Wufeng Formation Research Articles

Mineralogical and petrographic characteristics of the Ordovician-Silurian Wufeng-Longmaxi Shale in the Sichuan Basin and implications for depositional conditions and diagenesis of black shales

Depositional and diagenetic processes play important roles in controlling the fabric and petrophysical properties of black shales. In this study, an outcrop section of the Wufeng-Longmaxi Shale was studied through sedimentologic facies analysis, petrographic observations (both transmitted- and reflected-light microscopy and scanning electron microscopy), and X-ray diffraction analysis to investigate how depositional and diagenetic processes controlled the accumulation and evolution of organic-rich black shales. The studied Wufeng-Longmaxi Shale consists of over-mature black shales, with an equivalent vitrinite reflectance of 3.07%. Organic matter is dominated by pyrobitumen and graptolite, the former of which hosts abundant organic pores. Minerals mainly consist of quartz (average 42 wt %), carbonates (average 37.4 wt %), and clay minerals (average 15 wt %). Eighty percent of quartz is of biogenic origin, which occurs as siliceous fossils and microcrystalline quartz. Microcrystalline quartz forms a rigid framework and preserves organic pores from compaction. Clay minerals comprise dominantly (95 wt %) illite and mixed-layer illite/smectite. The vertical variations in lithofacies and quartz and carbonate mineral content exhibit three cycles, likely corresponding to three transgressive-regressive cycles. Within each cycle, quartz content first increases to reach a maximum at the maximum flooding surface, and then decreases. Although long being considered as deposited under quiescent and anoxic conditions, sedimentologic and petrographic analyses in this study indicate that the deposition of the Wufeng Formation and lower Longmaxi Formation was subject to intermittent reworking by bottom currents and bottom-water dysoxia. Meiofauna capable of tolerating low oxygen content could have lived in surficial sediments and disrupted the original fabric of sediments formed by bottom currents. Mineralogical and petrographic analyses in this study provide new insights into the accumulation mechanism of the organic-rich black shales of the Wufeng-Longmaxi Formations, calling for a critical reappraisal of other black shales that have conventionally been considered as deposited under mostly quiet and anoxic conditions. • Mineralogical and petrographic characteristics were studied. • Three stratigraphic cycles were identified. • Depositional conditions were not persistently quiescent and anoxic. • Intermittent reworking by bottom currents and bottom-water dysoxia existed.

Geochemical characteristics and resource potential of shale gas in Sichuan Basin, China

Shale gas is a green, low-carbon and clean unconventional natural gas energy. China is rich in shale gas resources. Accelerating shale gas exploration and development is of great practical significance to improve China's energy structure and achieve the goal of “2030 carbon peak and 2060 carbon neutralization”. The Ordovician Wufeng Formation to Silurian Longmaxi Formation in Sichuan Basin is the key strata for shale gas exploration and development in China. Based on the experimental analysis of shale gas in Wufeng Formation to Longmaxi Formation in Weiyuan, Changning, Zhaotong, Fuling and Weirong areas of Sichuan Basin, this paper systematically analyzes the geochemical characteristics of shale gas in Wufeng Formation to Longmaxi Formation, discusses the genesis of shale gas, the causes of hydrocarbon isotope inversion and the source of shale gas, and looks forward to the exploration prospects of shale gas resources in the basin. It has important guiding significances for shale gas exploration and development in Sichuan Basin and even in China. The results show that: (1) The shale gas of Wufeng-Longmaxi Formations is a typical dry gas, and the hydrocarbon isotopes are distributed in negative sequence. The carbon isotopes of alkane gas in Changning, Zhaotong and Fuling areas are heavier and have a higher degree of thermal evolution than those in Weiyuan and Weirong areas, and the rare gas is of crustal origin. (2) Shale gas of Wufeng-Longmaxi Formations is a thermogenic oil-typed gas at high-over mature stage, which is mainly the mixture of crude oil cracking gas and kerogen cracking gas. The hydrocarbon isotope inversion of alkane gas is mainly caused by the mixing of crude oil cracking gas and kerogen cracking gas at high-over mature stage, the exchange between formation water and methane at high evolution stage and so on. (3) The carbon isotopic values of methane in shale gas of Wufeng-Longmaxi Formations match the carbon isotopic values of kerogen in mudstones of Longmaxi Formation of Lower Silurian, which conforms to the law of carbon isotopic fractionation δ13Ckerogen>δ13Coil>δ13Calkane gas. (4) The total amount of marine, marine continental transitional and continental facies shale gas resources in Sichuan Basin is about 41.5 × 1012 m3, with broad exploration prospects in resource potential.

