Dolomite Reservoir Research Articles

Petrophysical properties prediction of deep dolomite reservoir considering pore structure

Petrophysical properties prediction of deep dolomite reservoir considering pore structure

Enrichment Mechanism and Prospects of Deep Oil and Gas

AbstractWith the deepening of oil and gas exploration, the importance of depth is increasingly highlighted. The risk of preservation of storage space in deep reservoirs is greater than that in shallow and medium layers. Deep layers mean older strata, more complex structural evolution and more complex hydrocarbon accumulation processes, and even adjustment and transformation of oil and gas reservoirs. This paper systematically investigates the current status and research progress of deep oil and gas exploration around the world and looks forward to the future research focus of deep oil and gas. In the deep, especially the ultra‐deep layers, carbonate reservoirs play a more important role than clastic rocks. Karst, fault‐karst and dolomite reservoirs are the main types of deep and ultra‐deep reservoirs. The common feature of most deep large and medium‐sized oil and gas reservoirs is that they formed in the early with shallow depth. Fault activity and evolution of trap highs are the main ways to cause physical adjustment of oil and gas reservoirs. Crude oil cracking and thermochemical sulfate reduction (TSR) are the main chemical modification effects in the reservoir. Large‐scale high‐quality dolomite reservoirs is the main direction of deep oil and gas exploration. Accurate identification of oil and gas charging, adjustment and reformation processes is the key to understanding deep oil and gas distribution. High‐precision detection technology and high‐precision dating technology are an important guarantee for deep oil and gas research.

Exploration field analysis and zone optimization of sinian, Tarim Basin, China

Carbonate rock from the Late Proterozoic to the Early Paleozoic is an important field of oil and gas exploration. The Sinian carbonate rock series in Tarim Basin are old, deep buried, with few drilling wells and various geological problems are unclear that restrict the exploration of deep carbonate rock. Based on the analysis of Sinian geological structure, by means of stratigraphic correlation and seismic prediction, the distribution of Sinian favorable source rocks, deep reservoir facies belts, and reservoir forming assemblages are studied to comb the exploration fields and favorable zones of Sinian in Tarim Basin. The findings reveal that slope-basin facies source rocks developed in Sinian. In the upper part of the Qigebulake Formation, high-quality dolomite reservoirs evolved, and mudstone from the Yuertus Formation at the foot of Cambrian can form favorable reservoir cap assemblage. The south slope of Tabei Uplift and the north slope of Tazhong Uplift are the most favorable exploration zones for Sinian dolomite, favorable exploration area of approximately 31,000 km2. The findings can provide a certain reference for the Sinian carbonate oil and gas exploration in the Tarim Basin.

Dolomite genesis and dolomitization mechanisms of the Ordovician lower Yingshan Formation, Gucheng area, Tarim Basin, China

The massive dolomite reservoirs of the Ordovician formations in the Tarim Basin have significant gas exploration potential, but their occurrence and distribution are not clearly understood. This study collected core samples covering regional dolomite reservoirs from the lower Yingshan Formation in the Gucheng area. A combination of petrology, fluid-inclusion assemblage (FIA) microthermometry, and isotope geochemistry data were used to define the dolomitization process and its impact on reservoirs. Petrological and geochemical analyses reveal that various dolomites have different dolomitization mechanisms. In (pene)contemporaneous (PC) stage, on the western side of the east Tarim Platform, laminated dolomicrite (LDm) of the third Member (3rd Mm) was formed by microbial and sabkha dolomitization, whereas dolomicrite (Dm) of the fourth Member (4th Mm) generated from sabkha dolomitization in the slightly elevated salinity setting in an evaporative sea environment. The residual structural dolomite (RSD) of the 3rd and 4th Mm was characterized by reflux fluids on the central platform margin. The shallow-middle burial (SMB) stage was dominated by reflux dolomitization, precipitated structureless dolomite (SD) primarily associated with RSD. Bimodal geochemical characteristics indicate that dolomitizing fluids gradually transitioned to pore water consisting of evaporated seawater reserved in the strata. In the middle-deep burial (MDB) stage, hydrothermal burial effects were superimposed and transformed the dolomites near the fractures. Leopard dolomite (LD) in the 4th Mm is limited to faults and has seawater and hydrothermal geochemical characteristics, controlled by the superimposition of pore water in SMB stage and hydrothermal fluids in MDB stage. Massive dolomite (MD) in 3rd Mm, which is not completely limited to the fracture system, was derived from multiple superimposed dolomitization events and transformed by terrestrial water. Finally, the effect of dolomitization on reservoirs was investigated. It is concluded that the sabkha and reflux dolomitization in PC stage was the primary dolomitization mechanism and was the basis for the pore formation.

