Lower Paleozoic Shale Research Articles

Implications of multi-stage deformation on the differential preservation of Lower Paleozoic shale gas in tectonically complex regions: New structural and kinematic constraints from the Upper Yangtze Platform, South China

The strength of tectonic deformation in tectonically complex regions, particularly multi-stage superimposed tectonic regions, can have a significant impact on shale gas preservation conditions. However, differential tectonic deformation has still not been quantitatively constrained, and its impact on the preservation conditions of shale gas is thus poorly understood in the northern Guizhou Province, southwest China. This study investigates the differential preservation conditions of marine shale gas in four eroded synclines (including Shixi, Daozhen, Zhongguan, and Fuyan Synclines) of the northern Guizhou Province. Interpretation of structures in field outcrops, geometry and kinematic analysis, and typical gas reservoir anatomy are combined to quantitatively explore the effect of superimposed deformation on the preservation conditions of shale gas. According to a set of new structural and kinematic constraints, the Riedel shear model for strike-slip faults is firstly conducted in the eroded synclines in the northern Guizhou Province, which innovatively established a linkage between detailed structural analysis and evaluation of shale-gas preservation conditions. We demonstrate that superimposing of deformations plays a role in differential accumulation in eroded synclines. The results indicate that (1) fault strike in the Shixi Syncline is more concentrated than Zhongguan and Fuyan Synclines, whereas fault strikes in the Zhongguan and Fuyan Synclines are more scattered. There is only one direction of maximum principal stress (σ1) in the Shixi and Daozhen Synclines. There are two maximum principal stress directions (σ1) in the Zhongguan and Fuyan Synclines. The Zhongguan and the Fuyan Synclines have tensional environments, whereas the Shixi and the Daozhen Syncline have compressional settings; (2) the formation dip angle, distance to a class II fracture, average detachment layer thickness, intensity of superimposed modifications, and interlimb angle may be positively correlated with gas content; (3) the progressive transformation pattern is established. These findings lay the foundation for the future research on the differential preservation mechanism of eroded synclines and may be useful for enhancing research on worldwide strike-slip faults controlling hydrocarbon accumulation.

Favorable tectonic patterns for shale gas preservation and enrichment in lower Paleozoic, southeastern Chongqing, China

Marine shale reservoirs have experienced intense tectonic movements in southeastern Chongqing, resulting in widely developed folds, faults and bedding slip deformation. The gas content is of a wide range while the organic matter content and thickness are sufficient for shale gas accumulation. Here, we review previous work and present outcrop and core data from 11 shale wells around southeastern Chongqing to analyze and summarize tectonic types and their combinations, as well as their influence on shale gas preservation and enrichment. Fault and bedding slips are prevalent in shale reservoirs, and they apparently influence shale gas content. Bedding slip crush is usually formed at the bottom interval where the total organic content is high from intense horizontal compression. Bedding slip crushing and well-developed high-angle natural fractures form a net fracture system where shale gas is enriched in an anticline structure or lost to air in a syncline structure laterally. Strata dip is the key geological factor that controls bedding slip deformation in shale reservoirs in certain areas. According to the established bedding slip geological model, when bedding slip occurred, the critical strata dips of the lower Silurian Longmaxi Formation were 13.34°, respectively. Bedding slip crushed anticlines without faults and synclines without faults and bedding slip crushing are the two tectonic patterns that are favorable for shale gas preservation and enrichment, which should be the focus of further explorations of lower Paleozoic shale gas in southeastern Chongqing.

