Vein Anthraxolite Genesis And Occurrence Regularity In The Western Margin Of Jiangnan Uplift

Many paleo oil reservoir evidences showed in the western margin of Jiangnan uplift, including vein anthraxolite was one of the important types. Vein anthraxolite and scatter anthraxolite existed in source rocks or reservoirare two types anthraxolite in research area. The biggest difference between two types anthraxolite was the occurrence characteristics of agglomeration. It was because of this, vein anthraxolite hold the conditions as solid fuel development, which provided a way to solve the shortage of coal resources in Xiangxi. In addition, as the final stage of the petroleum system evolution, anthraxolite underwent complex evolution and holded a wealth of information. Therefore, researching the anthraxolite genesis and spatial distribution laws could not only alleviate the situation of solid fuel shortage to some extent, but also guide the study of oil and gas and objectively promote the study of oil and gas in the Lower paleozoicin of south China.Vein anthraxolite as the object of study in this paper, which hosted in Cambrian strata in the area Guzhang—Jishou—Fenghuang of the western margin of Jiangnan uplift. In order to provide a better service to explore and develop the vein anthraxolite, comprehensive research and analysis were aimed at forming model and occurrence regularities of vein anthraxolite based on geological survey and experiments, and guided by structural geology, stratigraphy and geochemistry.The study area was located in the area of the west of Guzhang complex anticline to the east to west Paibi syncline in the western margin of the Jiangnan uplift. The main tectonic lines were NE, NNE direction.The regional faults were Guzhang-Jishou fault, Malichang fault and Huayuan—Zhangjiajie fault with the development of the secondary fractures of the trend of fracture density decreasing from southeast to northwest. The survey results showed that all the vein anthraxolite existed in Cambrian stratum and the mostanthraxolite veins exited in the Qingxudong formation of Lower Cambrian. The reduction of the upward trend showed the close relationship between anthraxolite and Early Cambrian paleo reservoir. The formation and evolution of paleo oil reservoir in study area controlled by Jiangnan uplift. The Yanshan movement overall damaged the paleo oil reservoir, at the same time, direct material sources for anthraxolite formation produced.Using a variety of methods analyzed the material source of vein anthraxolite. Firstly, anthraxolite vein strictly controlled by fractures morphology for changing thick or narrow along with the fractures morphology. At the same time, clear boundaries between veins and surrounding rock showed that anthraxolite underwent migration and negated the possibility vein anthraxolite came from surrounding rock. Secondly, there was genetic relationship between stone coal and shale of Early Cambrian and vein anthraxolite through analyzing the comparison of stable organic carbon isotope. In addition, the isotopic reversal phenomenon appeared may joint cause for kerogen, group composition and thermal evolution degree. Thirdly, analyzing of similarity comparison and principal component of trace elements proved the close relationship between coal, shale and anthraxolite.Pearson correlation analyzing of tungsten, molybdenum and radioactive elements, a series of samples as T2?T3?T4 and their surrounding rock in Jielian well, proved that there are no relationship between them, which conclusion was consistent with the vein macro characteristics observed. Fourthly, saturated hydrocarbon gas chromatography showed anthraxolite came from hydrocarbon source rock of lower marine plankton. However, there were differences between anthraxolite and coal from the appearance, composition and characteristics and infrared spectra features.The study indicated that vein anthraxolite was not directly formed from stone coal with plastic migration to fractures, while underwent the transition of the conventional reservoir evolution.FT-IR analysis showed that vein anthraxolite contained more aliphatic components compared with coal tar pitch, multiple absorption peaks of benzene and a large number double bonds of C=C and C=O formed by condensation for its high evolution degree. Raman spectra only appeared "G" and "D" peaks caused by the high thermal evolution degree and characteristics of anthraxolite, other carbon skeleton information disappeared or werecovered. Experiment resulted that the major components of natural solid bitumen in Xiangxi analyzed by XRD were asphaltene(20.96°,23.129°and 26.704°), quartz( 26.704°, 36.656°, 39.522°, 42.518°,50.203°, 54.935°, 60.056°and 67.799°), calcite(29.539°, 36.064°, 43.261°, 47.639°, 48.554°, 57.559°, 60.798°and 64.818°), pyrite(31.589°) and tungsten mineral(60.798°) and so on. Trace elements types and content of anthraxolite indicated overall performance of the enrichment of V, Ni elements, which might inherit the characteristics from hydrocarbon source rocks and continue to be reinforced in migration and evolution progress of oil and gas. In addition, the asphaltene characteristics of trace elements between small fault and big fault were not exactly the same. Except the surface weathering and pollution samples, the majority anthraxolite samples were similar to anthracite or bituminous coal by using coal industry analysis method. It is very important that the model determined based on above structure and composition analysis of anthraxolite from