Dalong Formation Research Articles

Comparative study on pore structure characteristics of marine and transitional facies shales: A case study of the Upper Permian Longtan Formation and Dalong Formation in the Lower Yangtze area, south China

The Permian shale reservoir in the Lower Yangtze area of South China is regarded as a target with a high potential for shale gas exploration. To investigate the pore structure characteristics and evaluate reservoir quality, 32 shale samples from the Upper Permian Longtan Formation and Dalong Formation in the Lower Yangtze area were studied using a suite of techniques including total organic carbon (TOC) content, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), nitrogen (N2) and carbon dioxide (CO2) gas physisorption, mercury injection capillary pressure (MICP), petrographic observations, and fluid inclusion analysis. Petrographic observations reveal that the Longtan Formation is rich in vitrinite and inertinite, while the Dalong Formation contains a large amount of solid bitumen. Primary organic pores (POPs) are the original cellular pores that develop in inertinite. Vitrinite particles did not show visible pores. Solid bitumen infills fractures and occupies the interparticle (interp) space between the mineral grains in the Dalong Formation. A small number of speckled secondary organic pores (SOPs) were observed in the solid bitumen with diameters less than 10 nm. Both the TOC and quartz contents exhibit negative relationships with the pore volumes (PVs) for the Longtan and Dalong Formations, owing to the poor development of pores in the organic matter (OM) and occlusion of most mineral pores by migrated bitumen. There is a positive relationship between clay minerals and PVs owing to the sheet-like meso-pores and macro-pores in the clay minerals. The evolution of organic porosity is related to the differences in maceral composition and thermal maturity. Thermal cracking of residual oil and bitumen and the expulsion of gaseous hydrocarbons contributed to the formation of SOPs in marine shales. In contrast, vitrinite and inertinite in gas-prone shales do not display noticeable morphological changes across the entire thermal evolution process, owing to their low hydrocarbon generation potential.

Petrographical and geochemical characterization of the Upper Permian Longtan formation and Dalong Formation in the Lower Yangtze region, South China: Implications for provenance, paleoclimate, paleoenvironment and organic matter accumulation mechanisms

The Upper Permian Longtan and Dalong Formations are considered prospective targets for shale gas exploration and production in the Lower Yangtze region, South China. However, the paleoclimate change and the mechanism of organic matter accumulation in the shales are rarely reported. In this study, the varying provenance, paleoclimate, paleoenvironment, primary productivity, and the mechanism of organic matter accumulation of the Longtan and Dalong Formations were investigated using X-ray diffraction (XRD), polished section and scanning electron microscope (SEM), pyrite morphology, and elemental geochemical data.Multiple geochemical proxies indicate that the Longtan Formation was deposited in marine–continental transitional facies under a highly humid climate and intermediate-to-strong weathering conditions. The water column was oxic with low paleoproductivity and high terrigenous influx. The rapid rise in sea level during the deposition of the lower Dalong Formation resulted in an anoxic water column, high productivity, and low terrigenous input under a semiarid–semihumid climate and weak-to-moderate weathering conditions. The oceanic upwelling elevated primary productivity and was crucial in the organic matter accumulation. Therefore, the total organic carbon (TOC) is high in the lower Dalong Formation. However, the later sea level fall at the upper Dalong Formation led to a dramatic change in the sedimentary environment, which is characterized by a moderately humid climate, intermediate weathering conditions, decreased productivity, a dysoxic–oxic water column, and enhanced detrital flux.The correlations between TOC content and geochemical proxies suggest that the enrichment of organic matter in the Longtan Formation was mainly affected by terrigenous influx and sedimentation rate. The high TOC enrichment in the lower Dalong Formation was because of oxygen-deficit water column, high productivity, and low terrigenous influx. In the upper Dalong Formation, a combined effect of the decrease in primary productivity and relatively oxygen-enriched conditions was not conducive to the organic matter accumulation. They gave rise to low concentration of organic matter. However, the positive correlations of TOC vs. Cu/Al and TOC vs. Ni/Al indicate that paleoproductivity was nevertheless the most critical factor controlling the enrichment of organic matter.

