Ordovician Majiagou Formation Research Articles

New understandings on gas accumulation and major exploration breakthroughs in subsalt Ma 4 Member of Ordovician Majiagou Formation, Ordos Basin, NW China

Geological conditions and main controlling factors of gas accumulation in subsalt Ma 4 Member of Ordovician Majiagou Formation are examined based on large amounts of drilling, logging and seismic data. The new understandings on the control of paleo-uplift over facies, reservoirs and accumulations are reached: (1) During the sedimentary period of Majiagou Formation, the central paleo-uplift divided the North China Sea in central-eastern of the basin from the Qinqi Sea at southwest margin of the basin, and controlled the deposition of the thick hummocky grain beach facies dolomite on platform margin of Ma 4 Member. Under the influence of the evolution of the central paleo-uplift, the frame of two uplifts alternate with two sags was formed in the central-eastern part of the basin, dolomite of inner-platform beach facies developed in the underwater low-uplift zones, and marl developed in the low-lying areas between uplifts. (2) From the central paleo-uplift to the east margin of the basin, the dolomite in the Ma 4 Member gradually becomes thinner and turns into limestone. The lateral sealing of the limestone sedimentary facies transition zone gives rise to a large dolomite lithological trap. (3) During the late Caledonian, the basin was uplifted as a whole, and the central paleo-uplift was exposed and denuded to various degrees; high-quality Upper Paleozoic Carboniferous—Permian coal measures source rocks deposited on the paleo-uplift in an area of 60 000 km2 providing large-scale hydrocarbon for the dolomite lithological traps in the underlying Ma 4 Member. (4) During the Indosinian—Yanshanian stage, the basin tilted westwards, and central paleo-uplift depressed into an efficient hydrocarbon supply window. The gas from the Upper Paleozoic source rock migrated through the high porosity and permeability dolomite in the central paleo-uplift to and accumulated in the updip high part; meanwhile, the subsalt marine source rock supplied gas through the Caledonian faults and micro-fractures as a significant supplementary. Under the guidance of the above new understandings, two favorable exploration areas in the Ma 4 Member in the central-eastern basin were sorted out. Two risk exploration wells were deployed, both revealed thick gas-bearing layer in Ma 4 Member, and one of them tapped high production gas flow. The study has brought a historic breakthrough in the gas exploration of subsalt Ma 4 Member of Ordovician, and opened up a new frontier of gas exploration in the Ordos Basin.

The moldic pore evolution of the Middle Ordovician sabkha dolostone in Ordos Basin, China: A study based on the petrographic and geochemical characteristics of pore fillings

In the Ordos Basin, the sabkha dolostone reservoir with moldic pores of the fifth Member in the Middle Ordovician Majiagou Formation (M5) dominated the gas production of the Lower Palaeozoic. Recent research studies revealed the complex history of the moldic pore evolution, which intensively affected the reservoir potential evaluation. The moldic pores were formed by dissolution of the gypsum concretion and then experienced a different extent of multi‐phase filling. In order to understand the evolution of the moldic pores and explain its affection on the development of reservoirs, comprehensive petrographic and geochemical analysis including the cathodoluminescent, scanning electron microscope, trace and rare earth elements in situ, and strontium isotopes have been utilized to study the moldic pore fillings. The results reveal the moldic pore evolution: (a) the gypsum concretions were dissolved and the moldic pores were formed in the early diagenetic stage; (b) the calcite fillings formed in the vadose zone and phreatic zone during the early diagenetic stage in the sabkha environment; (c) the ferroan dolomites formed during the process of conversion of montmorillonite to illite at the burial diagenesis stage, at the burial depth of 3,500 m–4,000 m in the middle‐Late Jurassic. Significantly, filling in the late burial period is more conducive to the pore preservation compared with the filling in the early diagenetic stage, and the formation of the sabkha dolostone reservoir lies in the preservation of pores in the early diagenetic stage. Consequently, the results of this study provide a framework to differentiate the petrographic and geochemical characteristics of kinds of pore fillings and to understand the moldic pore evolution and its affection on the development of reservoirs in similar sedimentary environments.

