Quantitative Analysis Of Volcanic Clastic Compaction

Compaction is an important dynamic factor that accompanies the basin filling process,and the results of its quantitative analysis provide important background values for other factors.Physical simulation is an effective method for single-factor characterization and is suitable for pyroclastic rocks with prominent deformation under compaction.Using the Quaternary volcanic ejecta of the Longgang and Changbaishan volcanoes as examples,physical compaction simulation experiments and rock pore casting experiments were designed based on scanning electron microscopy,porosity tests,and thin section identification.According to the results of the physical compaction simulation and various experiments,compaction curve of the volcanic ejecta under a single compaction factor was established,and compaction of the volcanic ejecta under overburden conditions was quantitatively analyzed.The factors influencing volcanic ejecta compaction were discussed,differences between the volcanic clastic compaction and sedimentary rock compaction curves were compared,and consequently,the causes were analyzed.The basic parameters and exponential relationship needed for the burial history simulation were determined,a volcanic clastic compaction model was established,and a treatment plan for pyroclastic rocks in the correction of stratum compaction was proposed,which was applied to the burial history restoration.The results showed that the field profiles of Changbaishan volcanoes were mostly accumulated pyroclastic flows,with reverse grain sequence bedding,mound bedding,and exhaust pipe structures;the clastic particles are sub-angular and sub-angular-sub-circular,indicating poor roundness.The accumulation section of volcanic debris flows develop trough bedding,and the clastic particles are sub-round,which was better.The volcanic ejecta in the study area were mainly alkali liquid pumice lithic,rough-surface pumice lithic,and basaltic pumice lithic,with a pore content of over 70%.After the compaction physical simulation experiment,most of the particles were in concave-convex contact,showing the squeezing and accumulation of fine pumice debris.The Longquan Longwan section of the Longgang volcanes exposed the heat base wave accumulation section,which developed grain sequence and mound bedding,and the clastic particle roundness was sub-angular-sub-round.The volcanic ejecta in the study area were mainly porous basaltic lithic material with a pore content of approximately 30%.After the physical compaction simulation experiment,most of the particles were in line and displayed concave-convex contacts.According to the porosity test results,the initial porosity of the pumice samples of the Changbaishan volcanoes was higher than that of the porous basaltic samples of the Longgang volcanoes.The initial porosity of the volcanic ejecta after sieving was higher than that of the in-situ volcanic ejecta.The higher the initial porosity of the volcanic ejecta,the greater the porosity reduction in the physical compaction simulation experiments.Compaction curves for volume-axial,cumulative compressibility-axial,porosity reduction-axial,porosity-axial,and permeability-axial compression were established based on the results of the physical compaction simulation experiments.Quantitative analysis of volcanic ejecta compaction showed that the relationship among porosity,porosity reduction,compaction,and axial compression was exponential.The relationship between the cumulative compressibility and axial compression was logarithmic.The analysis of the factors influencing volcanic ejecta compaction showed that the larger the particle size,the better the sorting,and the higher the pore content,the larger the curvature of the compaction curve of volcanic ejecta and more obvious the compaction of volcanic ejecta.The median particle size,sorting coefficient,and cutting content of volcanic ejecta were positively correlated with porosity reduction;the larger the particle size,the better the sorting,the higher the pore content,and the greater the porosity reduction.Additionally,the initial porosity and compaction coefficient were positively correlated with the median particle size,sorting coefficient,and cutting content.When the pyroclastic particle size was larger,sorting was better,the pore content was higher,and the initial porosity and compaction coefficient were higher.The quantitative analysis of volcanic ejecta compaction and a comparison with previous research results from the sedimentary rock mechanical compaction simulation revealed that the initial porosity and compaction coefficient of pyroclastic rocks were different from those of sedimentary rocks because of the influence of particle size,sorting,and lithic composition.The larger the particle size of the volcanic ejecta,the better the sorting,the higher the pore content of the cuttings,and the greater the initial porosity,compaction coefficient,and curvature of the pyroclastic compaction curve.However,the smaller the particle size of the sedimentary rock samples,the better the sorting,the lower the sand content,and the greater the initial porosity;the better the sorting,the higher the sand content,the smaller the compaction coefficient,and the smaller the curvature of the compaction curve of the sedimentary rocks.For the pure quartz sand samples,the larger the particle size and compaction coefficient,the greater the curvature of the compaction curve of the sedimentary rocks.For lithic fragment sandstone samples,the higher the lithic composition and content,the greater the curvature of the compaction curve.The compaction curvature of weathered basalt cuttings was higher than that of shale cuttings with different cutting compositions and the same sand content,and the compaction curvature of slate cuttings was the lowest.Therefore,in the mechanical compaction process,the factors influencing the initial porosity and compaction coefficient were different,leading to differences between the pyroclastic and sedimentary rock compaction curves.The difference in material composition between pyroclastic and sedimentary rocks,as well as the particle properties of pyroclastic rocks,lead to differences in their initial porosity,compaction coefficient,and compaction process.Based on the volcanic clastic compaction curves and laboratory porosity test results,the basic parameters and exponential relationship（initial porosity,compaction coefficient,porosity-depth relationship）needed for burial history simulation research were determined,and then the volcanic clastic compaction model and volcanic clastic compaction correction formula was established:φ（z）=0.48e(-0.109×10^-3z)A treatment plan for pyroclastic rocks in formation compaction correction was proposed.The compaction coefficient range of pyroclastic rocks was between 0.000055and 0.000142;the initial porosity range was between 33.75%and 60.45%.When recovering the burial history of a single well,the larger the granularity of the pyroclastic rocks,the higher the pore content of the cuttings,and the better the sorting;the selected compaction coefficient and initial porosity tended to 0.000142 and 60.45%,respectively.In contrast,the compaction coefficient and initial porosity were inclined to 0.000055 and33.75%,respectively.Based on the volcanic clastic compaction model,the Huoshiling Formation of Well LFS B213 was selected as the research object for the single well burial history restoration.According to the principle of stratum compaction correction,the stratum thickness of the Huoshiling Formation was calculated by the traditional lithologic classification method,and thickness of the Huoshiling Formation was 430.0m.When the volcanic rocks were considered in the thickness calculation of the Huoshiling Formation,the average,minimum and maximum thickness of Huoshiling Formation were 496.4m,485.1m and506.1m respectively.By comparing the traditional lithologic classification with the lithologic proportion of volcanic rocks,an error of 12.81-17.70%was found.This error cannot be ignored in the burial history restoration process of volcanic rocks.Therefore,it is necessary to conduct a single-factor quantitative analysis of volcanic clastic compaction.The unique properties of pyroclastic particles make their compaction processes to differ from those of sedimentary rocks.The single-factor quantitative analysis is of great importance to quantitatively characterize the burial evolution of volcanic reservoirs,to clarify the main factors controlling the compaction of volcanic rocks and determine the compaction model of volcanic rocks.The single-factor quantitative analysis also provides a basis for the quantitative characterization of reservoir evolution and compaction analysis of volcanic strata under the constraints of multiple factors in volcanic rocks.