of Lower Paleozoic carbonate in the Ordos Basin, petrography research and measurements of TOC, TOS , Ro , XRD, and comparative study with dolomite in modern Coorong Lake, it has been revealed that: ( i ) dolomitization may occur in micrite limestone, gypsum-halite and argillaceous sandstone, and it can be divided into three types: sedimentary penecontemporaneous-early diagenesis, late diagenesis of deep burial and catagenesis of uplift period. However, the crystal cell of the second type less than 35μm in size is most closely associated with gas pool; (ii) the highest content of organic matter (OM) is produced in samples from the argillic dolomite which may be formed by argillaceous fluid through gypsum-halite; (iii) in the evolution process from penecontemporaneous dolomite into stoichiometric dolomite, the crystal order of dolomite and the porosity of its host rock tend to increase, which is favorable to the formation of an available migration

Porosity enhancement potential through dolomitization of carbonate reservoirs, a case of study from the Euphrates Graben fields, East Syria

The dolostones and dolomitic limestones of the lower Cretaceous Judea formation are a key target ofhydrocarbons in most of the Euphrates Graben fields. Core materials investigation, thin sections petrographicallyexaminations, and petrophysical evaluations were obtained to determine enhancement ofthe porosity through dolomitization. Results showed that the lagoon-shallow marine carbonates of theJudea formation are subdivided into two main zones;the upper “limestone zone” is micritic limestonesdominated with no reservoir potential, and the lower “dolomite zone” is dolomitic limestones anddolostones dominated with good to very good reservoir potential. Dolomitization of the mudstones andwackestones of the micritic limestones resulted in formation of microcrystalline dolomicrite and earlyfabric destructive dolomites. Conversely, dolomitization of the packstones and grainstones resulted information of the fabric destructive and saddle dolomites. Based on petrography data, dolomitization ofthe “limestone zone” is interpreted by the seawater dolomitization mechanism at low-temperatures,while dolomitization of the “dolomite zone” is interpreted by the burial dolomitization mechanismunder high temperature and pressure conditions. The “limestone zone” is characterized by the poorestreservoir quality, while the “dolomite zone” is characterized by the best reservoir quality. The seawaterdolomitization did not significantly enhance the porosity, while the burial dolomitization contributes toenhancing the preserved secondary porosity. Stylolites microfractures and dissolution seams associatedwith dolomitization played as major factors in porosity enhancement of the dolostones and dolomiticlimestones and serving as pathways for hydrocarbon migration. Dissolution processes increased theporosity and more permeability unless they are filled with the precipitated dolomite and/or calcite.Calcification had significant effects on the porosity by blocking the cavities and channels and decreasedthe effective pore volume.

Hydrocarbon accumulation in deep ancient carbonate-evaporite assemblages

The Ediacaran–Ordovician strata within three major marine basins(Tarim,Sichuan,and Ordos)in China are analyzed.Based on previous studies focusing on the characteristics of the Neoproterozoic–Cambrian strata within the three major basins(East Siberian,Oman,and Officer in Australia)overseas,the carbonate–evaporite assemblages in the target interval are divided into three types:intercalated carbonate and gypsum salt,interbedded carbonate and gypsum salt,and coexisted carbonate,gypsum salt and clastic rock.Moreover,the concept and definition of the carbonate-evaporite assemblage are clarified.The results indicate that the oil and gas in the carbonate-evaporite assemblage are originated from two types of source rocks:shale and argillaceous carbonate,and confirmed the capability of gypsum salt in the saline environment to drive the source rock hydrocarbon generation.The dolomite reservoirs are classified in two types:gypseous dolomite flat,and grain shoal&microbial mound.This study clarifies that the penecontemporaneous or epigenic leaching of atmospheric fresh water mainly controlled the large-scale development of reservoirs.Afterwards,burial dissolution transformed and reworked the reservoirs.The hydrocarbon accumulation in carbonate-evaporite assemblage can be categorized into eight sub-models under three models(sub-evaporite hydrocarbon accumulation,supra-evaporite hydrocarbon accumulation,and inter-evaporite hydrocarbon accumulation).As a result,the Cambrian strata in the Tazhong Uplift North Slope,Maigaiti Slope and Mazatag Front Uplift Zone of the Tarim Basin,the Cambrian strata in the eastern-southern area of the Sichuan Basin,and the inter-evaporite Ma-4 Member of Ordovician in the Ordos Basin,China,are defined as favorable targets for future exploration.