Geochemistry and mineralogy of the Upper Ordovician Wufeng formation, Southwestern China: Implications for Paleoclimate construction

Wufeng Formation shale is an important source rock of unconventional hydrocarbons in the Lower Paleozoic shales of Sichuan Basin. However, the study on its provenance and paleoclimate is still relatively limited. In this study, mineralogical and geochemical data of the shales from the Upper Ordovician Wufeng Formation in southwestern China has been used to interpret the provenance and conditions of weathering and paleoclimate. The Wufeng shales have intermediate to high SiO2 (57.72–82.38 wt. %, av. = 68.84 wt. %) and Al2O3 (5.26–16.17 wt. %, av. = 10.62 wt. %), are rich in transition metal elements (i.e. V, Ni, Cu, Co and Cr) and Y as well as moderate depletion in Na2O and Sr, relative to the concentrations of the upper continental crust (UCC). In the chondrite-normalized (CN) rare earth elements (REE) distributions, these rocks display light REE (LREE) enrichment (La/YbCN = 6.69–12.63, av. = 9.28), flat heavy REE (HREE) (Gd/YbCN = 1.35–2.41, av. = 1.70), and clearly negative Eu anomalies (Euan = 0.50–0.66, av. = 0.58), showing similar characteristics with the CN post-Archean Australian Average Shales (PAAS). Wufeng Formation shales are immature composition without evident recycling sediments, and they are originated from an intermediate-felsic igneous source composed of tonalite–trondhjemite–granodiorite (TTG), granitic and andesitic igneous rocks. The chemical weathering conditions of studied shales decreased from moderate to low in the provenance region, suggesting a gradual cooling trend of the climate at Late Ordovician Thus, this article will be helpful to discern the provenance and variations of chemical weathering conditions and paleoclimate of Wufeng Formation shales.

The organic petrology of vitrinite-like maceral in the Lower Paleozoic shales: Implications for the thermal maturity evaluation

Vitrinite-like maceral (VLM) is a type of autochthonous, synsedimentary, dispersed organic matter, which is ubiquitous in many Lower Paleozoic marine shales. The VLM reflectance has been used as an alternative tool to evaluate the thermal maturity of pre-Devonian marine sediments lacking a classical vitrinite, however, the existing knowledge about its optical properties and reflectance characteristics is still relatively limited. In this study, the optical properties of VLM were examined by means of organic-petrological methods applied to a series of naturally and artificially matured VLM-rich shales samples and their reflectance crossplots. The strong positive correlation between the true maximum reflectance (Rmax) and bireflectance of VLM indicates the increasing anisotropy of VLM with increasing thermal maturity. VLM shows a linear coalification path in the studied shales. It displays biaxial neutral features in the samples with Rmax ≤ 2%, but shows obvious optically biaxial negative characteristics in the samples with Rmax > 2%. The shape of VLM reflectance indicating surface (VLMRIS) gradually becomes oblate with increasing maturity. These results indicate that VLM exhibits appearance and optical properties similar to vitrinite, and its reflectance can be used as a reliable indicator for thermal maturity evaluation. The equivalent vitrinite reflectance (EqVRo) values, which were calculated based on the mean measured maximum reflectance of VLM (VLMR¯max), evidence that the Niutitang Formation shales from China and Alum shales from Denmark are post-over mature, and the Yurtus Formation shales in Tarim Basin and the Alum shales from Sweden are still within the oil window.

Chemical structure and thermal maturation of the vitrinite-like maceral in the Lower Paleozoic shales: Insights from Raman spectroscopy of artificial pyrolysis residues

