Organic geochemical and mineralogical characterization of the lower Silurian Longmaxi shale in the southeastern Chongqing area of China: Implications for organic matter accumulation (original) (raw)
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Marine and Petroleum Geology, 2020
Organic matter (OM) is the material basis of shale gas accumulation, so understanding the mechanisms of OM accumulation plays a critical role in shale gas exploration. The marine shale from the Ordovician Wufeng−Silurian Longmaxi formations is thought to be a good shale gas source in China's northwestern Hunan Province in the Upper Yangtze platform. A total of 37 samples were collected from a well SY5, and OM accumulation mechanisms were analyzed based on geochemical proxies associated with shale composition. Total organic carbon (TOC) decreased upward from Wufeng to the lower and upper members of the Longmaxi. Average TOC contents were 2.64, 1.24, and 0.29%, respectively. Wufeng shale developed from migrated OM and biogenic quartz, whereas depositional OM and terrigenous quartz were widespread in the lower Longmaxi, and barely any OM was observed in the upper Longmaxi. The Fe−S−C systems and Mo−U covariation suggested that the oxygen level increased from Wufeng to the upper Longmaxi. The organic-rich Wufeng was deposited under anoxic conditions, and redox conditions during the lower Longmaxi deposition fluctuated greatly (suboxic to euxinic) and were mainly suboxic. The upper Longmaxi was deposited under oxic conditions. High excess silica concentrations and the C/P ratio of Wufeng shale implied high paleoproductivity, which may have contributed to phosphorus recycling under a strong reducing environment. P/Ti ratios showed that the lower Longmaxi was deposited under moderate paleoproductivity and the upper Longmaxi was deposited under low paleoproductivity. The Al and Ti concentrations increased and Zr/Al decreased from Wufeng to the upper Longmaxi, reflecting increased terrigenous flux. The high terrigenous flux occurred as the diluent decreased OM concentrations during the Longmaxi deposition. The strong reducing conditions, high paleoproductivity, and low terrigenous flux, which were favorable to OM production and preservation during Wufeng deposition, further determined the content and types of OM and quartz. The high OM and quartz contents were associated with favorable types (migrated OM and biogenic quartz), creating Wufeng shale high-quality reservoirs, which were conducive to shale gas accumulation. Ba and P did not act as paleoproductivity proxies under strong reducing conditions due to their redistribution, and the V-related ratios were unreliable due to high thermal maturity of marine shale. The element Si was not a proxy of terrigenous flux when the silica was biogenic in origin, but the excess silica concentration was a reliable proxy for paleoproductivity.
Petroleum Science, 2018
The Upper Ordovician-Lower Silurian mudstones (including the Wufeng, Guanyinqiao and Longmaxi Formations) in the Sichuan Basin are some of the most important shale gas plays in China. In order to enhance our understanding of the process of formation of organic carbon up to 10%, optical, microscopy and geochemical methods have been used to investigate the petrographic and geochemical characteristics of the formation. Firstly, three mudstone lithofacies were identified based on a wide variety of mudstone laminations. These are: (a) indistinctly laminated mudstone; (b) parallellaminated mudstone; and (c) nonparallel-laminated mudstone. Then, combining with the evidence from depocenter migration, Th/U ratios and total organic carbon, the abundant organo-minerallic fabrics suggest that organic carbon was preferentially deposited and preserved in anoxic, low energy and stagnant water conditions during deposition of the Wufeng and Longmaxi Formations. On the contrary, the Guanyinqiao Formation with poor organic carbon was deposited in oxic and high-energy water conditions.
AAPG Bulletin, 2019
The nanometer-scale architecture of organic matter (OM) and associated pores in highly mature gas shales from the lower Silurian Longmaxi Formation in the upper Yangtze platform of south China were investigated using field emission scanning electron microscopy (SEM), focused ion beam SEM, and lowpressure gas (N 2 and CO 2) adsorption bulk pore characterization. The Longmaxi shale comprises fine-grained siliciclastic rocks deposited in a marine shelf environment, which was dominated by quartz and clay minerals. Four porous OM types were found in the Longmaxi shale on the basis of the chemical composition and spatial occurrence of OM, including (1) isolated original OM particles, (2) OM-clay mineral complexes, (3) OM-heavy mineral complexes, and (4) secondary migrated bituminous OM. The pores in the particulate OM are not homogeneously distributed, and the processes leading to different pores depend on the specific OM type. The nature of OM-hosted pores is a result of several factors, such as primary porous kerogen, mechanical compaction, organic-inorganic interactions, gaseous and liquid hydrocarbon generation, retention, and expulsion. Pore volumes and specific surface areas of the Longmaxi shale derived from low-pressure N 2 and CO 2 adsorption experiments reveal positive linear relationships with total organic carbon contents, which indicates that the pore systems in the highly mature Longmaxi shale are dominated by OM-hosted pores.