Chemostratigraphic Analysis of Wufeng and Longmaxi Formation in Changning, Sichuan, China: Achieved by Principal Component and Constrained Clustering Analysis

The increasing proportion of unconventional worldwide energy demands have consistently promoted the necessity for exploring a precise, high-resolution, objective, and quantitative stratigraphic division method for macroscopically homogeneous mudstone successions. The chemostratigraphy can resolve this problem well, although it has been applied successfully in North America, but not systematically studied in China for shale gas exploration and development. This work has conducted a chemostratigraphic analysis of Wufeng and Longmaxi Formation on the Changning section of Sichuan Province, southwestern China, to testify its applicability for shale gas exploration in China. Principal component analysis (PCA) was first employed to reduce the dimensionality of datasets. Three chemofacies, including detrital (K, Ti, Fe, Al, Na, Mg, Cr, Zr, Rb), authigenic (Ca, Sr, Mn, Si, S, Ba), and redox-organic (P, V, Ni, Zn, Cu, TOC), were found. Subsequently, constrained clustering analysis was utilized for the zonation of each chemofacies into chemozones. Consequently, the whole Changning section was divided into twelve chemozones (CZ I–CZ Ⅻ). The geochemical interpretation for these chemozones can be resolved from the regional changes in paleogeography and paleoceanography during the Late Ordovician to Early Silurian period. Thus, a three-stage geochemical evolution along the Changning section can be classified: (1) the siliceous and anoxic deposits of Wufeng Formation (CZ I–CZ III) with high TOC contents; (2) the siliceous and anoxic sedimentary rocks of bottom Longmaxi Formation with still higher TOC (CZ Ⅳ); (3) the calcarous-detrital and oxic sediments for the rest of Longmaxi Formation (CZ Ⅴ–CZ Ⅻ). In considering their high content of TOC and abundant brittle siliceous minerals, the CZ (I–Ⅳ, 0 m–33.6 m) are thought to be the most preferable sweet spot for shale gas exploration.

Experimental Study on the Development Characteristics and Controlling Factors of Microscopic Organic Matter Pore and Fracture System in Shale

Research on microscopic pore and fracture system of shale is a hot spot in the field of unconventional petroleum geology. Micro- and nano-scale organic matter pores in shale play a vital role in the accumulation of hydrocarbons. Research on the types, evolution rules and controlling factors of organic matter pore-fracture system in shale reservoirs can provide scientific guidance for the prediction of shale “sweet spots”. In this paper, taking the shales from the Napo Formation, Oriente Basin, the Shahejie Formation, Zhanhua Sag, Bohai Bay Basin, and the Longmaxi and Wufeng Formations, Sichuan Basin as an example, the developmental characteristics of organic matter pore-fracture system were systematically studied using thin section, argon ion profiling scanning electron microscopy, X-ray diffraction, N2 adsorption and desorption, geochemistry experiments, and image processing technology. The types of shale organic matter pores were divided into kerogen-hosted pores, organic matter microfractures (intra-organic matter and organic matter edge microfractures), and asphalt pores (or intra-asphalt pores). The circumferences of organic pore were generally within 100 nm, and the areas of most pores were smaller than 1,000 nm2. Face rates of the organic pores were generally less than 1.5%, and the proportion of shale samples with a shape factor of 1 reached more than 50%. In addition, the deviation angles of organic matter pores at (0°, 45°) reached 90%, which showed that most of the organic matter pores tended to be oriented pores. The increase in the degree of thermal evolution provided driving force for the formation of circular pores in the organic matter. Internal factors (abundance of organic matter, kerogen types, and maturity) and external factors (diagenesis and mineral composition) controled the development of shale organic matter pores. Maturity, TOC content and inorganic minerals such as clay and pyrite content were positively correlated with the development of organic matter pores. However, brittle minerals caused a decrease in the face ratio of organic matter pores. Diagenetic compaction caused the organic matter pores being deformed or eventually disappeared. This research can provide scientific guidance for the high-efficiency exploration of hydrocarbons in shale.