Vitrinite-like macerals (VLM) are ubiquitous in many pre-Devonian marine shales. The thermal maturity of these sediments, which are devoid of classical vitrinite, has been commonly evaluated using the reflectance of VLM as a substitute for vitrinite reflectance, but the molecular structure of VLM itself has seldom been examined. In this study, the immature VLM-rich Cambrian Alum shales from Scandinavia were selected and pyrolyzed at 300–550 ℃ to acquire a series of artificial pyrolysis samples with different ranks. The molecular structures and thermal evolution of VLM in natural and artificial shales were investigated by Raman spectroscopy. These Raman spectra and optical properties were compared with those of vitrinite, graptolite, and solid bitumen. The Raman spectra of VLM in natural samples of variable thermal maturities and immature samples altered by pyrolysis were generally comparable. The reduction of WD (D band position) and FWHM-G (full width at half maximum of the G band) and the increase of WG (G band position) and AD/AG (area ratio) in VLM spectra, which parallels the increasing thermal maturity of the samples, indicated an increased degree of aromatic conjugation and more structurally ordered carbon moieties. The increase in wavenumber difference between the D and G bands positions in Raman spectra (Raman band separation, RBS) demonstrated the gradual removal of oxygen-containing functional groups, the loss of aliphatic side chains, and the increased degree of aromatic ring condensation. In this way, with progressing maturity, the molecular structure of VLM gradually evolved from heterocyclic aromatic compounds and amorphous carbon structures at low maturity levels to structurally better-ordered, larger, aromatic fused-ring units characteristic of post-to-over mature samples. These molecular structures and their changes associated with thermal maturation were similar to those known from classical vitrinite. Aside from its reflectance, the thermal maturity of VLM-rich sediments could be assessed by Raman spectroscopy, notably through the RBS parameter of VLM Raman spectra, which closely correlated with VLM random reflectance throughout the reflectance interval between 0.5% and 5.5%.

Depositional Environment and Organic Matter Enrichment in the Lower Paleozoic Shale from the Northeastern Margin of the Yangtze Platform, South China

In this study, twenty-six core shale samples were collected from the marine Lower Paleozoic shale in a well in the northeastern margin of the Yangtze Platform. Analyses of TOC content, mineral composition, major elements, along with trace and rare earth elements were conducted on the samples. The results were used to investigate the depositional conditions and their effects on organic matter accumulation and preservation. Generally, the sedimentation period of Niutitang Formation shale was in a cold and arid climate with anoxic marine environments, while the shale from Wufeng-Longmaxi Formation was formed in a warm and humid climate with oxic marine environments. In addition, the Wufeng-Longmaxi and Niutitang formations are characterized by low paleo-productivity. The organic matter enrichment for shale in this study could be simultaneously controlled by paleo-redox state and paleo-productivity. Organic matter enrichment of the Niutitang shale is mainly driven by preservation rather than productivity, while the dominant driving factor is the opposite for the Wufeng-Longmaxi shale. Additionally, palaeoclimate and terrestrial influx intensity were found to significantly impact the organic matter enrichment in the Wufeng-Longmaxi shale. The findings have implications for the understanding of the sedimentary processes, organic matter enrichment and preservation and shale gas potential of the study area.

Application of noble gas geochemistry to the quantitative study of the accumulation and expulsion of lower Paleozoic shale gas in southern China

The noble gases He and Ar are trace components in natural gas. Although their genesis is significantly different from that of hydrocarbon gases, they can provide key information on the accumulation and preservation of natural gas. In this paper, the relative and absolute abundances of He and Ar in lower Paleozoic shale gas samples from different areas of southern China were systematically analyzed, and the relationships between the concentrations of noble gases and the accumulation and expulsion of shale gas were evaluated. The evolution of He and Ar concentrations in shale gas system can be divided into three phases: the early accumulation stage, the hydrocarbon dilution stage and the preservation and enrichment stage. The generation of a large amounts of methane in the shale leads to the lowest relative concentrations of He and Ar in the hydrocarbon dilution stage. After that, the tectonic uplift causes the partial loss of the shale gas, and methane is no longer generated, but He and Ar are still produced. The concentrations of He and Ar present in shale gas are mainly controlled by the conditions of the preservation and enrichment stage. Tectonic uplift is the main reason for the loss of shale gas, and later uplift often results in higher gas content, which also leads to relatively low concentrations of He and Ar. The absolute abundance of residual Aratm and the He/Ar ratio of a shale gas can indirectly reflect the preservation conditions of that shale gas.