New cognition on pore structure characteristics of Permian marine shale in the Lower Yangtze region and its implications for shale gas exploration

Shale of the Middle–Upper Permian Dalong Formation and Gufeng Formation in the Lower Yangtze region is characterized by large thickness, high total organic carbon (TOC), wide distribution and moderate organic thermal evolution degree, so it may be the next important field of shale gas exploration. In order to point out the target and direction of shale gas exploration and development in this region, this paper selects the Dalong Formation and Gufeng Formation shale in the Xuanjing area of Lower Yangtze as the research object to quantitatively describe the development characteristics of shale pores of different scales by means of scanning electron microscopy (SEM), high pressure mercury injection, CO2 and N2adsorption. Then, the fractal dimension of sample pores is calculated by using the FHH (Frenkel–Halsey–Hill) model. Finally, combined with TOC and mineral compositions, the relationship between pore structure and fractal dimension is discussed. And the following research results are obtained. First, dominant lithofacies of Dalong Formation are argillaceous-rich siliceous shale and argillaceous-rich/siliceous mixed shale, which has smaller specific surface area and pore volume, larger average pore diameter. The pore development is mainly controlled by clay mineral content. Dominant lithofacies of Gufeng Formation is siliceous shale, which has a larger specific surface area and pore volume, and smaller average pore diameter. The pore development is controlled by organic matter and brittle mineral content. Second, the influencing factors of fractal dimension can be attributed to the controlling factors of micropore development. The fractal dimension D1of Dalong Formation ranges from 2.4515 to 2.5513 (average 2.5227), and D2ranges from 2.5817 to 2.6578 (averagely 2.6246). The fractal dimension D1of Gufeng Formation ranges from 2.5817 to 2.6578 (averagely 2.6246), and D2ranges from 2.7227 to 2.871 (averagely 2.813). Gufeng Formation shale presents the characteristics of more complex pore structure. Third, the fractal dimension D1is more sensitive to specific surface area, pore development and mineral composition, while D2is more effective in characterizing the average pore diameter. In conclusion, Dalong Formation shale with high D1and D2and Gufeng Formation shale with low D1and high D2can be taken as the favorable exploration targets of Permian marine shale gas in the Lower Yangtze region, and the regional overpressure zones (belts) with weaker structural deformation in hydrocarbon rich sags will be the favorable shale gas exploration targets in this area.

The geochemical characteristics, distribution of marine source rocks and gas exploration potential in the northwestern Sichuan Basin, China

In recent years, the northwestern part of the Sichuan Basin has become a popular location for natural gas exploration. However, up to this point, the geochemical characteristics and distribution of marine source rocks in this area have not been thoroughly investigated. In the northwestern Sichuan Basin, there are eight sets of marine source rock, including the Lower Sinian Doushantuo Formation, the third Member of Upper Sinian Dengying Formation, the Lower Cambrian Qiongzhusi Formation, the Lower Silurian Longmaxi Formation, the Middle Devonian, the Middle Permian Maokou Formation, the Upper Permian Longtan Formation/Wujiaping Formation, and the Upper Permian Dalong Formation, from which eight outcrop profiles were selected for systematic organic geochemical analysis. The Qiongzhusi Formation has the thickest high-quality source rocks, up to 80 m thick, followed by the Upper Permian Longtan/Wujiaping and Dalong formations, which have up to 20–30 m thick high-quality source rocks, whereas the thickness in other layers is relatively thin. By the abundance of organic matter, the Doushantuo, Qiongzhusi, Longmaxi, Longtan/Wujiaping, and Dalong formations developed high-quality source rocks with total organic carbon content (TOC) greater than 3%, while the third Member of Upper Sinian Dengying Formation, Middle Devonian and Maokou formations had low TOC. The kerogen of source rocks gradually enriched 13C from the type Ⅰ organic matter of the Lower Cambrian Qiongzhusi Formation to the Upper Permian Dalong Formation, and the δ13C values of Doushantuo Formation, the third Member of Dengying Formation, and Qiongzhusi Formation are less than −30.0‰. The δ13C values of Longtan Formation or Wujiaping Formation and Dalong Formation with type Ⅱ2 organic matter ranges from −28.0‰ and −26.0‰. Others lie somewhere in between type I and type II2. Except for Permian source rocks in Guangyuan and Chaotian areas, which are in the mature stage, source rocks in the northwestern Sichuan Basin are in the over-mature stage and largely in the thermal stage of dry gas. High abundance source rocks have provided rich marine natural gas resources. As a result, future gas exploration in the northwestern Sichuan Basin will focus on the gas-bearing system associated with the Qiongzhusi Formation and Upper Permian source rocks.

