Moderate Burial Depths Research Articles

Advancing Sustainable Geothermal Energy: A Case Study of Controlled Source Audio-Frequency Magnetotellurics Applications in Qihe, Shandong

Geothermal energy is a key part of sustainable and renewable energy strategies, especially for clean heating in northern regions. This study focuses on Qihe County in Shandong Province, applying a controlled source audio-frequency magnetotellurics (CSAMT) method to investigate deep karst geothermal reservoirs. This research addresses the complex geological conditions and electromagnetic interference in the region, aiming to improve sustainable geothermal resource development. The findings indicate that the geothermal reservoir in the study area primarily consists of Ordovician limestone, characterized by moderate burial depth, high water volume, and elevated water temperature. Integrating CSAMT with vertical electrical sounding (VES) and radiometric surveying has clearly defined the deep aquifer layers and major water-controlling fault structures. Drilling verification results demonstrate the significant effectiveness of the integrated geophysical methods employed, providing reliable technical support for deep geothermal exploration in similar regions. This study makes a significant contribution to the scientific and technical foundation necessary for the sustainable development and utilization of geothermal resources, supporting the broader goals of environmental sustainability and renewable energy.

Numerical and experimental modeling of wave-induced forces on submarine pipeline buried in the soil of different engineering properties

In the present paper, the wave-induced forces on the submarine pipeline buried in porous soil having different properties are studied. The mathematical problem is solved using the higher-order boundary element method, and the numerical results are validated with experimental results obtained through rigorous model tests. Four different types of Kuwaiti marine soil: Al-Khiran, Al-Koot, Sabiya, and Shuaiba are considered in the present study. The effect of burial depth of the submarine pipeline in the soil seabed, incident wave period and wave height, and hydraulic conductivity of the soils on the wave forces acting on the submarine pipeline is analyzed in a detailed manner. The study reveals that for moderate burial depth of the pipeline in the soil bed, the wave force acting on the submarine pipeline can be reduced up to 50% for soil having less hydraulic conductivity. The measured wave forces and numerical predictions are matching well for a wide range of wave, soil and structural parameters.

Study on the Bending Effect and Rock Burst Mechanism of Middle Rock Pillars in Extremely Thick Subvertical Coal Seams

Rock bursts occur in nearly vertical coal seam mines at shallow to moderate burial depths, which endangers safe mining. To study the rock burst mechanisms of nearly vertical and extremely thick coal seams, the characteristics of rock bursts were studied via on-site investigation, and a field test of in situ stress was carried out. The mechanical behavior of rock pillars in the middle of the B1+2 and B3+6 coal seams was analyzed using theoretical and numerical simulation methods. The results show that the horizontal maximum principal stress orientation and the nearly vertical coal seam strike were both 82°. The bending of the rock pillars occurred due to the horizontal unbalanced force, and a large amount of bending energy was accumulated within 50 m above the mining level. Rock pillars were bent toward the B1+2 mining goaf and exerted a reverse bending and squeezing effect on the B3+6 coal seam below the mining levels. In addition to the inclination and compression of the B3+6 coal seam roof, stress concentration zones formed in the B3+6 coal seam, where a large amount of elastic energy had accumulated in the coal-rock mass. Consequently, both the rock pillars and the B3+6 coal body at the mining level are in an unstable state undue to mining disturbance. Rock burst energy theory and numerical calculation results showed that in the stress concentration zones of the B3+6 coal seam, the energy density of the coal mass reached or exceeded its critical value before rock burst occurred, and rock bursts were prone to occur under mining disturbances. The in situ microseismic results showed that high-energy microseismic events were mainly concentrated in middle rock pillars around the mining levels and the coal mass in high-stress concentration zones.

Evolution History of Overpressured and Normally Pressured Shale Gas Reservoirs in Wufeng Formation–Longmaxi Formation, Sichuan Basin, China: An Analysis from the Perspective of Source and Seal Coupling Mechanism

