Cambrian Qiongzhusi Formation Research Articles

Origin and preservation mechanisms of organic matter in carbonate concretions from Lower Cambrian black shales in South China

Carbonate concretions are widely used in paleoclimate, paleoenvironmental, and paleontological studies, and even in the study of potential life on Mars. These petrological, elemental, isotopic, and lipid biomarker signals in Meso-Cenozoic carbonate concretions (relatively low thermal maturity) can effectively preserve details of seawater conditions and benthic ecosystems during the deposition of their host sediments/rocks. However, such research on Precambrian-Cambrian carbonate concretions under highly mature conditions remains scarce, and the ability of these ancient carbonate concretions to retain their original biogenic information remains uncertain. To achieve that, this study examines two Cambrian carbonate concretions, using samples from the center, transition, and rim of each, and their adjacent host black shales from the Lower Cambrian Qiongzhusi Formation in the Yangtze Block, South China. Organic and inorganic geochemical analyses were combined to elucidate the origin and preservation process of organic matter (OM) in these ancient Cambrian carbonate concretions. The results show that the thermal maturity of OM within these concretions (1.8 % EqVRo) is relatively low compared to their adjacent host shales (2.9 % EqVRo). Hopanes and steranes are detectable in both free and calcite-occluded hydrocarbons within these concretions, with concentrations of individual compounds ranging from 0.001 to 0.800 μg/g TOC, whereas kerogen-bound hydrocarbons lack detectable biomarkers. The results indicate that the two Cambrian carbonate concretions were formed mainly within the iron reduction and bacterial sulfate reduction zones, extending to depths of 10 to 38 m below the sediment-water interface. The OM within these concretions mainly inherited the initial unaltered signature of OM from the Qiongzhusi host shale. The carbonate concretions protected the internal OM from further thermal and secondary (e.g., biodegradation) alteration processes and might also prevent the formation of the conventional macromolecular skeletal kerogen manifested by the absence of bound biomarkers. The biomarkers, in both free and occluded forms, in the Cambrian carbonate concretions still retained their original source information, providing valuable insights into ancient biogeochemical processes during sediment burial and ancient seawater chemistry during the early Cambrian.

Three-stage hydrocarbon accumulations in the Middle Permian in the Central Sichuan Basin

The Middle Permain Maokou Formation (P2m) is a new region of natural gas exploration in the Sichuan Basin, is characterized by bioclastic limestone with localized dolomitization, and karst fractured-vuggy reservoirs. Currently, on the gas source, hydrocarbon accumulation process and control factors in the Sichuan Basin during the Permian are lacking. To bridge this gap, herein, we identified the filling sequence minerals inside the pores/vugs, along with the oil charge of the Maokou Formation using drill cores, thin sections, oil inclusion analysis, and U-Pb dating of calcite cements. The results showed that the reservoir space of the Maokou Formation was predominated by the residual dissolved pores/vugs, fractures, and dissolved fractures. The pores/vugs underwent four stages of mineral filling by very fine-fine (-crystalline, CC1) calcite → fine-medium calcite (CC2: from 256.4 ± 1.7 to 244.1 ± 6.3 Ma) → fibrous calcite (FC; ∼183.9 ± 8.2 Ma) → coarse-macro calcite (CC3; ∼171.5 ± 5.3 Ma). Combined with the homogenization temperature and salty of fluid inclusion, we considered that three stages of oil charge were present in the Maokou Formation reservoirs. The first stage involved the formation of paleo-oil reservoirs during the Late Permian to Early Triassic, corresponding to the high-maturity aqueous inclusions in CC2, with a homogenization temperature of 106.7°C–137.8°C. At that time, the oil generation from the Lower Cambrian Qiongzhusi Formation rocks peaked, and the generated hydrocarbons migrated upward into the Maokou Formation through the strike-slip faults in the basin center. The second stage involved the formation of paleo-oil reservoirs during the Early Jurassic. The Permian source rocks reached the oil generation window with hydrocarbon expulsion, which was consistent with the oil inclusions in FC. The third stage involved the formation of paleo-gas reservoirs during the Middle Jurassic to Early Cretaceous, corresponding to the high-density methane inclusions and bitumen inclusions occurring in CC3, with the homogenization temperature peaking at 151.9°C–178°C. The natural gas in the Middle Permian of the Central Sichuan Basin is predominantly sourced from the Lower Cambrian Qiongzhusi Formation mudstone and partially from the source rocks of the Middle Permian, indicating a significant source-reservoir conduit of the strike-slip faults in the basin center. The findings provide considerable baseline data to advance further research in the Sichuan Basin.