Main controlling factors of marine shale compressive strength: A case study on the cambrian Niutitang Formation in Dabashan Mountain

Lower Paleozoic shale is an important hydrocarbon source rock and natural gas reservoir in the Upper Yangtze Plate. In this study, compressive strength + acoustic emission (AE), low-pressure nitrogen adsorption and desorption (LP-N2A), low-field nuclear magnetic resonance (NMR), total organic carbon (TOC), X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM) and other experiments were carried out on the marine shale of the Cambrian Niutitang Formation in Fucheng Town, Dabashan Mountain. The results showed that: 1) The compressive strength of shale with an average of 58.38 MPa, and its rupture process can be divided into four stages; 2) The high proportion of mesopores in shale is significantly negatively correlated with compressive strength; 3) TOC in shale averaging 2.37 wt% was positively correlated with compressive strength; 4) The main element was SiO2 averaging 68.47 wt%, which is positively correlated with compressive strength. Meanwhile, Al2O3, Fe2O3 and CaO + MgO content were negatively correlated with compressive strength. To sum up, the organic and quartz content in shale jointly increase the compressive strength of shale, while the mesopores proportion has a constraining effect on the compressive strength. This study provides geological information and a theoretical basis for shale fracture development.

Geological Characteristics of Lower Paleozoic Shale Gas Accumulation in the Yuxi Region, Southern Sichuan Basin: In View of High-Density Methane Inclusions.

Twelve shale fracture veins and forty-nine fluid inclusion assemblages within the veins of 3800–4200 m in three wells located in different tectonic zones in the Yuxi Region, southern Sichuan Basin were selected in this study. The burial and thermal histories of single wells were reconstructed, and time–temperature–pressure of oil and gas filling were clarified using microscopy observation, Raman microprobe analyses, geochemical tests, and fluid inclusion microtemperature measurement. The shale fracture veins of the Wufeng-Longmaxi Formation in the Yuxi Region are mainly formed vertically and horizontally, where the vein-forming fluids are derived from endogenous fluids. A large number of methane inclusions, bituminous inclusions, and methane-bearing bituminous inclusions within the veins confirm the process of oil cracking gas and kerogen cracking gas. The homogeneous temperature (Th) of the aqueous inclusions contemporaneous with the bituminous inclusions ranges from 109.3 to 174.1 °C, which were trapped during 220 to 250 Ma. The homogeneous temperature of the aqueous inclusions contemporaneous with the methane inclusions ranges from 137.3 to 226.8 °C, which were trapped during 160 to 195 Ma and 51 to 56 Ma. The trapped pressure calculated by high-density methane inclusions (0.246–0.293 g/cm3) is between 82.9 and 140.1 MPa, with a pressure coefficient between 1.64 and 2.07. The formation pressure coefficient is nearly two, indicating that the current overpressure is inherited from the overpressure at the maximum burial depth. The earlier the fracture vein opening, the less the damage to the shale gas accumulation, and the more opening-closing phases, the lower the homogeneous temperature of the gas–liquid two-phase aqueous inclusions coeval with the high-density methane inclusions and the greater the degree of damage to the shale gas accumulation. The results provide a basis for further study on the genesis of overpressure and the migration of shale gas.

Storing characteristics and main controlling factors of connate water in lower Paleozoic shales in southeast Chongqing, China

Recent studies show that the shale gas content and storage space are influenced by connate water, while it is unclear how connate water occurs in different nanoscale pores in high and overmature shales, and research concerning the occurrence characteristics and factors controlling connate water in overmature shale gas formations is scarce. In this research, overmature lower Paleozoic shale samples from southeastern Chongqing were selected for connate water distribution and factor analysis. The results show that connate water in the lower Paleozoic shale occurs in organic matter and clay minerals, mainly occurs in clay minerals and tends to form clay-bound water in clay minerals. The effect of the connate water content (CIW) on the inorganic nonmicropore (mesopore + macropore) surface area (SA) is greater than that on the organic nonmicropore SA. The CIW mainly affects the micropore structure parameters of clay minerals, and the total organic carbon (TOC) content is the main factor controlling the shale micropore structure. Connate water occurs in shale micropores and nonmicropores, and adsorption and filling states simultaneously exist and are distributed in <10 nm mesopores and 0.4–0.6 nm micropores; this distribution reduces the pore structure parameters of inorganic matter in shale, especially the nonmicropore SA. The effect of the CIW on nonmicropore SA and micropore pore volume (PV) in inorganic minerals is significantly greater than that of organic matter. Connate water mainly occurs in inorganic mineral pores and mainly occupies the mineral nonmicropore SA. Connate water significantly reduces the inorganic pore structure parameters, while its effect on organic pores is limited.

