Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear.Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock r...Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear.Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock reservoir of the Fengcheng Formation in the Mahu Sag were analyzed by polarized and fluorescence thin sections,scanning electron microscope(SEM),electron microprobe(EMP),C,O and Sr isotopes analysis,and other techniques.(1)Dolomites were mainly precipitated in three stages:penecontemporaneous-shallow burial stage(early stage of the Middle Permian),middle burial stage(middle stage of the Middle Permian),and middle-deep burial stage,with the former two stages in dominance.(2)Dolomitization fluid was high-salinity brine originating from alkaline lake.In the penecontemporaneous-shallow burial stage,Mg^(2+)was mainly supplied by alkaline-lake fluid and devitrification of volcanic glass.In the middle burial stage,Mg^(2+)mainly came from the transformation of clay minerals,devitrification of volcanic glass and dissolution of aluminosilicates such as feldspar.(3)Regular changes of Mg,Mn,Fe,Sr,Si and other elements during the growth of dolomite were mainly related to the alkaline-lake fluid,and to different influences of devitrification and diagenetic alteration of volcanic materials during the burial.(4)In the penecontemporaneous stage,induced by alkaline-lake microorganisms,the micritic-microcrystalline dolomites were formed by primary precipitation,replacement of aragonite and high-Mg calcite,and other processes;in the shallow burial stage,the silt-sized dolomites were formed by continuous growth of micritic-microcrystalline dolomite and replacement of calcites,tuffs and other substances;in the middle burial stage,the dolomites,mainly silt-and fine-sized,were formed by replacement of volcanic materials.The research results are referential for investigating the formation mechanism and distribution patterns of tight dolomitic reservoirs in the Mahu Sag and other similar oil and gas bearing areas.展开更多
In order to discuss the relationship between dolomite reservoirs and diagenetic systems of the Changxing Formation, we studied carbon, oxygen and strontium stable isotopes, iron, manganese and strontium trace elements...In order to discuss the relationship between dolomite reservoirs and diagenetic systems of the Changxing Formation, we studied carbon, oxygen and strontium stable isotopes, iron, manganese and strontium trace elements and the Mg/Ca (mol%) ratio, dolomite order degree, and determined that burial dolomitization is the key to controlling the distribution of high quality dolomite reservoir in the Changxing Formation in the eastern Sichuan Basin. The dolomite of the Changxing Formation is divided into four diagenetic systems: (1) penecontemporaneous stage syngenetic brine diagenetic system, (2) early diagenetic stage strata seal brine diagenetic system, (3) middle-late diagenetic stage mixed hot brine diagenetic system and (4) tectonic uplift stage mixed hydrothermal fluid diagenetic system. New understanding of the controlling factors and distribution of dolomite reservoir development is discussed. Reef shoal facies belts controlled regional reservoir distribution and the scale of development. Burial dolomitization of a strata seal brine diagenetic system is the foundation of reservoir development, mainly developing pore reservoir. Burial dolomitization of mixed hot brine diagenetic system expanded the reservoir distribution and improved the reservoir quality, mainly developing pore-vug reservoir. Fracturing and dissolution of a mixed hydrothermal fluid diagenetic system is the key to improving the reservoir quality, mainly developing pore-vug-crack complex reservoirs.展开更多
Hydrothermal mineral assemblages and related hydrothermally enhanced fracturing are common in the Precambrian Dengying Formation of Central Sichuan Basin. Petrographic and geochemical analyses of core samples show tha...Hydrothermal mineral assemblages and related hydrothermally enhanced fracturing are common in the Precambrian Dengying Formation of Central Sichuan Basin. Petrographic and geochemical analyses of core samples show that the hydrothermal dolomite reservoirs of Dengying Formation consist of four main types of pores in the reservoir facies. These include: 1) hydrothermal dissolution vug(or pore), 2) intercrystalline pore, 3) residual inter-breccia vug(or pore), and 4) enlarged dissolved-fracture. There are three different fabrics dolomite