Study On Heterogeneity Of Longmaxi Formation Shale In Weiyuan Area, Southern Sichuan Basin

With the deepening of the shale gas geological evaluation,shale heterogeneity is becoming obviously.The Lower Silurian Longmaxi Formation marine shale in Weiyuan area,southern Sichuan Basin,has a great resource potential,studying the heterogeneity characteristics is helpful to figure out the Longmaxi Formation geological features,which provide theoretical basis for efficient exploitation of the area.Focusing on the S₁L₁¹,we divided it into four sequences based on the analysis of well-drilling,well-logging,well-cores and assay data.and conducted the heterogeneity by studying the lithofacies,organic geochemical,reservoir space,physical property and gas-content characteristics.Besides,the factors which affected the high yield or low yield of the shale gas well have been discussed.S₁L₁¹ in Weiyuan area is interpreted as four sequences,S₁L₁¹a and S₁L₁¹c are mainly made up with graptolite rich shale and graptolite rich silty shale,which deposited on the lean-anaerobic oxygen environment.S₁L₁¹b and S₁L₁¹d are composed of graptolite silty shale and graptolite calcium silty shale.which deposited on the normal-lean oxygen environment."The TOC + Mineral Composition" two-step method has identified eleven lithofacies in study area,among them,organic rich siliceous shale,organic rich carbonate-siliceous shale and organic rich siliceous-carbonate shale are favorable lithofacies.Vertically,S₁L₁¹a has a great lithofacies association.From the bottom up,organic lean clay shale has increased.Laterally,W4 well has thicker lithofacies association with the thick organic rich clay shale.In addition,mineral composition,TOC content,reservoir space,physical property and gas content characteristics show obvious heterogeneity.From S₁L₁¹a to S₁L₁¹d,clay minerals content increased,quartz and feldspar content reduced,which have resulted in the decrease of shale brittleness.TOC content,controlled by the ancient environment,also have changed from S₁L₁¹a to S₁L₁¹d.S₁L₁¹a shale have the highest TOC content with an average of 4.42%.From the bottom up,organic pore decreased to 41.00%from 60.00%,intergranular pore increased from 25.00%to 43.00%.Micro-fracture are abundant in S₁L₁¹a,which composed dominantly of structural fractures with subordinate horizontal fractures,mainly been filled in calcite.Porosity are greatly influenced by the TOC content,but up to S₁L₁¹d,the correlation diminished because of the increment of inorganic pore,besides,porosity heterogeneity between S₁L₁¹a and S₁L₁¹b is more obviously.Upward,gas content shows a trend of high-low-high-low,and its relationship between TOC content and porosity are relatively obvious,especially with the TOC content,which mainly through three aspects.In spite of the high porosity,S₁L₁¹d has a lower gas-filled porosity.The heterogeneity of each parameter are not only characteristic of vertical partition,but also of horizontal zoning.Shale heterogeneity has a great significance for shale gas production,horizontal segment choose the S₁L₁¹a-S₁L₁¹b or S₁L₁¹c-S₁L₁¹d which have a little level difference makes the yield vary widely.Through this study,we figure out that the drill encounter rate of S₁L₁¹a is one of the factors which control of the production.The higher encounter rate of S₁L₁¹a,the higher the test output.Lateral heterogeneity of the shale gas reservoir is another factor that makes the production different,it is considered that W2 and W4 well areas is the primary place to develop.