Pore Characteristics and Gas Preservation of the Lower Cambrian Shale in a Strongly Deformed Zone, Northern Chongqing, China

The Lower Paleozoic marine shale in southern China has undergone several strong tectonic transformations in an extensive region outside the Sichuan Basin. Although some shale strata underwent strong deformation, they still contain a significant amount of shale gas. The gas preservation mechanism in the strongly deformed shale has become the focus of attention. In this paper, the Lower Cambrian gas-bearing shale samples with a strong deformation taken from an exploration well in northern Chongqing, China, were investigated on their pore types and structure, with the aim to reveal the reason for the gas preservation. The pore types of the Lower Cambrian shale are dominated by microfractures and interparticle (interP) pores occurring mainly between clay minerals and between organic matter (OM) and clay minerals, while pores within OM that can be observed by FE-SEM (field emission-scanning electron microscopy) are rare. The shale has a low porosity, with an average of 1.56%, which is mainly controlled by the clay mineral content. The adsorption experiments of low pressure N2 (LPNA) and CO2 (LPCA) indicate that the shale is rich in micropores and small mesopores (&lt;2–3 nm) provided mainly by OM, but mesopores with a size range of 3–50 nm are underdeveloped. The shale, as revealed by LPNA data, has dominant slit-like or plate-like pores and an obvious low-pressure hysteresis (LPH), indicating a low gas diffusion. The deformed shale samples with a removal of OM by oxidation and their isolated kerogen further indicate that the LPH is completely related to OM, without any relationship with minerals, while an undeformed shale sample, taken from another well for a comparison, has no obvious LPH for both of its OM-removed sample and kerogen. Based on a comprehensive analysis of the relative data, it is suggested that the nanopores related to OM and clay minerals in the shale were significantly altered owing to the deformation, with a result of the pores being squeezed into the slit-like shape and converted into micropores. This extraordinary pore structure of the shale formed during the deformation process should be the main preservation mechanism of shale gas.

Comparison of joint sets orientation in the Lower Paleozoic shales exposed in Scania (SW Sweden) and concealed in N Poland: a multi-methodological approach

The Lower Paleozoic shales of SW Sweden and Eastern Pomerania (Poland) have a common history related to the depositional and tectonic evolution of the Baltic Basin. The major tectonic events are recorded, among others, as joints, which are either exposed in outcrops in SW Sweden or recognised in deep boreholes located in Pomerania. We present a comparison of the regional joint systems recognized by multiple methods in the studied region. In effect of a statistical compilation of measurements, five joint sets (named JS 1 to JS 5) were identified, traceable between Eastern Pomerania and SW Sweden. Our analysis showed a general consistency of joint set orientations, independent of their distance to the Tornquist tectonic zone passing through the study region. Three of the joint sets, JS 1 striking NNE, JS 2 striking WNW, and JS 3 striking NNW, are found to be the most frequent sets, occurring in most sites. Having more constant orientation, the JS 1 and JS 3 served as indicators of possible rotation of the tectonic block or stress field in the region. JS 1 and JS 2 are interpreted as an effect of late Carboniferous stress relaxation after Variscan collision, while JS 3 and the E–W striking JS 4 might be related to the Early Devonian Caledonian compressive stage.

