Jizhong Depression Research Articles

Determining complex petroleum migration pathways along faults and sandbodies: a new method and application

Introduction: The reservoir-forming rule in the paleo-slope zone outside petroliferous basins’ source kitchens of the eastern basin in China is unclear, manifested in the distribution of oil and gas in complex multi-layers. This study focuses on petroleum migration along faults and sandbodies to clarify this question and improve the exploration efficiency of petroleum in areas with similar geological conditions.Method: We summarized two models of petroleum migration based on two factors: the connected sandbodies and the vertical unsealing faults. Model I is conducive to the migration of hydrocarbons within a single layer, while Model II is conducive to the vertical migration of hydrocarbons across layers. Combining the means for evaluating fault sealing and sandbody connectivity, integrating these models could form a set of characterization methods for the distribution of the complex fluid migration pathways.Results: This method is applied to characterize the complex multilayer migration pathways distribution in the first and second members of the Shahejie Formation in the central-southern area of the Wen’an Slope in the Baxian Sag, Jizhong Depression, Bohai Bay Basin, China. The results show that, in the first and second members of the Shahejie Formation in the central-southern area of the Wen’an Slope, the area for Model II is mainly distributed in the central-eastern regions. The second member has a Model I area distributed in its western region, and the first member has a Model I area distributed in its centraleastern parts. The latter distribution area is larger than the former. According to hydrocarbon migration analysis, the first member is more conducive to large regional migration and accumulation of hydrocarbons than the second member.Discussion: The predicted result of petroleum migration is consistent with the hydrocarbon distribution found in the central-southern areas of the Wen’an Slope, which is that oil and gas distribution in the first member is significantly greater than that in the second member. Thus, our method is feasible for characterizing the regional distribution of complex multi-layer migration pathways in simple tectonic setting and oil source conditions. However, further research is still needed to increase the effectiveness of present petroleum migration analysis.

Analysis on Vertical Variation Characteristics of Eogene Formation Water in Jizhong Depression

Jizhong Depression is located in the middle and west of Bohai Basin. During the Tertiary period, the depression was in a relatively stable state, and the shape of the depression did not change much, but the internal ups and downs of the depression were still continuous. In the Tertiary period, it experienced three development stages: early division, middle communication and late contraction, and three different sedimentary facies appeared accordingly. At the same time, it also caused three shifts of sedimentary centers. In the medium-term communication stage, two large water invasion periods occurred, and in some areas, the sedimentary water medium changed from salty to light twice. Oil and gas in the depression are associated with formation water, and the formation history of formation water is closely related to the formation history of oil and gas reservoirs. They are both in a hydrodynamic system and exchange material components for a long time. The lower tertiary is the main oil-bearing interval in central Hebei, but it is affected by the late tectonic movement and its structural characteristics are complex. Therefore, the hydrochemistry and vertical distribution characteristics of tertiary strata are studied. By effectively studying the distribution characteristics of formation water, it is concluded that the salinity of formation water in the basin tends to increase from edge to center and from shallow to deep, mainly in alternate blocking zones and alternate stagnation zones. The water deposition environment is relatively stable and the general regularity is obvious, which is an effective means to study the migration and distribution of oil and gas.

Formation mechanisms and resource potential of carbonate geothermal reservoirs in the Beijing-Tianjin-Hebei plain