Genesis of Dolomite from Ma5^5-Ma5^10 Sub-members of the Ordovician Majiagou Formation in the Jingxi Area in the Ordos Basin

We clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma5^5-Ma5^10 sub-members of the Ordovician Majiagou Formation in the Jingxi area in the Ordos Basin： （1） Syngenetic microbial dolomitization is characterized by formation of dolomite with a mainly micrite structure and horse tooth-shape dolomite cements. （2） Seepage reflux dolomitization during the penecontemporaneous period superposed adjustment functions such as recrystallization and stabilization in the middle-deep burial stage, forming dolomites mainly consisting of micro crystal and powder crystal structure. （3） Powder dolomite, fine dolomite, and medium-coarse crystalline dolomite formed in pores and fractures in the middle-deep burial stage. The secondary concussive transgression-regression under a regressive background is an important condition for the occurrence of many stages of dolomitization in the study area. The basin was an occlusive epicontinental sea environment in the Ma5 member of the Ordovician Majiagou Formation sedimentary period. In the sediments, sulfate content was high, which is conducive to the preservation of microbial activity and microbial dolomitization. Micritic dolomite formed by microbial dolomitization provides good migration pathways for seepage reflux dolomitization. Affected by evaporation seawater with increased Mg/Ca ratio, seepage reflux dolomitization was widely developed and formed large-scale dolomite, and underwater uplifts and slopes are favorable areas for dolomite. In the middle-deep burial stage, dolomitizing fluid in the stratum recrystallized or stabilized the previous dolomite and formed a small amount of euhedral dolomite in the pores and fractures.

Role of tannin pretreatment in flotation separation of magnesite and dolomite

Flotation separation of magnesite and its calcium-containing carbonate minerals is a difficult problem.Recently,new regulat-ors have been proposed for magnesite flotation decalcification,although traditional regulators such as tannin,water glass,sodium carbon-ate,and sodium hexametaphosphate are more widely used in industry.However,they are rarely used as the main regulators in research because they perform poorly in magnesite and dolomite single-mineral flotation tests.Inspired by the limonite presedimentation method and the addition of a regulator to magnesite slurry mixing,we used a tannin pretreatment method for separating magnesite and dolomite.Microflotation experiments confirmed that the tannin pretreatment method selectively and largely reduces the flotation recovery rate of dolomite without affecting the flotation recovery rate of magnesite.Moreover,the contact angles of the tannin-pretreated magnesite and dolomite increased and decreased,respectively,in the presence of NaOl.Zeta potential and Fourier transform infrared analyses showed that the tannin pretreatment method efficiently hinders NaOl adsorption on the dolomite surface but does not affect NaOl adsorption on the magnesite surface.X-ray photoelectron spectroscopy and density functional theory calculations confirmed that tannin interacts more strongly with dolomite than with magnesite.

Impact and mechanism of bisphosphonate depressant 1-hydroxypropane-1,1-diphosphonic acid on flotation decalcification of dolomite-rich magnesite ore

Given the depletion of high-quality magnesite deposits and the rising demand for high-end magnesium materials,the separation and utilization of high-calcium magnesite ores have become essential.However,the similar surface properties and solubility of semi-soluble salt-type minerals,pose significant challenges for the utilization of dolomite-rich magnesite resources.In this study,1-hydroxypropane-1,1-di phosphonic acid(HPDP)was identified for the first time as a high-performance depressant for dolomite.Various tests,including contact angle measurements,ζ potential analysis,X-ray photoelectron spectroscopy,and atomic force microscopy,were conducted to elucidate the interfacial interaction mechanisms of HPDP on the surfaces of the two minerals at different scales.Additionally,molecular modeling calculations were used to detail the spatial matching relationship between HPDP and the crystal faces of the two minerals.It was emphasized that HPDP specifically adsorbed onto the dolomite surface by forming calcium phosphonate,ensuring that the dolomite surface remained hydrophilic and sank.Moreover,it was found that the adsorption strength of HPDP on the mineral surfaces depended on the activity of the metal sites and their spatial distribution.These findings provide a theoretical foundation for the molecular design of flotation reagents for high-calcium magnesite ores.