Identification of High-Quality Transverse Transport Layer Based on Adobe Photoshop Quantification (PSQ) of Reservoir Bitumen: A Case Study of the Lower Cambrian in Bachu-Keping Area, Tarim Basin, China

The Lower Paleozoic carbonate reservoir in the Tarim Basin is a hotspot area for deep oil and gas exploration in China. Although the Lower Cambrian of the Bachu Uplift has not encountered industrial oil flow, rich bitumen has been found there. As the most direct trace of petroleum migration, the effective identification of bitumen is the key to studying the hydrocarbon transportation path. In this study, the Adobe Photoshop quantification (PSQ) method is used to identify the bitumen content in the Xiaoerblak Formation in Well Shutan 1, and, combined with the bitumen characteristics of the Shihuiyao section, a high-quality petroleum transverse transport layer is determined. The results indicate the following: (1) In Well Shutan 1, bitumen is mainly concentrated in the middle and upper parts of the Xiaoerblak Formation with high porosity and high permeability. (2) The shale of the Yuertus Formation in the Shihuiyao section has low hydrocarbon generation potential. However, the overlying Xiaoerblak Formation has developed multistage bituminous veins and bitumen-encapsulated gravels, which is the result of multiple instances of horizontal hydrocarbon migration. (3) After combining the bitumen characteristics of Well Shutan 1 and the Shihuiyao section, it is confirmed that there are high-quality lateral transport conductors in the middle and upper parts of the Xiaoerblak Formation, and the Subsalt Cambrian dolomite reservoir has great exploration potential.

Fluid Inclusion Evidence for Oil Charge and Cracking in the Cambrian Longwangmiao Dolomite Reservoirs of the Central Sichuan Basin, China

The Cambrian Longwangmiao gas reservoirs in the Moxi area of the Sichuan Basin have a complex associated gas accumulation history. Based on core and thin section examination, fluid inclusion analyses and 1-D burial-thermal modelling, the diagenetic evolution and hydrocarbon accumulation processes in the Longwangmiao Formation have been reconstructed. Various diagenetic events were identified, making up a complete pore fill sequence as follows: solid bitumen/dolomite→ nonfluorescent solid bitumen → dolomite → quartz/yellow fluorescent oily bitumen → residual hole. Analyses of oil inclusions and bitumen-bearing inclusions are key to the understanding of the hydrocarbon accumulation processes. The Th values of the aqueous inclusions that are contemporaneous with hydrocarbon inclusions range from 74.3 to 214.3°C. In conjunction with burial-thermal history modelling results, the results indicate that there were two stages of oil charge and three stages of natural gas accumulation in the Longwangmiao carbonate reservoirs. The two stages of oil charge occurred in the Late Silurian and Middle Triassic, respectively. Three gas accumulation events occurred in the Middle to Late Triassic, Middle Jurassic to Early Cretaceous, and Late Cretaceous, respectively.

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.