Restoration of formation processes of dolomite reservoirs based on laser U-Pb dating: A case study of Ordovician Majiagou Formation, Ordos Basin, NW China

To address the issue of non-unique interpretation of dolomite reservoir diagenetic and porosity evolution in the previous qualitative or semi-quantitative studies, we investigate two dolomite reservoir types, i.e. weathering-crust karstic reservoirs and mound-beach reservoirs, in the Ordovician Majiagou Formation, Ordos Basin by using in-situ laser ablation U-Pb dating as well as carbon and oxygen isotopic composition analysis. The results show that: (1) According to the dating of 8 reservoir samples, the Majiagou Formation experienced 5 diagenetic stages (Stage 1: deposition of matrix dolomite or penecontemporaneous dolomitization, in 444.0–494.0 Ma; Stage 2: dogtooth- or blade-shaped dolomite cementation, in 440.0–467.0 Ma; Stage 3: dolomitic silt filling, in 316.5–381.0 Ma; Stage 4: crystalline dolomite filling, in 354.0 Ma; Stage 5: crystalline calcite filling, in 292.7–319.0 Ma). (2) Supra-salt weathering-crust karstic dolomite reservoirs went through several diagenetic processes including penecontemporaneous dolomitization, compaction, weathering-crust karstification, packing, and rupturing in succession. Gypsum mold pores formed in the phase of hypergenic karstification and were filled with such minerals as dolomitic silts and calcites, and thus the porosity decreased from 10%–40% to 3%–8%. (3) Sub-salt mound-beach dolomite reservoirs went through the diagenetic processes including penecontemporaneous dolomitization, compaction, subsea cementation, penecontemporaneous corrosion, infiltration backflow dolomitization, packing, and rupturing. The porosity of reservoirs was originally 10%–30%, decreased to 0–6% due to seawater cementation, rose back to 5%–15% owing to penecontemporaneous corrosion, and finally declined to 2%–6% as a result of crystalline dolomites and calcites packing. The above methodology for the restoration of dolomitization and porosity evolution may be helpful for the restoration of porosity evolution in other basins or series of strata.

Combination of inorganic and organic geochemical characteristics to study the causes of carbon isotopic anomaly of natural gas: A case study from the central gas field of the Ordos Basin, China

Carbon isotopic anomaly occurs in the central gas field of the Ordos Basin (δ13C1 > δ13C2), and natural gas is mainly stored in the Ordovician Majiagou Formation dolomite (OMFD). Inorganic geochemical characteristics of the OMFD, including oxygen, carbon, strontium isotopes, order degree, and rare earth elements (REEs) were measured. Organic geochemical parameters, including natural gas and source rock, were also investigated. The causes of the carbon isotopic anomaly were studied based on the combination of inorganic and organic geochemical characteristics. The δ18O of the OMFD ranges from −11.9 to −5.8‰ with an average −8.4‰. Some δ18O values are more depleted than the average value of the Ordovician seawater, indicating that the dolomite is high-temperature burial dolomite, which is consistent with the fluid inclusion. The 87Sr/86Sr varies from 0.7088 to 0.7369, which is higher than the contemporaneous seawater. This suggests that the OMFD is high-temperature burial dolomite. The order degree changes from 0.72 to 0.98 with an average value of 0.89, which also indicates that the OMFD may have formed in the burial diagenesis stage at higher temperature. The REEs distribution patterns illustrate that the OMFD was influenced by the high-temperature hydrothermal alteration in the later period. The δ13C of some OMFD are more depleted than the Ordovician seawater, indicating that there was organic carbon mixing in the process of dolomitization. Inorganic and organic geochemical characteristics confirm that carbon isotopic anomaly of natural gas is the result of high temperature and mixing of the OMFD organic carbon.

Petrography and origin of dedolomites of the Ordovician Majiagou formation in the southeastern Ordos Basin, China: implications for reservoir quality

