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The study on dolomite characteristics and its forming mechanism is of great significance for dolomitization theory and dolomite reservoir exploration. In recent years, the exploration of dolomite reservoirs in the Sichuan Basin has reached a bottleneck state. This study takes the Qixia Formation, Middle Permian in Northwest Sichuan basin as an example, the characteristics, distribution, diagenetic fluid, diagenetic environment of dolomite, and the dolomite formation mechanism were analyzed by means of observation and sampling of outcrops and cores, electronic scanning microscopy (SEM), and measurements of rare-earth elements (REEs), carbon isotopes, oxygen isotopes, and strontium isotopes. The results show that the dolomite exists in two forms, P-dol (dolomite in the dolomitic part of the mottled dolomitic limestone), and C-dol (crystalline dolomite). Core photos show that C-dol can be divided into two types: dolomite-A (dol-A) and dolomite-B (dol-B). The samples are low ∑REEs + Y, 13C (1.32‰–4.32‰), negative δ18O (− 5.87‰ to − 1.99‰) and δCe, and high in δEu and 87Sr/86Sr ratio (0.70724–0.70923). The partition pattern of REEs in the samples shows seawater characteristics of LREE loss, La enrichment, Ce weak loss, and Gd enrichment. These characteristics indicate that the diagenetic fluid of dolomite in the study area was a hybrid of seawater and freshwater and the diagenetic environment was at relatively high temperature. Moreover, after extending the “Mg cage effect” theory proposed by Folk, it is found that Mg2+ removed from sediments that were flushed by fresh water during the syndiagenetic stage facilitate the formation of dolomites. In accordance with the geological setting, a unique dolomitization model of the Middle Permian Qixia formation in the northwestern Sichuan Basin was built. The stratum in this model can be divided into four zones from top to bottom: freshwater vadose zone, mixed vadose zone, mixed phreatic zone, and seawater phreatic zone. The mottled dolomitic limestone is developed in the mixed vadose zone and the crystalline dolomite in the mixed phreatic zone. This study provides important evidences for the formation mechanism of dolomite and a reference to the study of ancient dolomite genesis and reservoirs of the Middle Permian Qixia formation in the northwest Sichuan Basin. [ABSTRACT FROM AUTHOR]

Great breakthroughs have been recently made in exploring oil and gas in the Middle Permian Qixia Formation, the southwestern Sichuan Basin, China. These discoveries in dolomitic reservoirs revealed abundant hydrocarbon reserves, agreeing on traditional predictions associated with dolomite prospect, which also left the controversy about the nature of dolomitization. Therefore, it is particularly important to research dolomite reservoirs characteristics and their genesis. In this study, comprehensive analyses, including pore-casted thin-section observation, scanning electron microscopy, physical property analysis, mercury injection measurement, fluid inclusion test, and isotopic measurement, are carried out to characterize Qixia Formation dolomites that are sampled from five outcrop sections and nine wells, followed by an investigation into controlling factors of these characteristics and establishment of pore evolution models. Granular dolomites, especially fine-grained and coarser dolomites, are demonstrated to be the best reservoir rocks, with their reservoir capacity positively related to dolomite grain sizes. There are mainly four types of pore-throat structures, namely large pores with wide throats, medium pores with medium throats, small pores with narrow throats, and fracture pore-throat structure. Dolomite reservoirs in the study area have experienced complex diagenetic changes, including the meteoric water dissolution in the penecontemporaneous stage, dolomitization in the shallow-burial stage, and hydrothermal- and formation water-related dissolution during the middle-deep burial stage. High-quality reservoirs are mainly distributed in the shoal facies deposits, especially where both dolomitization and multi-stage dissolution have occurred. [ABSTRACT FROM AUTHOR]