Investigating the formation mechanisms of carbonate geothermal reservoirs is of theoretical and practical significance for summarizing the formation pattern of geothermal resources and further guiding their effective exploitation. The Beijing-Tianjin-Hebei Plain (BTHP), predominantly located within the Jizhong Depression and Cangxian Uplift in the Bohai Bay Basin, serves as the primary region for geothermal exploitation and utilization in China. More than 1500 geothermal wells have been drilled therein, with water temperature at the wellhead ranging from 55 to 110 °C, single-well flow rate ranging between 80 and 120 m3/h, and cumulative heating area exceeding 100 × 106 m3. However, the exploration and research in the region remain limited overall. As per the previous geothermal and petroleum exploration results and the latest geothermal drilling data, this study comprehensively evaluated the geothermal resources of karst geothermal reservoirs. The results show that two suites of carbonate karst reservoirs, namely the Jxw Formation and the Ordovician strata, have primarily developed in the BTHP, and their formation and evolution can be divided into four stages: the Mesoproterozoic-Early Paleozoic stage with carbonate sedimentation and the development of interlayer karst, the Late Paleozoic stage with the development of direct sedimentary cover, the Mesozoic stage with compressional uplifting and development of buried hill karst, and the Cenozoic stage with regional cover deposition and the modification and finalization of karst geothermal reservoirs. Accordingly, the porosity evolution history of the geothermal reservoirs is composed of three stages, namely a significant decrease followed by a minor increase, a gradual decline, and then a small fluctuation from increase to decrease before slowly rising again. The karstification in geothermal reservoirs can be summarized into quasi-syngenetic karstification, epigenetic karstification, and burial karstification, which can be subdivided into seven subcategories. The carbonate geothermal reservoirs in the study area boast total geothermal resources of 53.821 × 109 GJ, or 184.155 × 109 t of standard coal equivalent (tce), and the annual exploitable geothermal resources in the urban area can heat an area of (400‒500) × 106 m3, indicating great potential of geothermal exploitation.

Hydrogeochemical Characteristics of Geothermal Water in Ancient Deeply Buried Hills in the Northern Jizhong Depression, Bohai Bay Basin, China

The Jizhong Depression boasts rich geothermal resources with a lengthy history of geothermal exploitation. Buried hill geothermal reservoirs, which serve as primary thermal sources for hydrothermal resource exploitation, are prevalent in this region and have advantages such as extensive development potential, significant geothermal reservoir capacity, superior water quality, and straightforward recharge. This study investigates the formation and evolution of deep geothermal water in the Jizhong Depression by analyzing the hydrochemical and isotopic data of geothermal water samples collected from buried hill geothermal reservoirs in the northern part of the depression. The findings reveal that the subsurface hot water samples from the carbonate geothermal reservoirs in this region were predominantly weakly alkaline water with a pH ranging between 6.61 and 8.87. The hot water samples collected at the wellhead exhibited temperatures varying from 33.9 °C to 123.4 °C and total dissolved solids (TDS) lying between 473.9 mg/L and 3452 mg/L. Based on the δ2H-δ18O stable isotope analysis, the geothermal fluids in the Jizhong Depression are predominantly sourced from atmospheric precipitation and exist in a somewhat isolated hydrogeological environment, exhibiting pronounced water–rock interactions and deep water circulation (with depths ranging from 1324 m to 3455 m). Through a comparison of various methods, it is deduced that the most appropriate geothermometer for deep karst geothermal reservoirs in the Jizhong Depression is a chalcedony geothermometer, and when using it, the deep reservoir temperature was estimated at 63–137.6 °C. The precipitation in the adjacent mountainous areas enables the groundwater to infiltrate and descend deep into the earth along piedmont faults. Subsequently, lateral runoff over extended periods replenishes the groundwater into the depression. This process allows for the groundwater to fully absorb heat from deep heat sources, resulting in the formation of the deep geothermal reservoirs in the northern Jizhong Depression. The insights obtained from this study offer a theoretical and scientific foundation for the exploitation and utilization of regional geothermal resources and the transformation of the energy structure in China.

Geothermal field model in the southeast of Shijiazhuang Region (China) deduced from magnetotelluric data

Shijiazhuang is located in the contact area between the Taihang Mountain Uplift and the Jizhong Depression of the North China Plain, adjacent to the eastern Taihang Mountain Fault. The major geothermal fields are located to the southeast of the Shijiazhuang region. Considering that the origin of these geothermal fields can be better delineated via geophysical methods, in this study, broadband magnetotelluric data were acquired from 51 stations. The aim of this study was to understand the underlying mechanism of the action of a geothermal system in terms of resistivity and other known information. Before inversion, the data were processed via remote reference technique, and the dimensionality and geoelectric strike directions were analyzed. 2D inversion of the TM-mode data was performed via the non-linear conjugate gradient method, and a deep electrical structure was imaged. Combining the data with previous data, the tectonic units and fault distribution of study area were comprehensively interpreted. The four components of the geothermal system have been identified: a heat source, channel, thermal reservoir, and caprock. The low-resistivity layer in the middle crust was considered as a deep heat source, and the fault was a heat diffusion channel. The high-porosity carbonate rock in the uplift area was considered as a thermal reservoir, and the overlying Neogene and Quaternary strata were considered as caprocks for thermal insulation. This study provides valuable reference basis for studying geothermal fields in this area.