On the evaluation of asphaltene adsorption onto dolomite surface: The roles of flow condition, composition of asphaltene, and dolomite size

Asphaltene precipitation/deposition is one of the main problems in petroleum industry at various stages. In this study, various controlling factors on the adsorption of different types of asphaltene on the surface of dolomite of different size, as one of the most abundant minerals in the reservoir is investigated. First, asphaltenes and adsorbents (dolomite) properties were analyzed via several analytical techniques. Then, adsorption of four asphaltene samples of various origins and chemical composition on dolomite at micron and nano size were carried out in both static and dynamic modes and results were examined following the adsorption process. Based on the static tests, the adsorption amount of asphaltenes on nano- and micro-dolomite was measured 3.52–11 mg/m2 and 0.82–2.74 mg/m2, respectively. The results revealed that the most significant parameters that would affect the asphaltene adsorption on dolomite as an adsorbent are the total nitrogen and sulfur (N + S) content and the aromaticity of the asphaltenes. Moreover, as the flow rate was increased three times in the dynamic experiments, a decrease of 15–25% in asphaltene adsorption was observed, which was attributed to physisorption of asphaltene on dolomite or due to higher flow velocities. Collectively, the findings of this study can enable us to better understand the mechanisms of asphaltene adsorption/precipitation on dolomite and lead to better management of oil production from dolomitic reservoirs.

Characteristics and Formation Mechanism of the Lower Paleozoic Dolomite Reservoirs in the Dongying Depression, Bohai Bay Basin

The Lower Paleozoic carbonate strata experience multi-stage tectonic activity and post-depositional volcanic activity in the Dongying Depression, Bohai Bay basin. These tectonic and magmatic activities have caused the reservoir to undergo severe diagenesis, resulting in strong reservoir heterogeneity. This study aims to identify the characteristics of dolomite, various reservoir spaces’ characteristics, the origin of different types of dolomite, and the porosity evolution. According to crystal size and morphology, dolomites can be divided into three kinds of matrix dolomites and four kinds of dolomite cements. The petrology and geochemistry of the dolomite suggests that matrix dolomite is formed from seawater. The medium-to-coarse-crystalline dolomite cement (D3) has a higher 87Sr/86Sr ratio (0.7119 to 0.7129) and a higher homogenization temperature (>125 °C), suggesting that the fluid for the precipitation of D3 is a mixed fluid formed by hydrothermal fluid eroding the 87Sr-rich feldspar sandstone. The strikingly negative δ18O values (−23.7 to −25.7‰ VPDB) of saddle dolomite (D4) indicate that D4 precipitated from hydrothermal fluids and the Mg2+ source may be due to dissolution of the host dolomite that formed in the evaporation environment. The reservoir spaces of the target strata in the study area mainly include fractures, dissolution vugs, intercrystalline pores, and moldic pores. Dissolution is the basis for forming high-quality dolomite reservoirs. The faults and fractures provided favorable conditions for dissolution. Hydrothermal fluid and organic acid were the main dissolution fluids for the dolomite reservoir, which were beneficial to the development of secondary pores. In the study area, organic acid dissolution was shown to contribute more than hydrothermal dissolution in the study area.

Characteristics and diagenetic evolution of dolomite reservoirs in the Middle Permian Qixia Formation, southwestern Sichuan Basin, China

Characteristics and diagenetic evolution of dolomite reservoirs in the Middle Permian Qixia Formation, southwestern Sichuan Basin, China

Processes controlling the hydrochemical composition of geothermal fluids in the sandstone and dolostone reservoirs beneath the sedimentary basin in north China

The Tianjin and Kaifeng geothermal areas are typical sedimentary geothermal systems located in North China Plain. These systems have great potential for geothermal development because the main reservoirs comprise dolomite and sandstone lithologies, distribute widely and have thick aquifers. This study investigated the water-mineral reaction processes that determine the composition of the thermal fluids in the dolomite and sandstone reservoirs. The results indicate that the reservoirs in Kaifeng and Tianjin are mid-low geothermal systems, with the highest reservoir temperatures 76.88 °C and 107.2 °C, respectively. The Kaifeng Neogene sandstone reservoirs could be treated as conductive geothermal systems, while the Mesoproterozoic dolomite reservoirs in the Tianjin geothermal area may be convective geothermal systems. Also, we conclude that the geothermal waters in the Tianjin and Kaifeng geothermal areas mainly originate from meteoric water, and isochemical dissolution of rocks is the main hydrochemical process in the dolomite or sandstone reservoirs, even though partial equilibrium existed in some cases. In the sandstone reservoirs, the K–Mg ion exchange system equilibrates more easily. However, the Ca–Mg equilibrium, determined by calcite-dolomite and chalcedony equilibria, seems common in the dolomite reservoirs. Additionally, in Kaifeng, apart from rocks dissolution, Ca and Mg are exchanged by Na adsorbed in the clay minerals, resulting in Na enrichment of the thermal fluids. However, in Tianjin's reservoirs, both alteration of Na-bearing silicate mineral and halite dissolution are the main sources of Na, although the Na-bearing minerals are rare in the dolomite reservoirs. Unlike Kaifeng, Ca and Mg in the geothermal fluids of Tianjin are primarily from the dissolution of calcite, dolomite, gypsum and fluorite. These minerals are also the main sources of F and SO4 in geothermal water, but the fluorite dissolution equilibrium is attained more quickly.