To elaborate genesis of the Ordovician Ma55 Sub-member dolomites in the Ordos Basin, this study implemented a series of analyses, including petrological investigation and geochemical (constant and trace elements, and isotopes) characterization. Three main types of dolomites were identified, namely micritic dolomite, micritic silty dolomite and silty dolomite. Specifically, the micritic dolomite is featured by dull red luminescence, low degree of order, high Sr isotope value, positive linear MgO–CaO correlation, high contents of Na and Sr elements, low contents of Fe and Mn elements, obvious negative δ13C excursion and slightly positive δ18O value, all of which indicate a syngenetic–penesyngenetic closed, high salinity environment. In comparison, the micritic silty dolomite is characterized by no or extremely dull red luminescence, moderate-to-high degree of order, higher δ13C value, slightly lower 87Sr/86Sr ratio, and sharp decrease in contents of K, Al, Fe and Mn elements, all of which indicate this type of dolomite to be the product of seepage–reflux dolomitization. Moreover, difference in geochemical characteristics between micritic silty dolomites in the depression and those in the platform flat suggests the Mg-rich brine from the tidal flat to be the dolomitization fluid for the platform flat while the Mg-rich brine from the overlying Ma54 Sub-member to be the dolomitization fluid for the depression. Meanwhile, the silty dolomite is observed to have no or extremely dull red luminescence, high degree of order, heavy δ18O value, high Sr isotope, lower Na content, and higher Fe and Mn contents, which are attributed to burial dolomitization in the fresh water environment. The distribution of dolomite was demonstrated to be controlled by sedimentary microfacies, and the dolomite in the West Jingbian dolomitic flat area turns out to have the largest distribution area and thickness as well as the best reservoir capacity. Massive fractures and vugs were formed in the dolomitization process, which were the basis of high-quality reservoir formation, involving generation of a relatively small amount of intercrystalline micropores during the syngenetic–penesyngenetic dolomitization, intercrystalline pores after the penecontemporaneous dolomitization, and intercrystalline and dissolution pores during the burial dolomitization. Moreover, dissolution during hypergene and burial periods contributed to most of the reservoir space, while filling effect during the burial period significantly reduced porosity. [ABSTRACT FROM AUTHOR]

This paper takes the thick and widely distributed crystal dolomite of Xixiangchi Formation, Upper Cambrian in Southeastern Sichuan basin as an example, the reservoir characteristics and main controlling factors of crystal dolomite have been analyzed systematically, and the genesis and reservoir formation mechanism of crystal dolomite have been discussed. It shows that the main reservoir rocks of crystal dolomite include powder crystalline dolomite and finely crystalline dolomites, and the intercrystal pores and dissolved intercrystal pores are the main reservoir spaces, with few dissolved vugs and fractures developed in the reservoir. The pore structure analysis shows that the throats of crystal dolomite reservoir are mainly lamellar, reflecting a good configuration between pores and throats, medium- low porosity and low permeability, representing typical porous-type reservoir. Dolomitization analysis indicates that the dolomitization occurred in the penecontemporaneous or early diagenetic stage. The degree of dolomitization decreased gradually from the top to the bottom in each sedimentary cycle, and distribution of dolomite in the study area shows that the content of dolomites decreases from the west to the east, which reflects the degree of dolomitization weakening eastward. Combined with the geochemical research of crystal dolomite, the C and O isotope data reflect the same characteristics of seawater during the late Cambrian. It indicates that the dolomitization might be related to the seepage-reflux of seawater in the early time. Recrystallization played an important role in the genesis of the crystal dolomite, because most of crystal dolomites remain residual granular texture. The intercrystal pores formed by seepage–reflux dolomitization acted as superior seepage channels for the reservoir transformation, and the recrystallization adjusted the previous pores and made them better, while the karstification was the key to the dissolved pore formation. [ABSTRACT FROM AUTHOR]

The Middle Triassic Leikoupo Formation in the northern Sichuan Basin is a new discovery sequence for strategy replacement in the gas exploration field. The carbonate paleokarst reservoirs of the Middle Triassic Leikoupo Formation in the northern Sichuan Basin provide a case study of gas exploration in carbonate paleokarst reservoirs. By conducting a comprehensive study of well drilling, drill cores, well loggings, and test data, combined with the analysis of geological background of the Sichuan Basin, it is considered that syndepositional, supergene, and burial paleokarstification can be identified in the Leikoupo Formation, and these types of karstification were vital for reservoirs formation. Syndepositional karstification controlled the formation and distribution of early reservoirs. This type of karstification was limited to high-energy grain shoals, and therefore its horizontal extent was limited by the distribution of this facies. During temporary episodes of relative sea level fall, carbonate shoals were exposed subaerially or within the fresh water lens, and significant selective dissolution occurs, forming moldic pores and intragranular dissolved pores. The reservoirs related to supergene karstification mainly occurred in the area of the buried hill containing carbonate rock at the top of the Leikoupo Formation, and are vertically limited within a depth of 70-90 m under the carbonate corrosion unconformity plane of the Leikoupo Formation. The reservoirs related to supergene karstification formed mainly in limestone breccia and vuggy limestone in surface karst zone, and the high-angle dissolved fracture and medium- and small-scale corrosion holes in vertical vadose zone, and medium and small corrosion holes, dissolved fractures, roof fracture-caves in horizontal karst zone. The karst monadnocks within karst slope are the most favorable microgeomorphic units for horizontal distribution of reservoirs. Burial karstification was related to dissolution of carbonate minerals by organic acids, and was the key factor in optimal transformation of reservoirs. Burial karstification corroded and expanded the pore system left after infilling of the earlier pore system, and mainly was to optimize the transformation the reservoir space redistribution of earlier pore system. The compact carbonate layers related to burial karstification only can be dissolved along the early cracks in the channel and formation cracked reservoirs, but development of this type of reservoir was limited. The reservoirs were influenced by syndepositional, supergene, and burial paleokarstification in the Leikoupo Formation, and carbonate paleokarst reservoirs were formed finally. [ABSTRACT FROM AUTHOR]