Biostratigraphy and reservoir characteristics of the Ordovician Wufeng Formation—Silurian Longmaxi Formation shale in the Sichuan Basin and its surrounding areas, China

Through graptolite identification in profiles, graptolite zone division, contour map compilation, and analysis of mineral composition, TOC content, lamina distribution features of shale samples, the biostratigraphic and reservoir characteristics of Ordovician Wufeng Formation—Silurian Longmaxi Formation in the Sichuan Basin and its peripheral are sorted out. There are 4 graptolite zones (WF1 to WF4) in Wufeng Formation and 9 (LM1 to LM9) in Longmaxi Formation, and the different graptolite zones can be calibrated by lithology and electrical property. The shale layers of these graptolite zones have two depocenters in the southwest and northeast, and differ in mineral composition, TOC, and lamina types. Among them, the graptolite zones of lower WF2 and WF4 are organic matter-poor massive hybrid shale, the upper part of WF1–WF2 and WF3 have horizontal bedding hybrid shale with organic matter, the LM1–LM4 mainly consist of organic-rich siliceous shale with horizontal bedding, and the LM5–LM9 graptolite zones consist of organic-lean hybrid shale with horizontal bedding. The mineral composition, TOC and lamina types of shale depend on the paleo-climate, paleo-water oxidation-reduction conditions, and paleo-sedimentation rate during its deposition. Deposited in oxygen-rich warm water, the lower parts of WF1 and WF2 graptolite zones have massive bedding, low TOC and silicon content. Deposited in cooler and oxygen-rich water, the WF4 has massive bedding, high calcium content and low TOC. Deposited in anoxic water with low rate, the upper part of WF2, WF3, and LM1–LM4 are composed of organic rich siliceous shale with horizontal bedding and high proportion of silt laminae. Deposited in oxygen rich water at a high rate, the graptolite zones LM5–LM9 have low contents of organic matter and siliceous content and high proportions of silt lamina.

Microscopic pore-fracture configuration and gas-filled mechanism of shale reservoirs in the western Chongqing area, Sichuan Basin, China

Taking the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation shale reservoirs in western Chongqing area as the study target, the argon ion polishing scanning electron microscope and nuclear magnetic resonance (NMR) experiments of different saturated wetting media were carried out. Based on the image processing technology and the results of gas desorption, the pore-fracture configuration of the shale reservoirs and its influence on gas-filled mechanism were analyzed. (1) The reservoir space includes organic pores, inorganic pores and micro-fractures and there are obvious differences between wells in the development characteristics of micro-fractures; the organic pores adjacent to the micro-fractures are poorly developed, while the inorganic pores are well preserved. (2) According to the type, development degree and contact relationship of organic pore and micro-fracture, the pore-fracture configuration of the shale reservoir is divided into four types. (3) Based on the differences in NMR T 2 spectra of shale samples saturated with oil and water, an evaluation parameter of pore-fracture configuration was constructed and calculated. The smaller the parameter, the better the pore-fracture configuration is. (4) The shale reservoir with good pore-fracture configuration has well-developed organic pores, high porosity, high permeability and high gas content, while the shale reservoir with poor pore-fracture configuration has micro-fractures developed, which improves the natural gas conductivity and leads to low porosity and gas content of the reservoir. (5) Based on pore-fracture configuration, from the perspective of organic matter generating hydrocarbon, micro-fracture providing migration channel, three types of micro gas-filled models of shale gas were established.

Machine Learning Method for TOC Prediction: Taking Wufeng and Longmaxi Shales in the Sichuan Basin, Southwest China as an Example

The total organic carbon content (TOC) is a core indicator for shale gas reservoir evaluations. Machine learning-based models can quickly and accurately predict TOC, which is of great significance for the production of shale gas. Based on conventional logs, the measured TOC values, and other data of 9 typical wells in the Jiaoshiba area of the Sichuan Basin, this paper performed a Bayesian linear regression and applied a random forest machine learning model to predict TOC values of the shale from the Wufeng Formation and the lower part of the Longmaxi Formation. The results showed that the TOC value prediction accuracy was improved by more than 50% by using the well-trained machine learning models compared with the traditionalΔLogRmethod in an overmature and tight shale. Using the halving random search cross-validation method to optimize hyperparameters can greatly improve the speed of building the model. Furthermore, excluding the factors that affect the log value other than the TOC and taking the corrected data as input data for training could improve the prediction accuracy of the random forest model by approximately 5%. Data can be easily updated with machine learning models, which is of primary importance for improving the efficiency of shale gas exploration and development.