in hydrothermal dolomite reservoirs, namely, saddle dolomite, fine-medium dolomite and micritic dolomite. Micritic dolomite is the original lithology of host rock. Saddle dolomite with curved or irregular crystal faces was directly crystallized from hydrothermal fluids(average temperature 192°C). Fine-medium dolomites are the products of recrystallization of micritic dolomite, resulting in abnormal geochemical characteristics, such as slight depletion of δ^(18)O, significant enrichment of Mn-Fe and ^(87)Sr/^(86)Sr, and positive Eu anomaly. A model for the distribution of various hydrothermal dolomite reservoir facies is proposed here, which incorporates three fundamental geological controls: 1) extensional tectonics and tectono-hydrothermal events(i.e., the Xingkai Taphrogenesis of Late Sinian-Early Cambrian, and Emei Taphrogenesis of Late Permian), 2) hydrothermal fluid storage in clastic rocks with large thickness(e.g., Nanhua System of Chengjiang Formation and part of Doushantuo Formation), and 3) confining bed for hydrothermal fluids(such as, the shale in Qiongzhusi Formation). The supply of hydrothermal fluid is critical. Large basement-rooted faults and associated grid-like fracture system may function as the channels for upward migration of hydrothermal fluid flow. The intersection of the above-mentioned faults(including the conversion fault), especially transtensional sags above negative flower structures on wrench faults can serve as a key target for future hydrocarbon exploration.展开更多
Based on a comprehensive study of texture,diagenetic behavior and evolution of dolomite in the Huanglong Formation,trace (e.g.,Fe,Mn and Sr) and rare earth element (REE) geochemistry,andisotopic characteristics (...Based on a comprehensive study of texture,diagenetic behavior and evolution of dolomite in the Huanglong Formation,trace (e.g.,Fe,Mn and Sr) and rare earth element (REE) geochemistry,andisotopic characteristics (e.g.,C,O and Sr),four types of diagenetic fluids are identified in the Huanglong Formation dolomite reservoirs of the Eastern Sichuan Basin,China:1):marine-derived pore waters in the marine diagenetic environment,2) sabkha compaction brine conserved in the early shallowburied diagenetic environment,3) strongly-oxidizing low-temperature meteoric water in the seepagesubsurface flow diagenetic environment,and 4) strongly reducing deeply seated mixed hot brine in the middle and deep burial diagenetic environment.The fluids developed hereditarily from one environment to another,which resulted in its respective characteristics.Fluid characteristics play an important role in the development of dolomite reservoirs:1) dolomitization by marine-derived pore water in the quasisyngenetic stage did not form an effective reservoir; 2) early diagenetic burial dolomitization by the sabkha compaction brine formed the basis for reservoir development; 3) meteoric water karstification in the paleo-epidiagenetic stage expanded both the distribution and the size of the reservoirs,and improved the reservoir quality; 4) deep-burial dissolution and tectonic fracturing in the reburial diagenetic stage further improved reservoir porosity and permeability.展开更多
This paper utilizes multi-methods,such as core observation,slice identification,isotope analysis,trace element analysis,fluid inclusion technique and so on,to study the causes of the dolomite in the Nanpu Sag and the ...This paper utilizes multi-methods,such as core observation,slice identification,isotope analysis,trace element analysis,fluid inclusion technique and so on,to study the causes of the dolomite in the Nanpu Sag and the origins of the dolomite reservoir.The study results show that the forming environment of dolomite in the Nanpu Sag is a fresh-water lake environment,and the dolomite is the product of dolomitization which is caused by volcanic thermal fluids in the Early Dongying period.With the development of intergranular pores in the dolomitization process,a lot of dissolved pores/vugs and fractures were formed by denudation in the later periods because of the influence of thermal fluids including the associated fluids of volcanic activities and the expelled hydrocarbon fluids of the source rocks.On the whole,these secondary dissolution spaces greatly enhance the reservoir ability of the dolomite,and there are enough reservoir spaces in the dolomite in the Nanpu Sag.展开更多