Among the shale gas fields in the Sichuan Basin, the Fuling, Changning, and Weiyuan fields with Wufeng Formation-Longmaxi Formation as the target formation have made significant contributions to the shale gas industry in China. However, several other areas in the basin which have witnessed gas flow from wildcat wells have not realized commercial development. In this study, we analyzed the Wufeng Formation–Longmaxi Formation shale gas reservoirs from the perspective of a coupling mechanism of source and seal, by considering three aspects, including the characteristics of source and cap rocks, evolution history of the source rock–reservoir caprock, and the differential enrichment of shale gas. The results indicate the following: (1) The occurrence of the regional caprock (Triassic gypsum-salt) determines the distribution of overpressured and normally pressured shale gas reservoirs. In addition, the erosion of the caprock and its integrity control the gas content and cost-efficiency of the normally pressured shale gas reservoirs. (2) For the overpressured reservoirs, the thickness of the siliceous and carbonate-bearing siliceous shales in the WF2-LM4 graptolite zone determines the gas content and producibility of the lower shale gas layer. In terms of the upper layers, the structural form plays a critical role. Generally, gas reservoirs in the upper layers that occupy the higher part of an anticline are more favorable, whereas those in a syncline or monocline exhibit poor productivity. (3) For the normally pressured shale gas reservoirs with poor caprock integrity, the enrichment and the commercial value of the gas depend mostly on the thickness of the WF2-LM4 graptolite zone. Therefore, we conclude that the areas near the core of synclines are more favorable and structurally stable, with relatively high formation pressure and moderate burial depths. Our findings can improve sweet spot prediction and offer valuable guidance for shale gas development in China and globally.

Multistage fracturing history in the Paleocene lacustrine shale oil reservoirs of the Subei Basin, Eastern China

This study examines sealed fractures in core samples from the Paleocene organic-rich lacustrine shales of the Gaoyou Depression, Subei Basin, China, using a multi-pronged approach to elucidate the interplay between fracture propagation and fluid circulation in shaly successions. Based on petrography, geochemistry, and geochronology of fracture-filling minerals, four generations of vein emplacement (denoted Types A-D) were identified. These were attributed to successive episodes of burial diagenesis, tectonism, and uplift occurring at different phases of basin evolution. These vein generations were then inserted into the temporal-spatial framework derived from basin modeling, leading to a reconstruction of the fracture diagenesis history in the Funing shales. Two of the vein generations contain evidence for two phases of oil migration marked by two types of oil inclusions with different fluorescence.It has been shown that the studied lacustrine shales were deposited at ca. 60.2–58.0 Ma. The earliest vein generation, represented by antitaxial beef calcite veins sub-perpendicular to shale bedding, propagated in incompletely consolidated shales within a regional thrust regime related to the Wubao Movement. The vein nucleation during a short-lived burial following initial deposition was also evidenced by the presence of monoliquid inclusions in the veins and calcite LA-ICP-MS U–Pb age (57.7 ± 3.7 Ma). Tectonic compressive stress together with overpressures arising from horizontal and vertical disequilibrium compaction were interpreted as being responsible for this fracturing episode. The second vein generation is bedding-parallel, oil inclusion-rich stretching calcite veins with a crack-seal texture, formed concurrently with kerogen maturation and early oil generation at moderate burial depths. The vein crack-seal fills containing yellow fluorescing oil inclusions provide evidence for episodic oil expulsion in response to cyclic rises and drops in pore pressure during the oil window. Bedding-parallel beef calcite veins characterized by antitaxial growth represent the next vein generation, emplaced during the maximum burial period (ca. 41.9 ± 2.7 Ma). The vein fibers have tracked the horizontal displacement of the wall rocks, likely related to lateral shortening before or at the initiation of Late Eocene basin inversion. The first three vein generations were generated in a relative closed fluid system, with vein fill sourced locally. Host-rock carbonate components and/or earlier diagenetic carbonates were dissolved and then mobilized into these veins. By contrast, the last vein generation with blocky calcite and barite cementation is associated with input of externally derived hydrothermal fluids. The evidence comes from abnormally high fluid-inclusion homogenization temperature (Th) and salinity data, as well as the fact that Ba and sulfate were sourced from the underlying evaporite layers rather than local shales. We thus infer that cross-stratal faulting during the uplift related to the Sanduo Movement resulted in the opening of initial hydraulically closed fluid system. This episode of tectonically-driven hydrothermal circulation from deeper locations in the basin into the Funing shales led to the mixing of oil-bearing pore fluids with exotic basinal hot brines. This also accounts for the presence of abundant blue-green fluorescing oil inclusions and significantly elevated fluid salinity. Overall, this study emphasizes that during basin evolution, differences in tectonic settings and the diagenetic boundary conditions under which veins formed results in different fracturing mechanisms and associated fluid-flow regimes in shale sequences, with consequent effects on vein attributes and mineralization.