Organic matter accumulation in the oil shale- and coal-bearing Huadian Basin (Eocene; NE China)
International Journal of Coal Geology, 2013
The Huadian Basin, located in northeastern China, is a fault-controlled basin filled by Eocene non-marine sediments assigned to the Huadian Formation. The formation is subdivided from bottom to top into three members: Pyrite Member, Oil Shale Member, and Carbonaceous Shale Member. Using bulk geochemical data from borehole Hd3 and core descriptions from a high number of wells, the factors controlling organic matter accumulation have been evaluated. The organic matter content of the Pyrite Member is relatively low (average TOC: 0.9 wt.%). Mudstones deposited in shallow lacustrine environments contain between 1 and 2 wt.% TOC (max. 6 wt.%) and a kerogen Type II (HI 400-600 mg HC/g TOC). An arid climate prevented the accumulation of thick coal. The overlying Oil Shale Member contains 13 oil shale layers, which are mined underground. The Oil Shale Member has been interpreted as a 3rd order sequence. Significant differences exist between oil shales deposited in different systems tracts: 2-m-thick coaly oil shale of low-quality (oil yield 3.5 wt.%) developed in the lowstand systems tract. Oil shale, up to 7 m thick, of moderate quality (oil yield 4.1-8.0 wt.%) was deposited during the transgressive systems tract (TST). Thinner (b 3 m) high quality oil shale (oil yield 6.4-19.8 wt.%) is found in the highstand systems tract (HST) and at the base of the regressive systems tract. The lateral extent of the oil shale layers reaches a maximum in the upper TST and the lower HST. The high number of oil shale layers deposited in an overall deep lacustrine environment notifies frequent changes in base level, controlled by tectonic movements, short-term climatic cycles, or cycles in organic matter production. The Carbonaceous Shale Member representing the balanced-fill to overfilled stage of basin evolution hosts economic coal seams, which are occasionally accompanied by shallow lacustrine shales with abundant hydrogen-rich organic matter. Otherwise non-coal layers are typically low in organic matter (average TOC: 0.9 wt.%). In contrast to the Huadian Basin, a very thick coal seam overlain by a single oil shale layer more than 100 m thick developed in the Eocene Fushun Basin. This indicates major differences in the evolution of basins with similar tectonic setting and age.
Fuel, 2019
Marine-continental transitional shale gas contains significant resources, but scant areas have been developed economically. The evolution of paleoenvironmental and potential of high-quality shales generation from the Carboniferous to Permian (mainly of Benxi, Taiyuan and Shanxi formations) in the east margin of Ordos Basin, China, was systematically studied by integrated analysis including rock-mineral composition, trace elements and organic geochemical testing. The results show that the Benxi Formation consisted principally by marine deposits, while the coexistence of marine and marine-continental transitional deposits are found in the Taiyuan Formation. The Shanxi and its upon Xiashihezi formations are mainly constituted by continental deposits. The Taiyuan Formation has been affected by frequent transgression and regression processes, while the sedimentary environment of the Shanxi Formation was relatively stable. The degree of oxidation gradually increased from the Benxi to the Xiashihezi Formation, and the paleoclimate transformed from humid and warm climate to aridity. The paleosalinity decreased gradually, even though the environment was generally maintained in a salt water environment with considerable rainfall and relatively active hydrodynamic conditions. The paleoproductivity increased gradually, and a generally anoxic environment in the surface water contributed to the enrichment of organic matter. The rock-mineral compositions of the shales are dominated by clay minerals and quartz, and the organic matter content of the shales is relatively high (averaging of 1.28%). The shales primarily contain type III kerogen, and the shales have generally entered a high maturity stage with good gas generation potential. Furthermore, the gas generated from the interbedded coal seams in the Upper Paleozoic is a stable gas supply for interbedded shales. The results provide a geochemical basis for further study of marine-continental transitional shales in the Ordos Basin and supply recommendations for the optimization of high-quality shales production in similar basins around the world.
Journal of Geochemical Exploration, 2012
The major processes responsible for the interplay of hydrothermal influence and paleo-depositional environments (paleoproductivity and plaeoredox condition) on the accumulation of organic matters can be better understood by integrating lithological and geochemical characteristics of samples from the well exposed Late Permian sediments in the Dalong Formation of the Shangsi Section, Northwest Sichuan Basin, South China: 1) The content of total organic carbon (TOC) ranging from 0.16% to 14.6% is closely related with the host lithology and high TOC contents prone to occur in intervals of siliceous rocks, dark shales, and laminated limestones that developed in deepwater intra-basin environments; 2) Non-detrital component elements and ratios (after Ti-normalized), e.g. Cu xs , V xs / (V xs + Ni xs ), for all samples and their correlations with TOC contents imprint the control of paleoproductivity level and paleoredox condition on the accumulation of TOC contents; 3) Characteristics such as original contents of Al, Fe, Mn, Th, U and their non-detrital origin values, non-biogenic contribution values (TiO 2 -normalized) of various elements, and the North America Shale Composite (NASC)-normalized Rare Earth Element (REE) Patterns of chert and shale samples are analyzed and compared to that of world average shales, classic hydrothermal and biogenic cherts, indicating hydrothermal contribution to siliceous rocks formation in the Dalong Formation of the Shangsi Section. Together with the discovery of volcanic ashes and bentonites in the Dalong Formation of the Shangsi Section, a new model has been proposed to explain the relationships of the hydrothermal activities and the organic matter accumulation process in the siliceous rocks in the Dalong Formation of the Shangsi Section: Hydrothermal contribution could cause a bio-thriving in surface water by providing nutrient elements and lead to a high primary productivity; With the organic matters sinking down, the anaerobic and anoxic conditions would control the organic matter accumulation during deposition and early diagenesis stage, which can imprint the organic carbon-rich siliceous rocks formation and the origin of high quality marine source rocks for hydrocarbon.
