Jizhong Depression Research Articles

A Method for Evaluating the Dynamic Sealing Efficiency of Underground Gas Storage Reconstructed by Aquifer

There are very few living examples of engineering test cases in domestic aquifer underground gas storage construction, however, engineering test analysis can greatly reduce the risk of evaluation. This paper systematically introduced interference well test method application of aquifer gas storage construction in Huabei oilfield, and take the target of the D5 aquifer construction in Jizhong depression as an example to analyze the reasonable water injection, excited period and interference test results. Interference well test result shows that the target cap rock of D5 aquifer is sealed, the fault vertical sealing ability is good, the sealing condition of the reservoir is provided, and also the horizontal connectivity of the reservoir is good. The research results are directive and with reference value for the construction of aquifer underground gas storage in China.

Present-day temperature–pressure field and its implications for the geothermal resources development in the Baxian area, Jizhong Depression of the Bohai Bay Basin

The Baxian area in the Jizhong Depression, west of the Bohai Bay Basin, is rich in geothermal resources, where Xiong County was built as the first smokeless city in China and the Xiong-county Model has become the geothermal resources demonstration model in China. In this study, the present-day geothermal gradient and the horizontal distribution characteristics of the temperature and pressure at different depths in the Baxian area were studied based on massive measured temperature and pressure data of the boreholes, and then the relationship was also discussed between the temperature–pressure field and the geothermal resources. The following findings were obtained. (1) The present-day geothermal gradient of the Baxian area is within the range of 21.8–73.5 °C/km, averaging 33.5 °C/km. (2) Its strata temperature increases as the depth increases. The horizontal variations of the geothermal gradient and strata temperature correspond to the basement relief very well. As the abnormally high temperature areas, the northern Niutuozhen Uplift, the Central Baxian Depression and the central East Langgu Depression possess huge geothermal resources and will be the most favorable exploration targets. (3) The formation pressure of the Baxian area is characterized by normal pressure and weak overpressure, and its horizontal distribution varies at different depths. The middle–strong overpressure generally developed at the depth of 4000 m in the southern Langgu Depression and southern Baxian Depression. This study is of important guiding significance for the exploration and development of geothermal resources in the Baxian area.

3-D geologic architecture of Xiong’an New Area: Constraints from seismic reflection data

Studying the geologic architecture of the Xiong’an New Area will provide important basis for the evaluation of crustal stability, urban planning and infrastructural projects in this region, and it is also of great significance in exploring the occurrence of oil and gas, geothermal, hot dry rock and other resources. The stratigraphic system of the study area is established by using latest high quality seismic reflection and deep borehole data. Characteristics of the major faults developed in the study area are finely depicted with the method of structural analysis. Tectonic evolution of Xiong’an and adjacent areas is reconstructed by using balanced geological cross-section technique. The tectonic activity of the study area is discussed on the basis of the development of secondary faults and the distribution of active earthquakes across the region. This study demonstrates that Xiong’an New Area is located at the transfer zone of the central and northern Jizhong Depression. There are three regional unconformities developed in this area, by which four structural layers are sub-divided. Controlled by the Taihang Mountain piedmont fault, the Daxing fault, the Rongcheng fault and the Niudong fault, the structural framework of the study area is characterized by intervening highs with sags. This structural pattern has an important controlling over the reservoir characteristics, hydrocarbon accumulation and the distribution of geothermal resources and hot dry rock within this region. Rifting in this area began in the early Paleogene, exhibiting typical episodic character and became inactive in Neogene. The development feature of secondary faults along with the distribution of active earthquakes indicate that Xiong’an New Area has been in a relatively stable tectonic setting since the Neogene, while the Baxian Sag and other structural units to the east of it have obviously been in a rather active environment.