Dedolomites, the replacement of dolomite by calcite, are widely distributed in the Ordovician Majiagou Formation of the southeastern Ordos Basin, China, which critically affects reservoir quality throughout the region. Two types of dedolomites were recognized in the upper 100 m of the Majiagou Formation, namely brecciated dedolomite and crystalline dedolomite. The petrographic and geochemical data reveal that the investigated dedolomites are often associated with clay minerals and dissolution-collapse breccia, indicating their close relationship with erosional unconformities. The relative negative δ18O and δ13C values and relative high Fe, Mn contents of dedolomite with respect to corresponding dolomite are interpreted as being result from meteoric phreatic water involvement. The dedolomitization process related to subaerial exposure is conventionally interpreted to be formed by dolomite dissolution and calcite precipitation. The rhombic shape crystals in the brecciated dedolomite was formed by an Mg2+-loss process under evaporite solution conditions, which retain the rhombic shape of the dolomite. The crystalline dedolomites, however, were formed by dissolution/precipitation and recrystallization process to form medium to coarse calcite mosaic with very low intercrystalline porosity. The resulting fabrics are mainly controlled by the paleo-topography. The precursor dolomite of the crystalline dedolomite is primarily recrystallized mudstone, which is more soluble and favorable for calcite precipitation. The crystalline dedolomite mostly occurs in the low paleotopographic locations, where the karst-saturated water with respect to CaCO3 is more concentrated, occluding the remaining porosity when excess calcium is supplied. It’s, therefore, recommended to implement drilling in tectonic highland and avoid low paleotopographic locations.

The Accumulation Characteristics of the Paleozoic Reservoir in the Central-Southern Ordos Basin Recorded by Organic Inclusions

The Permian tight clastic reservoir and Ordovician carbonate reservoir were developed in the central-southern Ordos Basin. This study investigated the fluid inclusion petrography, diagenetic fluid characteristics, formation process of natural gas reservoir, source rock characteristics, and reservoir accumulation characteristics of these Paleozoic strata by petrographic observations, scanning electron microscope imaging, fluid inclusion homogenization temperature, salinity, laser Raman spectrum, and gas chromatograph analyses. The results have suggested two phases of fluid inclusions in both the Permian sandstone and the Ordovician Majiagou Formation dolomite reservoirs, and the fluid inclusions recorded the history from the early thermal evolution of hydrocarbon generation to the formation, migration, and accumulation of natural gas. The early-phase inclusions show weak yellow fluorescence and recorded the early formation of liquid hydrocarbons, while the late-phase inclusions are nonfluorescent natural gas inclusions distributed in the late tectonic fractures and recorded the late accumulation of natural gas. The brine systems of the Permian and Ordovician fluid inclusions are, respectively, dominated by CaCl2-H2O and MgCl2-NaCl-H2O. The diagenetic fluids were in the ranges of medium-low temperature and moderate-low salinity. The natural gas hydrocarbon source rocks in the Ordos Basin include both the Permian coal-bearing rocks and the Ordovician carbonates. The process of the early-phase liquid hydrocarbon formation and migration into the reservoir corresponded to 220 Ma (Late Triassic). The late large-scale migration and accumulation of natural gas occurred at 100 Ma (early Late Cretaceous), which was close to the inclusion Rb/Sr isochron age of 89.18 Ma, indicating that the natural gas accumulation was related to the Yanshanian tectonic movement.

Potential causes of depleted δ13Ccarb excursions in Ordovician marine carbonates, Ordos Basin, China

Variations in the stable carbon isotope signature of marine carbonate rocks (δ13Ccarb) are utilized as an indicator for changes in depositional environment and tracers of diagenetic alteration. Various petrologic (textural changes) and geochemical indicators (trace elements) have been used to evaluate the effect of diagenetic alteration on the δ13Ccarb signature. However, how diagenetic processes involving interactions between organic matter and minerals impact the δ13Ccarb records of marine carbonate rocks has not been investigated thoroughly. In this study, systematic stable carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotope analyses were conducted on marine carbonates from the Middle Ordovician Majiagou Formation in the Ordos Basin, China to evaluate secondary overprint of the primary δ13Ccarb record. 496 carbonate samples from the gas reservoir (potential gas source rock) were selected from one well located in the natural gas exploration target—mid-east part of the Ordos Basin. A depleted δ13Ccarb excursion (i.e., a shift to more negative values) occurred in the 100 m-thick anhydrite-bearing carbonate sequence of this formation. The δ13Ccarb and δ18Ocarb values for the whole rock vary from −10.9‰ to 1.8‰ and −11.5‰ to −4.0‰, respectively. The calcite veins and vug-filling calcites also have depleted carbon isotope signature ranging between −21.1‰ and −4.4‰. The secondary calcites observed under the microscope were generated by the conversion from isotopically depleted organic matter and resulted in the depleted δ13Ccarb signature. The commonly occurring thermochemical sulfate reduction in the study area and the degradation of potentially enriched carboxylic acid salts in anhydrite-bearing carbonate setting are major factors contributing to the depleted δ13Ccarb values. The conversion from organic carbon to inorganic carbon through organic matter-involved diagenetic processes plays a significant role in explaining the genesis of depleted δ13Ccarb excursions in deeply buried marine carbonate sequences.