Enrichment control factors and exploration potential of lacustrine shale oil and gas: A case study of Jurassic in the Fuling area of the Sichuan Basin

Well Taiye 1 in the Fuling area of the eastern Sichuan Basin has obtained a high-yield industrial gas flow (7.5 × 104 m3/d gas and 9.8 m3/d oil) from the Middle Jurassic Lianggaoshan Formation, presenting a good test production effects, which means the realization of a major breakthrough in the exploration of Jurassic lacustrine shale oil and gas in the Sichuan Basin. In order to further determine the exploration potential of lacustrine shale oil and gas in this area and realize the large-scale efficient development and utilization of lacustrine shale oil and gas, this paper analyzes the geological conditions for the accumulation of lacustrine shale oil and gas in this area by using the drilling data of 10 key wells, such as wells Taiye 1 and Fuye 10. Then, the main factors controlling the enrichment of lacustrine shale oil and gas are discussed, and the exploration potential and favorable target zones of Jurassic lacustrine shale oil and gas in the Fuling area are defined. And the following research results are obtained. First, the quality Jurassic semi-deep lake shale in the Fuling area is characterized by high organic matter abundance, high porosity and high gas content, and it is the geological base of shale oil and gas enrichment. Second, the developed large wide and gentle syncline, good preservation condition and higher pressure coefficient (generally >1.2) are the key to the enrichment and high yield of shale oil and gas. Third, the developed microfractures in lacustrine shale are conducive to the enrichment and later fracturing of shale oil and gas. In conclusion, the Lianggaoshan Formation lacustrine shale in the Fuling area is widely distributed with moderate burial depth, developed microfractures and moderate thermal evolution, and its shale gas resource extent and shale oil resource extent are 1 922 × 108 m3 and 2800 × 104 t, respectively, indicating greater potential of shale oil and gas exploration, so shale oil and gas is the important field of oil and gas reserves and production increase in this area in the following stage.

In situ U-Pb dating and geochemical characterization of multi-stage dolomite cementation in the Ediacaran Dengying Formation, Central Sichuan Basin, China: Constraints on diagenetic, hydrothermal and paleo-oil filling events

Improved understanding of subsurface paleo-fluid circulation history can assist in reconstructing associated pore evolution of carbonate reservoirs. Multi-stage dolomite cements generated by basinal fluids over time were investigated using a combination of petrography, geochemistry, fluid-inclusion studies, LA-ICP-MS U-Pb geochronology, and seismic interpretation. This study aims to gain clues about diagenetic, hydrothermal and paleo-oil filling events within the Ediacaran Dengying Formation, Sichuan Basin, China.Four main phases of dolomite cementation were petrographically, geochemically and geochronologically distinguished that can be ascribed to major tectonic events throughout the basin evolution. The earliest dolomite generation occurs as fibrous dolomite crust (FDC) growing along the karst vug wall, interpreted as marine diagenetic cement formed in a submarine environment during the Late Ediacaran, most likely from the influx of seawaters following the uplift and subaerial exposure related to the Tongwang movement. In situ U-Pb ages obtained from the other three generations of cement dolomites are interpreted to coincide with the timing of repeated strike-slip fault movements as indicated by the seismic data, suggesting a direct link between cement emplacement and regional tectonic-thermal events. Sub-vertical extension dolomite veins (EDV), representing the second generation, are considered as being syntectonic, generated from slightly modified connate seawaters at a shallow to moderate burial depth in a transtensional stress regime during the Cambrian taphrogenesis. Two generations of saddle dolomite cements, SD1 and SD2, have recorded two major episodes of hot fluids circulating in the Dengying Formation. The SD1, filling in hydraulic fractures and vugs, formed during the third phase of cementation, as a result of a hydrothermal event associated with Caledonian exhumation. Deep-seated hot brines squeezed into the Dengying reservoirs along the sub-vertical strike-slip faults combined with some meteoric waters that percolated downward, are deemed responsible for precipitating SD1. The SD2 is the youngest cement present, which is interpreted to have formed during the ingress of deeper, hot, hypersaline brines arising from the late Permian hydrothermal activity likely triggered by the eruption of Emeishan flood basalts.Cross-cutting relationships between bitumen phases and different dolomite generations indirectly define the dates of two paleo-oil charging events. The first episode of paleo-oil charging occurred during the Late Ediacaran to Early Cambrian, likely sourced from underlying Ediacaran Doushantuo shales. Late oil emplacement occurred after the Late Permian, in agreement with the reported bitumen Re-Os age and the age inferred through fluid-inclusion analysis. Primary source rocks generating this phase of oil were considered as the Cambrian Qiongzhusi shales, because basin modeling-derived date of oil window matches with the second oil filling time.