Reservoir controlling differences between consequent faults and antithetic faults in slope area outside of source: A case study of the south-central Wenan slope of Jizhong Depression, Bohai Bay Basin, East China

Abstract The control effects of different occurrence faults on oil and gas accumulation and distribution in the outer slope area of oil and gas reservoirs were studied taking the south-central Wen'an slope of the Jizhong depression in the Bohai Bay Basin as an example. Based on 3D seismic data and the distribution of oil and water, the controlling differences between consequent fault and antithetic fault were analyzed and compared from the formation and evolution rule of faults and the formation mechanism of fault traps, including development positions of the consequent fault traps and antithetic fault traps, oil and gas distribution horizon adjusted by fault and formation period of fault traps. The differences between consequent faults and antithetic faults in controlling reservoirs have three main aspects: (1) Consequent fault traps and antithetic fault traps are in different positions, the consequent fault traps are at the segmented growing point in the hanging wall of “hard-linkage” faults, while the antithetic fault traps are developed in the position with the largest throw in the footwall because of tilting action; (2) The two kinds of faults result in different oil and gas distribution vertically, oil and gas adjusted by consequent faults is distributed in a single layer or multi-layers, while oil and gas adjusted by antithetic faults occur in single layers; (3) The two kinds of fault traps are formed in different periods, the consequent fault traps are formed at the time when the related faults enter the stage of “hard-linkage”, while the antithetic fault traps are formed at the beginning of the fault active period.

Evaluations of oil and gas lateral migration across faults: A case study of Shigezhuang nose structure of Wen'an slope in Baxian sag, Jizhong depression, Bohai Bay Basin, East China

Abstract To study the possibility and location of lateral migration of oil and gas across faults, a quantitative evaluation method for lateral migration of oil and gas across faults was established using the Shuangjing Knipe graphic method to identify the juxtaposition site and juxtaposition patterns of sand and sand in the upper and lower walls of the reverse faults and the consequent faults, combined with the oil and gas limit method of fault lateral seal in the test oil area. The quantitative evaluation method was applied to the first and two members of the Paleogene Shahejie strata (referred to as Es1 and Es2) of Shigezhuang nose structure of Wen'an slope in Baxian sag, Jizhong depression, Bohai Bay Basin, to determine the juxtaposition site of sand and sand in the upper and lower walls of the fault, and the shale gouge ratio (SGR) lower values are 26% and 29% respectively in the strata Es1 and Es2. Thus, the location of lateral migration of oil and gas across faults was determined. Based on the oil and gas distribution characteristics of the strata Es1 and Es2, and variation trend of nitrogen compounds in 4 wells in the strata Es1, the results were consistent with the quantitative evaluation of the location of lateral migration of oil and gas across faults, the feasibility of the evaluation method was preliminarily verified.

The lithologic differences between the third and fourth members of the Eocene Shahejie Formation in the Bohai Bay Basin and the associated climatic evolution

The Bohai Bay Basin straddled the Eocene boundary between arid and wet climates in eastern China. The basin was simultaneously affected by both arid and wet climates, and contemporaneous sediments developed under both extremely arid and extremely wet conditions. Data from every well drilled in the oil fields in the Bohai Bay Basin have been used to study the lower fourth member and the upper third member of the Shahejie Formation. The spatial distribution of climate indicators in the lower fourth member of the Shahejie Formation shows that the climate boundary extended from the northern Jizhong Depression to the Bozhong Depression. The portion of the basin to the north of this boundary mostly developed coalbeds, dark mudstone, and oil shale (indicative of a warm and wet climate), whereas the portion of the basin to the south of this boundary mostly developed gypsum, rock salt, and magenta dolomitic mudstone (indicative of a hot and arid climate). During the formation of the upper third member of the Shahejie Formation, the climate boundary shifted southward to the region between the southern part of the Jizhong Depression and the Jiyang Depression. The wet climate zone in the north expanded southward, and the arid climate zone in the south shrank. This shift is evident in the vertical lithologic characteristics of the members that formed before and after this shift in the climate boundary. The lithologic succession in the central part of the basin exhibits significant changes, whereas the lithologic successions in the southern and northern parts of the basin remain homogeneous and stable through time. Therefore, between the formation of the third and fourth members of the Eocene Shahejie Formation, the Bohai Bay Basin experienced a significant climate change. This change directly affected the pattern of the Eocene climate zones in eastern China and is indicative of changes in the paleoclimate zones near the Tropic of Cancer.