Characteristics of Pore Structure and Gas Content of the Lower Paleozoic Shale from the Upper Yangtze Plate, South China

This study is predominantly about the differences in shale pore structure and the controlling factors of shale gas content between Lower Silurian and Lower Cambrian from the upper Yangtze plate, which are of great significance to the occurrence mechanism of shale gas. The field emission scanning electron microscopy combined with Particles (Pores) and Cracks Analysis System software, CO2/N2 adsorption and the high-pressure mercury injection porosimetry, and methane adsorption were used to investigate characteristics of overall shale pore structure and organic matter pore, heterogeneity and gas content of the Lower Paleozoic in southern Sichuan Basin and northern Guizhou province from the upper Yangtze plate. Results show that porosity and the development of organic matter pores of the Lower Silurian are better than that of the Lower Cambrian, and there are four main types of pore, including interparticle pore, intraparticle pore, organic matter pore and micro-fracture. The micropores of the Lower Cambrian shale provide major pore volume and specific surface areas. In the Lower Silurian shale, there are mesopores besides micropores. Fractal dimensions representing pore structure complexity and heterogeneity gradually increase with the increase in pore volume and specific surface areas. There is a significant positive linear relationship between total organic carbon content and micropores volume and specific surface areas of the Lower Paleozoic shale, and the correlation of the Lower Silurian is more obvious than that of the Lower Cambrian. The plane porosity of organic matter increases with the increase in total organic carbon when it is less than 5%. The plane porosity of organic matter pores is positively correlated with clay minerals content and negatively correlated with brittle minerals content. The adsorption gas content of Lower Silurian and Lower Cambrian shale are 1.51–3.86 m3/t (average, 2.31 m3/t) and 0.35–2.38 m3/t (average, 1.36 m3/t). Total organic carbon, clay minerals and porosity are the main controlling factors for the differences in shale gas content between Lower Cambrian and Lower Silurian from the upper Yangtze plate. Probability entropy and organic matter plane porosity of the Lower Silurian are higher than those of Lower Cambrian shale, but form factor and roundness is smaller.

Mesozoic horizontal stress in the East Sichuan Fold-and-thrust Belt, South China: Insights for Lower Paleozoic shale gas retention

The Lower Paleozoic organic-rich shales are the primary targets of shale gas exploration in the East Sichuan Fold-and-thrust Belt (ESFB), South China. Horizontal compression during the Mesozoic orogeny caused the decollement of these shales and further influenced the retention of shale gas. In this study, seven sets of rock samples were recovered from outcrop sites in the ESFB, and the measured occurrences of the exposed strata in the ESFB were extracted from regional geological survey works. Acoustic emission tests under uniaxial compression were conducted on these outcrop samples. Magnitudes of the two principal horizontal effective stresses, SH-eff and Sh-eff, at the seven sampling sites, were acquired based on the Kaiser effect; magnitudes of the Mesozoic SH-eff experienced by the Lower Paleozoic shales in the different tectonic belts were then determined based on previous and newly measured paleo stress magnitudes, as well as their correlation with the measured dip angles of strata. Finally, the maximum depths of decollement (HD-max) of the shales were computed using the inferred paleo stress magnitudes. Measured SH-eff magnitudes of the outcrop samples ranged from 74.7 MPa to 117.0 MPa, and the ratios of SH-eff magnitudes to Sh-eff magnitudes fell between 1.10 and 1.55. The magnitudes of the paleo stress experienced by the Lower Cambrian shale were higher than those of the Lower Silurian shale due to a greater burial depth. The inferred maximum Mesozoic SH-eff magnitudes decreased from 180–200 MPa at the margin of the orogenic belt to 60–80 MPa at the margin of the basin, and the computed HD-max values decreased from more than 10 km to less than 5 km correspondingly. A lower magnitude of the Mesozoic horizontal stress and a shallower decollement contributed to better retention of the Lower Paleozoic shale gas inside the Sichuan Basin.

Anisotropy of Strength and Elastic Properties of Lower Paleozoic Shales from the Baltic Basin, Poland

The paper presents the results of laboratory studies on the strength–strain properties of shales representing four siltstone-claystone lithostratigraphic units occurring in the Baltic Basin. Laboratory studies in a triaxial stress state were conducted as single failure tests on cylindrical samples oriented parallel and perpendicular to lamination within the rocks. Mutually perpendicular samples were cut out from the same drill core sections in order to determine mechanical anisotropy. Samples oriented parallel to lamination were characterised by values of the static Young’s modulus twice as high as from samples oriented perpendicular to lamination. Similar variability was observed in the case of maximum differential stress values and Poisson’s ratio. Samples parallel to lamination registered notably lower axial strains, which influenced increased values of Young’s modulus and Poisson’s ratio. The rocks studied are characterised by VTI type (vertical transverse isotropy) internal anisotropy of the rock matrix, which significantly influences the anisotropy of their geomechanical properties.