Cd absorption characteristics of Suaeda salsa under different sediment burial and exogenous Cd input conditions in the Yellow River estuary, China.

Suaeda salsa (L.) Pall., a typical halophyte plant in the Yellow River estuary, has high enrichment capacity for heavy metals. However, few studies have investigated the Cd absorption characteristics of S. salsa under different sediment burial and exogenous Cd input conditions, especially following the water-sediment regulation scheme (WSRS), which brought sediment burial and exogenous substances to the estuary. So, we established a greenhouse pot culture experiment with four sediment burial depths (0 cm, 3 cm, 6 cm, and 12 cm) and exogenous Cd input levels (0 mg·kg-1, 0.5 mg·kg-1, 1.0 mg·kg-1, and 1.5 mg·kg-1) and analyzed the leaf, stem, root, and total biomass; leaf, stem, and root Cd content; and storage, sediment Cd content, accumulation factor, root/leaf (R/L), root/stem (R/S), and stem/leaf (S/L) ratios to study the Cd absorption characteristics of S. salsa under the different sediment burial and exogenous Cd input. Results showed that high Cd content in roots, stems, and leaves was harmful to S. salsa growth, and then led to a decrease in biomass (characterized by stem, leaf, and total biomass). Suaeda salsa exhibited a survival strategy to deal with Cd toxicity, which involved the roots absorbing Cd from the sediment and storing it in stems and leaves (stem and leaf Cd content peaked at 0.5 mg·kg-1 Cd input) at low Cd input, whereas roots stored more Cd and reduced Cd transport to stems and leaves at high Cd input. Therefore, we observed the maximum value of leaf (500.63 ± 19.15 g·m-2), stem (648.22 ± 50.08 g·m-2), and total biomass (1246.92 ± 55.49 g·m-2) in the treatment with 1.5 mg·kg-1 Cd input and 3-cm sedimentdepth due to the Cd content in leaves and stems being relatively low. The accumulation factors of leaves, stems, and roots varied (0.39-0.99, 0.19-2.58, and 0.80-20.45, respectively), and most of the accumulation factors for roots and leaves and the R/L and R/S ratios were >1, which indicated that S. salsa had high enrichment levels of Cd, which mostly accumulated in the roots. Shallow or moderate burial depth was beneficial to S. salsa growth, but sediment burial was not beneficial to Cd absorption because the sum of leaf, stem, and root Cd storage was higher at 0-cm depth compared with the other depths. Variance analysis showed that the influence of Cd input on leaf, stem, root, and sediment Cd content and stem and root Cd storage was significant (P < 0.05), whereas sediment burial, interaction of sediment burial and Cd input on Cd content, storage, and biomass were not significant (P > 0.05). Therefore, we concluded that more attention should be paid to the control of sediment burial and heavy metal input, especially during the WSRS, in the Yellow River estuary.

A novel authigenic magnetite source for sedimentary magnetization

AbstractWe report a novel authigenic nanoscale magnetite source in marine methane seep sediments. The magnetite occurs in large concentrations in multiple horizons in a 230 m sediment core with gas hydrate–bearing intervals. In contrast to typical biogenic magnetite produced by magnetotactic bacteria and dissimilatory iron-reducing bacteria, most particles have sizes of 200–800 nm and many are aligned in distinctive structures that resemble microbial precipitates. The magnetite is interpreted to be a byproduct of microbial iron reduction within methanic sediments with rapidly changing redox conditions. Iron sulfides that accumulated at a shallow sulfate-methane transition zone were oxidized after methane seepage intensity decreased. The alteration process produced secondary iron (oxyhydr)oxides that then became a reactive iron source for magnetite authigenesis when methane seepage increased again. This interpretation is consistent with 13C depletion in coexisting carbonate nodules. The authigenic magnetite will record younger paleomagnetic signals than surrounding sediments, which is important for paleomagnetic interpretations in seep systems. The microbial and possibly abiotic processes that caused these magnetic minerals to form at moderate burial depths remain to be determined.