Chemical Characterization and Genesis of Thermal Reservoir Water in the Southern Part of the Jizhong Depression

The Jizhong Depression is a typical Mesozoic and Cenozoic fault basin located in the northwestern part of the Bohai Bay Basin that has abundant hydrothermal and geothermal resources and enormous development potential. In this study, hydrochemical and isotopic analyses were conducted on water samples from the southern region of the Jizhong Depression. The formation and evolution processes of the deep geothermal water were analyzed, the circulation process of the deep geothermal water was determined, and the genetic mechanism of the geothermal systems was elucidated. The hydrochemical types of the geothermal fluids in the sandstone reservoirs in the research area are mainly Cl·HCO3−Na type, while the geothermal fluids in the carbonate reservoirs are mainly Cl-Na type and Cl·HCO3−Na type. The ion components in the geothermal water are mainly controlled by the dissolution of the carbonate rocks and the alternate adsorption of cations. The elevation of the geothermal water supply area is 763–1063 m, and the main source is precipitation from the mountainous areas in the western Taihang Mountains. The Na-K-Ca temperature scale and multi-mineral equilibrium method have relatively small errors and are suitable for the southern region of the Jizhong Depression, with average errors of 21.44 °C and 32.64 °C, respectively. The depth of the Jxw thermal storage cycle in the research area is 3033–5187 m, and the depth of the Ng thermal storage cycle is 1360–2862 m. The content of the main ions (Na+, K+, and Cl−) in the water samples of the study area is greater in the Jxw thermal storage than in the Ng thermal storage; the Jxw thermal storage water samples have lower γNa+/γCl− values than the Ng thermal storage; and the γSO42−/γCl− and γCl−/(γHCO3− + CO32−) values are greater than those of the Ng thermal storage, indicating that the Jxw thermal storage is located in a geological environment with better sealing, longer flow, slower water circulation, more complete leaching, and higher salinity than the Ng thermal storage. Part of the deep thermal storage is transmitted upwards through the rocks via thermal conduction, and part is transmitted upwards along fault channels via thermal convection, forming a convection–conduction-type geothermal system.

Study on the Controlling Factors and Laws of Deep Carbonate Geothermal Reservoirs in the Jizhong Depression

The absence of regional electrical data has constrained our comprehension of the deep geological structures in the Jizhong Depression. This limitation has impeded the exploration of factors and principles governing geothermal reservoirs. Historically, studies on these factors primarily centered on geothermal field attributes, such as anomalous geothermal flux, geothermal gradients, and deep Moho variations. In our research, we undertook an exhaustive interpretation of extended-period magnetotelluric readings. This was amalgamated with prior regional geological and geophysical studies to discern deep geological structural details pertinent to geothermal resource components, encompassing heat sources, pathways, reservoirs, and cap rocks. Our analysis spanned the foundational surface of the Cenozoic era, fault configurations, carbonate formation distribution, and layers with low velocity yet high conductivity. We also statistically evaluated geothermal wells, their density, and the water temperature attributes across various structural entities. This aided in comprehending how structural units influence thermal well features. Our research delineated the spatial interrelation between geothermal well distribution and deep geological structures, including carbonate rock distribution, Cenozoic thickness, fault patterns, and profound low-velocity, high-conductivity layers. Moreover, we scrutinized how structural unit types impact thermal well attributes, offering insights into the formation principles of deep carbonate geothermal reservoirs within the Jizhong Depression. Our findings suggest that the genesis of deep geothermal resources in the Jizhong Depression is swayed by regional geological conditions. Notably, the distribution of regional and especially deep geothermal wells is intimately tied to regional stratigraphy, structural designs, and profound geological structural traits. Furthermore, the temperature within geothermal wells correlates strongly with the burial depth, carbonate formation thickness, Cenozoic thickness, and proximity to fault channels responsible for fluid movement and heat transmission.