Application of U-Pb dating technique in the study of hydrothermal activities of dolomite reservoir: A case study on 3<sup>rd</sup> member of Yingshan Formation in Gucheng area, Tarim Basin, NW China

古城地区是塔东探区的重点勘探区块，下奥陶统鹰三段是塔东探区首要的勘探层系。探寻多期热液作用改造后的优质白云岩储层是扩大研究区勘探成果的关键。本文在储层岩石学特征判识的基础上，应用U-Pb同位素定年技术，结合碳氧锶镁同位素和稀土元素分析测试，分析了热液性质，明确了热液作用发生的确切时期及期次，探讨了热液作用对白云岩储层的影响。研究结果表明，古城地区鹰三段白云岩储层发育准同生-浅埋藏海水成因的粉晶-中晶白云岩及热液作用产物。热液作用可划分为二期：第一期为中-晚奥陶世贫镁热液，表现为鞍状白云石充填缝洞及重结晶次生加大的粗晶白云石，U-Pb测定年龄为464±12Ma到473.9±9.1Ma，氧同位素值比早奥陶世正常海相白云石明显偏负，锶同位素比值明显高于早奥陶世海水值，稀土元素Eu正异常，锶、铁、镁元素含量低；第二期为晚奥陶世-早志留世钙质热液，表现为方解石充填缝洞及对先存白云石的溶蚀，U-Pb测定年龄为448±15Ma到457.4±6.6Ma，氧锶同位素组成严重偏离同时期正常海相白云石范围，轻稀土元素明显富集，Eu正异常，锶、钙元素含量高，锰元素含量低。第二期热液作用是白云岩储层溶蚀的关键，同期NNE向断裂带作为其输导系统，是优质储层发育的有利部位，为有利区带预测提供依据。;The dolomite in 3^rd member of Yingshan Formation in Gucheng area is a significant exploration field in the east of Tarim Basin. High-quality dolomite reservoir reworked by hydrothermal activities is the key to expand exploration. Based on petrographic characteristics, this paper analyzed hydrothermal properties, clarified the precise dating and stages of hydrothermal process, and discussed hydrothermal effects on dolomite reservoir using U-Pb dating technique, combined with analysis of carbon and oxygen, strontium, magnesium isotopes and rare earth elements. The study shows that micrite-medium grained dolomites taken place in paracontemporaneous-shallow buried phase and hydrothermal products in research area. There are two stages of hydrothermal processes. The first stage hydrothermal process is very low in magnesium content in the Middle-Late Ordovician, at the result of saddle dolomite filling in fractures/vugs and coarse dolomite formed by recrystallization aged 464±12Ma~473.9±9.1Ma by U-Pb dating with oxygen isotope negative compared with normal marine dolomite, strontium isotope values higher than Early Ordovican sea values and positive Eu anomalies. The contents of strontium, ferrum and magnesium are relatively low. The second stage hydrothermal process is high in calcium content in the Late Ordovician-Early Silurian, at the result of calcite filling in fractures/vugs and dissolution of existing dolomites, aged 448±15Ma~457.4±6.6Ma by U-Pb dating with significantly negative oxygen isotope and enriched LREE and positive Eu anomalies. The strontium content is high while manganese content is low. Migration pathway of the second stage hydrothermal process as a key of dolomite reservoir dissolution is NNE fracture zones developing high-quality reservoir, which provides a basis for the prediction of favorable zones.