Structural patterns of the late Mesozoic crustal detachment system in the Raoyang Sag, Bohai Bay Basin, eastern China: New insights from 3D seismic data

Understanding the late Mesozoic tectonic origin and structural patterns of the Bohai Bay Basin (BBB) is of growing importance for its buried-hill petroleum exploration. This study revealed, based on 3D seismic and borehole data, that the Raoyang Sag in the Jizhong Depression, western BBB, was dominated by a crustal detachment system during the late Jurassic to the early Cretaceous. It was characterized by the WNW-dipping low-angle detachment fault F0 (namely Cangxi Fault), the structural dome cored by Archean Basement rocks at the footwall and supradetachment basins on the hanging wall. We suggested the late Mesozoic volcanic materials and coarse deposits accumulated in the supradetachment basins had locally diminished the petroleum prospect, but the syndetachment deformation and denudation had contributed to it by means of promoting petroleum migration from source rocks to reservoirs and improving porosity and permeability of the reservoir rocks by karstification and structural fracturing.

Tight oil accumulation of the redeposited carbonates in the continental rift basin: A case study from Member 3 of Shahejie Formation in Shulu sag of Jizhong depression, North China

Tight oil in the redeposited carbonates was mainly distributed in the Lower Submember of Member 3 of Shahejie Formation in Shulu sag of Jizhong depression, North China. Through high-resolution 3D seismic data, well logging data and drilling data, the Lower Submember of Member 3 of Shahejie Formation was divided into 5 third-order sequences and 15 parasequence sets. The redeposited marl and rudstone were major reserving horizons of tight oil, and ten reserving space types were developed and could be classified into two main categories, i.e., pores and fractures. Two types of tight oil reservoirs were established, i.e., the marl hydrocarbon reservoir of the source-reservoir integration and the rudstone hydrocarbon reservoirs of the source-reservoir paragenesis. The assemblage relationship among the high-quality source rocks, system tracts with the source-reservoir configuration was the major control factor for tight oil accumulation in the redeposited carbonates. The lacustrine transgressive system tracts and highstand systems tracts in SQ1 to SQ5 were the favorable horizons for development of the marl hydrocarbon reservoir, the lowstand system tracts in SQ1 to SQ3 were the favorable horizons for development of the rudstone hydrocarbon reservoir.

Characterization of Origin and Evolution of Formation Water in Buried Hill of Jizhong Depression, China, Using Multivariate Statistical Analysis of Geochemical Data

Groundwater samples from buried hill of Jizhong Depression were evaluated using two statistical analyses: hierarchical cluster analysis (HCA) and principal component analysis (PCA). The samples were classified into four clusters, C1–C4, in HCA and the hydrochemical types of C1–C4 are HCO3-Na, Cl·HCO3-Na, Cl-Na, and Cl-Na·Ca. From C1 to C2, C3, and C4, the water-rock interaction becomes increasingly intensive, and rNa/rCl gets lower while total dissolved solids and r(Cl-Na)/rMg get higher. Three components of PCA explain 86.87% of the variance. Component1 (PC1), characterized by highly positive loadings in Na+ and Cl−, is related to evaporation concentration. Component2 (PC2) is defined by highly positive loading in HCO3- and is related to influence of atmospheric water. With high positive loadings in Ca2+ and high negative loadings in Na+ and SO42-, component3 (PC3) suggests plagioclase albitization. The combination of HCA and PCA within the hydrogeological contexts allowed the division of study area into five dynamic areas. From recharge area to discharge area, the influence of atmospheric water gets weaker and water-rock interactions such as evaporation concentration and plagioclase albitization become intensive. Therefore groundwater in buried hill showed paths of hydrochemical evolution, from C1, to C2, C3, and C4. Buried hill reservoir in Jizhong Depression is mainly distributed in hydrodynamic blocking and discharge area; therefore the two regions can be the favorable areas for petroleum migration.

Meso–Cenozoic lithospheric thermal structure in the Bohai Bay Basin, eastern North China Craton