Dolomitization Controlled by Paleogeomorphology in the Epicontinental Sea Environment: A Case Study of the 5th Sub-Member in 5 Member of the Ordovician Majiagou Formation in Daniudi Gas Field, Ordos Basin

The 5th sub-member in 5 Member Ordovician Majiagou Formation in Daniudi Gas Field, Ordos Basin, is deposited in an environment consisting of an ancient epicontinental sea, where very fine crystalline dolostone reservoir has developed. In this study, based on the petrological and geochemical characteristics, the genesis of the dolomite developed in M55 were studied by analyzing the properties and sources of the dolomitization fluids, and the influence of the paleogeomorphology differences on the distribution of dolostone was also discussed in order to clarify the distribution of the dolostone developed in the lime flat of the epicontinental sea. The dolostone of the M55 had a crystal structure, mainly including microcrystalline and very fine crystalline. The content of MgO and CaO in dolomite was negatively correlated, indicating that it was the result of replacement. The dolomite was dark red under cathode luminescence, and the distribution mode of rare earth elements showed the negative anomaly of Ce and Eu, indicating that the dolomitization fluid was sea-sourced fluid. The δ13C, δ18O, and 87Sr/86Sr isotope range of limestone was similar to that of Ordovician seawater in the study area, whereas the δ13C, δ18O, and 87Sr/86Sr of dolostone were obviously more positive than that of limestone. The substitute index of the salinity (Z) of the dolomitization fluid was higher than 122, which is higher than limestone (Z = 120.5), indicating that the dolomitization fluid was slightly evaporated seawater. The wormholes observed on the core and the gypsum in the penecontemporaneous period observed in the thin sections indicated that the dolostone was formed in a period when the sea level was relatively low, and it was the result of seepage–reflux dolomitization. By analyzing the correlation between the thickness of dolostone and the paleogeomorphology of the M55 of the sedimentary period, it was found that the thickness of dolostone at relatively high altitude was significantly larger than that of other areas. The development of dolostone was controlled by sea level, and the local paleogeomorphology controls the distribution of dolostone during the period of low sea level. There were many more limestone–dolostone cycles and larger cumulative thicknesses of dolostone at relatively higher topography. This study provides a theoretical basis for the prediction of the distribution of dolostone reservoirs in the carbonate tidal flat environment dominated by lime flats under the background of the ancient epicontinental sea.

Minerals Filling in Anhydrite Dissolution Pores and Their Origins in the Ordovician Majiagou Formation of the Southeastern Ordos Basin, China

Mold pore cementation is the key factor constraining the reservoir property in the study area. The anhydrite dissolution pores in the Ordovician Majiagou Formation of southeastern Ordos Basin are commonly filled by minerals such as dolomite, calcite, pyrite, and quartz accounting for more than 90% of the total molds resulting in significant porosity volume reduction. The anhydrite dissolution pores in the Jingbian Gas Field in the middle east of the basin, however, are rarely filled by minerals with more than 30% molds, remaining open to become good reservoir space. Studies reveal that the calcite filling in anhydrite dissolution pores has a relatively negative δ18O value (-15.58‰~-8.96‰ VPDB) and negative δ13C value (-7.56‰~0.26‰ VPDB), which is interpreted to be caused by thermochemical sulfate reduction (TSR). The higher homogenization temperatures (140-234°C) and high salinity (19.13-23.18 wt.% NaCl equivalent) of the primary inclusions in calcite confirm the above interpretation. Dolomite is the second most abundant carbonate formed as by-product of TSR, which is promoted by the precipitation of calcite and resulted enriched in Mg2+/Ca2+ ratio in the pore water. Pyrite forms by the reaction of H2S released from TSR with the Fe2+ in the horizon, which is supported by its cubic habit and relatively high δ34S value (10.50‰~24.00‰VCDT). Quartz with relatively high homogenization temperature (113-154°C) is considered to precipitate in low-pH solution from calcite and pyrite precipitation after TSR. The southeastern Ordos Basin is much lower than the Jingbian Gas Field in paleogeographic location, which is submerged in the sea water of marine phreatic environments for a long time when sea water flooded from the southeastern direction. TSR occurs due to calcium sulfate enriched in pore water resulting in the minerals of dolomite, calcite, pyrite, and quartz filling in the molds leading to the low porosity and permeability of the study area.