An overview of the geology and production of the Fuling shale gas field, Sichuan Basin, China

The Fuling shale gas field in China is the largest shale gas field as well as the largest of its type discovered in any Lower Paleozoic formation. In this study, the geology and production of the upper and lower gas layers in the Fuling shale gas field are evaluated in terms of structure, shale quality, fault, initial production, and estimated ultimate recovery (EUR). The shale in the lower gas layer of the Jiaoshiba anticline is a high-quality reservoir, where the space is dominated by organic pores in kerogen, and the gas content is high. The shale gas wells reveal relatively high initial production and EUR. However, the shale in the upper gas layer of the Jiaoshiba anticline has reservoir space mainly composed of clay mineral pores and organic pores within bitumen, and the gas content is low. In terms of structure, primary gas migration may occur in the upper gas layer, resulting in free gas accumulation in the structural high, where the development effects are generally better than those in the structural low. The lower gas layer in the Pingqiao anticline, is the main interval for shale gas accumulation and development due to the high-quality shale. Under the influence of faults, the efficiency of exploration wells emplaced on top of the anticline is much lower a compared with those in the flanks. The residual synclines close to the Sichuan Basin, including the Baima and Baitao anticlines, are characterized by more recent uplifts, larger area, greater distance from the deep and large faults, and early fracture closure. Therefore, we recommend that the shale gas exploration and development should be carried out preferentially in areas close to the center of the residual synclines, featuring relatively high-pressure coefficient and moderate burial depth.

Mechanics and fracturing techniques of deep shale from the Sichuan Basin, SW China

Deep shale gas exploration and production in Fuling (Sichuan Basin, SW China) are confronted with hydraulic fracturing challenges owing to high stress, high fracture pressure, low pump rate and proppant concentration, as well as high closing pressure in deep strata. This study focused on the mechanical properties of shale core samples from Fuling through high-temperature triaxial rock mechanical tests and in-situ stress tests based on the Kessel effect of acoustic emission. Their mechanical property variations with depth were delineated using brittleness index calculated via simulating different depths and different confining pressures for the samples. The results showed that several parameters of deep shale reservoirs, i.e. brittleness index, fracture density, performance of self-propping, and flow conductivity, are lower than that of shale reservoirs with moderate burial depth. Thus, the current operating pressure in deep shale reservoir stimulation should be taken full advantage of, rather than channeling the focus on the propagation of fracture length. The objective is to increase the complexity of the near-hole fracture network for enhancing self-propping and flow conductivity of the fractures. This can be achieved by reducing the number of perforation clusters and cluster spacing, adopting variable-rate fracturing, decreasing proppant size, increasing sand volume, and optimizing the fracturing parameters. A field application showed that, compared with the neighboring wells, the test well had larger drainage area, doubling the gas yield.

Reservoir characteristics of and potential analysis to Lower Palaeozoic shale in Xiuwu Basin, Lower Yangtze Plate

ABSTRACT In order to evaluate reservoir characteristics and gas potential of organic-rich shale in Bottom-Lower Cambrian Hetang Formation and Upper Ordovician Xinkailing Formation, Xiuwu Basin, Jiangxi Province, Lower Yangtze Plate of China, researches regarding reservoir characteristics of organic-rich shale thickness, moderate burial depth, abundance and maturity of organic matter, fracturability, organic-matter pores and absorption performance have been conducted to Hetang Formation and Xinkailing Formation through field survey, shallow drilling, analysis to hydrocarbon-generation potential, scanning electron microscopy, measuring specific surface area and isothermal adsorption experiments. The results show that (1) Hetang Formation reservoir, although with extremely rich material basis, is of unfavorable hydrocarbon generation conditions due to late-stage thermal maturity, besides of undeveloped micropore structure, reservoir space development mainly by macroscopic crack, so further exploration should focus on locating favorable areas of low maturity and developed cracks, which is considered distributing in the core of Yanshan-Lingangshan-Shiziyan Mountains syncline; (2) Xinkailing Formation reservoir is of better organic matter abundance, pore development and absorption performance than Xinkailing Formation and hence further exploration should focus on surveying favorable areas of better sedimentary facies belt and stable structure for major breakthrough in shale-gas exploration, which is considered distributing in Wuning-Lixi and Jinkou-Tuolin areas.

Effects of Sediment Burial and Exogenous Cd Input on Biomass Allocation and Antioxidative Enzyme Activities of Suaeda salsa in the Coastal Wetland of the Yellow River Delta