The Bohai Bay Basin is a region where part of the North China Craton has been thinned and destroyed. It has experienced two periods of crustal thinning that occurred during the Cretaceous and Paleogene, but investigations of its Mesozoic and Cenozoic lithospheric thermal structure are limited. Therefore, in this study, the distributions of mantle heat flow, crustal heat flow, and Moho temperatures during the Meso–Cenozoic are calculated based on analyses of the thermal history of the Bohai Bay Basin. The results indicate that the ratio of mantle heat flow to surface heat flow peaked during the late stages of the early Cretaceous and during the middle to late Paleogene. The corresponding mantle heat flow was more than 65% of the surface heat flow. Moho temperatures reached three peaks: 900–1100 °C in the late stages of the early Cretaceous; 820–900 °C in the middle to late Paleogene; and (in the Linqing Depression, Cangxian Uplift, and Jizhong Depression) 770–810 °C during the early Neogene. These results reveal that the Bohai Bay Basin experienced significant geological change during the Cretaceous, including the transformation of lithospheric thermal structure from “cold mantle and hot crust” before the Cretaceous to “hot mantle and cold crust” after the Cretaceous. The results also indicate that the basin experienced two large-scale rifting events. Therefore, this work may provide the thermal parameters for further investigations of the geodynamic evolution of eastern China.

Re-exploration program for petroleum-rich sags and its significance in Bohai Bay Basin, East China

Abstract A comprehensive analysis is done to investigate the exploration maturity and exploration potential in the petroleum-rich sags in the Bohai Bay Basin, indicating that the re-exploration program is of great significance. The advances in geological theories, key engineering technologies and exploration methods involved in the re-exploration program are described. Taking the Raoyang sag as an example, the study documents how the program was conducted and discusses its promising future of application. Petroleum-rich sags, with an oil resource abundance of greater than 20×10 4 t/km 2 and a total oil resource of over 3×10 8 t, are the main domains for petroleum exploration and production in the Bohai Bay Basin. With a discovery maturity of more than 50%, these petroleum-rich sags have entered the high maturity exploration stage and discovery of new oil and gas reserves has become more and more difficult. However, the undiscovered oil resources in these sags are abundant and amount to 105×10 8 t, so that they are still the main exploration domains in the Bohai Bay Basin. With the petroleum-rich sags in the Jizhong depression of the Bohai Bay Basin as an example of the re-exploration program, the study presents a detailed documentation of oil accumulation in subsags, reservoir-forming in the subtle buried hills, and oil migration and accumulation in the weakly deformed structural slopes. It deals with the advances in the merged 3-D seismic prospecting over an entire sag, facies-based reservoir prediction, stimulation techniques for complex reservoirs and fast efficient drilling techniques. Furthermore, it proposes the re-exploration program, which comprises a new round of overall exploration philosophy, new evaluation and planning. It discusses the important roles played by the re-exploration program in sustaining the reserve additions in the highly explored petroleum-rich sags.

Seismic sedimentology of sub-clinoformal shallow-water meandering river delta: A case from the Suning area of Raoyang sag in Jizhong depression, Bohai Bay Basin, NE China

The seismic sedimentology characteristics of sub-clinoformal shallow-water meandering river delta can be comprehensively analyzed by core, well logging and 3-D seismic data. This paper summarizes the sedimentary pattern of sub-clinoformal shallow-water meandering river delta of Suning area in Raoyang sag, Jizhong depression of Bohai Bay Basin. In Suning area, there are eight fourth-order sequences recognized from the upper part of Sha 1 Member (Es1s) to Dong 3 Member (Ed3), reflecting periodical change of lake level. From the central lake to the shore, and from the transgressive to highstand systems tracts, the color of mudstones varies from grey to red, showing a drop of lake-level. The seismic reflection pattern indicates the drop of lake level: the shingled reflections in Es1s (SQ?3) indicate water depth of 40−60 m in the lake center, followed by sub-parallel (SQ?4-SQ?6) with lower continuity reflections, and disappearance of shingled reflections, showing water depth no more than 30 m. Analysis of core, well logging, and seismic amplitude strata slices reveal that the sequences SQ?4-SQ?6 of Es1s are sub-clinoformal shallow-water meandering river delta, different from the shingled reflection shallow-water meandering river delta in SQV3 of Es1s. The main seismic sedimentology characteristics are channel forms and lobate landforms advancing to the central lake by multiple zones, indicating numerous distributary channel and deltaic lobes depositions.