Based on seismic, drilling data and experimental analysis, the characteristics and main controlling factors of shoal dolomite gas reservoir in the third member of Ordovician Yingshan Formation of Gucheng area, Tarim b...Based on seismic, drilling data and experimental analysis, the characteristics and main controlling factors of shoal dolomite gas reservoir in the third member of Ordovician Yingshan Formation of Gucheng area, Tarim basin were examined.The study shows that the dolomite gas reservoir in Gucheng area is lithologic gas reservoir controlled by shoal and fault jointly,and its formation is mainly attributed to the following factors:(1) The continuously developing paleotectonic structure has been in the direction of gas migration and accumulation;(2) The large area of medium-high energy grain bank deposited in gentle slope environment is the material basis for the formation of dolomite reservoir;(3) Atmospheric water leaching and dolomitization and fluid dissolution in fault zone are the key factors for the formation of high-quality dolomite reservoir;(4)The natural gas comes from cracking of the ancient oil reservoir and hydrocarbon generation of dispersed organic matter in source rocks, and the NNE-trending strike-slip fault is the dominant channel for vertical migration of natural gas;(5) Limestone cap rocks in the first and second members of Yingshan Formation provide direct sealing for the formation of gas reservoir there. On the basis of comprehensive analysis, it is pointed out that the Gucheng area has three grain shoal zones in the third member of Yingshan Formation in nearly S-N direction, which together with seven strike-slip fault zones in NNE direction control the development of shoal dolomite gas reservoir.展开更多
Regarding to the problem on the reservoir-cap rock assemblage evaluation in the carbonate-evaporite paragenesis system,this study examined the dolomite and reservoirs genesis and the characteristics of reservoir-cap r...Regarding to the problem on the reservoir-cap rock assemblage evaluation in the carbonate-evaporite paragenesis system,this study examined the dolomite and reservoirs genesis and the characteristics of reservoir-cap rock assemblage.Based on the literature research of the global carbonate reservoirs and the case study on four profiles of carbonate-evaporite succession,together with geological and experimental work,three aspects of understandings are achieved.(1)Lithology of carbonate-evaporite paragenesis system is mainly composed of microbial limestone/bioclastic limestone,microbial dolomite,gypsum dolomite and gypsum salt rock deposited sequentially under the climatic conditions from humid to arid,and vice versa,and an abrupt climate change event would lead to the lack of one or more rock types.(2)There developed two kinds of dolomite(precipitation and metasomatism)and three kinds of reservoirs in the carbonate-evaporite system;and the carbon dioxide and organic acid generated during early microorganism degradation and late microbial dolomite pyrolysis process,and early dolomitization are the main factors affecting the development of microbial dolomite reservoirs with good quality.(3)In theory,there are 14 types of reservoir-cap rock assemblages of six categories in the carbonate-evaporite system,but oil and gas discoveries are mainly in four types of reservoir-cap rock assemblages,namely"microbial limestone/bioclastic limestone–microbial dolomite–gypsum dolomite–gypsum salt rock","microbial limestone/bioclastic limestone–gypsum salt rock","microbial dolomite–gypsum dolomite–gypsum salt rock"and"gypsum dolomite–microbial dolomite–tight carbonate or clastic rock".These four kinds of reservoir-cap rock assemblages should be related with the climate change rules in the geologic history,and have good exploration prospects.展开更多
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...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.展开更多
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 c...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.展开更多