Pore development characteristics and controlling factors of Lower Cambrian Qiongzhusi shale gas reservoir in Sichuan Basin, China

Recently, the shale gas exploration of Lower Cambrian Qiongzhusi Formation in Sichuan Basin has made a great breakthrough. However, the pore characteristics and controlling factors of the shale-gas reservoir in the Qiongzhusi Formation are not clear, which hinders further exploration and development of shale gas. This paper focuses on the Qiongzhusi Formation in the Lower Cambrian of the Sichuan Basin. On the basis of mineral composition and the lithofacies types of shale-gas reservoirs in Qiongzhusi Formation were classified by microscopic observation. Additionally, focused ion beam-scanning electron microscope (FIB-SEM), advanced mineral identification and characterization system (AMICS) analysis, large-field stitching scanning electron microscope technology (MAPS) and nitrogen adsorption experiment were employed to comprehensively determine the differential characteristics and controlling factors of pore development. The results indicate that the pore types in Qiongzhusi Formation are mainly composed of interparticle pores and intercrystalline pores. The pore structure of Qiongzhusi Formation is complex. Significant differences in pore structure parameters are observed among different lithofacies. Quartz and feldspar contribute to the preservation of primary pores due to their strong compressive resistance. Clay minerals and carbonate minerals impede pore development by occluding pores. The influence of organic matter on the pore development is relatively small. According to the mineral composition and pore parameters, laminated shale is a relatively favorable reservoir facies among different lithofacies types.

Characteristics and Controlling Factors of Pores in Different Shale Lithofacies Reservoirs of Lower Cambrian Qiongzhusi Formation, Southwestern Sichuan Basin, China

The shale reservoirs of the Lower Cambrian Qiongzhusi Formation are widely distributed in the Sichuan Basin and have abundant gas resources. However, the shale lithofacies of the Qiongzhusi Formation are complex due to frequent sea level changes. The reservoir pore structure characteristics and gas content of different shale lithofacies vary significantly, which makes identifying the ‘sweet spot’ a challenging task. In this study, core observation and X-ray diffraction (XRD) were used to analyze the lithofacies types and characteristics of the study area. The pore types of different shale lithofacies were observed using field emission-scanning electron microscopy. Pore structures were studied using low-temperature gas (including N2 and CO2) physisorption, and the pore volume (PV), specific surface area (SSA) and pore structure were systematically characterized. The primary factors influencing pore formation in different types of shale lithofacies were analyzed by combining geochemical experiments and mineral contents. The results indicate that the lithofacies of the Qiongzhusi Formation shale in the study area can be classified into five categories according to mineral compositions: Siliceous argillaceous shale (CM-1), Argillaceous siliceous mixed shale (M-2), Argillaceous siliceous shale (S-3), Siliceous rock (S) and Calcareous siliceous shale (S-2). Pores are abundant in S-3 shale, M-2 shale and CM-1 shale. The S-3 shale is more enriched in organic pores and clay mineral pores compared to other lithofacies shales, and the pore morphology is mainly wedge-shaped and plate-like. M-2 shale and CM-1 shale are rich in clay minerals and mainly develop clay mineral pores and are mainly wedge-shaped and plate-like. The S shale and S-2 shale mainly develop interparticle pores and clay mineral pores, which are mainly slit-like. The results show that TOC, pyrite content, quartz and feldspar mineral content, clay mineral type and content affect the pore structure in the study area. Quartz and feldspar content have a negative effect on micropore and mesopore volumes. TOCs have a weak positive correlation with micropore volume and micropore SSA. Clay mineral content has significant positive effects on the PV and SSA of micropores and mesopores, indicating that clay mineral content is the main factor affecting the pore structure of shale.

Geochemical properties, mechanism of formation, and source of solid bitumen in the Ediacaran Dengying Formation from the central to northern Sichuan Basin, China