Characteristics of multiple dolomitizing fluids and the genetic mechanism of dolomite formation in the Permian Qixia Formation, NW Sichuan Basin

The ultra-deep dolomite reservoir of the Permian Qixia Formation in northwest Sichuan is important for hydrocarbon reservoirs, but the mechanism of dolomite genesis is complex and has long been controversial, thus requiring further elucidation. By studying the dolomite with different types in the Permian Qixia Formation, northwest Sichuan Basin, this paper, based on outcrop sections observation and core description, seeks to find out the dolomitizing fluids' source and the dolomite's genetic mechanism by analyzing the dolomite's rock-mineral and geochemical characteristics. Various technologies are applied, including rock thin section observation, cathode luminescence, carbon and oxygen isotope analyses, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and inclusion homogenization temperature and salinity. Results show that the dolomite is mainly classified into silty-fine dolomite (Md1), fine-medium dolomite (Md2), medium-coarse dolomite (Md3), and saddle dolomite (SD) filled in the pores. Md1 is formed in the early shallow-burial stage; the dolomitizing fluids are mainly derived from the early seawater sealed in carbonate sediments; and the penesaline seawater is formed by the seepage reflux under a moderate evaporitic environment. Md2 and Md3 are formed in the middle and late shallow-burial stages. Of these two types, Md2 is formed earlier, and the dolomitizing fluids are primarily derived from the seawater sealed in the early stage and partially derived from the residual evaporated seawater and hydrothermal fluids. In contrast, Md3 is formed later, and the dolomitizing fluids are the mixture of the seawater sealed in the early stage and the hydrothermal fluids formed in the late stage. In addition, Md3 was affected by the hydrothermal process more significantly during the formation. SD is deposited and formed when the saturated hydrotherm upwells along with the fracture system in the late stage. This study provides a significant reference for understanding the source and action mechanism of multi-stage dolomitizing fluids and expands the exploration field of the deep dolomite reservoir.

Analysis of dolomite reservoir characteristics of Lower Permian Maokou Formation and dolomite origins in Chuanzhong Area

Dolomite of Lower Permian Maokou Formation has a strong heterogeneous formation in-plane and vertical in Chuanzhong Area. Its formation mechanism is not yet fully understood. Thus, this study focuses on the Maokou Formation core and conducts thin section analysis, isotope analysis, cathodoluminescence, and data analysis of fluid inclusion using a combination of geological and chemical analysis methods to summarize the characteristics of dolomite reservoir layer of Maokou Formation and its formation mechanism. The Maokou Formation is believed to be the result of a deformity of dolostones, medium–fine grained dolomite, and a few gray dolomites, with the exception of gypsum layer. In the reservoir space of dolostones, pore type occupies the first layer, and dissolved seam and particle-dissolved pores are in the second layer. The dolomite value of δ 13 C was distributed in the range of 1.78‰–3.95‰(PDB), and the value of δ 18 O was distributed in the range of −7.39‰ to −3.8‰(PDB) with above 120 salinity. Thus, the dolomite reservoir was formed in a normal marine environment. Inclusions show that dolomite in the study area have a multi-phase fluid filling and a homogenized temperature of the maximum value of above 160 °C, which is not within the value that the normal temperature gradient can reach. Thus, buried dolomitization, seepage flow, Penesyndiagenesis, and Sabha genetic model are ruled out from the study area of Maokou Formation. The fault development that connected the basal layer with the stratum of Maokou Formation was distributed in the G3 well area, which provides a channel for the upper channeling of the basal hydrothermal fluid. In the Chuanzhong Area, the dolomite of the Lower Permian Maokou Formation is considered to be mainly of tectonic-hydrothermal origin.