Experimental studies on nanopore evolution in organic matter-rich shales

The evolution of nanopores when highly-matured or over-matured shales encounter secondary hydrocarbons was characterised in two groups of Paleozoic marine shale samples. The samples were subjected to hydrous pyrolysis to investigate the evolution of organic-hosted pores and mineral-hosted pores. The samples were collected from the Upper Permian Dalong Formation (sample JG) and the Lower Silurian Longmaxi Formation (sample CN) in the Sichuan Basin, Southwest China. X-ray diffraction, low-pressure CO2 and N2 isotherm adsorption, and scanning electron microscopy (FE-SEM) were utilized to determine the mineralogical compositions and the nanoporosity evolution characteristics of the samples. FE-SEM observations showed that the inorganic pores and organic pores in the JG samples increased as the pyrolysis temperature increased from 400 °C to 525 °C, while the pores in the CN sample did not change significantly. The maximum N2 and CO2 adsorption capacities of the JG shale sample peaked at the pyrolysis temperature of 450 °C. Meanwhile, the maximum N2 adsorption capacity of the CN shale sample reached its maximum at the pyrolysis temperature of 500 °C. The maximum CO2 adsorption of the CN samples generally decreased with increasing thermal simulation temperature, reaching a minimum at 525 °C. The pore volumes of the original and pyrolyzed samples of JG and CN were calculated based on the N2 and CO2 adsorption data. The experimental results are summarized as follows. (1) The pore volumes of the shale samples with different maturities all increased in the pyrolysis simulation, but pore volume increases observed in the matured sample were higher than those in the over-matured sample. (2) Laboratory pyrolysis experiments can increase organic matter (OM) micropores when the gases are generated from the kerogen/bitumen/oil. (3) As maturity increases further, OM micropores become connected together by the formation of new micropores, forming mesopores or even macropores. (4) In the processes of hydrocarbon generation, the kerogen type and carbonate mineral recrystallization or dissolution also have a great influence on the evolution of pores.

Upwelling-induced organic matter enrichment of the Upper Permian Dalong Formation in the Sichuan Basin, SW China and its paleoenvironmental implications

The organic-rich siliceous rock was developed worldwide during the Late Permian which contains plenty of information about paleoenvironment. This study reveals the vertical variation in organic matter enrichment of the Upper Permian Dalong Formation in the northern Sichuan Basin, which was related to the expansion of the Paleo-Tethys Ocean induced transgression and upwelling in the northern continental margin of the Upper Yangtze. The Dalong Formation was divided into three units from bottom to top on the basis of lithology and organic matter abundance: the lower calcareous mudstone (Unit A), the middle organic-rich siliceous shales (Unit B), and the upper marlstones (Unit C), with average TOC contents of 4.09%, 6.72%, and 1.20%, respectively. The Unit A was deposited at the early stage of the transgression under seasonal upwelling settings with moderate paleoproductivity evidenced by the moderate-high Cd/Mo ratios and concentration of P, Cu and Zn. The Unit B shows extremely high Cd/Mo ratios and Ni concentration indicating the perennial upwelling and high paleoproductivity induced by the extensive transgression. The Unit C was deposited under similar hydrographic conditions with the Unit A but lower paleoproductivity due to the effect of the latest Permian mass extinction. In addition, the redox conditions varied with the paleoproductivity and positive correlations between P and redox proxies (e.g. VEF, NiEF, ZnEF and CuEF) were observed (the Unit A: moderate paleoproductivity and suboxic-anoxic conditions, the Unit B: high productivity and anoxic-euxinic conditions, the Unit C: low-moderate productivity and oxic-suboxic conditions), which indicate that nutrients brought to primary producers by the upwelling during the transgression stimulate blooms and accelerate the consumption of oxygen leading to the anoxic-euxinic bottom water conditions. During the transgression, differences in types and strength of the upwelling result in the variation of paleoproductivity and redox conditions, and thus the organic matter abundance.

Pore formation and evolution of organic-rich shale during the entire hydrocarbon generation process: Examination of artificially and naturally matured samples

Shale pore type and variation have significant influences on natural gas storage and are of vital importance for shale gas resource evaluation. To better understand the formation and evolution of shale pores during hydrocarbon generation process, organic geochemistry, X-ray diffraction, field emission scanning electron microscopy (FE-SEM), and low-pressure nitrogen gas adsorption (N2GA) were applied to samples created by thermal simulation and to naturally evolved samples of the Upper Permian Dalong Formation in the northwestern Sichuan Basin. The results show that the development of organic matter (OM) pores is controlled by kerogen type and thermal maturation level and begins at equivalent vitrinite reflectance (eqvRo) value of 0.91%. Two types of OM pores are developed during OM evolution stage, as bubble pores that are formed at the oil window stage and spongy pores which occcur at the dry gas stage. Maximum pore volume, particularly mesopore volume, occurs within two opportune development periods with the corresponding eqvRo values of 1.11–1.53% and 2.5–2.9%, respectively. The periods are known to be the two peak stages of shale pore development. The first peak corresponds to the late oil generation stage, and the second corresponds to the cracking peak of extractable OM to gas, during which numerous OM micropores and mesopores are generated. OM pore generation is generally resulted from OM decomposition, and pore structure parameters exhibit significant positive correlations with OM reduction for simulated samples. For naturally matured samples, TOC content is positively correlated with pore structures. Extractable OM generated from primary kerogen fills OM pores and mineral-related pores and further reduces shale pore volume. In terms of analysis on naturally matured samples with similar maturities, OM pores in type II1 kerogen are inferior to those in type I kerogen but more developed than those in type III kerogen. Shale pore evolution is also controlled by diagenesis. Compaction and cementation have adverse effects on shale pore preservation during early and late diagenesis stages, which leads to a significant decrease in shale pore volume, particularly that of mesopores. It should be noted that OM pores are better developed and preserved even when the eqvRo value exceeds 3.25%, which is of vital importance for extending the field of shale gas exploration of over-mature shale reservoirs.