Several large gas reservoirs, including the Weiyuan and Anyue gas fields, have been discovered in the Ediacaran Dengying Formation in the Sichuan Basin, indicating vast opportunities for deep-petroleum exploration. Understanding the origin and evolution process of these gas reservoirs is crucial for further petroleum exploration and development. Although the burial history of the Sichuan Basin and its surrounding areas is basically dominated by Paleozoic burial and preservation, and Meso-Cenozoic uplift and erosion. However, the source of the Dengying Formation natural gas in the Sichuan Basin is still a subject of controversy between the Ediacaran and Cambrian source rocks due to high thermal maturity, secondary alteration, and multistage tectonic activities. This study aims to examine the investigation on the properties and genetics of Dengying solid bitumens that existed in the central to northern Sichuan Basin, in order to provide insights into the source and evolution history of the Dengying paleo-oil reservoirs. For this purpose, petrologic, elemental, and organic geochemical analyses were used to investigate the properties, origin, and formation mechanism of solid bitumens. The results indicate that these solid bitumens have suffered intense thermal alteration (EqVRo 2.1%–5.3%), and are classified as pyrobitumens. Solid bitumens in the Gaoshiti bulge and the north slope area were subjected to tectonic thermal events (e.g., Emeishan igneous province) and relatively strong thermochemical sulfate reduction (TSR) alteration. These solid bitumens were assumed to be pyrolysis products of light (condensate) oil. In contrast, solid bitumens in the Weiyuan bulge and the Micangshan uplift area were not influenced by tectonic thermal events but experienced weak TSR alteration, and originated from either normal or heavy crude oil. Moreover, a systematic bitumen-source correlation was carried out using δ13Corg values, bound steranes, and trace element distributions. Results reveal that the solid bitumens in the central Sichuan paleo-uplift were mainly derived from the lower Cambrian Maidiping Formation, those in the north slope area originated from the lower Cambrian Qiongzhusi Formation, while those in the Micangshan area were sourced from the Ediacaran Doushantuo Formation. Structural and burial history analysis implies that the differences in the source of the Dengying paleo-oil reservoir (i.e., solid bitumen) between these study areas are attributed to the distribution of source rocks and their contact relationship with reservoirs. This research successfully obtained insights into the source and genetic mechanism of paleo-oil reservoirs by analyzing the properties of high-maturity solid bitumens, which can guide further exploration and development of deep petroleum resources.

Fluid history of the lower Cambrian Longwangmiao Formation in the Anyue gas field (Sichuan Basin, SW China)

The Anyue gas field in China is an important source of natural gas supporting the country energy transition to the goal of carbon-neutrality. However, the fluid P–T history, which will provide a better insight into gas accumulation and probable leakage in the main reservoir (the Lower Cambrian Longwangmiao Formation) has still yet to be fully understood. This study combines detailed petrographic observations with transmitted light, ultraviolet light, reflected light and cold cathodoluminescence, fluid inclusion microthermometry, Raman spectrometry, 1D basin modeling and fluid P–T modeling techniques to reconstruct the fluid-involved basin evolution of the Longwangmiao Formation in the Anyue region. The results showed that the paragenesis of the Longwangmiao Formation includes dolomite matrix, dolomite 1, dolomite 2, dolomite 3 (saddle dolomite), dolomite 4, quartz and calcite. The first generation of bitumen occurs between dolomite 1 and dolomite 2, corresponding to the primary oil generation of the Lower Cambrian Qiongzhusi Formation at the burial maximal in the Silurian. The second generation of bitumen, corresponding to intense oil cracking into gas, has occurred between dolomite 4 and quartz at a geological period between the Middle Jurassic and Early Cretaceous (early Yanshanian movement). Fluid inclusions trapping pressure modeling explains that the trapping pressure increases from dolomite 4 to quartz due to burial and the oil cracking into gas, then stays constant from quartz to calcite due to the sealing of fractures. The open fractures formed after calcite partially reduced the overpressure condition of the Longwangmiao Formation. Collectively, this study will provide necessary information for a better understanding of deeply buried reservoirs to support more efficient development of such reservoirs in higher quality zones and reduce exploration risks and operational costs.

Tracking multiple paleo oil charges in the Precambrian dolomite reservoirs of the central Sichuan Basin, China

In the Precambrian dolomites in the central Sichuan Basin, a substantial paleo-oil system previously existed, which has since evolved into a dry gas system that is currently in place. The genesis of this paleo-oil system remained poorly understood, leaving uncertainties regarding the timing of oil charging and associated source rocks. Integration of Re–Os geochronometry with basin modeling allows temporal-spatial constraints on the ancient petroleum system to be established.Two texturally distinct generations of reservoir bitumen were identified. Late-generation bitumen yielded a geologically meaningful Re–Os age of 189 ± 20 Ma (MSWD = 10.4, 2σ), distinct from the previously documented Re–Os ages such as the 414 ± 44 Ma reported by Shi et al. (2020). This new age, along with the ∼414 Ma, aligns within uncertainties with the two relative timings derived from our independent basin model (ca. 450–390 Ma and 260–180 Ma), revealing two temporally distinct oil generation-migration events. The date of ∼189 Ma also lends support to the previous oil charge timing inferred from fluid inclusions. We evaluated the imprecisions of these two Re–Os ages and considered them to result from a narrow spread in 187Re/188Os, and variations in initial Os isotopic compositions (Osi) due to the giant size of the petroleum system and a long duration of oil generation from source kitchen over a large geographical area in this play. Effects of asphaltene precipitation, due to uplift-induced decompression of the migrating oils, may also have contributed to the scatter. Separating the samples based on Osi produced a higher precision age (184.1 ± 5.7 Ma, Model 1) with an MSWD of 0.97.The first oil charge is relatively weak as shown by the basin model and observed bitumen volumes, and it had not been well preserved due to significant reservoir exhumation by Caledonian tectonism. Consequently, the younger oil charge is considered to be the principal precursor to the current dry gas accumulations. Os isotope histories for four potential source units were constructed using publicly available Re–Os data. Comparisons between bitumen and rock Os compositions tend to support the lower Cambrian Qiongzhusi Formation as the predominant source unit for the two major paleo-oil charges. However, contributions from the Duoshantuo, Dengying, and Maidiping source rocks can not be precluded. Overall, these new observations allow temporal-spatial constraints to be placed on the complex evolution, from paleo-oil generation to destruction and dry gas generation.

