Daniudi Gas Field Research Articles

The damage mechanism of spent acid in acid fracturing and damage reduction measures in Daniudi gas field

In the process of acid fracturing, spent acid after acid-rock reaction will adsorb and block matrix pores, damaging reservoir, resulting in damage to the reservoir. In order to clarify the influence of spent acid damage on carbonate reservoir of Majiagou Formation in Daniudi Gasfield, the spent acid damage experiments on matrix, natural fracture and acid etched fracture were carried out through the configuration of field spent acid, and the damage mechanism was analyzed from micro and macro perspectives. In addition, the potential sediment test of acid rock chemical reaction of carbonate rock cores containing gypsum in Daniudi Gasfield has been carried out. The experimental results show that spent acid has a high damage rate to matrix. Spent acid enters the matrix in the form of pore clogging to contaminate the core, with a failure rate of about 30%. The spent acid remains on the surface of natural- fracture, and the reservoir with high permeability is less damaged by the spent acid. Spent acid damage rate is between 10 and 20% in the natural fracture. But acid etched fracture is almost unaffected by spent acid. Therefore, the damage of spent acid is mainly caused by blockage, and the adsorption damage is relatively low. On the other hand, Gypsum has no obvious damage in acidizing fracturing. Finally, in order to reduce the spent acid damage, acid optimization measures were taken. The optimized acid can reduce the damage of spent acid by less than 10%.

Formation mechanism of the upper Paleozoic tight sandstone gas reservoir in the Daniudi gas field, Ordos Basin, China

The upper Paleozoic gas reservoir in the Daniudi gas field, Ordos Basin, is one of the typical tight sandstone gas reservoirs in China. Research into the sequential order of reservoir densification and gas accumulation is the key to revealing the formation mechanism of tight sandstone gas reservoirs. However, this scientific issue is still a subject of debate in the Daniudi gas field. Based on a series of reservoir experiments and in combination with stratigraphic burial and thermal maturity history, a comprehensive study is conducted on the formation and natural gas accumulation processes of the upper Paleozoic sandstone reservoir in the Daniudi gas field. Study results indicate that the main causes of sandstone reservoir densification of the study area include long-term compaction, multistage quartz cementation and carbonate cementation. In accordance with the time order of reservoir densification and gas accumulation, the formation of tight gas reservoirs can be divided into two types: self-sourced and near-source accumulations. In the former, before sandstone reservoir densification occurs, low-maturity natural gas generated from coal rocks and dark mudstones enter the reservoir quickly, migrate towards structural highs as driven by buoyancy, and then accumulate and form gas reservoir in favourable lithologic traps. In the latter, after tight sandstone reservoirs are formed, high-maturity gas migrates along fractures formed in the Late Cretaceous and accumulate in the nearby reservoir with micro-fractures. The study results provide a new understanding of the formation process of tight sandstone gas reservoirs and provides a reference for the exploration and development of the same type gas reservoirs.

Provenance of Volcanogenic Deposits from the Shanxi Formation of the Daniudi Gas Field, Ordos Basin, and Its Tectonic Implications

Through detailed drill-core description, petrography, cathodoluminescence imaging, and geochemistry analysis, sedimentary deposits with a high content of volcanogenic components (27.6%) were discovered in the Shanxi Formation of the Daniudi Gas Field, Ordos Basin. The volcanogenic components include volcanogenic quartz, lava fragments (dominated by rhyolite fragments), tuff fragments, and tuffaceous matrix. In addition, at least two volcanic ash layers were discovered. Although the metamorphic rocks of the Precambrian succession in the ancient land of Yin Mountain are usually considered the primary source of the Shanxi formation, it is most likely that the volcanogenic components were derived from intermediate-acid volcanic materials that accumulated in the Daqingshan area during the Late Carboniferous to Early Permian. Those volcanogenic components are mainly transported by flowing water rather than by air from origin to basin. In addition, the accumulation of volcanic materials in the Daqingshan area was controlled by volcanic eruptions occurring solely in this area, rather than by the Inner Mongolia Orogenic Belt. Those volcanic eruptions were related to the southward subduction of the Paleo-Asian Ocean Plate beneath the northern margin of the North China Plate in the Late Carboniferous to Early Permian. The active continental margin related to the subduction of the Paleo-Asian Ocean Plate lasted at least until the Early Permian, and the closure of the Paleo-Asian Ocean must have occurred later than the Early Permian.