Controlling Factors and Formation Models of Organic Matter Accumulation for the Upper Permian Dalong Formation Black Shale in the Lower Yangtze Region, South China: Constraints from Geochemical Evidence.

The black shale in the upper Permian Dalong Formation is considered as an excellent source rock in the Lower Yangtze region. However, mechanisms of organic matter (OM) accumulation in such a setting are scarcely understood. Here, the characteristics of total organic carbon (TOC) and elemental geochemistry of 33 rock samples from GD1 well are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that the lower and middle parts of Dalong Formation (section A) display high TOC contents ranging from 1.19 to 6.45% (average 3.19%), whereas the upper part (section B) exhibits medium TOC contents varying from 1.18 to 4.90% (average 2.09%). These data also elucidate that the target shales were deposited in a complex paleoenvironment with moderate to strong water-mass restriction that was characterized by warm and semiarid–semihumid paleoclimate, high biotic productivity, fluctuating plaeoredox conditions, and a relatively high sedimentary rate. Compared to the organic-rich shales from section A mainly developed under an anoxic condition, shales from section B formed in an oxic-to-dysoxic water environment exhibited a comparatively higher sedimentary rate. Moreover, among all these factors that might affect OM accumulation, the paleoredox conditions appear to be the dominant controlling factors for section A, whereas the biotic productivity, paleoredox conditions, and sedimentary rate are the main controlling factors for section B. Finally, two formation models for OM accumulation in Dalong Formation shale in the Lower Yangtze region are proposed. The “preservation model” for OM accumulation in section A emphasizes that the reducing deep-water environment, which was mainly caused by the regional sea level rise, is favorable for OM preservation. The “integrated model” for OM accumulation in section B stresses that greater biotic productivity intensifies respiratory oxygen consumption in a water column and a higher sedimentary rate can greatly shorten OM exposure time for respiration by oxygen, both of which cause OM accumulation under an oxidizing water environment. These findings also add to our knowledge that despite the oxygenated water environment during shale deposition, TOC contents are not necessarily lower.

Evolution characteristics of maturity-related sterane and terpane biomarker parameters during hydrothermal experiments in a semi-open system under geological constraint

In order to accurately assess the thermal maturity of highly/over-mature shales in Paleozoic strata in southern China, hydrothermal experiments of shale samples from the Dalong Formation from bed 18 of well-exposed Shangsi PTB Section was conducted in a semi-open system under various conditions of geological constraint. The saturated extracts of pyrolyzed shale samples were subjected to gas chromatography and mass spectrometry (GC-MS) analyses. The results indicated that hydrothermal experiments of Dalong shales in the presence of fluid pressure (FP), lithostatic stress (LS), water, catalyst of clay minerals and trace elements, is partially equivalent to catalytic hydropyrolysis in terms of the catalytic hydrogen effect for the release of bound biomarkers from geomacromolecules of kerogen and asphaltene. The evolutional trend of maturity-related biomarkers were classified into the following three categories: (a) the most common maturity parameters (e.g., Ts/(Ts + Tm) and C29S/(S + R)) initially increased and then decreased with a pyrolysis temperature (450 °C) at the reversal point during maturation; (b) the parameters (e.g., C32S/(S + R)) primarily increased or decreased and reached a temperature (400 °C) of tuning point, and then remained unaltered with increasing temperature (c) the parameters (e.g., C27DS/S); firstly increased, and then declined at a specific turning point temperature (425–500 °C), and thereafter continued to increase until the end of heating process. As a result, the threshold temperature for the release of bound sterane or terpane occurred in the range 400–500 °C. Most maturity-related sterane and terpane biomarker parameters cannot be applied to evaluate the maturity of highly/over-mature shale. However, the C30M/C30H ratio and S/T ratio, which are strongly correlated with temperature, may be employed to assess extremely mature shale from these Paleozoic strata due to the integrated effects of multiple factors in the thermal evolution process of biomarkers. This is very important for the exploration and development of shale gas in southern China. Additionally, the values of several source-related biomarker parameters derived from C27–29 regular steranes are relatively constant during maturation, such as the ternary plot of the C27–29 αααR sterane versus the C27/C29 αααR sterane ratio. These are useful indicators for distinguishing the genetic source of highly/over-mature shales.