different aspects.The analysis of anthraxolite organic geochemical properties was very necessary for the close relationship between anthraxolite and oil and gas. Determining the anthraxolite grade of maturity was harder than coal, especially in high or over grade of maturity. The determination of true thermal evolution degree influenced by heterogeneity and high or over grade of maturity index effectiveness. Therefore, a joint analysis of multiple means was made in this paper to determine the true anthraxolite maturity.Rb, Tmax and Raman spectrum characteristics of anthraxolite were obtained through experiments, and then were converted into the equivalent vitrinite reflectance using correlated contrast or formula. Thereinto, the equivalent vitrinite reflectance of Rb ranged from 1.9 to 2.31, that of Tmax showed Ro>2.0 and that of Raman spectra was 3.6. Comprehensively, the vein anthraxolite has been in the over grade of maturity. Group composition analysis showed that the relative content of saturated hydrocarbon in most samples exceeded 50%, while the asphaltene content was not as high as expected. The vast majority of anthraxolite group component relative content was that saturated hydrocarbon>non hydrocarbon>aromatic> asphaltenes>chloroform bitumen "A", but which in surrounding rock was that saturated hydrocarbon>aromatic>non hydrocarbon>asphaltenes>chloroform bitumen "A". Rock pyrolysis experiments results of anthraxolite are as follows: Free hydrocarbon content(S1) was very low with the range of 0.02~0.32mg/g, and residual hydrocarbon(S2) value was alsonot high with the range of 0.03~3.32mg/g, but residual carbon content was very high with the range of 110.21~515.03 mg/g(removal of abnormal samples) which the majority samples were 330mg/g above.HI, one of the indicators of hydrocarbon generation ability was low which basically below 10. The above parameters showed that vein anthraxolite hydrocarbon generation ability was poor, so it could not be evaluated as hydrocarbon source rock.There are two major problems about the forming model of vein anthraxolite: one was the material source, the other was the forming process. A variety of methods including positive and negative aspects used to determine the close relationship about the problem of material source between stone coal shale and vein anthraxolitein in the Early Cambrian. But characteristics as appearance, composition and FT-IR indicated that there were obvious differences between them, so vein anthraxolite was not directly evolved by stone coal or shale. There was an important issue ignored by previous research that products would be different when the reservoir damaged in different evolution stages. Specifically, liquid oil existed in the reservoir when itdamaged in the conventional evolution stage, but there must be heavy asphalt formed and gaseous hydrocarbon if the ancient oil reservoir was destroyed in dry gas stage. Considering the structure evolution process experienced in study area, strong liquidity gaseous hydrocarbon was gradually lost and only residual the heavy asphalt because the cover has been eroded during the Indosinian movement. Therefore, it was impossible for liquid oil to inject the fractures when the paleo oil reservoir damaged in the study area. There should also be anthraxolite inporosity for liquid oil flow easily, but the actual situation was not so. Again, the composition would produce differentiation if multiple stage charging, but the experimental tests showed that the anthraxolite composition in different zones were basically same, which was clearly inconsistent with the model.In fact, the vein anthraxolite forming progress in study area was mainly through three processes. The first stage was that the deposit of good hydrocarbon source rock as stone coal and black shale which played the foundations for paleo oil reservoir when Jiangnan old land has not been formed in the early metaphase of Early Paleozoic. The study area was shelf to slope facies of relatively deep water. The second stage was the Caledonian to Indosinian which characterized by turbulence in the early stage of uplift and created the good conditions for oil and gas migration and accumulation. Hydrocarbon generation and expulsion was over when the marine deposit end and the cover also lifted by erosion in late Indosinian. The light part dissipated and resided the oil heavy asphalt which provided direct source for vein anthraxolite. Reservoir trap conditions are destroyed, and then the semi-solid bitumen migrated to fill the crack systems in a relatively short time, and then solidified into anthraxolite veins. Combined with the evolution of the Jiangnan uplift and petroleum system, anthraxolite forming progress was mainly divided into three stages: diagenesis(source rock)-accumulation(paleooil reservoir)-vein(anthraxolite). Yanshan movement was the direct reason for paleo reservoir damaged of all the tectonic movements and influenced the formation and distribution of vein anthraxolite. Vein anthraxolite forming model and the survey results showed that the occurrence fundamentally controlled by three factors as source rock distribution, thermal evolution degree and structural features. More importantly, it was the matching relation between time and space of thermal evolution and tectonic evolution. Prediction of favorable location of anthraxolite veins was the combined position of anticline flank, high angle fracture and outcrop of Niutitang Formation. Under the guidance of this theory, two favorable areas of occurrence were zoned.