Evaluation of carbon dioxide storage in the deep saline layer of the Ordovician Majiagou Formation in the Ordos Basin

Geologic CO2 storage (GCS) is one of the preferred solutions to reduce greenhouse gas emissions. Comparing with other geological reservoirs, the deep saline aquifers have huge storage potential and currently the most promising one. Since technical and economical viability of geologic CO2 storage depends highly on the CO2 storage capacity, this paper mainly uses the storage mechanism method to estimate the CO2 storage capacity of the Ordovician Majiagou Formation in order to give data support for GCS project in the Ordos Basin. The results show that the total effective CO2 storage capacity of the deep brine layer of the Ordovician Majiagou Formation in the Ordos Basin is 15.98-109.22Gt. The Majiagou Formation has sufficient storage capacity to accommodate decades of CO2 emissions generated by multiple coal-fired power plants in the Ordos Basin. For CO2 geological storage, the structural trapping mechanism contributes the largest amount of CO2 storage, and the amount of CO2 storage contributed by the two mechanisms of bound gas trapping and solubility trapping is almost negligible. The above results provide methods and references for CO2 estimation of GCS project in this region, and provide data support for the site selection and injection of CO2 sequestration in deep salt layers of the Ordos Basin.

Dolomitization of the Middle Ordovician Ma55 Sub-member of the Majiagou Formation and implications for hydrocarbon exploration in the northern Ordos Basin, NW China

The Middle Ordovician Majiagou Formation in the northern Ordos Basin consists of a sequence of subtidal and peritidal carbonates that are extensively dolomitized. The diagenetic evolution of the Ma55 Sub-member was investigated to determine the factors controlling reservoir quality in the northern Ordos Basin. Dolomite in the Ma55 Sub-member is classified into three main types on the basis of crystal size: dolomicrite (D1), micritic–silty and fine silty dolomite (D2), and silty (and coarser) dolomite (D3). The Dolomite D1 was produced by dolomitization in a low-salinity diagenetic environment and transformed by near-surface freshwater leaching during shallow burial. The Dolomite D2 developed as a result of post-penecontemporaneous seepage–reflux dolomitization and was affected by later burial dolomitization. The Dolomite D3 was produced during burial dolomitization. Although the original porosity of the Dolomite D1 was low, freshwater diagenesis increased the porosity significantly. As a result of the transformation of montmorillonite to illite, there was a deficiency in the Mg2+ necessary for Dolomite D2 formation, resulting in the nonuniform dolomitization of this type of dolomite. The intercrystalline pores of the Dolomite D2 are mostly filled with montmorillonite and illite, reducing its porosity and permeability and consequently making the Dolomite D2 a poor reservoir. The Dolomite D3 has a higher porosity and contains various types of dissolution and intercrystalline pores, generating a higher permeability. This dolomite is a potential reservoir in the study area and a possible target for oil and gas exploration. Overall, the results of the study suggest that the quality of a potential dolomite reservoir is strongly controlled by the diagenetic history of the host carbonates.

New Discovery and Significance of Microbialites of the Middle Ordovician Majiagou Formation in Liujiang Basin, Hebei, China

The concept, classification, genesis, and significance of microbialites have been constantly improving for over a century, but they are still hot spots in the field of geology because of the strong attractions and many unsolved mysteries such as the mechanism of the microbial deposition process, the microbial induced sedimentary structures, and so on. At the same time doubt and confusion arises among students and beginners because of the mysteries. Generally, any findings of various stromatolites, oncolites, and thrombolites are all classified under microbialites. Moreover, microbialites have also been discovered from the middle Ordovician Majiagou formation in Liujiang Basin, Hebei, China. As an important geological practice base for thousands of students and beginners every year, in order to enhance the interest in geosciences the further research of the microbialites found in Liujiang Basin is necessary. In this study, the classification scheme of microbialites has been summarized. On this basis, the study shows that the main microbialites in the study area are microbial carbonate rocks, which include wavy, columnar, conical-shaped stromatolite carbonate rocks and oncolite carbonate rocks. The variety, macroscopic characteristics, and clear internal features of microbialite carbonate rocks are all have considerable significance in judging hydrodynamic conditions, sedimentary environment, paleogeographic characteristics, and biological evolution. And the evolutionary process of different kinds of microbialites has been discussed. However, the roles of microbialites in the formation mechanism and ecological significance are still need further study. With the new discovery of microbialites from the middle Ordovician Majiagou formation in Liujiang Basin, a new window of study microbialites has been opened for everyone. More findings and detailed characteristics of microbialites in this area are urgently needed during geological practice.