The deeply buried Lower Cambrian Longwangmiao Formation and Upper Ediacaran Dengying Formation from the Sichuan Basin,China,have a total natural gas reserve up to 3×10^(12)m^(3).The complex diagenetic evolution a...The deeply buried Lower Cambrian Longwangmiao Formation and Upper Ediacaran Dengying Formation from the Sichuan Basin,China,have a total natural gas reserve up to 3×10^(12)m^(3).The complex diagenetic evolution and their impacts on the present-day reservoir quality have not been systematically elucidated,hampering the current exploration.Crucially,the integration and comparation diagenetic study on these two formations,which may be able to shed new lights on reservoir formation mechanism,are yet to be systemically evaluated.By compiling geochemistry data,including carbonate U-Pb ages and petrophysics data,coupled with new petrology,trace elements,and strontium isotope data,of various types of diagenetic carbonates,this study aims to decipher the potential links between diagenesis and reservoir development of both formations.Intriguingly,similar diagenetic sequence,which contains five distinctive dolomite phases,is established in both formations.The matrix dolomite(D1)and early dolomite cement(D2)were likely formed by reflux dolomitization,as inferred by their nearly syn-depositional U-Pb ages and elevatedδ^(18)O caused by seawater evaporation.The subsequent moderate burial dolomite cement(D3)was most plausibly the product of burial compaction as indicated by its lighterδ^(18)O and slightly younger U-Pb ages compared with D1 and D2.Whereas deep burial dolomite cements(D4 and D5)yield markedly depletedδ^(18)O,elevated ^(87)Sr/^(86)Sr,along with much younger U-Pb ages and higher precipitation temperatures,suggesting that they were likely linked to hydrothermal fluids.Despite the wide occurrence of meteoric and organic acids leaching and thermochemical sulfate reduction,they may have only played a subsidiary role on these reservoirs development.Instead,superior reservoir quality is tightly linked to tectonics as inferred by higher reservoir quality closely related to the well-developed fractures and faults filled with abundant hydrothermal minerals.Notably,good reservoirs in both formations are mainly attributed to high permeability caused by tectonics.Hence,this new contribution emphasizes the crucial role of tectonics on spatially explicit reservoir prediction of deep to ultra-deep(up to>8000 m)carbonates in the Sichuan Basin,as well as other sedimentary basin analogues in China.展开更多
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 t...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 shoalµbial 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.展开更多
Ancient dolomite reservoirs play an increasingly important role in deep oil and gas exploration.The mechanism of formation and preservation of dolomite reservoirs is complex,which is always the key issue.With the disc...Ancient dolomite reservoirs play an increasingly important role in deep oil and gas exploration.The mechanism of formation and preservation of dolomite reservoirs is complex,which is always the key issue.With the discovery of deep oil and gas in the Ediacaran dolomites of the world,the upper Ediacaran Qigebrak Formation in the Tabei area has begun to attract attention,but its reservoir space difference and formation mechanism have yet to be clarified.Based on ultra-deep drilling cores and field outcrops in the Tabei area,the lithofacies,reservoir space,and formation mechanism are systematically analyzed by macro to micro,and qualitative to quantitative petrology:(1)The types of dolomite can be divided into five major categories,including microbial dolomite,granular dolomite,residual granular dolomite,crystalline dolomite and karst breccias.(2)The main types of reservoir space are microbial-framework pores,microbial-mold pores,and non-fabric selective dissolution pores.Spongiomicrobialite,karst breccias,and fine-grained dolomite are the dominant reservoir rock types.(3)High-frequency sedimentary cycles and meteoric dissolution are the key factors of reservoir formation.Two sets of large-scale reservoirs are present:the first set is mainly controlled by the supergene karst of the Keping movement,and the second set is mainly controlled by high-frequency sedimentary cycles in the penecontemporaneous period.The reservoirs formed at the shallow burial stage and were preserved until the deep burial stage.(4)The quality of a deep reservoir depends on the geological events that affect the processes of pore reduction and increase.Cementation,compaction and pressure solution are the main destructive diagenetic processes;however,the reservoir space can still be effectively preserved under the influence of constructive diagenetic processes,such as meteoric dissolution and early dolomitization.This research has important theoretical and practical significance for revealing the formation mechanism of upper Ediacaran deep dolomite reservoirs in the Tarim Basin.展开更多
基金Supported the Major National Oil and Gas Projects of China(2016ZX05046-006).