Control of oil and gas formation in the Shahejie Formation by the Li Xian Slope Fracture in the Jizhong Depression

Combining the information on the period of fault activity, hydrocarbon source rock distribution and reservoir distribution within the Lixian slope of the Jizhong Depression, the study was conducted on the controlling role of fractures on hydrocarbon formation. The results of the study show that: 1. the influence of fractures on the formation of traps, (1) the nose-like tectonic zone controls the formation of lithological traps of broken nose traps; (2) traps are formed at the segmentation point of the lower plate of the positive fault Gao Yang fault, which is located at the boundary of the external and external zones; (3) Fault activity controls the formation of trap closure; 2. The influence of fracture on oil and gas transportation, the role of oil and gas in the process of vertical transportation, combined with the location of hydrocarbon source rock development and the activity of the fault, fracture controls the vertical transportation of oil and gas.

High-Speed Railways Interference Signal Characteristics and Multiple Remote References Denoising of Magnetotelluric Data in Jizhong Depression, China

In the economically developed Beijing–Tianjin–Hebei region, magnetotelluric data are susceptible to contamination from cultural noise, which can be caused, for example, by urban stray currents, high-speed railways, or high-voltage lines. The multiple remote references method is an effective tool that can be used to suppress interference and improve signal-to-noise ratios. Therefore, this paper first introduces the basic principles of multiple remote references and then takes high-speed railway noise as an example. The characteristics of the time domain and frequency domain of the high-speed railway noise signals are analyzed. Then, we use two remote reference stations (with a single remote reference and multiple remote references) to process the data interfering with the high-speed railway and compare the results. Finally, the multiple remote references method is used to process the data for the entire section. Coupled with the known geological and seismic data, the inversion results well-reflect the deep underground geological structure.

Influencing factors of rock thermal conductivity and applicability evaluation of its mixing law predictive models

The thermal conductivity (TC) of rocks is a very important parameter in geothermal theory and applied research, its value is related to rock composition, mineral grains characteristics, rock porosity, and pore fluid etc., and simultaneously is affected by formation thermodynamic environment (temperature, pressure,etc.). Statistical analysis results of the thermophysical property parameters of borehole samples from Jizhong Depression indicated that the TC is mainly controlled macroscopically by lithology, compared with sandstone, mudstone, conglomerate, and gneiss, dolomite has a greater TC. In the dry state, the TC of dolomite samples showed a decrease with increasing temperature, the values of TC decreased by 12.3–27.5% in the temperature interval of 30–130 °C. Moreover, under the same temperature conditions, excluding the effects of water content and mineral fraction, the TC of medium–coarse crystalline dolomite, medium–fine crystalline dolomite, microcrystalline dolomite, and mud-powder crystalline dolomite decreased successively, reflecting the influence of the mineral grain size on TC. The validity and applicability of six commonly used mixing law predictive models for TC were evaluated based on X-ray diffraction (XRD) data from 70 borehole samples. The results showed that Geometric mean model displayed the best fit (goodness of fit, R2 = 0.878) for rocks, and Voigt-Ruess-Hill average also showed a good fit, with an R2 of 0.868, and had the best prediction for mudstone (AME = 10.16, RMSE = 0.25). In cases where the calculated values deviate significantly from the measured values, it may reflect the limitation that the mixing models consider only the composition of minerals but ignores the structural characteristics of rocks.

Multi-Mode Surface Wave Tomography of a Water-Rich Layer of the Jizhong Depression Using Beamforming at a Dense Array

Urban structure imaging using noise-based techniques has rapidly developed in recent years. Given the complexity of the cross-correlation function in high-frequency signals, here, the beamforming (BF) method was used to analyze one data set taken from a dense array in the Jizhong Depression and obtain multi-mode dispersion curves. Multi-mode surface waves improved inversion stability, reduced non-uniqueness, and yielded a one-dimensional shear wave (S-wave) velocity model. Interpolation yielded a high-resolution three-dimensional (3D) S-wave velocity model for the study area. The model shows that velocity gradually changed in the horizontal direction and greatly increased in the vertical direction, which is largely consistent with changes in the sedimentary environment related to the continuous subsidence of the Jizhong Depression since the Quaternary. A low-velocity anomaly at a depth of ~300–400 m was revealed and determined to be caused by either a deep-buried ancient river course or low-lying area. This study demonstrates the potential of the BF method for processing dense array data sets of urban exploration. The high-resolution 3D S-wave velocity model provides a new reference for studying the Quaternary structure of the Jizhong Depression, as well as groundwater resources, urban infrastructure, and underground spaces.