Longwangmiao Formation in Lower Cambrian is one of the key petroleum systems in Sichuan basin. The reservoir is well developed in the Longwangmiao Formation which was deposited in the restricted carbonate platform. It is indicated that the karstification occurred in the reservoir of Longwangmiao Formation during the Caledonian tectonic movement on the basis of seismology, petrology, sedimentology and geochemistry. Distinguishable markers of the paleo-karstification include: the lost strata and unconformable contact occurring between Middle-Upper Cambrian carbonates and Lower Permian Liangshan Formation; the occurrence of collapse breccias and fillings in caves and karrens; the evident difference between cave fill dolostones and matrix dolomite in the Sr/Ba ratio; and the log response of cavernous formations. The karstification lasted for a long time. According to the residual thickness between the top of Longwangmiao Formation and the bottom of the Permian, paleo-geomorphology during the Caledonian karstification had been recovered. In that time, Longwangmiao Formation was not directly exposed to the surface in central Sichuan basin; lots of meteoric fresh water flowed through faults and cracks from Gaotai Formation and a series of vertical dissolved caverns and karrens were formed. Meanwhile, due to the denudation window of Leshan-Longnvsi paleo-uplift lying in the west of the study area, meteoric fresh water flowed from the denudation window and the Longwangmiao Formation became a karstic confined aquifer. Under the pressure, diagenetic fluid migrated along the formation. Consequently, elongated dissolved pores and vugs formed and bedding karst reservoir developed in Longwangmiao Formation. The intensity of the karstification weakened with the distance from the paleo-uplift increasing, which indicates that the karstification was deeply influenced by the paleo-uplift. Finally, the supergene karstification model of Longwangmiao Formation has been established on the basis of the synthesis of the above-mentioned research results. [ABSTRACT FROM AUTHOR]

It is indicated in this paper that the karstification occurred in Block 12 of Tahe oilfield during Early Hercynian stage on the basis of identification features obtained through the latest research on seismology, petrology, and geochemistry. Distinguishable markers of the paleo-karstification mainly include: the lost strata and unconformable contact occurring between Lower-Middle Ordovician carbonates and Lower Carboniferous Bachu Formation, the occurrence of collapse breccias and fillings in subsurface stream, the evident difference between cave-fill calcite and matrix carbonate in the Sr/Ba ratio, and the log response for cavernous formations. The paleo-geomorphology of Early Hercynian karstification can be divided into four units: karst highland, I- and II-level karst slope, and karst basin in Tahe area, and the Block 12 was basically located in II-level karst slope. Three karst cycles developed in the Block 12 according to Early Hercynian tectonism characterized by multiphase pulse-type uplift movement and three sets of cavernous formation formed. II- and III-level cavernous formations mainly developed in Block 12. The Early Hercynian karstification is distributed on the north area of the pinch-out line of Upper Ordovician Sangtamu Formation in the plane, and is vertically developed in the Lower-Middle Ordovician carbonates. The karstification is essentially controlled by the paleo-geomorphology and fault. Fissures and caves formed by the karstification are the keys to affecting reservoir quality. The evolution model of Early Hercynian karstification has been established on the basis of the synthesis of the above-mentioned research results. It can be divided into three stages: rapidly uplifting and denudation stage, three episodes of uplifting and karstification, and subsiding and reworking stage. [ABSTRACT FROM AUTHOR]