Siliceous Origin of the Cambrian Qiongzhusi Formation Shale in the Middle Part of the Upper Yangtze Platform: Significance of Organic Matter Enrichment.

The early Cambrian Qiongzhusi Formation shale is rich in organic matter and is a high-quality marine Source rock. However, the origin of Qiongzhusi Formation siliceous rocks is unknown, and the role of siliceous rocks in the process of organic matter enrichment or preservation is also lacking. This study combines thin section, scanning electron microscopy, SEM/EDS, major and trace element analysis, and N2 adsorption experiments to analyze and evaluate the shale of the Qiongzhusi Formation in the central region of the Sichuan Basin. The quartz types in the shale of the Qiongzhusi Formation are divided into four types, namely, bioclastic siliceous rocks, terrestrial detrital quartz, siliceous microcrystalline quartz particles, and microcrystalline quartz aggregates; at the same time, according to petrographic and geochemical parameters, the content of authigenic quartz in Qiongzhusi Formation shale decreases from top to bottom, and terrigenous detrital quartz tends to increase, and biogenic silicon accounts for the majority of authigenic quartz components; autogenous quartz has a positive impact on the pore structure of shale, providing sufficient pore space for the development of organic pores and protecting the internal pore network by forming intergranular pores as rigid frameworks. At the same time, it plays a crucial role in the enrichment and preservation of organic matter.

Metal elements potential for hydrocarbon–source correlation

Inorganic geochemical approaches (i.e., trace elements) for hydrocarbon–source rock correlations in sedimentary basins are a frontier area of research in petroleum geochemistry. This study used inorganic geochemistry to the over mature petroleum systems in the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin of China. Our results show that the BaN–NiN–CuN ternary diagram, VN–NiN–MoN ternary diagram and V/Ni–U/Th diagram (related to paleo-productivity and redox conditions), rare earth element (REE)–Y patterns, Ce/Ce*–La/La* diagram, and Ce/Ce*–La/La*–(LREE/HREE)N ternary diagram (related to fluid interactions) can robustly distinguish different source rocks. In particular, V, Ni, and Mo have a strong relationship to organic matter in source rocks, while their occurrence is most likely related to porphyrins in petroleum. Therefore, the VN–NiN–MoN ternary diagram can be used for hydrocarbon–source rock correlations. In contrast, Ba, Cu, U, Th, and REE–Y have different and complex geochemical behaviors, limiting the application in hydrocarbon–source rock correlations. The application in the Sichuan Basin shows that the hydrocarbons in the Dengying reservoir were mainly derived from the lower Cambrian Qiongzhusi Formation with minor contribution from the lower Cambrian Maidiping and Sinian Doushantuo formations. Compiled data from Lower Paleozoic crude oils in the Tarim Basin show that the VN–NiN–MoN ternary diagram can also be applied to low-maturity crude oil systems. Therefore, V, Ni, and Mo have strong relationship to organic matter in petroleum systems and the potential for hydrocarbon–source correlation, while other elements should be used under given conditions.