Geochemical characteristics and their implication of light hydrocarbons in natural gas from the Lower Paleozoic carbonate reservoirs in the Daniudi gas field, Ordos Basin

The genetic types and source of the Lower Paleozoic natural gas in the Ordos Basin in China are controversial, and the geochemical characteristics of light hydrocarbons of natural gas in the fifth member of the Ordovician Majiagou Formation (O1m5) are studied by taking the Daniudi gas field as an example, in order to reveal the gas origin and source. The C5−7 light hydrocarbons are dominated by iso-alkanes rather than the n-alkanes, and the methyl cyclohexane (MCH) content is mostly higher than the nC7 content in C7 light hydrocarbons. The K1 and K2 values range from 1.06 to 1.16 and from 0.33 to 0.81, respectively. The geochemical characteristics of light hydrocarbons indicate that the Lower Paleozoic natural gas in the Daniudi gas field is mainly coal-derived gas which has been generated by the humic source rocks in the Upper Carboniferous Taiyuan Formation (C3t), with insignificant contribution by the O1m sapropelic source rocks. The O1m5 gas pools were accumulated through downward and lateral migration in the free phase. The aromatic contents in C6-7 and nC7/MCH ratios are easily affected by the factors such as water solubilization, adsorption in migration, evaporation, and migration fractionation, therefore, secondary alteration needs to be considered when using these parameters to identify the gas origin.

Impact of coal evolution on formation of tight sandstone reservoirs: A case study in the Daniudi gas field, Ordos Basin, China

Tight sandstone gas in coal-bearing strata has become an important unconventional gas worldwide. This research aims at understanding the impacts of coal evolution on diagenesis and reservoir quality of sandstones by means of thin section, cathode luminescence (CL), scanning electron microscope (SEM), stable isotope analysis, and fluid inclusions. The results show that carbonate cement was the main type of cement developed in tight sandstone reservoirs, of which the carbonate ions were jointly provided by coal layers and reaction in adjacent mudstones. For sandstones close to coal layers, their carbonate ions were mainly controlled by coal evolution and there was various carbonate cement developed due to cations supplied by seawater and the intense dissolution of volcanic fragments and feldspars. Combined with coal thermal simulation and burial history, it implies that at different maturity stages of coal evolution, its impact on sandstone diagenesis was different. In the early stage, the main product was CO2, which caused the dissolution of early calcite and promoted intense compaction. In the mature stage, CO2 and other hydrocarbon gases were released, resulting in more dissolution and silica cementation. In the over-mature stage, the main product was methane, and CO2 that did not spill out preserved by forming carbonate cement. The coal evolution had a strong impact on the reservoir quality of sandstone with large grain sizes, but little effect on medium- and fine-grained sandstones. High porosity and low permeability reservoirs were usually formed in small conglomerates and coarse-grained sandstones away from coal layers. While low porosity and high permeability reservoirs were distributed close to coal layers. This study aids in understanding the formation mechanism of tight sandstone reservoirs in coal measures and provides theoretic support for further exploration.

Factors controlling the occurrence, distribution and physical properties of the Ordovician palaeokarstic reservoir in the Ordos Basin Daniudi gas field