The Yellow River Delta has been facing the threat of functional degradation during the recent years. The Water-Sediment Regulation Project not only supplements abundant freshwater, but also alters the sediment burial and heavy metal levels, which affects vegetation growth. Thus, we selected the pioneer species Suaeda salsa, to study the effects of different sediment burial depths (0, 3, 6, 12 cm) and exogenous Cd inputs (0, 0.5, 1.0, 1.5 mg·kg-1) on biomass allocation and activities of antioxidative enzymes in the coastal wetlands of the Yellow River delta. The results showed that a shallow or moderate burial depth had a stimulatory effect on chlorophyll content, while an excessive burial depth inhibited the growth of Suaeda salsa and chlorophyll content. With increasing Cd input, chlorophyll content and dry mass decreased. At a lower Cd input and moderate burial depth, activities of CAT and SOD increased, and at high levels, SOD activities decreased, while activities of CAT at a 12 cm burial depth and 1.0 mg·kg-1, 1.5 mg·kg-1 Cd input were higher than those for the control (62.66% and 58.56%). CAT activities reached high values (15.76 U·mg-1) at a high Cd input (1.5 mg·kg-1) and burial depth (12 cm). Analysis of variance showed that Cd input had a significant effect on protein content, and CAT and SOD activities, and sediment burial depth had a significant effect on the protein content and SOD activities. Interaction between Cd input and sediment burial depth had a significant effect on CAT and SOD activities (P<0.05). These results demonstrated that sediment burial depth and Cd input had a great influence on the growth of Suaeda salsa, and to some extent, Suaeda salsa could change its biomass allocation and antioxidative enzyme activities to adapt to severe environments.

Geology and hydrocarbon potential of offshore SE Somalia

Interpretation of a 20 550 line km 2D seismic survey acquired in 2014 by Soma Oil and Gas in the deep water offshore area of SE Somalia has identified three previously undocumented sedimentary provinces – Jubba Deep, Mogadishu Deep and Mid Somalia High – all of which have distinctive geological characteristics. Well and stratigraphic controls are limited, with inferred lithologies largely based on seismic stratigraphic interpretation. The Jubba Deep has a thick Late Cretaceous–Early Tertiary deltaic section deformed by major gravitational collapses in the Paleocene-age Kismaayo Thrust Belt (KTB) and the Pliocene-age Baraawe Thrust Belt (BTB). It is proposed that the KTB has significant hydrocarbon potential in deltaic and pro-delta sands trapped in thrust anticlines and sourced with oil from Mid-Cretaceous mobile shales. The Mogadishu Deep Basin has a thick Mesozoic and Tertiary section but is missing the thick deltaics seen in the Jubba Basin. Volcanics are present in this basin. The Mid Somalia High has a relatively thinner sedimentary section where Cretaceous and Jurassic reservoirs and potential source rocks are at moderate burial depths. An extensive post-rift Mid–Late Jurassic carbonate platform is developed here with potential hydrocarbon targets in interpreted reefs and shoal facies. Supplementary material : Additional seismic examples and map figures are available at https://doi.org/10.6084/m9.figshare.c.3902650

Carbonate and silicate cementation of siliciclastic sediments of the New Jersey shelf (IODP Expedition 313): relation with organic matter diagenesis and submarine groundwater discharge

The New Jersey continental shelf extends 150 km off the shoreline. During IODP Expedition 313, siliciclastic deposits of late Eocene to late Pleistocene age were drilled down to 631, 669 and 755 m below seafloor at sites 27A, 28A and 29A respectively in very shallow waters (33.5 to 36 m depth). Pore water salinities display multilayered brackish-salty-brine units 10 to 170 m thick, where low-salinity water is preferentially stored in fine-grained sediments. The sharp boundaries of these buried aquifers are often marked by cemented layers a few centimetres thick. The mineralogy and scanning electron microscope observations of these layers show two phases of cementation by authigenic minerals: (1) the early carbonate cement is frequently associated with pyrite, and (2) the late silicate cement infills the residual porosity. The isotopic compositions of the carbonate cements vary widely: −2.4 < δ18O ‰ VPDB < +2.8; −15.1 < δ13C ‰ VPDB < +15.6. The δ18O values indicate that the carbonate cements precipitated with pore waters comprising variable mixtures of seawater and 18O-depleted fresh water originating from submarine groundwater discharge. The δ13C values of the carbonate cements are related to organic matter diagenesis, providing 13C-depleted dissolved inorganic carbon during bacterial sulphate reduction and anaerobic oxidation of methane, and 13C-rich dissolved inorganic carbon during methanogenesis. The diagenetic cementation processes included chemical weathering of reactive silicate minerals by the CO2-rich pore waters issued from organic matter diagenesis that released bicarbonate, cations and dissolved silica, which were further precipitated as carbonate and silicate cements. The estimated range of temperature (18±4 °C) during carbonate precipitation is consistent with carbonate cementation at moderate burial depths; however, silicate cementation occurred later during diagenesis at deeper burial depths.