The Sedimentary Characteristics of Suning-Dawangzhuang Carbonate Bar in Ji Zhong Depression

The Sedimentary Characteristics of Suning-Dawangzhuang Carbonate Bar in Ji Zhong Depression

Core evidence of paleoseismic events in Paleogene deposits of the Shulu Sag in the Bohai Bay Basin, east China, and their petroleum geologic significance

The Shulu Sag, located in the southwestern corner of the Jizhong Depression, Bohai Bay Basin of east China, is a NE–SW trending, elongate Cenozoic half-graben basin. The lowermost part of the third member of the Shahejie Formation in this basin is characterized by continental rudstone and calcilutite to calcisiltite facies. Based on core observation and regional geologic analysis, seismites are recognized in these lacustrine deposits, which include soft-sediment deformation structures (sedimentary dikes, hydraulic shattering, diapir structures, convolute lamination, load-flame structures, ball-and-pillow structures, loop bedding, and subsidence structures), synsedimentary faults, and seismoturbidites. In addition, mixed-source rudstones, consisting of the Paleozoic carbonate clasts and in situ calcilutite clasts in the lowermost submember of Shahejie 3, appear in the seismites, suggesting an earthquake origin. A complete representative vertical sequence in the lowermost part of the third member found in well ST1H located in the central part of the Shulu Sag shows, from the base to the top: underlying undeformed layers, synsedimentary faults, liquefied carbonate rocks, allogenetic seismoturbidites, and overlying undeformed layers. Seismites are widely distributed around this well and there are multiple sets of stacked seismites separated by undeformed sediment. The nearby NW-trending Taijiazhuang fault whose fault growth index is from 1.1 to 1.8 and the NNE-trending Xinhe fault with a fault growth index of 1.3–1.9 may be the source of the instability to create the seismites. These deformed sedimentary layers are favorable for the accumulation of oil and gas; for example, sedimentary dikes can cut through many layers and serve as conduits for fluid migration. Sedimentary faults and fractures induced by earthquakes can act as oil and gas migration channels or store petroleum products as well. Seismoturbidites and mixed-source rudstones are excellent reservoirs due to their abundant primary or dissolved pores.

Sequence stratigraphy and reservoir potential in the Wenan inner-slope of the Baxian sag, Bohai Bay Basin, eastern China

The high-resolution sequences, depositional environments, and lithostratigraphic trapping styles of an Eocene lacustrine rift succession in the Wenan inner-slope of the Baxian sag in Jizhong depression of Bohai Bay Basin were studied using newly processed pre-stack seismic 3-D data, complemented by well logs and cores of new drilled well XL1. Four third-order sequences were identified in the study interval (the 3rd and 4th members of the Shahejie Formation). The lower two sequences mainly contain transgressive system tracts (TST) and highstand system tracts (HST), whereas the upper sequence only contains lowstand system tracts (LST). Seismic stratal slices demonstrate that the deltaic deposits were widely developed in the Wenan inner-slope. AVO inversion showed that the net sandstone of the study stratigraphic units decreased in thickness in the depositional-dip-oriented direction. A new potential reservoir model of the 4th member of the Shahejie Formation was built for the development of abundant undiscovered lithostratigraphic reserves and four major lithostratigraphic trapping styles were identified.

Evaluation of geological features for deep coalbed methane reservoirs in the Dacheng Salient, Jizhong Depression, China

The Dacheng Salient of the northeast Jizhong Depression is one of the most promising areas for deep coalbed methane (CBM) development in China. This work investigates the Permo-Carboniferous (C-P) Taiyuan and Shanxi Formation coals and CBM geology and gas accumulation characteristics in the Dacheng Salient based on field data from 24 wells and laboratory measurements of eight coal samples and eight mudstone and sandstones from exploration well D1-1.The results show that the coal rank in the Dacheng Salient is a high-volatile C bituminous coal to low-volatile bituminous coal (0.54–1.85% Ro) controlled by the magmatism in the region. The coals vary from 2.94 to 8.58% in porosity and 0.263m2/g to 2.6m2/g in BET surface area. The in-situ permeability is low, normally less than 0.5×10−3μm2.Methane isothermal adsorption measurements of five coal samples reveal that their maximum adsorption capacity (on an as-received basis) varies from 17.13 to 25.98m3/t, with an apparent relationship with ash yield. The roof and floor of coal seams are mainly mudstone, with minor quantities of sandy mudstone and carbonate; all have the ability to be suitable cap rocks with adequate sealing capacity and large breakthrough pressure. The in-place gas content range from 10.23 to 1.41m3/t with a decreasing trend from No. 3 to No. 6 coal seam group in the D1-1 well due to effects by the active ground water near the east Jinghai-Dacheng fault. However, the gas content and the gas saturation increase with burial depth and ground water salinity, and decrease with coal rank. Finally, based on a comprehensive analysis of coal thickness, gas content, hydrogeology conditions, roof, floor, and depth properties, this study reveals that the best prospective target areas for CBM production are the south-central parts of the Dacheng Salient at1200~1600m in depth