文摘Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear.Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock reservoir of the Fengcheng Formation in the Mahu Sag were analyzed by polarized and fluorescence thin sections,scanning electron microscope(SEM),electron microprobe(EMP),C,O and Sr isotopes analysis,and other techniques.(1)Dolomites were mainly precipitated in three stages:penecontemporaneous-shallow burial stage(early stage of the Middle Permian),middle burial stage(middle stage of the Middle Permian),and middle-deep burial stage,with the former two stages in dominance.(2)Dolomitization fluid was high-salinity brine originating from alkaline lake.In the penecontemporaneous-shallow burial stage,Mg^(2+)was mainly supplied by alkaline-lake fluid and devitrification of volcanic glass.In the middle burial stage,Mg^(2+)mainly came from the transformation of clay minerals,devitrification of volcanic glass and dissolution of aluminosilicates such as feldspar.(3)Regular changes of Mg,Mn,Fe,Sr,Si and other elements during the growth of dolomite were mainly related to the alkaline-lake fluid,and to different influences of devitrification and diagenetic alteration of volcanic materials during the burial.(4)In the penecontemporaneous stage,induced by alkaline-lake microorganisms,the micritic-microcrystalline dolomites were formed by primary precipitation,replacement of aragonite and high-Mg calcite,and other processes;in the shallow burial stage,the silt-sized dolomites were formed by continuous growth of micritic-microcrystalline dolomite and replacement of calcites,tuffs and other substances;in the middle burial stage,the dolomites,mainly silt-and fine-sized,were formed by replacement of volcanic materials.The research results are referential for investigating the formation mechanism and distribution patterns of tight dolomitic reservoirs in the Mahu Sag and other similar oil and gas bearing areas.
基金funded by PetroChina Southwest Oil and Gasfield Company Scientific and Technological Projects "The Research of Changxing Organic Reef Bioherm and Reservoir Development Characteristics at Kaijiang-Liangping East Trough" (Number XNYQT-XNS02-2007-TS-5777)
文摘In order to discuss the relationship between dolomite reservoirs and diagenetic systems of the Changxing Formation, we studied carbon, oxygen and strontium stable isotopes, iron, manganese and strontium trace elements and the Mg/Ca (mol%) ratio, dolomite order degree, and determined that burial dolomitization is the key to controlling the distribution of high quality dolomite reservoir in the Changxing Formation in the eastern Sichuan Basin. The dolomite of the Changxing Formation is divided into four diagenetic systems: (1) penecontemporaneous stage syngenetic brine diagenetic system, (2) early diagenetic stage strata seal brine diagenetic system, (3) middle-late diagenetic stage mixed hot brine diagenetic system and (4) tectonic uplift stage mixed hydrothermal fluid diagenetic system. New understanding of the controlling factors and distribution of dolomite reservoir development is discussed. Reef shoal facies belts controlled regional reservoir distribution and the scale of development. Burial dolomitization of a strata seal brine diagenetic system is the foundation of reservoir development, mainly developing pore reservoir. Burial dolomitization of mixed hot brine diagenetic system expanded the reservoir distribution and improved the reservoir quality, mainly developing pore-vug reservoir. Fracturing and dissolution of a mixed hydrothermal fluid diagenetic system is the key to improving the reservoir quality, mainly developing pore-vug-crack complex reservoirs.