Theory and practice of unconventional gas exploration in carrier beds: Insight from the breakthrough of new type of shale gas and tight gas in Sichuan Basin, SW China

Unconventional gas in the Sichuan Basin mainly includes shale gas and tight gas. The development of shale gas is mainly concentrated in the Ordovician Wufeng Formation–Silurian Longmaxi Formation, but has not made any significant breakthrough in the Cambrian Qiongzhusi Formation marine shale regardless of exploration efforts for years. The commercial development of tight sandstone gas is mainly concentrated in the Jurassic Shaximiao Formation, but has not been realized in the widespread and thick Triassic Xujiahe Formation. Depending on the geological characteristics of the Qiongzhusi Formation and Xujiahe Formation, the feedback of old wells was analyzed. Then, combining with the accumulation mechanisms of conventional gas and shale gas, as well as the oil/gas shows during drilling, changes in production and pressure during development, and other characteristics, it was proposed to change the exploration and development strategy from source and reservoir exploration to carrier beds exploration. With the combination of effective source rock, effective carrier beds and effective sandstone or shale as the exploration target, a model of unconventional gas accumulation and enrichment in carrier beds was built. Under the guidance of this study, two significant results have been achieved in practice. First, great breakthrough was made in exploration of the silty shale with low organic matter abundance in the Qiongzhusi Formation, which breaks the traditional approach to prospect shale gas only in organic-rich black shales and realizes a breakthrough in new areas, new layers and new types of shale gas and a transformation of exploration and development of shale gas from single-layer system, Longmaxi Formation, to multi-layer system in the Sichuan Basin. Second, exploration breakthrough and high-efficient development were realized for difficult-to-produce tight sandstone gas reserves in the Xujiahe Formation, which helps address the challenges of low production and unstable production of fracture zones in the Xujiahe Formation, promote the transformation of tight sandstone gas from reserves without production to effective production, and enhance the exploration and development potential of tight sandstone gas.

Thermal maturity calibration of extremely high-mature pre-Devonian strata: A case study from the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin, South China

Thermal maturity assessment of pre-Devonian strata is challenging due to the absence of humic vitrinite, and conventional organic geochemical methods (i.e., Rock-Eval Tmax, atomic H/C, biomarker parameters) are usually ineffective for the highly-over mature source rocks. The Lower Cambrian Qiongzhusi Formation shale in the Sichuan Basin, as the main high-quality source rock, is an important exploration target for shale gas. In this study, optical reflectance and Raman spectroscopy measurements were performed on eight Qiongzhusi Formation core samples from well JY1 in conjunction with basin modelling to calibrate to equivalent vitrinite reflectance and constrain thermal maturity. The results reveal solid bitumens/pyrobitumens as the predominant organic constituents in the Qiongzhusi Formation. As burial depth increases from 3,300m to 3,600m, the solid bitumen reflectance acquired from optical measurement and Raman calculation increase from 2.53% to 3.86%, and from 2.88% to 3.49%, respectively, along with increasing optical anisotropy. The Raman-calculated solid bitumen reflectance data are consistent with the optical reflectance data at depths <3,500m, while the former become lower than the latter at depths >3,500m and are consistent with reflectance values constrained by basin modelling. We propose that the surface quality and granularity of solid bitumen, hydrothermal fluid alteration, and varied thermal conductivity of mineral matrix result in the higher optical reflectance of solid bitumen occurrence in the Qiongzhusi carbonates. The multidisciplinary calibration approach integrating optical reflectance measurement, Raman spectroscopy and basin thermal modelling is considered effective and more accurate for thermal maturity assessment.

Geochemistry of marine black shale of the Cambrian Qiongzhusi Formation, Yangtze Plate, SW China: implications for provenance and paleoweathering

Geochemistry of marine black shale of the Cambrian Qiongzhusi Formation, Yangtze Plate, SW China: implications for provenance and paleoweathering

Natural Gas Accumulation and exploration potential of permian Volcanic Rocks in Southwest Sichuan Basin

To figure out gas enrichment laws in Permian volcanic rocks, Sichuan Basin, an association among reservoir-forming elements was explored to analyze forming modes and point out exploration prospects in accordance with reservoir characteristics, gas source conditions, and fault elongation in Zhougongshan and Jianyang areas. Results show that there mostly exist porous volcaniclastic rock and lava reservoirs of explosive facies. In Jianyang with Lower Cambrian Qiongzhusi Formation as the principal source rock, early faults offered pathways to assemble ample initial oil and gas. Following an increase of burial depth, oil started cracking vastly to create oil-cracked gas. These elements constitute one reservoir-forming mode of Cambrian hydrocarbon supply–fault transporting–in-situ cracking in Jianyang. While in Zhougongshan with Lower Permian Maokou 1 Member as the main source, active faults during hydrocarbon generation and expulsion acted on trap preservation. Late kerogen-cracked gas gathered at structural highs far from giant faults. Thus, the other mode exhibited in Zhougongshan is Permian hydrocarbon supply–fault adjustment–effective trap accumulation. Reservoir-forming conditions in volcanic rocks present certain disparity among all kinds of tectonic settings inside the basin, giving rise to various volcanic gas reservoirs developed. The Permian volcanic rocks are expected to be a major domain for reserve and production increase in Sichuan Basin.