Eogenetic karst and its control on reservoirs in the Ordovician Majiagou Formation, eastern Sulige gas field, Ordos Basin, NW China

Abstract To further ascertain the origin of the Ordovician Majiagou Formation reservoirs in the Ordos Basin, the M54-M51 sub-members of the Ordovician Majiagou Formation in the eastern Sulige gasfield of Ordos Basin were taken as examples to analyze the vertical development characteristics of eogenetic karst and to discover the dissolution mechanism and its control on reservoirs through observation of a large number of cores and thin sections. According to detailed analysis of petrologic characteristics, the reservoir rock types include micritic dolomite, grain dolomite and microbialite which have mainly moldic pore, intergranular (dissolved) pore, and (dissolved) residual framework pore as main reservoir space respectively. The study area developed upward-shallowing sequences, with an exposure surface at the top of a single upward-shallowing sequence. The karst systems under the exposure surface had typical exposure characteristics of early dissolution and filling, indicating these reservoirs were related to the facies-controlled eogenetic karstification. With the increase of karstification intensity, the reservoirs became worse in physical properties.

Reasons for carbon isotope rollover in the Yan'an gas field of the southern Ordos Basin, China: Evidence from the geochemical comparison of gas from fluid inclusions with wells

Samples for inclusion analysis were collected to investigate reasons for carbon isotope rollover in the Yan'an gas field (YAGF). Homogenization temperature of fluid inclusions, constituents and carbon isotope of gases in fluid inclusions were measured. Geochemical parameters of gas in wells and fluid inclusions in the northern gas fields (NGF) and YAGF were compared. The results show that geochemical characteristics of well and fluid inclusions gas in the NGF are almost the same, suggesting that it was originated from one set of source rocks. The direct reason for carbon isotope rollover in the YAGF is the depleted ethane carbon isotopes, while the methane carbon isotopes do not change much compared with the primary gas. There was a period of high temperature fluid filling and accumulation (140–160 °C) in the YAGF. The higher thermal maturity has disparate effects on methane and ethane. Higher thermal maturity is an important inducement of carbon isotope rollover. Mixture of oil-derived gas generated from the marine Ordovician Majiagou Formation will also result in carbon isotope rollover.

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 have δ13C 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 lower δ18O 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 lowest δ18O 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.

Differences in evaporite geochemistry of the greater Ordos Ordovician salt basin, China: evidence from the M56 submember of the Majiagou Formation

Ordovician evaporites are very rare in the global geological record. Thick salt deposits in the Ordovician Majiagou Formation are present in the eastern greater Ordos Basin, China. Potash mineralization layers and a thin industrial grade potash layer occur in the sixth submember of the fifth member of the formation, thus indicating a good potash prospect. The eastern and western salt depressions are separated by an uplift and have been divided into five secondary salt sags (East-1, East-2, West-1, West-2, and West-3). The ratio of the salt layer thickness to that of the whole succession was very low in the West-2 sag and the salt depositional center was the East-2 sag, which implied that seawater entered the basin from the western sags and led to desalination and the formation of deep-water sediments. Analyses indicated that the potash formation conditions were obviously different in the salt sags. Although a thin potash layer was present in the East-1 sag, findings indicated that this area was not favorable for potash accumulation. The geochemical and sedimentary environment of the East-2 sag was similar to that of typical potash basins globally. Rounded sylvine grains occurred between salt crystals. The preservation characteristics indicated that evaporite deposits had reached the KCl bitterns stage. An eastward input of terrestrial materials carried a large amount of clay, and the clay layer protected previously deposited sylvine particles, thereby avoiding further potassium dissolution. Sedimentary, geochemical, and petrological evidences indicate good prospects for potash exploration in the East-2 salt sag.