Genesis Mechanism of Deep Carbonate Reservoir in Jizhong Depression

Abstract The occurrence condition of geothermal resources in Jizhong depression is good for development, but the thermal genetic mechanism of thermal reservoir has been lack of systematic and theoretical research, which leads to large number of failure cases in the process of geothermal well location. In this paper, the thermal genetic mechanism of thermal reservoir in Jizhong depression is explored in the aspects of heat source, heat source channel, and thermophysical structure by means of core thermophysical property test, magnetotelluric sounding, and natural seismic P wave velocity imaging. According to the study, the heat of deep carbonate reservoir in Jizhong depression mainly comes from the deep mantle. Heat from the mantle enters deep thermal reservoir in the form of thermal conduction and thermal convection which flows through deep faults such as Niudong-Hexiwu fault. Cenozoic sandstone and mudstone caprock whose thermal conductivities are low is more than 1000 meters, which is favorable for heat storage. In comparison, the thermal conductivity of deep carbonate thermal reservoir is much higher than that of sandstone and mudstone. Therefore, differences in thermal conductivity and thermal convection from deep faults are the reasons why the heat reservoir temperature in the uplift area is higher than that in the subdepression area at the same depth.

The Estimation of Geothermal Reservoir Temperature Based on Integrated Multicomponent Geothermometry: A Case Study in the Jizhong Depression, North China Plain

The coal-dominated energy structure in the Beijing–Tianjin–Hebei region has caused serious air pollution and contradicts the construction of a clean, low-carbon, safe and efficient energy system. Substituting geothermal energy for fossil energy such as coal can effectively alleviate this problem. Located in the hinterland of the Beijing-Tianjin-Hebei region, the Jizhong Depression is rich in geothermal resources and has great development potential, though the degree of current development and utilization is not high. Vigorously developing geothermal energy can not only effectively alleviate the air pollution problem in the Beijing–Tianjin–Hebei region, but also optimize the regional energy structure. Geothermal reservoir temperature determines the development and utilization value of geothermal resources, and accurate evaluation of the geothermal reservoir temperature of geothermal resources can provide a reliable basis for the subsequent development of geothermal resources in the Jizhong Depression. Aiming at the commonly used sandstone geothermal reservoir and carbonate geothermal reservoir in the Jizhong Depression, this paper collected 24 sandstone geothermal reservoir geothermal fluids and 14 carbonate geothermal reservoir geothermal fluids in the central-southern area of the Jizhong Depression and a water chemistry test was carried out. According to the test results of water chemistry, the temperature of the geothermal reservoir is estimated by using the cation geothermometer, the SiO2 geothermometer and the multi-mineral equilibrium method, and it is compared with the actual temperature measurement results of the boreholes. The results show that the direct use of a geothermal geothermometer for calculation will cause large errors. Through water–rock balance analysis, the use of a Na-K-Mg balance diagram, SiO2 and 1000/T relationship diagram and Na/K and 1000/T relationship diagram can determine whether the geothermal fluid is suitable for the geothermometer, which can effectively reduce the error. The chalcedony geothermometer in the central and southern part of the Jizhong Depression is the most suitable. The multi-mineral balance method, the Na-K geothermometer and the K-Mg geothermometer have also achieved good results, while the quartz and Na-K-Ca geothermometers are not suitable for the south-central Jizhong Depression area.

Characteristics, Diagenesis, Controlling Factors and Formation Mechanism of Deep-Burial Ordovician Carbonate Reservoirs in the Yangshuiwu Area, Jizhong Depression, Bohai Bay Basin

Characteristics, Diagenesis, Controlling Factors and Formation Mechanism of Deep-Burial Ordovician Carbonate Reservoirs in the Yangshuiwu Area, Jizhong Depression, Bohai Bay Basin

Genetic Mechanism of Geothermal Anomaly in the Gaoyang Uplift of the Jizhong Depression