Examining and applying the theory of “exploring petroleum inside source kitchens” for continental shale oil: A case study from the Kong 2 member of the Cangdong sag in the Bohai Bay Basin, China

Continental shale oil to be studied urgently is an important unconventional “source rock oil and gas” and key domain of “exploring petroleum inside source kitchens”. Based on the “source rock oil and gas” theory and the thought of “exploring petroleum inside source kitchens”, this paper established the ”six determination” exploration workflow for continental shale oil (i.e., determining the background, source rock, reservoir, play, resources, and sweet spot), which geological characteristics and exploration workflow are significantly different from conventional hydrocarbon. A case study was conducted by the workflow in the Kong 2 member (Ek2) of the Cangdong sag in the Bohai Bay Basin, China. Semi-deep to deep lacustrine facies belts and many gentle slope structures (belts) were widespread during Kongdian Formation depositing, thus providing favorable conditions for the formation and large-area accumulation of shale oil. Ek2 developed broader-distributed, and thick-bedded dark mudstone and oil shale, which exhibit high TOC, a good kerogen type, and great hydrocarbon-generating potential. Siltstone to fine sandstone and mud to micritic dolomite reservoirs were found within the low-dip slopes and central zone, which are thick, overlapped, abundant high-angle fractures, and exhibit low porosity and ultra-low permeability. Sandstone-type and dolomite-type shale oil of Ek2 characterized source–reservoir integration and five types of sources–reservoir configurations. They mainly overlapped at the front end of the braided river delta in the slope zone and in the sag center, where are superimposed vertically, and are connected horizontally with conventional structural–lithological oil pools in the up-dip slope. The median geological resources of the two classes shale oil were 5.26×108/t, with the remaining resources accounting for 90% of the total and moderate buried depths, thus the exploration potential is great. Several special exploration wells have obtained industrial oil flow. There are two types of sweet spots: low permeability sandstone layers and interbedded layers of oil shale and dolomite. A few favorable sweet spots were identified as targets for subsequent exploration. Then, three aspects of thinking and recommendations to solve the next exploration direction and technical problems were proposed, which will provide important reference and theoretical guidance for the further exploration of the same type of shale system in China and the world.

Sweet spots prediction through fracture genesis using multi-scale geological and geophysical data in the karst reservoirs of Cambrian Longwangmiao Carbonate Formation, Moxi-Gaoshiti area in Sichuan Basin, South China

AbstractFractures play a vital role in reservoir transformation, but the distribution of faults and fractures is difficult to obtain by observing cores, which ultimately limits the effective development of gas dolomite reservoirs. We propose an integrated method that incorporates thin-section observations, three-dimensional (3D) seismic data, and image logs to interpret the distribution of faults and fractures of Cambrian Longwangmiao Carbonate Formation to predict potential development areas in the Moxi-Gaoshiti area of the Sichuan Basin, South China. Firstly, the faults were well interpreted by using the automatic tracking and 3D visualization technique based on the new seismic combination attribute of symmetry and ant tracking. Secondly, a comprehensive analysis was conducted using the thin sections, paleogeomorphology, and in situ test results to determine the fracture types (corrosion and structural fractures). The results help us to find potential sweet spot zones with good permeabilities, which is of great significance in reducing the risk of water production of drilled wells in the field development.

Influence of crystal nucleus and lattice defects on dolomite growth: Geological implications for carbonate reservoirs

Dolomitization is controlled by numerous interacting factors, causing the mechanism of dolomite reservoir formation to remain unclear. In particular, the influences of lithological minerals and crystal lattice characteristics on dolomitization may be the main driver of large-scale dolomite development. In this study, physical laboratory experiments involving the dissolution, precipitation, and transformation of carbonate minerals were conducted, along with real-time testing and analysis of rock samples and water samples, to determine the mechanism underlying metasomatic dolomitization. Additionally, multiple two-dimensional and three-dimensional homogeneous and heterogeneous numerical models were established. Changes in mineral content, ion concentration, and porosity were calculated using multiphase flow numerical simulation technology, and dolomitization was comparatively analyzed under various lithological conditions. The combination of physical experiments and numerical simulation effectively demonstrated the influence of dolomite crystal nucleus and lattice defects on dolomite formation. The presence of a dolomite crystal nucleus can promote the dissolution of calcite and allow more Mg to enter the calcite crystal lattice, promoting further dolomitization. Dolomitization occurs preferentially at lattice defects, and calcite grain boundaries break easily. The results of this study verify that surface conditions with low temperature and pressure are unsuitable for the development of dolomite, but microbes may produce small dolomite nuclei, leading to the large-scale development of dolomite. In the deep layer, because of defects in the crystal lattice, numerous pores form through dissolution. Because the lattice of deep dolomite is less stable than the lattice of limestone, dolomite has better physical properties than limestone after dissolution.