The effect of hydrothermal fluids on Ordovician carbonate rocks, southern Ordos Basin, China

Hydrothermal processes have an important influence on the spatial distribution and formation of carbonate reservoirs, and play a critical role in certain deep reservoirs where hydrothermal fluid-rock interaction may result in permeability creation due to carbonate dissolution or permeability destruction due to the deposition of cementing minerals. This study analyzed certain hydrothermal indicator minerals (quartz, celestite, multi-stage calcareous fillings) in carbonate rocks from the Ordovician Majiagou Formation, southern Ordos Basin, China. The petrological and geochemical characteristics of these indicator minerals imply that the Majiagou Formation was affected by two metasomatic events related to the Hercynian–Indosinian and Yanshanian orogenies and linked to magmatic-hydrothermal fluids derived from basaltic melts. Most of the δCe and δEu values obtained from matrix dolomite samples were lower than one, but U/Th and V/C ratios showed positive anomalies. The δ13C and δ18O values of the matrix dolomite were concentrated between −12.8‰ and −6.0‰, whereas δ13C showed a bipolar distribution at the same depth. Elemental geochemistry results showed that the Ordovician carbonate rocks were subjected to freshwater leaching followed by evaporation and widespread hydrothermal dissolution successively. As a result of the multi-episodic hydrothermal activity that affected the southern Ordos Basin and the path of migration channel, a parental magmatic-hydrothermal fluid (fluid 1) that had been generated during the underplating of the southern North China Craton by Mesozoic alkaline basalt melts, separated into two types of fluids with different chemical properties: an acidic, hydrocarbon-bearing fluid (fluid 2a) mixed with Cambrian oil and gas and a weakly to moderately alkaline brine (fluid 2b), not in contact with hydrocarbons. Both fluids 2a and 2b then interacted with the Ordovician carbonate rocks with fluid 2a having undergone hydrothermal sulfate reduction with SO42- in the layers in which gypsum was developed, and convection of fluid 2a having resulted in brine formation. The reaction products of H2S and CO2 together with organic acid co-dissolved carbonate minerals in the Ordovician strata played a major role in the formation of a high porosity reservoir. Conversely, owing to the progressive reduction of temperature and solubility during fluid ascent, fluid 2b precipitated quartz-calcite fillings in the residual pore spaces, resulting in the localized destruction of the original reservoir permeability. In summary, this study analyzed the petrological and geochemical characteristics of 60 carbonate rock drill core samples, established the likely formation mechanism of fluids 2a and 2b, and determined the effects of these hydrothermal fluids on the Ordovician Majiagou Formation carbonate reservoir.

Ichnofabrics and their roles in the modification of petrophysical properties: A case study of the Ordovician Majiagou Formation, northwest Henan Province, China

The carbonate rocks of the Ordovician Majiagou Formation from the northwestern part of Henan Province and their diverse trace fossils provide an outcrop analogue for bioturbated, low porosity and low permeability carbonate reservoirs. Three ichnofabrics can be defined based on the ichnotaxonomy, ichnodiversities, individual abundances and overall bioturbation indices. The Balanoglossites ichnofabric represented medium to relatively high-energy sedimentary environments such as intertidal zone and subtidal shoals in the carbonate platform. The Planolites ichnofabric suggested relatively low-energy, protected sedimentary environments such as lagoons in a restricted carbonate platform, but also occurs in the deeper-water depressions in the open carbonate platform. The Chondrites ichnofabric characterized low-energy and reduced oxygen conditions, with burrows penetrating below a shallow redox interface in deeper-water settings that include the distal slope as well as local depressions in the open carbonate platform. Our analysis suggests that the modifications of petrophysical parameters are controlled or influenced by the ichnofabric characteristics (burrow architecture, burrow cross-cutting relationship, degree of bioturbation and burrow connectivity), the permeability contrast between burrow-fills and matrix, and the dolomitization of burrow-fills. The micritic host sediments have negligible porosity and permeability. The bioturbated sediments have significant differences in fabric, texture, porosity and permeability. The burrow-fills are very commonly dolomitised. Each of the burrow-types can enhance reservoir properties, but the greatest differences are associated with the Balanoglossites ichnofabric, in which porosity and permeability can reach over 2% and 70 mD. Preliminary stratigraphic studies indicate that reservoir potential in the subsurface will be linked to an understanding of the distribution of shoal and intertidal facies as well as their associated Balanoglossites ichnofabrics.