Geological characteristics of shale gas in the Dalong Formation in Well Xiangye 1 in the Lianyuan Sag, Xiangzhong Depression#br#

Geological characteristics of shale gas in the Dalong Formation in Well Xiangye 1 in the Lianyuan Sag, Xiangzhong Depression#br#

Geochemistry of the upper Permian sandstones in the Dalong Formation in the South Yellow Sea Basin, East China: implications to provenance, weathering, and tectonic setting

Geochemistry of the upper Permian sandstones in the Dalong Formation in the South Yellow Sea Basin, East China: implications to provenance, weathering, and tectonic setting

Multiple sulfur isotope chemostratigraphy across the Permian–Triassic boundary at Chaotian, China: Implications for a shoaling model of toxic deep‐waters

AbstractPrevious studies on multiple sulfur isotopes (32S, 33S, and 34S) in sedimentary pyrite at the end‐Permian suggested a shoaling of anoxic/sulfidic deep‐water contributing to the extinction. This scenario is based on an assumption that the sedimentary sulfur cycle was largely controlled by benthos activity, though a stratigraphic correlation between the sulfur records and ichnofabrics of the sediments at the end‐Permian has not yet been examined. We report the multiple sulfur isotopic composition of pyrite in the Permian–Triassic boundary interval at Chaotian, South China. Our data can be generally explained by a mixing of sulfur in sulfide from two different sources: one produced via sulfate reduction in an open system with respect to sulfate and the other produced in a closed system. In particular, the former with the substantially low δ34S (<−40 ‰) and high ∆33S (up to +0.100 ‰) values was likely produced via water‐mass sulfate reduction or via sulfate reduction in oxic sediments with common burrows. The frequent occurrence of small pyrite framboids (mostly <5 μm in diameter) in the Lopingian (Late Permian) Dalong Formation of deep‐water facies supports the enhanced water‐mass sulfate reduction in an anoxic deep‐water mass. The negative ∆33S values are observed only in the oxic limestones, and no substantial ∆33S change is observed across the extinction horizon despite of the disappearance of bioturbation. Our results are apparently inconsistent with the previous shoaling model. We expand the model and infer that, when the deep‐water was sulfidic and its shoaling rate was high, a substantial amount of hydrogen sulfide (H2S) was supplied onto the shelf via the shoaling; that resulted in the positive ∆33S value of the bulk sediments. The observed ∆33S variation on a global scale suggests a substantial variation in H2S concentration and/or in upwelling rate of shoaling deep‐waters during the Permian–Triassic transition.

Organic-rich siliceous rocks in the upper Permian Dalong Formation (NW middle Yangtze): Provenance, paleoclimate and paleoenvironment

The organic-rich shales of the upper Permian Dalong Formation in the middle Yangtze area are considered an important shale gas exploration horizon. However, the origin of siliceous matter and the mechanism of organic matter enrichment are still controversial. Based on geochemical data, we studied the provenance, paleoclimate, paleoenvironment, primary productivity and hydrological conditions of the Dalong Formation in western Hubei Province and discussed the mechanism of organic matter enrichment. The shales of the Dalong Formation can be divided into two units, organic-rich and organic-lean shales, with average TOC contents of 5.81 wt% and 0.45 wt%, respectively. The western Hubei Basin was a strongly restricted basin during deposition, and the provenance of the Dalong Formation had a continental margin sedimentary background, mainly from mixed felsic and basic sources. Terrigenous clastic quartz, biological quartz and hydrothermal quartz are recognized in the shales of the Dalong Formation. Based on U, V, and Mo concentrations and V/(V + Ni) ratios, the organic-rich shales were deposited in anoxic-sulfurized conditions. In contrast, the organic-lean shales were deposited under oxic-suboxic conditions. The Dalong Formation had moderate to high primary productivity and controls on the redox conditions in the basin. The decrease in primary productivity in organic-lean shales may be related to the transition from a warm and humid climate to a hot and dry climate. The sedimentary model shows that the formation of organic-rich siliceous rocks in the Dalong Formation was controlled by primary productivity. The decrease in primary productivity driven by the hot and dry climate in the latest Permian is the main factor controlling the enrichment of organic matter and the formation of bioquartz.