Genetic Relationship between Mississippi Valley-Type Pb–Zn Mineralization and Hydrocarbon Accumulation in the Wusihe Deposits, Southwestern Margin of the Sichuan Basin, China

The coexistence of numerous Mississippi Valley-type (MVT) Pb–Zn deposits and oil/gas reservoirs in the world suggests a close genetic relationship between mineralization and hydrocarbon accumulation. The Wusihe MVT Pb–Zn deposits are located along the southwestern margin of the Sichuan Basin. Based on the spatiotemporal relation between Pb–Zn deposits and paleo-oil/gas reservoirs, ore material sources, and processes of mineralization and hydrocarbon accumulation, a new genetic relationship between mineralization and hydrocarbon accumulation is suggested for these deposits. The Wusihe Pb–Zn deposits are hosted in the Ediacaran Dengying Formation dolostone, accompanied by a large amount of thermally cracked bitumen in the ore bodies. The Pb–Zn deposits and paleo-oil/gas reservoirs are distributed along the paleokarst interface; they overlap spatially, and the ore body occupies the upper part of the paleo-oil/gas reservoirs. Both the Pb–Zn ore and sphalerite are rich in thermally cracked bitumen, in which µm sized galena and sphalerite may be observed, and the contents of lead and zinc in the bitumen are higher than those required for Pb–Zn mineralization. The paleo-oil/gas reservoirs experienced paleo-oil reservoir formation, paleo-gas reservoir generation, and paleo-gas reservoir destruction. The generation time of the paleo-gas reservoirs is similar to the metallogenic time. The source rocks from the Cambrian Qiongzhusi Formation not only provided oil sources for paleo-oil reservoirs but also provided ore-forming metal elements for mineralization. Liquid oil with abundant ore-forming metals accumulated to form paleo-oil reservoirs with mature organic matter in source rocks. As paleo-oil reservoirs were buried, the oil underwent in situ thermal cracking to form overpressure paleo-gas reservoirs and a large amount of bitumen. Along with the thermal cracking of the oil, the metal elements decoupled from organic matter and H2S formed by thermochemical sulfate reduction (TSR) and minor decomposition of the organic matter dissolved in oilfield brine to form the ore fluid. The large-scale Pb–Zn mineralization is mainly related to the destruction of the overpressured paleo-gas reservoir; the sudden pressure relief caused the ore fluid around the gas–water interface to migrate upward into the paleo-gas reservoirs and induced extensive metal sulfide precipitation in the ore fluid, resulting in special spatiotemporal associated or paragenetic relations of galena, sphalerite, and bitumen.

Astronomical forced sequence infill of Early Cambrian Qiongzhusi organic-rich shale of Sichuan Basin, South China

Early Cambrian Qiongzhusi Formation has attracted great attention due to shale gas finding. However, this organic-rich shale formation is yet no robust high-resolution stratigraphic sequence framework. Here the evidence on Milankovitch cycles in three wells drilled through Qiongzhusi (QZS) Formation in Sichuan Basin has been reported. The logging data made it possible to develop a floating timeline calibrated to stable eccentricity cycles of 405-kyr within ~10.92 and ~7.34 Myr intervals in intracratonic rift and low bulge of Sichuan Basin, respectively. In this research, astronomical time scale was anchored to the UPb Zircon age of QZS Formation/Maidiping Formation boundary (526.86 ± 0.16 Ma), developing an absolute timescale for considered intervals extending from 526.86 to 515.94 Ma. Average spectral misfit and correlation coefficient analyses were applied to quantitatively measure the fitting of the witnessed sedimentary cycles to astronomical periods and provide a possible sedimentation rate range. A sedimentary noise model was chosen to detect high-resolution sea-level variations under the control of orbital forcing and variations of sea-level were linked to obliquity modulation cycles of about 1.2 Myr. The orbital obliquity modulation cycles witnessed for early Cambrian QZS Formation (~1.2 Myr) were correlated to eustatic variations of sea level generated based on sedimentary noise model (DYNOT and ρ1). Furthermore, correlations between sequence stratigraphy and cyclostratigraphy were discussed, and the third-order sequence boundaries well corresponded to minima ~1.2 Myr orbital obliquity modulation curve and high values of DYNOT. The previous sequences framework was demonstrated to be isochronous and credible.