Effects of gas saturation and reservoir heterogeneity on thermochemical sulfate reduction reaction in a dolomite reservoir, Puguang gas field, China

Varying concentrations of H2S have been found in the carbonate reservoir, Puguang gas field, China. Thermochemical sulfate reduction (TSR) is considered the most important process to generate a high level of hydrogen sulfide (H2S) in the reservoir. Although TSR has been intensively studied in this field, it is still not clear on (1) the controls of H2S concentration variations in the reservoir, (2) effects of variable gas and water saturations and reservoir heterogeneity on the TSR reactions, (3) reservoir quality changes during the TSR reactions, (4) potential sources and sinks of sulfur and carbon and their quantification. In this study, we conducted reactive transport modeling to quantitatively simulate the TSR and H2S generation processes in the Upper Permian Changxing Formation and Lower Triassic Feixianguan Formation in Puguang gas field, using log-based depth and thickness, paleo mineralogy assemblages and paleo temperature in late Jurassic, and hydrostatic pressure. The modeling results are relatively reliable as the predicted H2S and CO2 concentrations are comparable with the observed ones in the wells after calibration. The results indicated that both mineralogical composition and gas saturation in the reservoir affect the local efficiency and extent of TSR reactions, of which the abundance of effective anhydrite is an important control on the extent of the TSR reactions. The modeling durations of TSR reactions were ∼6 m.y. and TSR had a small impact on the reservoir porosity (porosity changes generally less than 0.1 porosity unit). The sources and sinks of carbon and sulfur were locally balanced with each other, respectively, and the minor imbalances were likely due to the molecular diffusion. The calibrated models have predictive potential on the H2S and CO2 concentrations changes and porosity modifications during the TSR processes in the subintervals with limited samples. The validated TSR models will facilitate the investigation of organic-inorganic interactions and their impacts on both non-hydrocarbon (H2S and CO2) generation and reservoir qualities.

Genesis and pore evolution of dolomite reservoir in the Majiagou Formation, Ordos Basin, China

In gypsum–carbonate rock assemblages, multistage and complex fluids control the formation of dolomite reservoirs that are a focus of hydrocarbon exploration. It is difficult to determine the types of dolomite reservoirs and their formation mechanisms due to the diverse rock assemblages and multiple stages of diagenesis. In this study, we investigated the petrology, reservoir physical properties, and geochemistry of the 6th sub-member of member five of the Majiagou Formation (i.e. Ma56) in the Ordos Basin, China. These data were used to determine the nature and types of gypsum–carbonate rocks, and constrain their reservoir characteristics and diagenetic history, and fluid-related mechanisms that led to dolomite reservoir development and preservation. The Ma56 was deposited on a restricted evaporatic platform in the North China Craton, and contains three main types of dolomite reservoirs with variable types of reservoir space. Dolomite reservoir formation was closely related to penecontemporaneous dolomitization, karstification, and differential cementation. Early large-scale dolomitization produced dolomitized carbonate sediments that were resistant to compaction and dissolution, which was conducive to the preservation of primary and secondary pores. The intermittent exposure and dissolution of mound–shoal facies sediments, due to high-frequency sea-level fluctuations, was the dominant mechanism for formation of secondary dissolved pores and high-quality reservoirs. During burial, differential cementation occurred due to interaction between fluids and pore size, which determined the extent of reservoir preservation. In general, the studied dolomite reservoirs have undergone multistage diagenesis and alteration, which led to complex and multistage development of the reservoir porosity. However, the reservoir lithology and pore space developed mostly in the depositional to penecontemporaneous stages. Our results provide new insights into the origins of deeply buried dolomite reservoirs in carbonate–evaporite successions.

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.