Research On The Pore Structure Of Xishanyao Formation Coals In Santanghu Basin

In this thesis,we focused on the coals from Xishanyao Formation in Santanghu Basin and makes full use of the theoretical methods such as sequence stratigraphy of coal-bearing strata to discuss the coal accumulation of the Xishanyao formation.On this basis,the degree of coalification,types of macerals,content of inorganic matters have been studied.We used complementary experiments to characterize the multiscale pore structure of the coals and discuss the influence of coal composition and sequence stratigraphic on the pore structure.The main conclusions can be summarized as follows:1.Detailed stratigraphic analyses of data from boreholes for the Xishanyao formation have been conducted in Santanghu basin and it was divided into 2 third-order sequences.Lowstand systems tract（LST）,lake transgressive systems tract（EST）,highstand systems tract（HST）was subdivided by the stacking patterns of stratigraphy,changes in lithology and lithofacies.A series of sequence-specific paleogeographic maps have been constructed based on the contours of lithological parameters.The paleogeographic units include braided river,braided river delta and lacustrine sedimentary system facies.The preferred sites of coal accumulation is the transition region between braided river delta and lacustrine from the margin to the center.Furthermore,there are a range of thick coal seams at the contact interface between the delta front and the delta plains.The coal-accumulation features in sequence stratigraphic framework are clarified.Through analyzing coal seam change features,coal-rich zone,the accommodation rate in relation to peat production would have controlled on coal formation and preservation.During the EST deposition,coal seams are thick and continuous while only thin,discontinuous and scattered coal seams are formed in the HST.The comprehensive coal-accumulation model is established based on the researches mentioned and these results are of practical significance for after coal resources exploration in Santanghu Basin.2.Through maceral analysis,industrial analysis and element analysis,the content of coal composition was obtained.The maceral composition of these investigated coals is dominated by vitrinite,with trace amount of inertinite.The content of liptinite is unstable,dominated by cutinite and macrosporinite.The vitrinite reflectance of coal ranges from0.54 to 0.89,and the types of coal are low rank and medium rank bituminous coal.The results of elemental analysis showed that the content of organic C in coal is high and the content of H is very low.The inorganic mineral content is obviously higher in EST.In contrast,the content of inorganic mineral and ash content is relatively low in the HST and LST.The content of FC_ad and V_ad is lower in EST,but higher in HST and LST.C and H are important constituent elements of organic matter in coal,the variation trend of which and ash content also showed opposite trend.The content of organic maceral is lower at EST and higher at LST and HST.3.According to the adsorption loop analysis,there are many fracture-shaped and wedge-shaped holes in the coal samples of the Santanghu Basin.In addition,some coal samples contain cylinder holes with one end closed.Based on the results from the low temperature liquid nitrogen adsorption analysis,the smaller the pore diameter is,the greater the contribution to the pore specific surface area is.That is,the specific surface area of micropores（2-10nm）occupies the majority of the specific surface area（>2nm）.On the contrary,the larger the pore diameter is,the larger the pore volume increment will be.Based on the results from CO₂ adsorption analysis,the specific surface area of super micropores obtained by DFT model is much larger than that of the micropores and the transition pores.Distribution curves of pore volume and specific surface area show the same multimodal characteristics.D₁ and D₂ are used to characterize the pore surface and the pore structure fractal dimension,respectively.4.The contents of vitrinite and inertinite are negatively correlated with the BET specific surface,BJH pore volume.On the contrary,inorganic minerals in coal have an important influence on pores（2nm-100nm）.This indicates that the organic matter has little influence on the part of pore,and it presents a negative correlation with it.The DFT volume and specific surface increased with the increase of vitrinite content.This indicates that the organic matter in the low-rank coal contains a large number of adsorption pores,and most of them are super micropores.The correlation of industrial analysis components to microporos and transition pores shows an opposite trend to that of super micropores.The fixed carbon content and volatile content are negatively correlated with BET specific surface area,while positively correlated with DFT specific surface area in coal.The past research reveal that the formation and thickness of coal seam are con controlled by sedimentary environment.Meanwhile,environment influences the composition of coal,so the sedimentary environment also indirectly controls the pore structure of coal reservoir.During the EST,the inorganic mineral content and ash yield in the coal were higher,and the BET specific surface area and BJH volume were larger correspondingly.On the contrary,in the HST and LST,the ash content is relatively low,resulting in relatively lower BET specific surface area and BJH volume.Similarly,there is a similar rule for fractal dimension D₂,which is relatively large in coal reservoirs in the EST.