Geological difference and its significance of marine shale gases in South China

Abstract Organic-rich marine shales are developed in both the Cambrian Qiongzhusi Formation and the Ordovician Wufeng Formation–Silurian Longmaxi Formation in South China, but are different in the drilling results of shale gas exploration. Comparing the differences in shale gas formation conditions between Qiongzhusi and Wufeng-Longmaxi has practical and theoretical significance. This study reveals: (1) in the Sichuan Basin, the Wufeng-Longmaxi Formation has slightly higher TOC than the Qiongzhusi Formation, whereas Qiongzhusi Formation has some local high TOC areas outside of the Sichuan Basin; (2) the Qiongzhusi Formation has much higher thermal evolution degree than the Wufeng-Longmaxi Formation; (3) with undeveloped organic pores, the Qiongzhusi Formation has a 1/3 to 1/2 porosity of the Wufeng-Longmaxi Formation; (4) Qiongzhusi shale has a lower gas content, only 1/2 of that in Wufeng-Longmaxi shale; (5) the Qiongzhusi Formation is mainly composed of siliceous shale and the silica is hot water origin, whereas the Wufeng-Longmaxi Formation consists mainly of calcareous siliceous shale and the silica is biogenic origin; (6) the Wufeng-Longmaxi Formation has overpressure, while the Qiongzhusi Formation is normal in pressure. The reasons for the differences are: (1) different sedimentary environments affect TOC and shale thickness; (2) the Qiongzhusi Formation is over-mature, which caused depletion of hydrocarbon generation, organic carbonization, porosity reduction, and gas content drop; (3) the bad roof and floor conditions of the Qiongzhusi Formation are not good for shale gas preservation; (4) Wufeng-Longmaxi Formation is located in the slope and syncline accompanied with overpressure, and is in favor of preservation and high production of shale gas; (5) the uranium content in the Qiongzhusi Formation is twice that of the Wufeng-Longmaxi Formation, which is the main reason of its higher thermal evolution degree. It is concluded that shale gas enrichment in the marine shale in South China requires favorable geological conditions: organic-rich intervals, moderate thermal evolution, rich organic pores, high gas content, good roof and floor preservation conditions, and moderate burial depth. The Wufeng-Longmaxi Formation has better shale gas enrichment conditions and higher resource potential, whereas the Qiongzhusi Formation has poorer shale gas accumulation conditions and limited favorable areas.

Thermal evolution characteristics of Triassic coal in Chuxiong Basin and its geological significance

Chuxiong Basin in Yunnan is a typical Mesozoic foreland basin which is enriched in widely distributed Triassic coal resources with thick deposits and of important strategic significance. By applying vitrinite reflectance measurement, inclusion thermometry, fission track dating and EASY% Ro numerical simulation, the Triassic coal thermal evolution history of the Chuxiong Basin was analyzed, and the results were concluded. The vitrinite reflectance of Chuxiong Basin is higher in the west and south in general. Vertically, in the east, west, and north of the basin, the vitrinite reflectance increases with increasing depth, and in the northern part, exceptionally high values occur, and there is no significant regularity in the east. The formation of inclusions inside quartz veins in Chuxiong Basin Triassic coal are unrelated with magmatic hydrothermal fluid, and there are multi-phase inclusions formed in three or four sections of tectonic movements. The main heating period (apparent age) of the Triassic coal is concentrated in the late Cenozoic, during which the coal was subjected to repeated thermal disturbance, resulting in a multimodal distribution of the fission track data, which reveals mild burial features of the early stages of the Late Cenozoic. The Triassic coal of Chuxiong Basin has experienced two major temperature increasing processes, which occurred in the early-mid Yanshan and the early Himalayan, respectively. The first hydrocarbon generation period of coal organic matter occurred in the formation stage of the foreland basin, during which the south and west of the basin generated large amounts of hydrocarbon, but little was preserved. The second generation stage in the Early Himalayan had conditions suitable for high gas accumulation, especially in the western and southern regions. The upper Triassic coal is of moderate burial depth and is less affected by the strike-slip effect. There are key areas of Chuxiong Basin oil and gas exploration, such as the Yanfeng Basin in the north-central, Yongren and Yunlong areas.