Quantitative Characterization and Determination of the Main Factors That Control Fracture Development in the Lower Paleozoic Shale in Southeastern Chongqing, China

The Longmaxi and Niutitang Formations are typical shale reservoirs in southeastern Chongqing. Since the Paleozoic, southeastern Chongqing has experienced multistage tectonic movement and diagenesis, resulting in the formation of a large number of natural fractures. In shale reservoirs, fractures not only provide seepage channels for oil and gas migration but also play an important role in the oil and gas reservoir space. Natural fractures provide a flow path from source rock to reservoir during oil filling and connect hydraulic fractures, matrix pores, and boreholes during production. Therefore, identifying the main factors that control the development of natural fractures in the Longmaxi and Niutitang Formations in southeastern Chongqing has a guiding significance for the efficient development of shale reservoirs in this area. By considering the Longmaxi and Niutitang Formation shales as case studies, using field outcrops, drilling cores, and other data, and conducting X-ray diffraction (XRD) analysis as well as total organic carbon (TOC) measurements, the quantitative parameter characteristics and main factors that control the development of natural fractures in reservoir shales were examined in detail. The results obtained showed that there are three types of fractures in the lower Paleozoic shale in southeastern Chongqing, which are structural, diagenetic, and abnormal high-pressure fractures. Among them, the fractures in the Longmaxi Formation shale, which are relatively abundant, predominantly consist of low-angle and high-angle inclined fractures, while the Niutitang Formation predominantly consists of high-angle fractures. Additionally, the investigation of fracture size and fracture density, as well as correlation analyses, showed that the fractures of the Lower Paleozoic shale are predominantly micro fractures that play a key role in improving reservoir seepage. It was also noted that the development of fractures is affected by several factors, including tectonic stress, mineral composition, organic carbon content, and rock thickness. The degree of fracture development was found to be positively correlated with brittle mineral and quartz contents, and high organic matter contents also significantly favored the formation micro fractures.

Magnetic susceptibility variations in lower Paleozoic shales of the western Baltic Basin (northern Poland): A tool for regional stratigraphic correlations and the decoding of paleoenvironmental changes

Magnetic susceptibility variations in lower Paleozoic shales of the western Baltic Basin (northern Poland): A tool for regional stratigraphic correlations and the decoding of paleoenvironmental changes

Maturity evolution of Lower Cambrian Qiongzhusi Formation shale of the Sichuan Basin

The Lower Cambrian Qiongzhusi Formation source rock is one of the main sources of oil and gas in the Sichuan Basin. Owing to the controversial thermal history and the complex structural pattern in the Sichuan Basin, it is difficult to characterize the differences in maturity evolution and the timing of hydrocarbon generation periods of Qiongzhusi hydrocarbon source rocks in different tectonic zones. Based on the comprehensive analysis of the previous thermal history and denudation studies from the tectonic units of the basin, the thermal histories and denudation amount in main period of tectonic movement of a five-part tectonic development are obtained in this paper. Although the thermal history of different tectonic units varies, generally, the heat flow increased slowly in the early Paleozoic, and it experienced a peak heat flow at the end of the Middle Permian and decreased subsequently. Based on new thermal history and denudation data, the maturation of the Lower Cambrian Qiongzhusi Formation source rocks in the Sichuan Basin were modeled and their evolution can be classified into three patterns, which are: (1) continuous heating pattern in southern Sichuan, (2) Silurian-Permian heating cessation pattern in western Sichuan and northwest central Sichuan, and (3) Silurian-Permian and Triassic heating cessation pattern in the central uplift, northern structural zone and western part of the eastern structural zone. In addition, the key hydrocarbon generation periods of different tectonic units were determined in this study. Tectonic and sedimentary burial, together with high heat flow values during the Late Permian, were the main factors controlling levels of thermal maturity and thermal evolution models of the Qiongzhusi Formation. The present-day thermal maturity of the Qiongzhusi Formation depends on the thermal history and maximum burial depth before uplift in Late Cretaceous. However, the maturity evolution pattern affecting the key hydrocarbon generation stage was mainly related to the burial histories of source rocks in the Paleozoic. The thermal modelling of this study provides new evidence for the source rock evolution and new insight into Lower Paleozoic shale gas.