The Daniudi gas field in China's Ordos Basin is located in the transitional zone between palaeo‐topographic highs and slopes, whereby dissolution‐collapse breccias are widespread within the palaeokarstic reservoir. Abundant cores, well logs and thin sections were used to study palaeo‐topography, palaeokarstic breccia types, diagenesis and reservoir pore characteristics. The results indicated that reservoir quality is controlled by lithology, palaeo‐topography and diagenesis. The extent of palaeokarstic reservoir dissolution was shown to be determined by lithologic differences, whilst only gypsiferous dolostones and finely crystalline dolostones emerged in reservoir formation. Palaeo‐topography was also found to regulate reservoir development. The reservoirs are primarily located 20 m below the Ordovician unconformity in palaeo‐topographic hills and palaeo‐topographic slopes, whilst their depth is palaeo‐topography dependent. Lithology and palaeo‐topography also coordinate the distribution of palaeokarst breccia facies. Clast‐supported and matrix‐supported chaotic breccias were found in argillaceous dolostone strata. However, crackle–mosaic breccias occurred in gypsiferous dolostones and crystalline dolostones, located in palaeogeomorphic residual hills and slopes. Reservoirs generally develop in crackle–mosaic breccias of Ma51 and Ma52 sub‐Members, the quality of such reservoirs is dependent on the development and distribution of palaeokarst breccia facies and diagenesis. Dissolution of dolomite and anhydrite nodules was found to result in moldic pores and dissolution‐enlarged pores. Overall, the most damaging effect on reservoir quality resulted from calcite cement infilling moldic pores, enlarged pores and fractures.

An Improved Acid Fracturing Design through RoqSCAN Technology: A Case Study from Daniudi Gas Field in Ordos Basin, China

The concept of geological engineering-integrated fracturing is widely applied in the conventional and unconventional reservoirs, aiming to achieve efficient and economic development. There is no doubt that fine reservoir evaluation is the basis and core of geological engineering-integrated fracturing. Therefore, in this paper, we performed a series of investigations on the basis of formation fine evaluation by using RoqScan technology. Firstly, the physical parameters of the target well located in Daniudi Gas Field were evaluated by a laboratory test and logging interpretation. Next, the composition and function of the RoqSCAN tool were introduce in detail. Furthermore, the elements, minerals, and rock mechanical parameters of the target well were evaluated by the RoqSCAN tool. Finally, the fine reservoir model was established on the basis of fine formation evaluation. Moreover, we conducted a series of research on porosity and principal stress of the target formation to compare the difference between the fine model and the conventional model. Based on the fine reservoir model, the acid-fracturing design and numerical simulation were further investigated. The results of experiments and numerical simulation mainly achieved the following findings: 1. The porosity and permeability of the target formation indicated that it was a tight, low-permeability reservoir; 2. According to the comparison of the distribution for different parameters in the target well, the modeling with fine reservoir evaluation had higher accuracy than the traditional modeling; 3. Based on the fine evaluation results, acid fracturing is recommended as a stimulation method to enhance the productivity of the target formation. This study provides an essential method for the integrated fracturing design of geological engineering.

How to improve machine learning models for lithofacies identification by practical and novel ensemble strategy and principles

Typically, relationship between well logs and lithofacies is complex, which leads to low accuracy of lithofacies identification. Machine learning (ML) methods are often applied to identify lithofacies using logs labelled by rock cores. However, these methods have accuracy limits to some extent. To further improve their accuracies, practical and novel ensemble learning strategies and principles are proposed in this work, which allows geologists not familiar with ML to establish a good ML lithofacies identification model and help geologists familiar with ML further improve accuracy of lithofacies identification. The ensemble learning strategy combines ML methods as sub-classifiers to generate a comprehensive lithofacies identification model, which aims to reduce the variance errors in prediction. Each sub-classifier is trained by randomly sampled labelled data with random features. The novelty of this work lies in the ensemble principles making sub-classifiers just overfitting by algorithm parameter setting and sub-dataset sampling. The principles can lower the bias errors in the prediction. Two issues are discussed, videlicet (1) whether only a relatively simple single-classifier method can be as sub-classifiers and how to select proper ML methods as sub-classifiers; (2) whether different kinds of ML methods can be combined as sub-classifiers. If yes, how to determine a proper combination. In order to test the effectiveness of the ensemble strategy and principles for lithofacies identification, different kinds of machine learning algorithms are as sub-classifiers, including statistical classifiers (LDA, NB, KNN, ID3 tree and CART), kernel method (SVM), and ensemble learning algorithms (RF, AdaBoost, XGBoost and LightGBM). The experiments used a published dataset of lithofacies from Daniudi gas field (DGF) in Ordes Basin, China. Based on a series of comparisons between ML algorithms and their corresponding ensemble models using the ensemble strategy and principles, conclusions are drawn: (1) not only decision tree but also other single-classifiers and ensemble-learning-classifiers can be as sub-classifiers of homogeneous ensemble learning and obtain a higher accuracy; (2) the ensemble principles for the introduced homogeneous and heterogeneous ensemble strategy are effective to improve ML in lithofacies identification; (3) in practice, heterogeneous ensemble is more suitable for building a more powerful lithofacies identification model, though it is complex.