The Gaoyang uplift is rich in geothermal resources, but there are few studies on the regional geothermal genetic mechanism. A large number of geothermal wells fail in position calculation because of a shortage of basis. By using the methods of P-wave velocity structure imaging and magnetotelluric sounding, it draws the following conclusions: 1) The crustal thickness in the Gaoyang uplift area is relatively thin, about 32 km; 2) The cutting depth of the Gaoyang East fault exceeds 28 km and has reached the bottom of the lower crust. It is a large tensile fault that provides a channel for mantle heat flow into the shallow crust; 3) The Gaoyang uplift and its surrounding depressions form a concave-convex base fluctuation mode, which is conducive to the accumulation of heat flow to the uplift; 4) The deep carbonate thermal reservoir in the Gaoyang uplift is overlaid with Cenozoic sand and mudstone strata, with a thickness of more than 3000 m and low thermal conductivity, which is conducive to the preservation of thermal storage heat. Therefore, the shortening of the heat conduction path caused by regional crustal thinning, convective heat conduction of large faults, concave-convex structure, and thick Cenozoic caprock are the reasons why the heat reservoir temperature in the Gaoyang uplift is higher than that around, which provides a theoretical basis for geothermal development and utilization.

Natural gas origin of Langgu Sag in Jizhong Depression, Bohai Bay Basin: Insight from hydrous pyrolysis experiments of gold tube

Two mudstone samples in low-maturity from the E2s3 source rock interval were used to hydrous pyrolysis experiments of gold tube with isothermal heating in order to better understand natural gas origins of Langgu Sag. The results displayed that the E2s source rocks in Langgu Sag attained the peak yield of nearly 350 mg/g TOC at about 0.9% of Ro for C6+ compounds due to thermal cracking occurrence of them if Ro higher than 0.9%, and C6-14 compounds would reach the maximum yield at about 1.3% of Ro owed to C14+ compounds preferentially cracking to them. Hydrocarbon gases were dominated by kerogen cracking gases at Ro lower than about 1.3% (referred to oil associated gases) and higher than about 1.8% (interpreted to residual kerogen cracking gases) but by secondary gases (belonged to early-generated liquid hydrocarbon cracking gases) if Ro at about 1.3%–1.8% for E2s source rocks. Furthermore, the yield of kerogen cracking gases would reach 90–100 ml/g TOC at 1.3% of Ro, far greater than 20 ml/g TOC as the lower limit standard of an effective gas source rock, even the kerogen cracking gas yield could reach 20 ml/g TOC if Ro at about 0.8%, therefore, as long as the E2s source rocks in Langgu Sag reach mature level, they can become effective gas source rocks.Considering their calculated Ro mainly distributing between 1.0% and 1.3% except Ordovician buried-hill reservoirs of the D2 oil-gas field, the proved natural gases in Langgu Sag mainly originated from kerogen cracking gases at Ro lower than 1.3%. And the gases from Ordovician buried-hill reservoirs of the D2 oil-gas field, with calculated Ro ranging about 1.3%–1.6%, essentially derived from residual liquid hydrocarbon cracking gases of the E2s4 source rocks, because the Ro of E2s3 source rock is mainly lower than 1.1% but it reaches about 1.8% for E2s4 source rock at present. The natural gases in shallow reservoirs experienced different degrees of bacterial degradation and formed methane-dominated gas reservoirs, consequently, the C1/C1-5 values displayed a range of 0.96–0.99 for bacterial degradation gas reservoirs and of 0.67–0.89 for natural gas reservoirs without bacterial reworking.

Multiscale Full-Waveform Inversion with Land Seismic Field Data: A Case Study from the Jizhong Depression, Middle Eastern China

The Jizhong depression contains several geothermal reservoirs that are characterized by localized low-velocity anomalies. In this article, full-waveform inversion (FWI) is used to characterize these anomalies and determine their extent. This is a challenging problem because the reservoirs are quite small and the available data have usable frequencies only down to 5 Hz. An accurate-enough starting model is carefully built by using an iterative travel time tomography method combined with a cycle-skipping assessment method to begin the inversion at 5 Hz. A multiscale Laplace–Fourier-domain FWI with a layer-stripping approach is implemented on the starting model by gradually increasing the maximum offset. The result of overlapping the recovered velocity model on the migrated seismic profile shows a good correlation between the two results. The recovered model is assessed by ray tracing, synthetic seismogram modeling, checkerboard testing and comparisons with nearby borehole data. These tests indicate that low-velocity anomalies down to a size of 0.3 km × 0.3 km at a maximum depth of 2 km can be recovered. Combined with the well log data, the resulting velocity model allows us to delineate two potential geothermal resources, one of which was previously unknown.