基金funded by the National Science and Technology Major Project(grant No.2016ZX05052)the National Natural Science Foundation of China(grant No.41072102)
文摘Hydrothermal mineral assemblages and related hydrothermally enhanced fracturing are common in the Precambrian Dengying Formation of Central Sichuan Basin. Petrographic and geochemical analyses of core samples show that the hydrothermal dolomite reservoirs of Dengying Formation consist of four main types of pores in the reservoir facies. These include: 1) hydrothermal dissolution vug(or pore), 2) intercrystalline pore, 3) residual inter-breccia vug(or pore), and 4) enlarged dissolved-fracture. There are three different fabrics dolomite in hydrothermal dolomite reservoirs, namely, saddle dolomite, fine-medium dolomite and micritic dolomite. Micritic dolomite is the original lithology of host rock. Saddle dolomite with curved or irregular crystal faces was directly crystallized from hydrothermal fluids(average temperature 192°C). Fine-medium dolomites are the products of recrystallization of micritic dolomite, resulting in abnormal geochemical characteristics, such as slight depletion of δ^(18)O, significant enrichment of Mn-Fe and ^(87)Sr/^(86)Sr, and positive Eu anomaly. A model for the distribution of various hydrothermal dolomite reservoir facies is proposed here, which incorporates three fundamental geological controls: 1) extensional tectonics and tectono-hydrothermal events(i.e., the Xingkai Taphrogenesis of Late Sinian-Early Cambrian, and Emei Taphrogenesis of Late Permian), 2) hydrothermal fluid storage in clastic rocks with large thickness(e.g., Nanhua System of Chengjiang Formation and part of Doushantuo Formation), and 3) confining bed for hydrothermal fluids(such as, the shale in Qiongzhusi Formation). The supply of hydrothermal fluid is critical. Large basement-rooted faults and associated grid-like fracture system may function as the channels for upward migration of hydrothermal fluid flow. The intersection of the above-mentioned faults(including the conversion fault), especially transtensional sags above negative flower structures on wrench faults can serve as a key target for future hydrocarbon exploration.
基金supported by the National Natural Science Foundation of China(Grant No.41002033)National Major Science and Technology Specific Project of China(2011ZX05030-003-02)+1 种基金Natural Science Key Project of Education Department in Sichuan(13ZA0058)Foundation for Fostering Middle-aged and Young Key Teachers of Chengdu University of Technology
文摘Based on a comprehensive study of texture,diagenetic behavior and evolution of dolomite in the Huanglong Formation,trace (e.g.,Fe,Mn and Sr) and rare earth element (REE) geochemistry,andisotopic characteristics (e.g.,C,O and Sr),four types of diagenetic fluids are identified in the Huanglong Formation dolomite reservoirs of the Eastern Sichuan Basin,China:1):marine-derived pore waters in the marine diagenetic environment,2) sabkha compaction brine conserved in the early shallowburied diagenetic environment,3) strongly-oxidizing low-temperature meteoric water in the seepagesubsurface flow diagenetic environment,and 4) strongly reducing deeply seated mixed hot brine in the middle and deep burial diagenetic environment.The fluids developed hereditarily from one environment to another,which resulted in its respective characteristics.Fluid characteristics play an important role in the development of dolomite reservoirs:1) dolomitization by marine-derived pore water in the quasisyngenetic stage did not form an effective reservoir; 2) early diagenetic burial dolomitization by the sabkha compaction brine formed the basis for reservoir development; 3) meteoric water karstification in the paleo-epidiagenetic stage expanded both the distribution and the size of the reservoirs,and improved the reservoir quality; 4) deep-burial dissolution and tectonic fracturing in the reburial diagenetic stage further improved reservoir porosity and permeability.
文摘This paper utilizes multi-methods,such as core observation,slice identification,isotope analysis,trace element analysis,fluid inclusion technique and so on,to study the causes of the dolomite in the Nanpu Sag and the origins of the dolomite reservoir.The study results show that the forming environment of dolomite in the Nanpu Sag is a fresh-water lake environment,and the dolomite is the product of dolomitization which is caused by volcanic thermal fluids in the Early Dongying period.With the development of intergranular pores in the dolomitization process,a lot of dissolved pores/vugs and fractures were formed by denudation in the later periods because of the influence of thermal fluids including the associated fluids of volcanic activities and the expelled hydrocarbon fluids of the source rocks.On the whole,these secondary dissolution spaces greatly enhance the reservoir ability of the dolomite,and there are enough reservoir spaces in the dolomite in the Nanpu Sag.