The effect of meteoric water on the very fine crystalline dolomite reservoir in the shallow burial zone: A case study of the Ma55 submember of Majiagou Formation in Ordos Basin

The meteoric water has obviously changed the physical properties of dolostone reservoirs in the vertical vadose zone and the horizontal phreatic zone, but its influence on the dolostone reservoirs in the shallow burial zone beneath the phreatic surface is still unclear. This study aims to reveal the effect of meteoric water on the dolostone reservoirs in the shallow burial zone through X-ray diffraction, cathodoluminescence, C, O, and Sr isotope using the sample from Majiagou Formation in the Daniudi gas field, Ordos Basin. The diagenesis and paragenesis of the Ma 55 submember were identified and interpreted through petrological study, combined with data from electron probe, X-ray diffraction analysis, and geochemical parameters of diagenetic minerals. The color of the very fine crystalline dolomite under the cathodeluminescence is dark red and red. The order degree of dolomite ranges from 0.54 to 0.91, showing the origin of early seepage-reflux dolomitization. There are a large number of different calcite cements as fills within the pores and fractures. The color of the calcite cement under the cathodoluminescence is orange-yellow, with a zonal structure. Hydrothermal fluid during late diagenesis could be identified by the authigenic fluorite filling in the fractures. According to the assembly of diagenetic minerals, the very fine crystalline dolostones have experienced the seepage-reflux dolomitization, meteoric water dissolution, shallow burial cementation and late cementation. The void spaces of the very fine crystalline dolostones are intercrystalline pores and microfractures. Although a large number of dissolved pores and caves developed in the period of meteoric water dissolution, these caves and dissolved pores has been mostly filled by multi-stages of cementation. Therefore, the effect of meteoric water on dolostone reservoirs in the shallow burial zone beneath the phreatic surface is not obvious. The main controlling factor for the quality of dolostone reservoir was dolomitization. This study provides a new understanding of the influence of meteoric water on reservoir quality in the shallow burial zone during the paleokarst period.

A deep kernel method for lithofacies identification using conventional well logs

How to fit a properly nonlinear classification model from conventional well logs to lithofacies is a key problem for machine learning methods. Kernel methods (e.g., KFD, SVM, MSVM) are effective attempts to solve this issue due to abilities of handling nonlinear features by kernel functions. Deep mining of log features indicating lithofacies still needs to be improved for kernel methods. Hence, this work employs deep neural networks to enhance the kernel principal component analysis (KPCA) method and proposes a deep kernel method (DKM) for lithofacies identification using well logs. DKM includes a feature extractor and a classifier. The feature extractor consists of a series of KPCA models arranged according to residual network structure. A gradient-free optimization method is introduced to automatically optimize parameters and structure in DKM, which can avoid complex tuning of parameters in models. To test the validation of the proposed DKM for lithofacies identification, an open-sourced dataset with seven conventional logs (GR, CAL, AC, DEN, CNL, LLD, and LLS) and lithofacies labels from the Daniudi Gas Field in China is used. There are eight lithofacies, namely clastic rocks (pebbly, coarse, medium, and fine sandstone, siltstone, mudstone), coal, and carbonate rocks. The comparisons between DKM and three commonly used kernel methods (KFD, SVM, MSVM) show that (1) DKM (85.7%) outperforms SVM (77%), KFD (79.5%), and MSVM (82.8%) in accuracy of lithofacies identification; (2) DKM is about twice faster than the multi-kernel method (MSVM) with good accuracy. The blind well test in Well D13 indicates that compared with the other three methods DKM improves about 24% in accuracy, 35% in precision, 41% in recall, and 40% in F1 score, respectively. In general, DKM is an effective method for complex lithofacies identification. This work also discussed the optimal structure and classifier for DKM. Experimental results show that (m1,m2,0) is the optimal model structure and linear SVM is the optimal classifier. (m1,m2,0) means there are m1 KPCAs, and then m2 residual units. A workflow to determine an optimal classifier in DKM for lithofacies identification is proposed, too.