Effective hydrocarbon-bearing geological conditions of the Permian strata in the South Yellow Sea Basin, China: Evidence from borehole CSDP-2

For nearly 60 years, petroleum geologists have been puzzled by oil and gas exploration in the South Yellow Sea basin, which is considered to have broad oil and gas prospects, but no commercial hydrocarbon reservoirs have been obtained. The hydrocarbon-bearing geological conditions of the Permian strata with good exploration prospects also have never been systematically investigated. To solve the problem, a series of experiments was conducted on core samples obtained from borehole CSDP-2, the first exploration well with continuous coring in the South Yellow Sea Basin, including petrology analysis, geochemical testing, pore-throat testing, hydrocarbon-charging analysis, and sealing-ability evaluation. Vitrinite reflectance (Ro), total organic carbon (TOC), pyrolysis, extractable organic matter (EOM) and kerogen microscopic examination were carried out to evaluate the quality of the Permian source rocks. Porosity, permeability, thin section, scanning electronic microscopy (SEM) and rate-controlled mercury injection (RCMI) analyses were combined to verify the effectiveness of sandstone reservoirs. The sealing capacity of caprock was mainly evaluated by the displacement pressure. And according to the fluid-inclusion analysis and oil-source correlation, the hydrocarbon accumulation events were determined. After comprehensive analysis, we clarified the effective hydrocarbon-bearing geological conditions and favorable prospects in the Permian of the South Yellow Sea Basin. Furthermore, the main controlling factors pertaining to favorable source rocks and tight sandstone reservoirs were ascertained in this paper.The results show that two sets of the Lower Permian Qixia and Gufeng Formation (P1q-g) and the Upper Permian Longtan and Dalong Formation (P2l-d) source rocks, along with the P2l-d tight sandstone reservoirs, compose two types of hydrocarbon accumulation assemblages, which are effectively sealed by the Lower Triassic Qinglong Formation (T1q) roof caprock. One type represents the ‘lower-generation and upper-accumulation’ between the P1q-g source rocks and the P2l-d sandstones, and the other type represents the ‘self-generation and self-accumulation’ in the Upper Permian strata. Vertical stacking between the Permian source rocks and sandstone reservoirs ensures the near-source hydrocarbon accumulation, and faults and microcracks can be regarded as beneficial channels for hydrocarbon migration. In addition, we find that the quality of source rocks is improved by large-scale marine transgression, high bio-productivity, and tectonic–thermal events. The quality of tight sandstone reservoirs is more dependent on throats with relatively larger radius than pores. This study provides important enlightenment for exploring marine hydrocarbons in the South Yellow Sea Basin, and can be useful in understanding the dominant geological factors affecting favorable source rocks and sandstone reservoirs.

Tectonic control of Guadalupian-Lopingian cherts in northwestern Sichuan Basin, South China

Abstact Cherts have been considered to be formed mainly through processes involving biosilicification, riverine inputs and hydrothermal activities. In the Permian, massive cherts are widely distributed in South China, bearing significant implications for the reconstruction of paleogeography and paleoceanography of the eastern Paleo-Tethys Ocean. The origin of the Permian cherts in South China, however, remains inconclusive due to a lack of systematic petrographic and geochemical study. In this article, we analyze the petrologic and geochemical characteristics of the cherts collected from five outcrops in the northwestern Sichuan Basin, South China. These cherts are found from the Upper Guadalupian Maokou Formation (ca. 268–259 Ma) and Upper Lopingian Dalong Formation (ca. 254–252 Ma). Field and microscopic observations suggest that cherts from the Maokou Formation occur mainly in nodular forms, and consist of microcrystalline quartz, silicified bioclasts and siliceous fossil debris. Both petrographic observation and geochemical analysis suggest that the Maokou cherts are mainly of hydrothermal origin near deep faults in the southwest and of biogenetic origin with less hydrothermal influence in the northeast of the studied area. The Dalong cherts occur in both nodular and bedded forms, with more abundant siliceous fossil debris than the Maokou cherts. Major, trace and rare earth element data support that the Dalong cherts are influenced by both biosilicification and hydrothermal activities. The spatial heterogeneity of the cherts origin in South China is largely controlled by tectonic activities associated with the Emeishan large igneous province (ELIP) event and the closure of the Paleo-Tethys Ocean. The troughs formed by the extensional tectonic activity linked to the ELIP could provide accommodation space and activate deep faults allowing the rise of silicon-rich hydrothermal fluids from the deep and nutrient-rich water from volcanogenic upwellings. Subsequent cooling may have facilitated the precipitation of large amounts of silica and the formation of cherts in the study region. This work demonstrates that the origin of the Guadalupian-Lopingian cherts in South China is predominantly controlled by tectonic and biological activities, which sheds light on the global “Permian Chert Event”.