Origin and evolution of formation water in North China Plain based on hydrochemistry and stable isotopes (2H, 18O, 37Cl and 81Br)

Origin and evolution of formation water are of great hydrogeochemical interest, the research on which can provide indicator for genesis and production of oilfield. Although many studies were carried out to investigate the origin and evolution of formation water in North China Plain (NCP), the research is still insufficient because of its complicated history. Considering these, based on hydrogeochemistry and stable isotopes (2H, 18O, 37Cl and 81Br), the origin and evolution of formation waters in North China Plain were further discussed. Results indicate that formation waters both in Jizhong Depression and Huanghua Depression in NCP are derived from meteoric water (river water and/or lake water) and the primary evolution processes are evaporation and mixing with seawater. The difference between two tectonic units is that the evolution of formation waters in Jizhong Depression is similar to that of salt lake in arid area, which underwent more evaporation, while formation waters in Huanghua Depression show more trace affected by seawater. This geochemical difference reflects the tectonic difference between two depressions. It can also be concluded that 81Br behaves more conservatively than 37Cl in the evolution of groundwater under certain circumstances.

Permo-Carboniferous Sedimentary Facies Analysis in Northeast Jizhong Depression

This document studies the deposition condition in northeast Jizhong depression by using relevant data of cores, logging and scanning electron microscope. The result indicates that the study area has 7 kinds of lithofacies, including conglomerate, sandstone, siltstone, mudstone, bauxite, carbonate rock and coal seam . Bottom-up development in the study area cover tidal flat, lagoon, delta, meandering river and braided river, a total of five kinds of sedimentary facies. Benxi Formation, Taiyuan Formation and Shanxi Formation deposited in transitional facies, which has high generation potential of Hydrocarbon. Lower and upper Shihezi Formation is mainly a set of river deposition as a good reservoir. The top of upper Shihezi Formation and Shiqianfen Formation are mainly mudstone, which is conducive to seal oil and gas.

Organic geochemistry and reservoir characterization of the organic matter-rich calcilutite in the Shulu Sag, Bohai Bay Basin, North China

Although extensive studies have been conducted on unconventional mudstone (shales) reservoirs in recent years, little work has been performed on unconventional tight organic matter-rich, fine-grained carbonate reservoirs. The Shulu Sag is located in the southwestern corner of the Jizhong Depression in the Bohai Bay Basin and filled with 400–1000 m of Eocene lacustrine organic matter-rich carbonates. The study of the organic matter-rich calcilutite in the Shulu Sag will provide a good opportunity to improve our knowledge of unconventional tight oil in North China. The dominant minerals of calcilutite rocks in the Shulu Sag are carbonates (including calcite and dolomite), with an average of 61.5 wt.%. The carbonate particles are predominantly in the clay to silt size range. Three lithofacies were identified: laminated calcilutite, massive calcilutite, and calcisiltite–calcilutite. The calcilutite rocks (including all the three lithofacies) in the third unit of the Shahejie Formation in the Eocene (Es3) have total organic carbon (TOC) values ranging from 0.12 to 7.97 wt.%, with an average of 1.66 wt.%. Most of the analyzed samples have good, very good or excellent hydrocarbon potential. The organic matter in the Shulu samples is predominantly of Type I to Type II kerogen, with minor amounts of Type III kerogen. The temperature of maximum yield of pyrolysate (Tmax) values range from 424 to 452 °C (with an average of 444 °C) indicating most of samples are thermally mature with respect to oil generation. The calcilutite samples have the free hydrocarbons (S1) values from 0.03 to 2.32 mg HC/g rock, with an average of 0.5 mg HC/g rock, the hydrocarbons cracked from kerogen (S2) yield values in the range of 0.08–57.08 mg HC/g rock, with an average of 9.06 mg HC/g rock, and hydrogen index (HI) values in the range of 55–749 mg HC/g TOC, with an average of 464 mg HC/g TOC. The organic-rich calcilutite of the Shulu Sag has very good source rock generative potential and have obtained thermal maturity levels equivalent to the oil window. The pores in the Shulu calcilutite are of various types and sizes and were divided into three types: (1) pores within organic matter, (2) interparticle pores between detrital or authigenic particles, and (3) intraparticle pores within detrital grains or crystals. Fractures in the Shulu calcilutite are parallel to bedding, high angle, and vertical, having a significant effect on hydrocarbon migration and production. The organic matter and dolomite contents are the main factors that control calcilutite reservoir quality in the Shulu Sag.