Research on architecture of multi-type sand bodies inside distributary channel of delta plain: a case study in the Daniudi Gas Field, Ordos Basin

Research on architecture of multi-type sand bodies inside distributary channel of delta plain: a case study in the Daniudi Gas Field, Ordos Basin

Hierarchical Multiple-Point Geostatistical Modeling Method and Application Based on Braided River Reservoir Architecture

The Lower Shihezi Formation of the Daniudi gas field in the Ordos Basin is a typical reservoir of a braided river system in an alluvial plain, characterized by extensive braided river development, parallel development from the near source to the center of the basin, and frequent interweaving and cut stacking, as well as a complex deposition process that has seen frequent river channel changes. The braided river belt, braided channel, channel bar inside the river, and interlayer within the channel bar constitute a hierarchical and complicated architectural feature, which poses a great challenge to accurately characterize this type of reservoir for modeling. We proposed a hierarchical, level-by-level embedding, and progressive multiple-point geostatistical modeling strategy that is refined layer by layer according to a 5–3 level architectural unit hierarchy, with the modeling results of each level providing constraints for the next level modeling. The hierarchical geological model based on the combination of qualitative architectural anatomy and quantitative pre-architecture unit scale is critical in guiding the efficient development of the remaining gas in the braided river reservoir in Daniudi.

Reservoir Modeling of Braided River Reservoirs Based on Geological Knowledge Database: A Case Study of P1x Formation of the Daniudi Gas Field, Ordos Basin, China

Abstract The sandy braided river depositional system developed in the Lower Shihezi Formation of the Daniudi gas field, Ordos Basin. It has the characteristics of frequent migration and oscillation of braided channels and large well spacing, making it challenging to portray the braided river sand body in this area, bringing uncertainty to the 3D geological modeling of the reservoir. This study takes the primary gas reservoir H1 member as an example. It establishes a quantitative geological knowledge database for the reservoir by statistically fitting the correlation equations between the braided channel and channel bar in the planes and profiles to reduce the uncertainty of reservoir modeling. This study combines the multisource and multiscale information from modern sedimentation, field outcrops, and tank simulation experiments of the braided river. From the data, the distribution intervals for the thickness and width of the braided channel sand body in the H1 member are 1–22 m and 7–320 m, respectively, and the thickness, width, and length distribution intervals of the sand body in the channel bar are 3–30 m, 80–1500 m, and 240–4200 m, respectively. A 3D training image is established using the object-based simulation method based on the H1 member’s well data and combining the quantitative parameters of various microfacies in the geological knowledge database. The multiple-point geostatistical modeling method is applied to establish a sedimentary microfacies model. The model’s uncertainty is reduced through multi-information fusion constraint modeling, providing a reliable basis for guiding the prediction of the remaining gas in the Daniudi gas field.

Optimization of Acid Fracturing Process for Carbonate Reservoirs in Daniudi Gas Field