基金Supported by the National Natural Science Foundation of China(Grant No.U20A201009 and 41972157)PetroChina Science and Technology Major Project(Grant No.2016E-0204)。
文摘Based on seismic, drilling data and experimental analysis, the characteristics and main controlling factors of shoal dolomite gas reservoir in the third member of Ordovician Yingshan Formation of Gucheng area, Tarim basin were examined.The study shows that the dolomite gas reservoir in Gucheng area is lithologic gas reservoir controlled by shoal and fault jointly,and its formation is mainly attributed to the following factors:(1) The continuously developing paleotectonic structure has been in the direction of gas migration and accumulation;(2) The large area of medium-high energy grain bank deposited in gentle slope environment is the material basis for the formation of dolomite reservoir;(3) Atmospheric water leaching and dolomitization and fluid dissolution in fault zone are the key factors for the formation of high-quality dolomite reservoir;(4)The natural gas comes from cracking of the ancient oil reservoir and hydrocarbon generation of dispersed organic matter in source rocks, and the NNE-trending strike-slip fault is the dominant channel for vertical migration of natural gas;(5) Limestone cap rocks in the first and second members of Yingshan Formation provide direct sealing for the formation of gas reservoir there. On the basis of comprehensive analysis, it is pointed out that the Gucheng area has three grain shoal zones in the third member of Yingshan Formation in nearly S-N direction, which together with seven strike-slip fault zones in NNE direction control the development of shoal dolomite gas reservoir.
基金Supported by the China National Science and Technology Major Project(2016ZX05004-002).
文摘Regarding to the problem on the reservoir-cap rock assemblage evaluation in the carbonate-evaporite paragenesis system,this study examined the dolomite and reservoirs genesis and the characteristics of reservoir-cap rock assemblage.Based on the literature research of the global carbonate reservoirs and the case study on four profiles of carbonate-evaporite succession,together with geological and experimental work,three aspects of understandings are achieved.(1)Lithology of carbonate-evaporite paragenesis system is mainly composed of microbial limestone/bioclastic limestone,microbial dolomite,gypsum dolomite and gypsum salt rock deposited sequentially under the climatic conditions from humid to arid,and vice versa,and an abrupt climate change event would lead to the lack of one or more rock types.(2)There developed two kinds of dolomite(precipitation and metasomatism)and three kinds of reservoirs in the carbonate-evaporite system;and the carbon dioxide and organic acid generated during early microorganism degradation and late microbial dolomite pyrolysis process,and early dolomitization are the main factors affecting the development of microbial dolomite reservoirs with good quality.(3)In theory,there are 14 types of reservoir-cap rock assemblages of six categories in the carbonate-evaporite system,but oil and gas discoveries are mainly in four types of reservoir-cap rock assemblages,namely"microbial limestone/bioclastic limestone–microbial dolomite–gypsum dolomite–gypsum salt rock","microbial limestone/bioclastic limestone–gypsum salt rock","microbial dolomite–gypsum dolomite–gypsum salt rock"and"gypsum dolomite–microbial dolomite–tight carbonate or clastic rock".These four kinds of reservoir-cap rock assemblages should be related with the climate change rules in the geologic history,and have good exploration prospects.
基金Supported by the China National Science and Technology Major Project(2016ZX05004-002)PetroChina Science and Technology Project(2019-5009-16).
文摘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.
基金This study was supported by the National Science and Technology Major Projects of China(Grant Nos.2016ZX05004006-001-002 and 2016ZX05004002-001)PetroChina Science and Technology Project(Grant No.2019B-0406)the China Scholarship Council(No.201908080005)。
文摘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.
基金supported by grants from the National Natural Science Foundation of China(41972149,41890843).