Paleoenvironmental Reconstruction and Organic Matter Accumulation of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin, South China

Paleoenvironmental Reconstruction and Organic Matter Accumulation of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin, South China

Superimposed hydrocarbon accumulation through multi-source and multi-stage evolution in the Cambrian Xixiangchi Group of eastern Sichuan Basin: A case study of the Pingqiao gas-bearing anticline

The Xixiangchi Group in eastern Sichuan Basin has great potential for natural gas exploration. However, there is a lack of in-depth studies of the hydrocarbon sources and the formation and evolution of gas reservoirs in this Group. Systematic investigation about the gas reservoir in Pingqiao anticline was consequently carried out in terms of characteristics of reservoir bitumen, the geochemical characteristics of natural gas, diagenetic minerals, and fluid inclusions. Based on this, combined with the reconstruction of the burial history, thermal evolution history and uplifting history of strata, and analysis of the regional tectonic settings, the hydrocarbon sources were identified and the formation and evolutionary processes of the gas reservoirs in Xixiangchi Group was revealed in this study. It was shown that the gas reservoirs have mixed gas sources from the shale source rocks in the Lower Cambrian Qiongzhusi Formation and in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, and experienced several evolutionary stages, including the paleo-oil reservoir stage from the Late Siliurian to the Middle Permian, the paleo-gas reservoir stage from the Late Permian to the Early Cretaceous, and the superimposed accumulation and mixed-source gas reservoir stage since the Late Cretaceous. The mixed-source gas reservoir is formed by the adjustment of the Xixiangchi Group paleo-gas reservoirs and depressurization of the overpressure Wufeng-Longmaxi shale gas reservoirs and the charging of gas into the Xixiangchi Group reservoir of the Pingqiao anticline since the Late Cretaceous, which show obvious superimposed accumulation characteristics. There are different accumulation patterns in different geological periods. The accumulation pattern of the “old source - young reservoir” (i.e. hydrocarbons generated from older source rocks accumulating in younger reservoirs) dominates before the Late Cretaceous, and that of “juxtaposed young source - old reservoir” (i.e. hydrocarbons generated from younger source rocks accumulating in juxtaposed older reservoirs) dominates after the Early Cretaceous. Moreover, faults acted as critical vertical pathways for hydrocarbon migration during the evolution of the Xixiangchi Group gas reservoirs. This model provides new insights and theoretical basis for evaluation and mapping of the Xixiangchi Group play fairway in eastern Sichuan Basin.

Comparison of the Ediacaran and Cambrian petroleum systems in the Tianjingshan and the Micangshan uplifts, northern Sichuan Basin, China

Several different types of petroleum occurrences (including solid bitumens, oil sands, and oil seeps) are observed in various strata (e.g., the Ediacaran, Cambrian, Devonian, Permian, Triassic, and Jurassic strata) of the Tianjingshan and the Micangshan uplifts, northern Sichuan Basin, China. However, the sources of these petroleum occurrences are still in dispute, because of the severe biodegradation, strong thermal alteration, and multiple tectonic movements. At present, such uncertainty in the potential source rock systems has hindered oil and gas exploration in the study areas. In this study, methods including catalytic hydropyrolysis and trace elements analysis were introduced into the oil-source correlation study, to avoid the influence of biodegradation and/or thermal alteration. The results show that the petroleum shows that occur in the Cambrian-Jurassic strata of the Tianjingshan area are characterized by low maturity (equivalent vitrinite reflectance (EqVRo) < 1.0%), light organic carbon isotopic ratios (δ13Corg values ranging from −34.0‰ to −38.4‰), a predominance of C29-steranes, enrichment in V and Mo, and strong depletion in Bi elements. In contrast, the solid bitumens in the Ediacaran Dengying Formation in the Micangshan area have high maturity (EqVRo > 2.0%), slightly heavier δ13Corg values ranging from −27.5‰ to −36.0‰, a predominance of C27-steranes, weak enrichment in Mo and strong depletion in Rb and Bi elements. In addition, the Ediacaran and the Lower Cambrian source rocks can be discriminated by the bound sterane distributions, trace element concentrations, and bulk δ13Corg values. Further oil-source correlation indicates that the petroleum shows in the Cambrian-Jurassic strata in the Tianjingshan area were dominantly derived from the Lower Cambrian Qiongzhusi Formation, while those in the Ediacaran Dengying Formation in the Micangshan area were mainly sourced from the Ediacaran Doushantuo Formation. Finally, the results of oil-source correlation, combined with the tectonic evolution history of the investigated areas further elucidate the occurrence and distribution of petroleum, which can assist in oil and gas exploration in the investigated uplifts and the adjacent areas.