The Daniudi gas field is located in the Ordos Basin’s northern section of the Yishan slope. The intertidal–subtidal depositional environment dominates the lithology of the Ma55 sub-member, resulting in a stable, thick-layered dark gray–gray–black limestone and lime dolomite. The stratum is stable laterally as well as dolomite, with an average thickness of 26.8 m. Fractures, dissolution expansion pores, and inter-crystalline dissolved pores are the primary reservoir space kinds, with a minor number of karst caves and fractures generated as well. The main distribution ranges for porosity and permeability are 1–8 percent and 0.01–1 mD, respectively. Low porosity, tightness, and ultra-low permeability are common characteristics, and a single well typically has no natural productivity. Production stimulation technologies like pre-fluid acid fracturing, compound sand addition acid fracturing, and multi-stage injection + temporary plugging volumetric acid fracturing have been gradually optimized using the horizontal well development method, and breakthroughs in the development of tight and low-permeability carbonate rock reservoirs have been made. However, the conditions of different types of reservoirs are quite different, and the acid fracturing process is not matched and imperfect, resulting in large differences in the productivity of different horizontal wells after fracturing, as well as a high proportion of low-yield wells, which cannot meet the needs of cost-effective and effective development of this type of gas reservoir. In light of the aforementioned issues, a series of laboratory tests have been carried out to explore the stimulation effects of acid fracturing on different types of reservoirs and to optimize the acid fracturing process in the Daniudi gas field. The results show that the rock mechanical performances and the acid etching conductivities of the rock specimens are related to the types of reservoirs. The rock mechanical properties can be deteriorated after acidizing, but different types of reservoirs have different degrees of deterioration. According to the results of acid etching conductivity of different types of reservoirs, conductivities obtained by high and low viscosity and cross-link-gelled acid (two stage injection) processes are higher than those of high viscosity systems. The experimental results of process suitability suggest adopting high and low viscidity acid systems for pore type and fracture-dissolved pore type reservoirs, and cross-linked acid systems for fracture-pore type reservoirs. The findings of this study can help form a better understanding of the performance of different types of reservoirs under the various acidified conditions that can be used for the optimization of acid fracturing processes in carbonate formations.

The Effect of Single Sandstone Stacking Pattern on the Sandstone Reservoir Physical Properties—A Case Study from the Shanxi Formation in the Daniudi Area, Northeastern Ordos Basin

The role of the single sandstone stacking pattern in controlling the physical properties of the sandstones deposited in the distribution channels of the deltaic plain is unclear. This study aims to reveal the effect of the single sandstone packing patterns on the reservoir qualities of sandstones from the Shanxi Formation in the Daniudi gas field of Ordos Basin. Based on the core observation, 2D-image analysis, and thin section identification, the lithofacies were identified, the stacking patterns of the single sandbody were divided, and the differences in minerals composition and diagenesis of different sandstone stacking patterns were analyzed. According to the sedimentary facies analysis, 10 types of lithofacies have been identified in the Shanxi Formation in the study area. The single sandstone stacking patterns include mixed stacking patterns of coarse to medium-grained sandstone, fining upwards stacking patterns of coarse- to medium-grained sandstone, and coarsening upwards stacking patterns of fine- to coarse-grained sandstone. Among these single sandstone stacking patterns, there is a greater percentage of lithofacies with high reservoir quality in the fining upwards stacking patterns of coarse- to medium-grained sandstone. Through a comparative study of the differences in minerals composition, the degree of compaction, and dissolution of sandstones, it is suggested that the high porosities and permeabilities of the coarse- to medium-grained sandstone lithofacies in the fining upwards stacking patterns are caused by the low content of the matrix in sandstone, relatively weak compaction, relatively high amounts of primary pores, and strong dissolution. The relatively high content of rigid clastic particles with coarser-grained size was favorable for the preservation of primary pores. The relatively high primary porosity could provide favorable passages for the late diagenetic fluid, leading to the development of dissolved pores. The study can provide an important basis for the exploration of high-quality sandstone reservoirs in the distributary channels of the delta plain.