Constraints of sedimentary structures on key parameters of shale gas reservoirs: A case study of the Wufeng and Longmaxi formations of the Ordovician‐Silurian transition in Luzhou area, Sichuan Basin, southwestern China

There are various sedimentary structures developed in the black shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in Luzhou area, southern Sichuan Basin, southwestern China. In order to evaluate the controlling effects of sedimentary structures on the key parameters of shale reservoir properties, collected classification of sedimentary structures, scanning electron microscope observation, analysis of shale fabric and porosity characteristics, low‐pressure nitrogen adsorption, methane isothermal adsorption, and conventional triaxial compression test were carried out based on shale samples from Wells A‐H1, B‐H1, C‐H1 and D‐H1 in the study area. The sedimentary structures recorded in these shales are categorized into five types: homogeneous bedding, weak horizontal bedding, strong horizontal bedding, discontinuous ripple bedding, and gravel‐bearing massive bedding. In general, the palaeo‐hydrodynamics and oxygen content of water columns were weak when homogeneous bedding shale was deposited. The organic matter is the most abundant and the total organic carbon content is up to 4.01% on average. The average quartz content is 43.5%, and the brittleness index is the highest. The porosity and adsorption capacity were the highest, with an average of 4.57% and 79.5 scf/ton, respectively. Combined with its mechanical properties of low Poisson's ratio and high Young's modulus, homogeneous bedding sedimentary structure is associated with the shale units possessing the highest organic matter abundance, the strongest reservoir property, and the best fracability, which is the priority target for exploration and development of the Wufeng to lower Longmaxi Formation in the Sichuan Basin, southwestern China.

Sedimentary tectonic pattern of Wufeng and Longmaxi Formations in the northern margin of Sichuan Basin, South China

ABSTRACT Organic-rich black shales in the Sichuan Basin from the Upper Ordovician and Lower Silurian are considered excellent source rocks and unconventional reservoirs of hydrocarbons in South China. The Wufeng and Longmaxi Formations in the Micangshan – Dabashan areas of the northern margin of Sichuan Basin were taken as the research objects. The purpose of this study is to reconstruct the sedimentary tectonic pattern of the Wufeng and Longmaxi Formations in the study area. First, the stratigraphic framework correlation and sedimentary facies analysis of Wufeng and Longmaxi Formations in the study area were carried out. Then, Mo/TOC and Al-Co[EF]×Mn[EF] geochemical indicators were used to identify the degree of water restricted during the deposition period. It was found that the thickness of black shale was relatively small near the uplift area. However, far away from the uplift area, from west to east, the black shale thickened and the TOC content increased gradually, which were controlled by the upwelling of continental margin and intracontinental depression. The results show that: (1) At the front edge of Micangshan, during the deposition period of Wufeng Formation, Fucheng and Qiaoting profiles were located in the intersection area of upwelling formed by two kinds of monsoon, which led to the long-term influence of upwelling in this area. The sedimentation period of Longmaxi Formation was mainly affected by ‘Xixiang Rise’, which led to the uplift of Wangcang and Nanjiang areas into islands. (2) At the front edge of Dabashan and Western Hubei – Eastern Chongqing areas, the Wufeng Formation presented a tectonic pattern of alternating uplift and depression. Due to the continuous compression of the Cathaysian Block from southeast to northwest, the Shizhu – Wuxi areas continued to subside during the deposition period of Longmaxi Formation, and the black shale was quite thick.

Multi-scale multi-dimensional characterization of clay-hosted pore networks of shale using FIBSEM, TEM, and X-ray micro-tomography: Implications for methane storage and migration