Water contents and electrical conductivity of peridotite xenoliths from the North China Craton: Implications for water distribution in the upper mantle

The eastern North China Craton (NCC) experienced significant lithospheric thinning and widespread magmatism in the Mesozoic, and is characterized by high surface heat flow and high conductivity layers (HCL) in the upper mantle. An integrated study of petrology and petrophysics will improve our understanding of the relationships between the electrical structure, thermal structure and chemical compositions (iron content and water content) of the upper mantle. Nine spinel peridotite xenoliths were collected from four Cenozoic basalts (Yangyuan, Hannuoba, Hebi and Nushan) in the Eastern Block and the Taihang Mountains of the NCC. These samples show compositional variations from depleted harzburgites to fertile lherzolites, representing the relic Archean lithosphere, modified Proterozoic lithosphere and newly accreted lithosphere after the lithospheric thinning event. The water contents of the peridotite samples were analyzed using Fourier transform infrared spectrometry. The water contents in olivine are very low (2–13ppm H2O), hence the whole-rock water concentration is controlled by orthopyroxene (Opx) and clinopyroxene (Cpx). Different hydration states of peridotites are distinguished according to the water contents in Opx: saturated (350ppm), water-rich (>120ppm), water-poor (40–90ppm) and dry (~1ppm). Using a piston cylinder press and Solartron 1260 phase-gain analyzer, the electrical conductivity of sintered peridotites was measured at pressures of 1–2GPa and temperatures of 350–1150°C. The electrical conductivity (σ) follows an Arrhenius equation: σ=σ0·exp(−ΔH/kT), where T is in Kelvin and k is the Boltzmann constant. The pre-exponential factor (σ0) and activation enthalpy of electric conductivity (ΔH) of spinel peridotites vary in the range of 100.65–102.38S/m and 1.03–1.45eV, respectively. Based on electrical conductivity of mantle minerals, we proposed a new equation to model the effect of iron content and water content on the conductivity of olivine, Opx and Cpx. The calculated conductivity using the Hashin–Shtrikman average can match the measured values of the peridotite samples by assuming a mixture of dry olivine and hydrogen-bearing Opx and Cpx. This demonstrates the contribution of both small polaron conduction and proton conduction mechanisms to the bulk conductivity of peridotites, and the dominant contribution of olivine to the bulk conductivity. Based on the thermal state of the lithosphere in different tectonic units, we obtained in situ conductivity of peridotites and pyroxenites with different hydration states. A comparison between in situ conductivity of mantle rocks and the magnetotelluric (MT) data in the eastern NCC suggests that in most regions, the shallow lithospheric mantle consists of both water-rich and water-poor peridotites, while the underlying lithospheric mantle is dry. The HCL in the upper mantle could be caused by partial melting in the lithospheric mantle (e.g., beneath the northern Taihang Mountains), or by water-rich peridotites near the lithosphere–asthenosphere boundary (e.g., beneath the Jizhong depression and the Luxi uplift) and in the asthenosphere (e.g., beneath the southern Taihang Mountains). Combined with water contents in peridotite xenoliths, we propose that the lithospheric mantle of the Eastern Block has been further dehydrated by the early Cretaceous giant igneous event and the Cenozoic volcanism. The dehydration process was accompanied with the lithospheric thinning and gradually increased the strength of the lithosphere, which finally resulted in the lithospheric thickening in the late Tertiary and Quaternary beneath the Eastern Block. In contrast, the Taihang Mountains are subjected to progressive lithospheric thinning since late Miocene and the lithospheric mantle has been significantly dehydrated. Our study not only provides an example to trace the heterogeneous water concentration in the continental upper mantle, but also highlights the contribution of magmatism-induced dehydration to the evolution of cratons.