Gas and water distribution in the tight gas sands of the northwestern daniudi gas field, ordos basin, China: Impact of the shale barrier

The production of gas from the Upper Paleozoic tight sands in the Daniudi gas field, Ordos Basin, China, is hindered by a lack of knowledge of the factors controlling water distribution. This study aims to clarify the origin of the water, reduce unwanted water production, and detect probable locations of “sweet spots”. A geological study during field development was conducted in the northwestern region of the gas field by integrating production data with other core, well log, mud log, and geological data. The results showed that the gas and water distribution was apparently related to gas charge, with a close negative correlation identified between the thickness of shale barrier and the cumulative gas production on a per well basis. Therefore, the shale barrier has the significant influence on the fluid distribution in the study area. In addition, the shale barrier was found to play a dual role in hindering gas charge toward the upper reservoir and in sealing trapped gas in the lower reservoir, given the configuration of “gas generation below and gas storage above”. This implies the wax and wane relationship. Insights on the shale barrier’s effect contribute to an alternative strategy of “sweet spot” prediction for resource development in unconventional gas reservoirs, i.e., if improved technologies are currently lacking to discriminate between gas-bearing and water-bearing zones in tight gas sands, the spatial distribution of continuous shale can be predicted with 3D seismic data as an initial step, which is thought to be less difficult. The attempts to detect “sweet spots” should subsequently focus on the local area where the thinner shale barrier exists below the producing zone. This work not only can enrich the theory of hydrocarbon accumulation in the Ordos Basin, but also presents an analogue to tight sand gas for worldwide sedimentary basins.

Geochemical Characteristics of Helium in Natural Gas From the Daniudi Gas Field, Ordos Basin, Central China

Helium-bearing gas is accumulated in the Lower Ordovician, Upper Carboniferous, and Lower Permian reservoirs of the Daniudi gas field in Ordos Basin, and the helium concentrations and isotopic compositions are investigated in order to reveal the abundance and origin of helium. Geochemical characteristics indicate that the natural gas from the Daniudi gas field has helium concentrations of 0.0271–0.1273%, with R/Ra ratios of 0.007–0.072. The 4He/20Ne ratios range from 848 to 17,000, which are substantially higher than the ratio of air or air saturated water. The proved helium reserves of the Daniudi gas field exceed 100 × 106 m3, suggesting an extra-large helium gas field. Helium in the field is of crustal origin and derived from the radioactive decay of U and Th in the rocks and minerals, with no significant contribution by atmospheric or mantle-derived helium. The natural gas in the Daniudi gas field displays the characteristics of typical crustal helium, which is consistent with the gases from cratonic basins (Ordos, Sichuan, and Tarim) in China, whereas the gases from rift basins (Songliao, Bohai Bay, and Subei) have experienced a significant addition of mantle-derived helium.

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.

Physical threshold of high quality reservoir of Upper Paleozoic in Daniudi gas field, Ordos Basin

Combined with the gas content and physical property data of the reservoirs, physical threshold of the Upper Paleozoic high quality reservoirs in Daniudi gas field, Ordos Basin is defined through the definition of effective reservoirs, using the method of oil test and distribution function. The results show that the threshold of porosity and permeability is 8% and 0.6×10−3μm2, respectively. The distribution function method shows that the physical threshold of the low-producing reservoir (with a daily gas production of more than 5 000 m3) is 8∼9% porosity and 0.6∼0.8×10−3μm2 permeability. Based on comprehensive analysis, porosity of 8% and permeability of 0.6×10−3μm2 are set as the physical threshold of high quality reservoir.

Sedimentary facies of clastic-platform carbonate sediment strata of epicontinental sea in the Daniudi Gasfield, Ordos Basin

Mixed deposition is a sedimentary phenomenon of special significance to the tectonic movement, material supply, sea level change, and paleoclimate of the sedimentary environment. The sediments of the Late Carboniferous-Early Permian System in the North China Craton, which are composed of clastic rock and limestone, are mixed clastic-platform carbonate sediments in the epicontinental sea. Based on the geological and well logging data of nearly 100 exploratory wells and hundreds of development wells in the region, the lithologic association and cycle features of the Upper Carboniferous-Lower Permian were determined. From this data, the Benxi Formation and the Taiyuan Formation of the Daniudi Gasfield are considered to belong to a mixed clastic coastline-carbonate platform sedimentary system and coastal plain sedimentary system in the epicontinental sea. Based on the six transgressions and regressions during the Late Carboniferous-Early Permian epochs in the region, combined with the regional evolution history, this study analyzed the sedimentary evolution history of the Daniudi Gasfield from the Late Carboniferous to the Early Permian.