| Shale gas,as a new energy source,has drawn attentions of scholars from all over the world for its huge potential.There are many Paleozoic to Cenozoic black shales in South China,among which the reserves of shale gas are approximate those in the U.S.Though industrial air flow has been obtained from the Early Paleozoic black shale,present theories on shale gas in China are still insufficient,which is restricting the development of exploration on shale gas.More studies on geological characteristics of the black shale,especially the distribution and preservation of the organic matters in southern China are needed.In this study,two sections of successive deep water sediments in the Upper Permian Dalong Formation in the Shangsi and Ganxi Area were measured.The distribution and preservation of the organic matters in the Dalong Formation were detected via scanning electron microscopy and the sporopollens were analyzed.Combining with the trace element geochemistry of the shales,the influences of paleo-environments on preservation of organic matters were discussed.Based on this study and previous data,the patterns of the organic matters were concluded for evaluating the gas in the shales.The controlling factors on preservation of organic matters in modern marine sediments and black shales were concluded.It is found that the controlling factors are corresponding in both geological historial and modern sediments,i.e.types of organic matter(the composition of organic matter),productivity and preservation environment,and clay mineral adsorption and protection.31 samples from the Shangsi Section and 21 samples from the Gansi Section were collected to detect the types and distributions of organic matters.The brightness,morphology and energy spectrum of organic matters in secondary electron(SE)and backscattered electron(BSE)images are significant for distinguishing their types.The results showed that organic matters in the Dalong Formation can be divided into three types: morphological organic matter,dispersed organic matter and asphalt.All types of organic matter in the backscattered picture are black,but the form of morphological organic matter is a kind of highly stable particle organic matter with biological structure,preserving the original information of the organism,so that it is shining in the secondary electronic picture.Dispersed organic matter is the product of the interaction between clay minerals and organic matter,which can be seen flaky mineral polymer in the secondary electron image,and black patches in backscattering mode.Dispersed organic matter has characteristics of high C contents and low O,Si and Al contents.The block-like asphalt is transported into the sample hole system,with edges and fissures.The energy spectrum data show that the asphalt got extremely high contents of C,and extremely low contents of O,Si and Al.The results also showed that the effect of diagenesis on the storage and distribution of organic matter was mainly reflected in the dispersion of organic matter and asphalt,and had no obvious effect on the organic matter preserved in its stability.Clay mineral morphology of dispersed organic matter is more obvious and the lower C content can be tested in the samples of deeper diagenesis area(Ganxi section).The composition and morphology of some bitumen are also affected by diagenesis.The sporopollen organic matters were analyzed to make comparison with the SEM results.In the transmission light,organic matters contain morphological organic matters and amorphous organic matters.Morphological organic matter is organic particle that retains the biological form,which consists of spores(including sporopollen fossils and other algae)and structural organic matter(including plant and animal).The amorphous organic matter is mainly composed of sponge amorphous organic matter and granular amorphous organic matter.The author identified well-preserved spherical,ellipsoidal fossils Dictyotidium,Leiosphaeridia,chain-related fossils Reduviasporonites,sporopollen fossils Cycadopites,Lundbladispora and lining of foraminifera,and some algal spores and plant tubules.The morphology organic matters in the transmission light and SEM is very similar,corresponding to SEM morphological organic matter.The sponge amorphous organic matter is the main part of dispersed organic matter,often existing in the form of polymer,which may be the result of organic poly-condensation,the quantitative results show the same variation with TOC,meaning this kind of organic matter can be associated with the dispersion of organic matter,also shows that the sponge amorphous organic matter(organic matter)is the main component of TOC.The particle amorphous organic matter can only exist alone,but also occurred with sponge amorphous organic matter together.This kind of organic matters often have three-dimensional properties,which can be associated with the asphalt.Previous studies have shown that reduced water column and high productivity are important factors to affect the preservation of organic matters in the Dalong Formation,but the specific mechanism was not described in detail.The authors considered the high-precision trace element geochemistry,biology and sea level change data,and divided the profile into several intervals,then discussing productivity and redox environment on the impact of organic matter preservation in detail.The reduction of the water environment can promote the storage of organic matters,like the interval II of the Shangsi section,the interval I of the Ganxi section and the Interval III of the Ganxi section.They are he main organic matter accumulation intervals(TOC average is more than 3%),deposited in the reduced water colum.When the water column is in the state of hypoxia,the influence of the redox environment on the storage of organic matter will be decreased,and the productivity become the key factor to control the change of TOC content.In he interval II of Shangsi section and the interval III of Ganxi section,TOC has an excellent correlation with the productivity index(R2?0.79),but he correlation with the redox environment index is weaker(R2?0.58).When the water column is in oxidized-suboxidized state,like the interval IV of the Shangsi section,the interval IV of the Ganxi section,the interval V of the Ganxi section,which has the lower TOC level,the average value is less than 1%.Under this condition,the effect of factors on organic matters may vary according to the situation.If the contents of morphological organic matters are high,the controlling effect of productivity on the storage of organic matters is more obvious,such as the interval II of Ganxi section,the interval IV of Ganxi section.Conversely,he redox environment of water column will be the main control factor,such as the interval I of Shangsi section and the interval IV of Shangsi section.Finally,the author concluded all the data and summarizes the depositional model of the Dalong Formation as follows: The increase of the paleo-productivity of the Dalong Formation is mainly caused by the two transgression events during the Late Permian.The volcanic activity of Upper Yangtze started to be frequent in the latest Middle Permian,and the weathering made a large number of nutrient elements and clays accumulate in the shelf lowlands.With the sea level rises rapidly during the Late Permian,the accumulated nutrients were brought into the ocean system,leading to increased productivity in adjacent oceans and the subsequent stratified seawater.The hypoxic environment tends to spread from the middle of the seawater,and as the dissolved oxygen in the seawater is further reduced,the minimum zone of oxygen is expanded to the underlying water body to form a stable reduced environment.At this time,the remaining organic matter are prone to interact with clay mineral to form dispersed organic matter so that they can be preserved in sediments quickly,so the surface water productivity is the main factor in controlling the preservation of organic matters.The reduced environment lasted until the end of the second transgression,and most of the nutrient elements had been consumed during the first transgression.Therefore,the productivity of the second transgression began to decrease,and the water environment changed from sub-oxidation to hypoxic environment.The TOC is controlled by the water redox environment and the productivity.The organic matter type is controlled by the productivity and the clay mineral contents.The contents of the organic matters decreased,and a large amount of organic matter is lack of clay mineral adsorption is decomposed,only morphological organic matter are preserved.Understanding the types of organic matter in muddy source rocks is helpful to understand hydrocarbon source formation during the diagenetic process and is also helpful for the unconventional oil and gas exploration.Morphological organic matter is the outer wall of spores,pollens or or acritarchs,which has high stability,but it is not directly related to TOC content.The dispersed organic matter is the main component of amorphous organic matter,and it has the same variation trend with TOC.Therefore,the author believes that the dispersed organic matter is the main body of TOC in organic-matter-rich siliceous mudstones.Dispersed organic matter has the potential as a deep hydrocarbon parent material,showing that TOC in the siliceous mudstone can generally reflect the preservation of the original organic matter.Asphalt is the result of the organic matter migration.When the amount of asphalt is present,the TOC can not accurately reflect the productivity level of the ocean,but the residual asphalt has the ability to decompose in deep conditions.The author thinks that the organic matters of the Dalong Formation,have the potential of hydrocarbon production,but the maturity of those in the Guangyuan area is relatively moderate,indicating that the Dalong Formation in the Jianchang area,Hubei Province is better for shale gas exploration. |
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