Pore Structure and Fractal Characteristics of Different Shale Lithofacies in the Dalong Formation in the Western Area of the Lower Yangtze Platform

The purpose of this article was to quantitatively investigate the pore structure and fractal characteristics of different lithofacies in the upper Permian Dalong Formation marine shale. Shale samples in this study were collected from well GD1 in the Lower Yangtze region for mineral composition, X-ray diffraction (XRD), and nitrogen adsorption–desorption analysis, as well as broad-ion beam scanning electron microscopy (BIB-SEM) observation. Experimental results showed that the TOC (total organic carbon) content and vitrinite reflectance (Ro) of the investigated shale samples were in the ranges 1.18–6.45% and 1.15–1.29%, respectively, showing that the Dalong Formation shale was in the mature stage. XRD results showed that the Dalong Formation shale was dominated by quartz ranging from 38.4% to 54.3%, followed by clay minerals in the range 31.7–37.5%, along with carbonate minerals (calcite and dolomite), with an average value of 9.6%. Based on the mineral compositions of the studied samples, the Dalong Formation shale can be divided into two types of lithofacies, namely siliceous shale facies and clay–siliceous mixed shale facies. In siliceous shale facies, which were mainly composed of organic pores, the surface area (SA) and pore volume (PV) were in the range of 5.20–10.91 m2/g and 0.035–0.046 cm3/g, respectively. Meanwhile, the pore size distribution (PSD) and fractal dimensions were in the range 14.2–26.1 nm and 2.511–2.609, respectively. I/S (illite-smectite mixed clay) was positively correlated with SA, PV, and fractal dimensions, while illite had a negative relationship with SA, PV, and fractal dimensions. I/S had a strong catalytic effect on organic matter for hydrocarbon generation, which was beneficial to the development of organic micropores, so I/S was conducive to pore structure complexity and the increase in SA and PV, while illite easily filled organic pores, which was not beneficial to the improvement of pore space. In clay–siliceous mixed shale facies, which mainly develop inorganic pores such as intergranular pores, SA and PV were in the range of 6.71–11.38 m2/g and 0.030–0.041 cm3/g, respectively. Meanwhile, PSD and fractal dimensions were in the range of 14.3–18.9 nm and 2.563–2.619, respectively. Quartz and I/S showed weak positive correlations with SA, PV, and fractal dimensions. The various compact modes between quartz particles and the disorder of I/S were conducive to the complexity of pore structure and the improvement of SA and PV. The research findings can provide a reference for the optimization and evaluation of shale gas favorable area of the Lower Yangtze Platform.

A Changhsingian (late Permian) nautiloid assemblage from Gujiao, South China

AbstractA c. 0.30 m thick cherty limestone bed in the Dalong Formation at Gujiao (Guizhou) has yielded a highly diverse Changhsingian nautiloid assemblage. Its age is late Changhsingian, indicated by the co‐occurring ammonoid Pseudotirolites sp. This assemblage is composed of nine species and five taxa in open nomenclature in nine genera, including one new genus and three new species: Neotainoceras zhaoi sp. nov., Nodopleuroceras gujiaoense sp. nov. and Gujiaonautilus longliensis gen. et sp. nov. Sholakoceras is for the first time recorded in South China. Compared with literature data from 52 sites in various regions worldwide, the material described here presents the most diverse Changhsingian nautiloid assemblage. Among the genera of the Gujiao assemblage, Pleuronautilus, Tainoceras and Liroceras are cosmopolitan, while the others are more endemic or genera known so far only from South China.

Effect of Organic Matter Type and Maturity on Organic Matter Pore Formation of Transitional Facies Shales: A Case Study on Upper Permian Longtan and Dalong Shales in Middle Yangtze Region, China