Gas shales contain a variety of clay-rich materials with multifarious pore networks. Clay-hosted porosity is an essential component and considered to play a crucial role in establishing the original hydrocarbon molecules in place and transport characteristics of the shale gas reservoir. To better understand the petrophysical basis of gas storage and migration mechanisms, we aim to visualize and quantify the nature of the clay-hosted pore networks and across micrometer and nanometer length scales. Core and outcrop samples are provided of various clay-rich shales (Qiongzhusi, Longmaxi, Wufeng, and Shihezi Formation), which are characterized using a synergistic multi-scale multi-dimensional workflow by FIBSEM, TEM, and X-ray micro-tomography (mCT). Clay-hosted pores are observed in three significant modes of occurrence depending on imaging of pores at the 2D-FIBSEM/TEM level. The first pore type is found between clay domains in clay matrix as interparticle pore, with pore size smaller than 1 μm, and includes six subtypes: type a, type b, type c, type d, type e, and type f. The second associated with the admixture belongs to inter-aggregate pore that can be further sub-divided into three classes: type g (organic-clay), type h (pyrite-clay), and type I (organic-pyrite-clay), with pore size above 50 up to 500 nm. The third associated with the clay nanoplatelets is intraparticle pore. The upper pore-size range of such pores is generally less than 1 nm, with most pores being less than 0.4 nm. Using 3D-mCT and 3D-FIBSEM, the data sets were reconstructed, clay structures were segmented and visualized, revealing the well-connected clay-hosted pore networks within the heterogeneous clay matrix and quantitatively computing pore size, pore volume, and porosity at micrometer and nanometer scales. These visual results highlight the significance of clay-hosted pore networks in shale gas reservoirs because they are the dominant controls on the petrophysical properties. The application of this workflow to worldwide clay-rich shale deposits will allow essential insights into estimating porosity and permeability of shale formations and provide insight to the storage and transport of hydrocarbon molecules from shale matrix to predict total gas resources.

Types of biogenic quartz and its coupling storage mechanism in organic-rich shales: A case study of the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the Sichuan Basin, SW China

Biogenic quartz in the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation (Wufeng– Longmaxi) shale layers in the Sichuan Basin and its periphery is qualitatively analyzed and quantitatively characterized by organic petrologic, mineralogic, and geochemical methods to find out the coupling effect between organic matter and quartz. (1) There are two types of biogenic quartz in the shale layers: Type I quartz is submicron quartz appearing in clusters around the organic matter. Type II quartz is in nano-scale grain size and floats in spherical shape on organic matter, with grains in point-to-point or surface-to-surface contact; this type of quartz is mainly biologic origin and slightly affected by hydrothermal activity in local parts. (2) The reservoirs in the Wufeng–Longmaxi formations is consistent in distribution with biogenic silica content in them, and mainly concentrated at the bottom of the Wufeng–Longmaxi formations, and is thinner in the Changning and Weiyuan regions, while thicker in the Fuling region. (3) The biogenic quartz in the Wufeng–Longmaxi shale worked through the entire evolution process of hydrocarbon generation. The presence of biogenic quartz can enhance the development of organic matter pores and microcracks, and can effectively preserve the organic matter pores and residual intergranular pores, forming “biological silicon intergranular pores, organic pores and micro-fractures”. This would benefit later hydraulic fracturing and result in high production/stable production of well. The coupling effect between biogenic quartz development and organic matter evolution and hydrocarbon generation is a critical factor for high-quality shale reservoir development.

Self-diffusion flow and heat coupling model applicable to the production simulation and prediction of deep shale gas wells

Clarifying the flow laws of shale gas under high temperature and high pressure is the prerequisite to accurately predicting the productivity of deep shale gas wells. In this paper, a self-diffusion flow model of flow field and temperature field coupling (referred to as self-diffusion flow and heat coupling model) was established based on the previously proposed self-diffusion flow model, while considering the influence of the temperature field change. Then, its calculation result was compared with that of the flow model based on Darcy's law and Knudsen diffusion (referred to as modified Darcy flow model). Based on the self-diffusion flow and heat coupling model, the self-diffusion flow behaviors of deep shale gas under the influence of temperature field change were analyzed, and the influence of bottomhole temperature on the degree of reserve recovery of deep shale gas was discussed. Finally, the self-diffusion flow and heat coupling model was applied to simulate the production of one shale-gas horizontal well in the Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in the Changning Block of the Sichuan Basin. And the following research results were obtained. First, at the same parameters, the shale gas production calculated by the self-diffusion flow and heat coupling model is higher than the result calculated by the modified Darcy flow model. Second, when temperature field change is taken into consideration, the self–dviffusion coefficient profile presents a peak, the gas density profile presents a valley and the data points corresponding to the peak/valley move synchronously to the internal formation, which indicates that the self–diffusion coefficient influences the gas mass transfer rate, and the influence range of near well low temperature on gas self-diffusion increases continuously as the production continues. Third, when the bottomhole temperature is lower than the formation temperature, the self–diffusion coefficient of the gas near the well decreases and the gas is blocked near the well, which reduces the gas well production. Fourth, the production simulation result of the case well shows that the self-diffusion flow and heat coupling model can predict the production of deep shale gas more accurately if temperature field change is taken into consideration. In conclusion, the self-diffusion flow and heat coupling model established in this paper is of higher reliability and accuracy and can be used for productivity simulation and prediction of deep shale gas wells. The conclusion of this paper has certain guiding significance for deep shale gas production and gas well productivity prediction.