Evolution of diagenetic system and its controls on the reservoir quality of pre-salt dolostone: The case of the Lower Ordovician Majiagou Formation in the central Ordos Basin, China

The evolution of diagenetic system and its impact on the carbonate reservoir quality are of paramount importance. New investigation of the Lower Ordovician Majiagou Formation, central Ordos Basin, China, has revealed that the diagenesis and quality of pre-salt dolostone reservoirs were controlled by the evolution of diagenetic system. Three different reservoir types (grain, crystalline and algal dolostone) and five kinds of reservoir space (intergranular pore, intragranular pore, intercrystalline pore, dissolution pore and vug in karst system, and parallel dissolution pore of laminar algae) are identified in the Majiagou Formation pre-salt dolostone of the central Ordos Basin, based on the petrographic characteristic. The diagenetic process of pre-salt dolostone can be divided into four stages: the penecontemporaneous diagenesis, the early diagenesis, the epidiagenesis and the mesodiagenesis-late diagenesis. Petrographic and geochemical analyses of the studied dolostone reservoir indicated that the diagenetic system was open during the penecontemporaneous diagenesis and became closed during the early diagenesis. Open and closed diagenetic system existed in the west and east of the central Ordos Basin during the epidiagenesis, respectively. After that, the western and eastern pre-salt dolostone reservoirs both evolved into a closed diagenetic system during the mesodiagenesis-late diagenesis. Subaerial meteoric dissolution in an open diagenetic system (secondary pore formation), and subsequent pore preservation in a closed diagenetic system, are the critical factors in the improvement of pre-salt dolostone reservoir quality. Although two cycles from open to closed diagenetic system developed in the western dolostone reservoir, the pore space there was filled with overlying weathering mud in the second open diagenetic system during the epidiagenesis, which reduced the reservoir quality. The eastern dolostone reservoir experienced just one cycle from open to closed diagenetic system, the reservoir quality was improved and then preserved. Thus only a good match among the diagenetic systems during different diagenetic stages will be good for the reservoir formation and preservation. Therefore, the potential high-quality reservoirs in the pre-salt dolostones mainly located in the eastern part of the central Ordos Basin.

Genesis of dolomite in the upper assemblage of the Ordovician Majiagou Formation in the southeastern Sulige gas field, Ordos Basin, China: Evidence from C, O, and Sr isotopes and major and trace elements

Typical dolomite reservoirs exist in the upper assemblage of the Ordovician Majiagou Formation in the southeastern Sulige gas field, Ordos Basin, however, the current understanding of dolomite genesis needs to be clarified. This study confirms the dolomitization characteristics, diagenetic environment, and genesis of dolomite through the use of core observations, thin section identification, and geochemical data (carbon and oxygen stable isotopes, strontium isotopes, and major and trace element contents). The results showed that dolomite samples from the study area includes micritic dolomite, powder crystal dolomite, and micrite to powder crystal dolomite, in which the main reservoir space consisted of intergranular pores, intercrystalline pores and various dissolved pores. The δ13C value of the dolomite samples ranged from −7.26‰ to1.28‰ with an average of −0.45‰, which is equivalent to that of seawater during the Ordovician. The δ18O value ranged from −9.94‰ to −6.32‰ with an average of −7.86‰, which is clearly more negative than that of the Ordovician seawater. The 87Sr/86Sr ratio varied from 0.70867 to 0.71033 with an average of 0.70946, which is significantly more positive than that of Ordovician seawater. The Mg/Ca ratio was lower than that of the seawater and close to 1:1. Overall, all of the samples were characterized by high Fe and Mn contents, while low Sr and Na contents. These results clearly indicate that the dolomitized fluid was closely related to seawater or a sea-source fluid. Furthermore, the restored paleosalinity, paleotemperature, and paleodepth also indicate that the dolomite was formed in a marine reducing environment. Combined with the petrological characteristics and geochemical indicators, two dolomitization models (mixed water dolomitization and burial dolomitization) were established for the upper assemblage of the Ordovician Majiagou Formation in southeastern Sulige gas field. We infer that mixed water dolomitization was dominant before the Carboniferous, whereas burial dolomitization was dominant after the Early Permian.