Evaluation of the Modification Potential for Carbonate Geothermal Rocks by considering Multiple Mechanics Properties and Acid Dissolution in the Jizhong Depression, China

Abstract Evaluation of geothermal reservoir modification potential is of great significance for improving the efficiency of geothermal exploitation and reducing costs. However, it is difficult to accurately evaluate the potential of geothermal reservoir modification given the limited number of parameters used in practical studies. Therefore, an evaluation method that considers the potential of geothermal development by applying multiple parameters is essential. In this study, five types of rock samples are collected in the field from the Fujunshan Formation, Gaoyuzhuang Formation, Majiagou Formation, and the second and fourth groups of the Wumishan Formation. The uniaxial compressive strength (UCS) and uniaxial tensile strength (UTS) of rock samples were obtained by uniaxial compression tests and Brazilian splitting tests, followed by the calculation of the brittleness index utilizing UCS and UTS. Then, the joint roughness coefficient and crack width were obtained by 3D scanning and postprocessing. In addition, the fracture extension degree of each lithology was obtained via acid dissolution experiments. Analytical hierarchy processes (AHP) associated with expert experience were adopted to evaluate the modification potential of rock samples under hydraulic stimulation and acid etching based on the above parameters. The results show that the carbonate rocks of the Majiagou Formation are prone to cracking, efficient for flow and heat exchange, sensitive to acid dissolution, and thus have the highest potential for modification. The proposed evaluation method and implemented case study can provide a practical reference and solid base for further applications.

Characteristics and formation mechanism of mesogenetic dissolution: A case study of Ordovician carbonate in the western slope of the Shulu Sag, Jizhong Depression, Bohai Bay Basin

Mesogenetic dissolution has the potential to enhance the quality of carbonate reservoirs, and thus it is critical to identify the source of corrosive fluids and establish the mechanism that leads to mesogenetic dissolution. An integrated study using cores, thin sections, seismic profiles, and geochemistry was performed to investigate the characteristics, discuss the possible corrosive fluids, and establish the formation mechanism of mesogenetic dissolution in Ordovician carbonates in the western slope of the Shulu Sag, Bohai Bay Basin. The Shulu Sag underwent a four-stage growth history, and the carbonate rocks in the study area experienced three episodes of diagenetic alteration (i.e., syngenetic, telogenetic, and mesogenetic). The micro- and macroscopic occurrences of mesogenetic dissolution were ubiquitous, including, for example, stylolites, solution seams, tiny pores, laminar dissolution pores, dissolution pores and vugs associated with stylolites, dissolution enlarged fractures, erosion of late minerals (e.g., saddle dolomite), and related hydrocarbon inclusions. The partition patterns of trace elements normalized to Upper Crust Continent (UCC) and rare earth elements (REEs) normalized to Post-Archean Australian Shale (PAAS) were consistent between eroded and uneroded samples. Compared with uneroded samples, eroded samples had higher contents of some trace elements (e.g., Co, Cr, Cu, Ni, V, and Zn) and REEs, and relatively higher average δ13C values and 87Sr/86Sr ratios. These characteristics indicate that the most likely corrosive fluids causing dissolution were organic acids in hydrocarbon-bearing fluids, which may have been derived from the maturation of organic matter in marlstone in the lower part of the Sha3 member (Es3l). The combination of the tilted basin structure and presence of good conduits (i.e., unconformity, stylolites, and fractures) promoted the migration of corrosive fluids and the occurrence of mesogenetic dissolution in Ordovician reservoirs from the Neogene-Quaternary period to the present. A geologic model for the formation mechanism of mesogenetic dissolution has been proposed, which provides new insight for future hydrocarbon exploration.

Rock Types and Reservoir Characteristics of Shahejie Formation Marl in Shulu Sag, Jizhong Depression, Bohai Bay Basin

Rock Types and Reservoir Characteristics of Shahejie Formation Marl in Shulu Sag, Jizhong Depression, Bohai Bay Basin