Volcanism-induced late Boda warming in the Late Ordovician: Evidence from the Upper Yangtze Platform, South China

Late Boda warming was the last warm episode in the Late Ordovician, and little is known about its cause. Volcanism was a hypothetical candidate that triggered this warming event owing to its potential to enhance the atmospheric CO2 level. Here, we conducted a high-resolution petrographic and geochemical study on the bentonite beds-bearing intervals from the Upper Ordovician Wufeng Formation in South China. This investigation enables us to find the detailed process of paleoenvironmental repercussions induced by volcanism from both short-term and long-term perspectives. The short-term effect was revealed by a 25-cm-long drill core bracketing three cycles of volcanic activity. Three nearly identical cycles have been identified, and each cycle is comprised of a bentonite bed, a siliceous clay-rich mudstone interval, a radiolarian bed, and a siliceous mudstone interval with intercalations of thin pyrite layer. During the eruption stage, a series of environmental and biogeochemical disruptions were brought to the local system, including ocean fertilization, the elevation of paleoproductivity, the onset of euxinia/intensified ocean anoxia, a warmer climate, and enhanced weathering. With the waning of volcanic activities, the volcanic impacts weakened, and a relatively stable water-column chemistry, microbial populations, diversity, and community structure were established until the onset of the subsequent volcanic event that broke equilibrium. The long-term effect was illustrated by the correlation between the distribution of bentonite beds and coeval global sea-level fluctuations. Our results show that intense and frequent volcanic activities in South China, probably contributed by coeval volcanic activities from other regions worldwide, exerted a cumulative heating effect on the global system, contributing to late Boda warming.

Geochemical Characteristics of Late Ordovician Shales in the Upper Yangtze Platform, South China: Implications for Redox Environmental Evolution

Changes to the redox environment of seawater in the Late Ordovician affect the process of organic matter enrichment and biological evolution. However, the evolution of redox and its underlying causes remain unclear. This paper analyzed the vertical variability of main, trace elements and δ34Spy from a drill core section (well ZY5) in the Upper Yangtze Platform, and described the redox conditions, paleoproductivity and paleoclimate variability recorded in shale deposits of the P. pacificus zone and M. extraordinarius zone that accumulated during Wufeng Formation. The results showed that shale from well ZY5 in Late Ordovician was deposited under oxidized water environment, and there are more strongly reducing bottom water conditions of the M. extraordinarius zone compared with the P. pacificus zone. Excess silica (SiO2(exc)) and substitution index of paleoproductivity (Y) indicated that the P. pacificus zone had higher paleoproductivity whereas the M. extraordinarius zone was lower. The high productivity level controlled O2 release in the shallow water area as well as the oxidation degree of the P. pacificus zone. The decrease of productivity and the relatively stagnant water mass of the inner Yangtze Sea controlled the formation of relatively reduced water conditions in the M. extraordinarius zone. The chemical index of alteration (CIA) results suggested that palaeoclimatic conditions changed from warm and humid to cold and dry climate from the P. pacificus to the M. extraordinarius zones in the study area. A comparative analysis of the published Fe-S-C data for the Xiushan Datianba section showed that in the P. pacificus zone of the inner Yangtze Sea, warm and humid climate conditions drove high productivity, sulphate flux and low reactive iron flux, which promoted the expansion of oxic ocean-surface waters and mid-depth euxinic waters. In the M. extraordinarius zone, the cold and dry climate with significant uplift of the Xiang’e Submarine High led to the relative sea level decline, resulting in low productivity, sulfate flux and high reactive iron flux, which promoted the expansion of the mid-depth ferruginous waters and the shrinkage of oxic ocean-surface waters. The results offered new insights into the co-evolution of continents and oceans, and explained the role of continental weathering and uplift of the Xiang’e Submarine High in the exchange of sulfate flux and nutrients in the redox environment change of inner Yangtze Sea during the Late Ordovician.