文摘The deeply buried Lower Cambrian Longwangmiao Formation and Upper Ediacaran Dengying Formation from the Sichuan Basin,China,have a total natural gas reserve up to 3×10^(12)m^(3).The complex diagenetic evolution and their impacts on the present-day reservoir quality have not been systematically elucidated,hampering the current exploration.Crucially,the integration and comparation diagenetic study on these two formations,which may be able to shed new lights on reservoir formation mechanism,are yet to be systemically evaluated.By compiling geochemistry data,including carbonate U-Pb ages and petrophysics data,coupled with new petrology,trace elements,and strontium isotope data,of various types of diagenetic carbonates,this study aims to decipher the potential links between diagenesis and reservoir development of both formations.Intriguingly,similar diagenetic sequence,which contains five distinctive dolomite phases,is established in both formations.The matrix dolomite(D1)and early dolomite cement(D2)were likely formed by reflux dolomitization,as inferred by their nearly syn-depositional U-Pb ages and elevatedδ^(18)O caused by seawater evaporation.The subsequent moderate burial dolomite cement(D3)was most plausibly the product of burial compaction as indicated by its lighterδ^(18)O and slightly younger U-Pb ages compared with D1 and D2.Whereas deep burial dolomite cements(D4 and D5)yield markedly depletedδ^(18)O,elevated ^(87)Sr/^(86)Sr,along with much younger U-Pb ages and higher precipitation temperatures,suggesting that they were likely linked to hydrothermal fluids.Despite the wide occurrence of meteoric and organic acids leaching and thermochemical sulfate reduction,they may have only played a subsidiary role on these reservoirs development.Instead,superior reservoir quality is tightly linked to tectonics as inferred by higher reservoir quality closely related to the well-developed fractures and faults filled with abundant hydrothermal minerals.Notably,good reservoirs in both formations are mainly attributed to high permeability caused by tectonics.Hence,this new contribution emphasizes the crucial role of tectonics on spatially explicit reservoir prediction of deep to ultra-deep(up to>8000 m)carbonates in the Sichuan Basin,as well as other sedimentary basin analogues in China.
基金Supported by the National Natural Science Foundation of China(U22B6002)National Project for Oil and Gas Technology(2016ZX05-004)CNPC Science and Technology Project(2023ZZ02).
文摘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 shoalµbial 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.
基金supported by the National Natural Science Foundation of China(Grant Nos.41821002,41902131)the Program of the Major Science and Technology Program(Grant No.ZD2019-183-002)。
文摘Ancient dolomite reservoirs play an increasingly important role in deep oil and gas exploration.The mechanism of formation and preservation of dolomite reservoirs is complex,which is always the key issue.With the discovery of deep oil and gas in the Ediacaran dolomites of the world,the upper Ediacaran Qigebrak Formation in the Tabei area has begun to attract attention,but its reservoir space difference and formation mechanism have yet to be clarified.Based on ultra-deep drilling cores and field outcrops in the Tabei area,the lithofacies,reservoir space,and formation mechanism are systematically analyzed by macro to micro,and qualitative to quantitative petrology:(1)The types of dolomite can be divided into five major categories,including microbial dolomite,granular dolomite,residual granular dolomite,crystalline dolomite and karst breccias.(2)The main types of reservoir space are microbial-framework pores,microbial-mold pores,and non-fabric selective dissolution pores.Spongiomicrobialite,karst breccias,and fine-grained dolomite are the dominant reservoir rock types.(3)High-frequency sedimentary cycles and meteoric dissolution are the key factors of reservoir formation.Two sets of large-scale reservoirs are present:the first set is mainly controlled by the supergene karst of the Keping movement,and the second set is mainly controlled by high-frequency sedimentary cycles in the penecontemporaneous period.The reservoirs formed at the shallow burial stage and were preserved until the deep burial stage.(4)The quality of a deep reservoir depends on the geological events that affect the processes of pore reduction and increase.Cementation,compaction and pressure solution are the main destructive diagenetic processes;however,the reservoir space can still be effectively preserved under the influence of constructive diagenetic processes,such as meteoric dissolution and early dolomitization.This research has important theoretical and practical significance for revealing the formation mechanism of upper Ediacaran deep dolomite reservoirs in the Tarim Basin.