Burial depth and diameter of the rhizome fragments affect the regenerative capacity of a clonal shrub

Clonal plants in highly disturbed habitats are often broken into small fragments of various sizes and buried at various soil depths. As a storage organ, rhizome fragments play an important role in enabling plants to survive in such habitats. But few studies have been concerned about the regenerative capacity of rhizome fragments of clonal shrubs of different rhizome diameter and at different burial depths. Here, we investigated whether deeper burial decreased, and diameter of the rhizome fragment increased, the regenerative capacity of a clonal shrub. Research samples of rhizome fragment (rhizome diameters of 2, 5, 10, 15, and 20mm) of the clonal shrub Calligonum arborescens were buried at different depths (0, 1, 5, 10, and 20cm). Increasing the diameter of the rhizome fragments significantly increased the survival rate of fragments, and increased the above-ground, below-ground and total biomass production of fragments. Vegetative reproduction ability also increased with an increase in diameter of the rhizome fragments. With an increase in sand burial depth, above-ground, below-ground, total biomass production and vegetative reproduction ability first decreased and then increased, and no fragments survived at the 0cm burial depth. These results indicate that sand burial depth and diameter of the rhizome fragments significantly affected the regeneration capacity of C. arborescens. Sand burial is one of the essential prerequisites for C. arborescens rhizome fragments’ survival. Moderate burial depth (5cm) and larger fragment diameter (20mm diameter) were more suitable for biomass production and vegetative reproduction. These results indicate that reserves stored in rhizome fragments can contribute greatly to the regeneration capacity of the C. arborescens—responses that are very important for C. arborescens survival and establishment in frequently disturbed habitats.

Factors controlling permeability of cataclastic deformation bands and faults in porous sandstone reservoirs

Improving the prediction of sub-seismic structures and their petrophysical properties is essential for realistic characterization of deformed sandstone reservoirs. In the present paper, we describe permeability contrasts induced by cataclastic deformation bands and faults in porous sandstones (766 data synthesized from field examples and the literature). We also discuss the influence of several factors, including tectonic regime, presence of a fault, burial depth, host sandstone porosity, and grain size and sorting for their initiation and permeability. This analysis confirms that permeability decrease is as a function of grain-crushing intensity in bands. Permeability reduction ranges from very limited in crush-microbreccia of compaction bands to high permeability reduction in cataclasites and ultracataclasites of shear-dominated bands, band clusters and faults. Tectonic regime, and especially normal-fault regime, with its tendency to localize strain and generate faults, is identified as the most important factor, leading to the formation of cataclastic bands with high permeability contrasts. Moreover, moderate burial depth (1–3 km) favors cataclastic bands with high permeability contrasts with respect to the host sandstone. High porosity, coarse-grain size and good grain sorting can slightly amplify the permeability reductions recorded in bands.

A developmental model of lacustrine shale gas genesis: A case from T3y7 shale in the Ordos Basin, China

Exploration for gas producing lacustrine shales in the Ordos Basin has identified considerable capacity in T3y7 shale in the Upper Triassic. The T3y7 shale is a black color, siliclastic organic rich lacustrine mudrock. The mineral components primarily include clay minerals, quartz and feldspar, with the brittle mineral (quartz, feldspar and carbonate) content accounting for 56 wt. %. The presence of type II kerogens, the high TOC contents (most 2%–4%), thermal maturation reaching the oil/gas window and constant organic-rich shale thickness indicate that optimum conditions for shale gas have been reached. The maximum adsorption gas content of T3y7 shale reaches 1 cm3/g at 0.5 MPa, and the value of most samples was more than 2 cm3/g and, at times, even reached 4.6 cm3/g at 5 MPa pressure, which is higher than the main gas shale in the U.S. Organic carbon isotope suggests that the natural gas in T3y7 shale is associated with oil-prone and thermally mature gases. According to the simulation product, three stages can be identified in the shale thermal maturation and hydrocarbon expulsion process: the oil window (0.5% < Ro < 1.1%), wet gas window (1.1% < Ro < 1.8%) and the dry gas window (Ro > 1.8%). These results indicate that the natural gas in T3y7 shale is characterized as wet gas. The relationship between the gas/oil ratio and the conversion degree of organic matter (TRHI) was established in this study. Comprehensive analysis of basin evolution, the development of organic-rich shale and burial history, shale gas – shale oil/gas – shale oil – shale gas was regularly distributed from the bottom to the upper strata. Simultaneously, shale oil – wet gas – shale gas developed from the depression center to the basin slope and, subsequently, to the basin margin. In the lake, the zonation of organic types was obvious, which was manifested as Type I-prone in the semi-deep and deep lake while Type III-prone in the shallow lake. The organic matter of target shale was primarily Type-II. The shale gas of T3y7 shale was mainly pyrolysis yield, which was most associated with oil, and the wet gas appears broadly. Compared with the marine-continent transition and marine shale, T3y7 shale is characterized by a moderate burial depth and thickness, high TOC content, low porosity, high gas content, mostly adsorbed gas and wet gas.