Organic matter properties and shale gas potential of Paleozoic shales in Sichuan Basin, China
Journal of Natural Gas Science and Engineering, 2016
In this study, Lower Paleozoic shale samples collected from Lower Cambrian Niutitang Formation, Upper Ordovician Wufeng Formation, and Lower Silurian Longmaxi Formation in different regions in the Sichuan Basin were analyzed using geochemical and petrophysical methods to characterize the difference in organic matter properties (including abundance, type and thermal maturity), pore development, mineralogy to shale gas resources potential. The studied marine shales all displayed excellent, highquality organic matter richness and could be the strata for shale gas generation over geological time. There are four systematic geochemical and petrophysical variation trends that indicate that the overmature source rocks of the Sichuan Basin constitute a special shale gas reservoir system: (1) The measured d 13 C values for sedimentary organic carbon (TOC) presents a distinct trend indicative of 13 C enrichment, which indicates that the TOC may be related to the diversity of preserved phytoplankton in the different shale strata in the Sichuan Basin. This biont diversity and organism replacement process were confirmed by the biomarker distribution patterns in the sediments. (2) The bitumen "A" contents display negative correlation with sedimentary age and TOC, suggesting that most of the residual liquid hydrocarbons in those shales have been transformed into shale gas due to higher thermal maturity during the diagenetic transformation of the organic matter burial process, and the shale gas in reservoirs in those types of shales were mostly generated from the cracking of residual bitumen during a stage of relatively high thermal evolution. (3) The quartz and TOC present strongly positive relationship, suggesting that the increased quartz in most of the marine shales is a biogenic silica signal. (4) Total porosity displays a negative relationship with TOC and the quartz contents in the three Paleozoic marine shales, suggesting that re-precipitated pyrobitumen created by oil cracked to gas in overmature source rocks could be the reason leading to the lowest porosity and smaller pores in the most aged but most TOCabundant shales. Skewing toward smaller pores will reduce the pore volume and result in larger internal surface areas and greater sorption energies, which should reduce the productive capacity of shale gas.
Energy Exploration & Exploitation, 2021
Organic matter heterogeneity exerts an important impact on the generation, evaluation, and exploitation of shale oil resources. In the past, only a limited number of analytical samples that represented the contribution or influence of high or low organic matter abundance intervals were used to represent an entire set of thick source rocks, and based on this limitation, occasionally researchers have reached incorrect conclusions. Here, the heterogeneity of organic matter and its significance to shale oil in the Triassic Zhangjiatan shale of the Ordos Basin were discussed based on sedimentary and geochemical analyses. Our results indicate that (1) the Chang 73 shale within the study area is characterized by high abundance, good quality, strong heterogeneity, and segmented enrichment in the vertical profile. The primary organic matter type was type II1, and this was followed by types I, II2, and III that exhibited three changing trends in the vertical profile. Organic matter maturity i...
Minerals
Lower Silurian Longmaxi formation (LSL) shale is widely and continuously distributed in the northeastern Sichuan Basin and, based on structural analogies with the gas producing LSL formation in the southeastern Sichuan Basin, has significant potential for shale gas exploration. However, limited research has been performed to evaluate the shale gas potential in this region. Samples from a recently completed exploratory well (Well-WQ2) in the northeastern Sichuan Basin indicate that the LSL shale has a vertical property sequence that closely resembles the vertical property sequences in wells in the gas-producing sections of the southeastern Sichuan Basin. The continuous sampling and analyses of Well-WQ2 have allowed a detailed investigation of the vertical variations in lithofacies, mineral characteristics, pore structures, and organic geochemical characteristics. The Longmaxi formation was divided into two third-order sequences (SQ1 and SQ2) based on systematic core observations and well logging analyses. Both SQ1 and SQ2 include a transgressive system tract (TST) and a high-stand system tract (HST). The lithofacies exhibit an upward decrease in the organic content. From SQ1 to SQ2, the quartz content, in situ graptolite content, total organic carbon (TOC) content, and brittleness index decrease, but the clay mineral content increases. The LSL shale sections from depths of 1204 to 1214 m and from 1271 to 1282 m possess well-developed fractures and high permeability. Additionally, the average porosity and permeability in SQ1 are higher than those in SQ2. In addition, the positive correlation between the TOC and quartz contents of the assayed samples suggests that much of the quartz is of biogenic origin. Changes in the sedimentary and diagenetic environments during deposition are two key factors that contribute to the observed vertical heterogeneity of the Longmaxi formation. In conclusion, the shale sections of the lower part of the SQ1, like their analogs in the southeastern Sichuan Basin, are the most favorable targets for shale gas production in the northeastern Sichuan Basin.