Organic matter (OM) nanopores developed in transitional facies shales, i.e., the Upper Permian Longtan and Dalong Formations in the Yangtze Platform, China, were investigated to determine the corre- sponding influence of thermal maturity and OM types within the geological conditions. A suite of 16 core samples were taken from Type-III Longtan shales and Type-II Dalong shales from two wells covering a ma- turity (Ro, vitrinite reflectance) ranging from 1.22% to 1.43% and 2.62% to 2.97%, respectively. Integrated analysis of the shale samples was carried out, including field-emission scanning electron microscopy (FESEM), low-pressure N2 and CO2 adsorption, high-pressure CH4 adsorption, and mercury intrusion capillary pressure (MICP) analysis. The fluid inclusions of liquid and gas hydrocarbons trapped in calcite vein samples in Dalong shales of two wells were studied using laser Raman and fluorescence spectroscopy. FE-SEM images indicated that OM pores in different formations varied substantially in terms of shape, size, and distribution density. OM pores in Type-II Dalong shales of Well XY1 were mainly micropore, sparsely distributed in the gas-prone kerogen with a spot-like and irregular shape, while bitumen rarely developed observable pores. In contrast, the morphology of OM pores in Type-III Longtan shales were significantly different, which was due to differences in the OM type. The primary OM pores in some terrestrial woody debris in Longtan shales had a relatively larger pore diameter, ranging from hundreds of nanometers to a few micrometers and were al- most all rounded in shape, which might be one of the factors contributing to larger pore volume and gas adsorption capacity than Dalong shales of Well XY1. Comparing Dalong shales of Well XY1 with relatively lower thermal maturity, there were abundant spongy-like pores, densely developed in the pyrobitumen in Type-II Dalong shales of Well EY1, with an irregular shape and diameter ranging from several to hundreds of nanometers. Many blue fluorescent oil inclusions and a small number of CH4 inclusions mixed with C2H6 could be observed within calcite veins in Dalong shales of Well XY1, whereas only CH4 inclusions could be identified within calcite veins in Dalong shales of Well EJ1. Therefore, thermal maturity not only controlled the type of hydrocarbons generated, but also makes a significant contribution to the formation of OM pores, resulting in larger pore volumes and adsorption capacity of Type-II shale samples in the dry gas window.

Impact of hydrothermal activity on organic matter quantity and quality during deposition in the Permian Dalong Formation, Southern China

Abstract The very high temperature at the center of a hydrothermal system is detrimental for the deposition of organic-rich sediments. However, it's not known how the petroleum source rock develops at the boundary of a hydrothermal system where the seawater temperature was only slightly elevated. The petroleum generation potential, using pyrolysis-GC, GC-MS and FT-ICR MS techniques, of eight organic-rich Permian shale samples from Shangsi Section China were systematically investigated. Principal component analysis reveals that the quantity and quality of the organic matter in the shale is dominated by biomass productivity, the redox condition, and terrestrial organic matter input. Rare earth element distribution pattern reveals that the two samples with highest TOC contents show a slight Eu anomaly which indicates the sample was influenced by a hydrothermal system to some degree. A depositional model showing how the influences from hydrothermal event promote the development of petroleum source rock is summarized: (1) At the edge of a hydrothermal system, the seawater-temperature can be elevated by 5–10 °C which further promotes the development of phytoplankton and zooplankton. (2) An upwelling can be intrigued and chemical weathering will be accelerated which both introduce excessive nutrients and increase the biota productivity. (3) Flux with the terrestrial advanced plant can also be accelerated to the shelf area by the volcano-induced climate change. (4) The consumption of the extensive biomass induced by the hydrothermal activity would expand the oxygen minimum zone, and further enable more organic matter to be preserved in shale.

Genetic types of natural gas and gas-source correlation in different strata of the Yuanba gas field, Sichuan Basin, SW China

The chemical compositions and stable carbon and hydrogen isotopic compositions of natural gases in different strata of the Yuanba gas field in the Sichuan Basin were analyzed to investigate their genetic types and source. Natural gases from the Permian Changxing and Triassic Feixianguan formations have dryness coefficients higher than 0.995 and contain H2S in most gas samples, having a positive correlation between the H2S and CO2 contents. The identification of gas origin and gas-source correlation from this study indicate that natural gas in the Ziliujing Formation and most gas samples from the Xujiahe Formation are coal-type gas produced from primary cracking of kerogen, and they are self-sourced reservoirs in relatively independent systems. The natural gases in the 1st and 2nd members of the Xujiahe Formation were mixed with minor amounts of oil-type gas from argillaceous carbonate rocks in the Leikoupo Formation. In addition to the self-accumulated oil-type gas in the Leikoupo Formation, natural gas mainly came from the overlying or lateral source rocks in the Xujiahe Formation. The H2S-bearing natural gases in the Changxing and Feixianguan formations were produced by the cracking of crude oil from source rocks in the underlying Wujiaping Formation, with partial contributions from certain local source rocks in the Permian Dalong Formation. The H2S-bearing natural gas has undergone the thermochemical sulfate reduction (TSR) which were dominated by heavy hydrocarbons. The high δ13C values of CO2 in these gas reservoirs mainly result from the interaction between acidic fluids and carbonate rocks in the reservoirs.