An integrated study of the petrographic and geochemical characteristics of organic-rich deposits of the Wufeng and Longmaxi formations, western Hubei Province, South China: Insights into the co-evolution of paleoenvironment and organic matter accumulation

Enhanced productivity and/or widespread anoxia have been postulated as the dominant controls on organic matter (OM) accumulation in South China during the Ordovician-Silurian (O-S) transition. However, their roles appear to have varied in space and time, and causal mechanisms linking OM accumulation with changing paleoenvironmental conditions remain poorly constrained. Here we report on results of a multi-faceted investigation of the Wufeng and Longmaxi formations in a drilled core from Xianfeng County, western Hubei Province, South China. The roles played by paleoredox conditions, paleoproductivity, paleoclimate, terrigenous input, upwelling, and volcanism on OM accumulation and preservation during the O-S transition are considered. Our results suggest that the depositional history of OM accumulation as recorded by the studied succession can be subdivided into five phases, each one represented by a lithostratigraphic unit. Unit 1 (average TOC = 3.28%) comprises siliceous shale deposited under generally suboxic conditions interrupted by brief episodes of anoxia beneath very productive surface water during the late Katian time (~447.62 Ma). Seasonal upwelling-driven productivity appears to have exerted fist-order control on OM accumulation. Overlying siliceous organic-rich shale of unit 2 (average TOC = 4.93%) records anoxic bottom water conditions from the latest Katian to Hirnantian time. Enhanced productivity and OM preservation may have been maintained by phosphorous cycling. Calcareous shale of the overlying Guanyingqiao unit 3 (TOC = 2.8%) accumulated under suboxic conditions and low to moderate water column productivity during the late Hirnantian Stage. Climate-driven sea level fall was responsible for diminished OM accumulation at this time. Siliceous clay-rich shale of unit 4 (average TOC = 5.53%) was deposited beneath highly productive surface water under anoxic bottom water conditions at the beginning of the Rhuddanian. The presence of volcanic ash and enriched Hg concentrations in these deposits suggests that productivity was enhanced by volcanism. Overlying silty shale of unit 5 (average TOC = 1.71%) was deposited in the middle to upper Rhuddanian under dominantly suboxic conditions, declining productivity levels, and increased delivery of terrigenous sediments. Accumulation of these deposits was contemporaneous with a period of falling sea level related to the Kwangsian Orogeny that would have curbed OM accumulation. Our results, considered in tandem with those of studies from shallow to deep water regions of the Yangtze Block, reveal a level of spatial heterogeneity of redox conditions and productivity levels across the Yangtze Sea during the O-S transition manifested by regional differences in OM accumulation mechanisms and histories.

Classification and Evaluation on Shale Gas Reservoir for Wufeng-Longmaxi Formation in Chuannan Area, Sichuan Basin

Abstract Based on the comprehensive statistic and analysis on some representative geological and physical data, the classification criteria on net pay for shale gas reservoir of the Wufeng-Longmaxi formation in Shunan area, Sichuan Basin, are proposed, which include porosity (φ), gas saturation (Sg), density of rock (DEN), brittleness index (BI), and gas content (Vt). When the porosity, gas saturation, brittleness index, and gas content are larger than 3%, 30%, 40%, and 1 m3/t, respectively, and the density of rock is lower than 2.7 g/cm3, then this formation can be seen as the net pay. The application of two key parameters, gas content and brittleness index, could reflect the reservoir resource basis and fracability, respectively. The gas content has a positive correlation with porosity and total organic volume, and the brittleness index has a positive correlation with siliceous and carbonate content. According to the range of these two parameters, the net pay can be classified into three types. For type I, the gas content and brittleness index are larger than 4 m3/t and 50%, respectively. For type II, either the gas content or the brittleness index is lower than 4 m3/t and 50%. For type III, the gas content should be larger than 1 m3/t and lower than 4 m3/t, and the brittleness index is between 40% and 50%. The field application case indicates that the Wufeng formation and low member of the Longmaxi formation have good quality and mainly consist of type I and II formations. In addition, it is found that there is a positive correlation between the penetration ratio for type I formation and the testing production and estimate ultimate recovery (EUR). While this ratio is larger than 50%, the testing production rate and EUR will be over 15×104 m3/d and 8000×104 m3 with a probability of 92%, which meet the requirement of exploitation with reasonable economic benefits.