Assessing the Gas Potential of the Lower Paleozoic Shale System in the Yichang Area, Middle Yangtze Region

This study aims to characterize the geological properties and to evaluate the gas potential of the lower Paleozoic shales in the Yichang area by integrating a series of analyses on geology and geochemistry, well logging data, and 2D seismic interpretation. The thickness of the lower Cambrian Shuijingtuo shale is less than 10 m in the southeast but may exceed 120 m in the southwest area. The thickness of the upper Ordovician Wufeng shale and the lower Silurian Longmaxi shale (hereafter referred to as the Wufeng-Longmaxi shale) gradually increases in thickness from <10 m in the south to 40–50 m in the northeast. The organic matter of the above two sets of black shale is mainly dominated by type I and II1 kerogen. The thickness of black shale units with TOC > 2% is more than 10–20 m, commonly located at the bottom of the shale intervals. Black shales near the Huangling anticline have low thermal maturity due to its relatively shallow burial. The predominant minerals are quartz and clay minerals. Carbonate contents of the Shuijingtuo shale are relatively high compared to the Wufeng-Longmaxi shale. Pores are more developed in the Wufeng-Longmaxi shale than those in the Shuijingtuo shale. Natural fractures are common in the Shuijingtuo shale but are rare in the Wufeng-Longmaxi shale. The well logging and lab analytical results both indicate that the natural gas content in the black shales is relatively high. The prospective area of the Shuijingtuo interval for shale gas production is nearly 670 km2, located in the southwestern region. In contrast, the prospective area of the Wufeng-Longmaxi interval for shale gas production is approximately 1590 km2, situated in the eastern area.

Alteration of organic macerals by uranium irradiation in lower Paleozoic marine shales

Measuring the degree of organic matter (OM) alteration caused by uranium (U) irradiation is important in the effective evaluation of the hydrocarbon potential and thermal maturity of U-rich source rocks. This study investigates OM alteration in the Baltoscandian U-rich, lower Paleozoic Alum shale obtained from three research wells in the thermally immature parts of eastern Sweden and western Estonia. The results show a marked increase in the proportion of refractory, non-generative organic carbon (NGOC) in the U-rich shale. Fluorescence spectrometry of unicellular alginites and lamalginites show a reduction of up to 95.6% in fluorescence relative intensity (RI) with increase in U content from 29 to 401 ppm. Fluorescence spectra of the liptinites exhibit a marked “red shift” as expressed by increase in red(R)/green(G) quotients. This change in fluorescence properties of the liptinites is associated with up to 77.6% decrease in hydrogen index (HI) and hence loss of hydrocarbon generation potential. Furthermore, organic molecules show significant loss of aromatic moieties as well as aliphatic constituents resulting in a more condensed macromolecular structure. The measured random reflectance of solid bitumen (BRo) appears to be significantly elevated in micro scale proximity to the U-containing minerals. The heterogeneous distribution of U-containing minerals and the contact with solid bitumen attributes to the wider range of BRo values and lead to the overall increase in mean BRo in samples with high U contents. In contrast, Ro values of zooclast macerals (GRo, e.g., graptolite, chitinozoans and vitrinite-like fragments) appear to be less influenced by U irradiation and hence are a more reliable maturity indicator in U- rich lower Paleozoic shales.