Depositional Paleo-Environments of Lower Cambrian Qiongzhusi Formation in the Western Middle Yangtze Block and Its Controlling Effect on the Organic Matter Enrichment

No systematic comparative study has been conducted on the factors controlling organic matter enrichment in the different depositional environments of the Lower Cambrian Qiongzhusi Formation in the western Middle Yangtze Block, leading to a large discrepancy in our understanding. Based on organic geochemical and elemental analyses of core, outcrop, rock, and mineral samples from the slope, deep-water shelf, and shallow-water shelf, in this study, comparative analysis of the organic matter content, sedimentological characteristics, and depositional paleoenvironments of the Lower Cambrian Qiongzhusi Formation in the western Middle Yangtze Block was conducted, and the main controlling factors and models of the organic matter enrichment were investigated. The results revealed that the organic matter enrichment in the Qiongzhusi Formation was jointly controlled by redox conditions, water restriction, upwelling currents, terrigenous inputs, and paleo-productivity, but the main factors controlling the enrichment during the different periods were significantly different. (1) During the deposition of the Qiong 1 Member, the extensional rifting was strong, and the sea level was always high. The low degree of terrigenous dilution and anoxic conditions favored organic matter preservation. In this period, the upwelling currents were the main factor controlling organic matter enrichment. The paleo-productivity decreased as the intensity of the upwelling currents gradually weakened from the slope to the shelf, leading to a decrease in the total organic carbon (TOC) content and thereby a gradual decrease in the biogenic silica content of the shale. (2) During the deposition of the Qiong 2 Member, the extensional rifting weakened, and the sea level continued to drop. The upwelling currents, terrigenous input, and redox conditions were all important factors controlling the organic matter enrichment in the region. From the slope to the shelves, the conditions favorable for organic matter enrichment gradually worsened, and the TOC content gradually decreased, with the lithofacies gradually transitioning from biogenic siliceous shale to clayey shale or clayey-calcareous shale. (3) During the deposition of the Qiong 3 Member, the Yangzi Platform underwent a filling and leveling-up process, and the redox conditions played a major role in controlling the organic matter enrichment. The entire region was dominated by an oxygen-rich environment, and the conditions were no longer favorable for organic matter preservation, leading to a low average TOC content. Overall, the spatial variability of the TOC content was closely associated with changes in the depositional paleoenvironment caused by sea-level changes.

Characteristics and environmental significance of concretions in the Lower Cambrian Qiongzhusi Formation in the Middle–Upper Yangtze region, China

Based on the outcrop and drilling data of the Sichuan Basin and its periphery, this paper reveals the concretion development characteristics of the Lower Cambrian Qiongzhusi Formation and its significance to the environment. Three preliminary conclusions are drawn as follows. (1) The exposure layers and lithofacies characteristics of the concretions in the Qiongzhusi Formation are very different from area to area. In the eastern Sichuan-western Hubei-Hunan & Guizhou and southern Sichuan depression areas, the concretionsare developed in the section of SQ1, and are mainly composed of calcareous siliceous mixed shale facies, siliceous shale facies, dolomitic shale facies and dolomite facies. The clay content is generally less than 25% and is significantly less than the surrounding rocks, and the TOC is generally higher in the lower concretion layers and less in the upper concretion layers. In the central Sichuan and northern Sichuan depression areas, the concretions appear in the sections of SQ1 and SQ2, and are dominated by calcareous shale facies and marl facies. The calcic content is obviously higher than that of surrounding rocks, and the clay content is less than that of surrounding rocks, and the TOC is less than 1.50%. Because mineral composition difference is high, most of the GR curves of the concretions show trough response characteristics, reflecting the central region enriched with calcium or siliceous, and the edge enriched with clay and organic matter. (2) The concretion layers generally occur in carbonaceous shale, silty shale and siliceous shale with a TOC over 0.7%, and are especially developed in carbonaceous shale and siliceous shale of SQ1, and occasionally appear in grey silty shale with a TOC over 0.2%, showing that the concretion layers are basically symbiotic with grey shale (especially with high-quality shale). (3) The concretions are a kind of important sedimentary response of rifting activities in Yangtze marine basin. According to the distribution characteristics of concretions and black shale in the Qiongzhusi Formation, the rifting activities of Yangtze marine basin can be divided into three periods including development period (SQ1), adjustment period (SQ2) and abortion period (SQ3). In the period of SQ1, controlled by massive rifting and upwelling ocean currents in Yangtze marine basin, the concretions and organic-rich shale were widely developed in the Lower Qiongzhusi Formation. In the periods of SQ2 and SQ3, while rifting activity of Yangtze marine basin decreased and aborted, and while sea level fell and upwelling ocean currents receded to the southeast, the concretions and organic-rich shale just appeared in the local depression areas in small scale.