A composition-based model for methane adsorption of overmature shales in Wufeng and Longmaxi Formation, Sichuan Basin

The Wufeng-Longmaxi Formation shale in Sichuan Basin is the main strata of shale gas exploration and development in China. Adsorbed methane is an important part of the total shale gas content. The influence of shale composition on adsorption characteristics is complex, and it is difficult to obtain a comprehensive understanding without a sufficient amount of data. In this work, the total organic carbon (TOC), X-ray diffraction (XRD) mineral analysis, and the excess methane adsorption isotherms with pressure up to 50 MPa at 40 °C of 106 overmature shale samples with TOC of 0.8%-9.0% and clay content of 2%-58% that belong to Wufeng-Longmaxi formation from 10 shale gas wells in Sichuan Basin, China are experimentally investigated and analyzed. New correlations found between compositions and methane adsorption characteristic parameters guide the establishment of a composition-based methane adsorption model that the excess adsorption isotherm of shale is the content-weighted sum of equivalent unit-content excess isotherms for organic matter and clay. With this model, the differences of excess isotherms of these two compositions and the differences of clay-contribution to methane adsorption for different sublayers are disclosed. It is beneficial to improve the accuracy of shale reservoir evaluation of studied blocks under the condition of limited coring and deepening the understanding of the underlying mechanism of methane adsorption characteristics.

Classification Evaluation of Gas Shales Based on High-Pressure Mercury Injection: A Case Study on Wufeng and Longmaxi Formations in Southeast Sichuan, China

Classification Evaluation of Gas Shales Based on High-Pressure Mercury Injection: A Case Study on Wufeng and Longmaxi Formations in Southeast Sichuan, China

Three-Dimensional Morphology and Connectivity of Organic Pores in Shale from the Wufeng and Longmaxi Formations at the Southeast Sichuan Basin in China

Organic pores play an important role in shale reservoirs. Organic pores occur where shale gas was produced and accumulated. However, there is little scientific understanding of the distribution and connectivity of organic pores. Organic pore types and their structural characteristics were studied using a total organic carbon (TOC), thin section, focused ion beam scanning electron microscope (FIB-SEM), and nano-CT. The samples were from the Wufeng Formation in the Upper Ordovician and Longmaxi Formations from the lower Silurian. The results show that organic matter is mainly concentrated in the Wufeng Formation and the bottom of the Longmaxi Formation and that the middle and upper parts of the Longmaxi Formation contain a low amount of organic matter. The shale of the Wufeng-Longmaxi Formation has high maturity, and its organic pores are well developed. There are three types of organic pores: algae, graptolite, and pyrobitumen pores. The pore connectivity of shale with a high organic content is better than that of shale with a low organic content. The volume of the organic pores accounts for more than 50% of the volume of the organic matter. Majority of the organic pores have an aperture smaller than 100 nm and are round, nearly circular, and elliptical in morphology. Most of the organic pores in a shale formation are developed in pyrobitumen, and most of the larger organic pores are concentrated at the center of solid pyrobitumen. The organic pores in pyrobitumen have the best connectivity and are the most favorable reservoir spaces and migration channels for shale gas, which is a crucial point of reference for future research of shale gas.

Quartz crystallinity index: New quantitative evidence for biogenic silica of the Late Ordovician to Early Silurian organic-rich shale in the Sichuan Basin and adjacent areas, China

Quartz crystallinity index (QCI) was used to reflect the crystallisation of silica in the Late Ordovician Wufeng (WF) and Early Silurian Longmaxi (LM) Formation shale, as well as the airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale, to distinguish the two types of silica. The silica in different graptolite biozones exhibited different crystallisation. The WF2–3, LM1–4 graptolite biozones showed obviously lower QCI values than the LM5–9 graptolite biozones and the Lucaogou Formation samples. The graptolite organisms played the role of adsorption, fixation, and precipitation in silicon accumulation and enrichment in stratum. The biogenic origin caused the poorest quartz crystallisation in WF2–3 and LM1–4 graptolite biozones samples. The airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale exhibited relatively poor quartz crystallisation because of weaker diagenesis intensity. Generally, although the WF2–3 and LM1–4 graptolite biozones underwent strong diagenesis and contained a small amount of detrital quartz, the silica still exhibited lower QCI values than the airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale. The biogenic silica crystallisation was much poorer than that of the airborne volcanic ash-derived silica. QCI is an effective quantitative index to demonstrate the biogenic silica in the organic-rich and silica-rich shale.