| The Anyue gas field was discovered in the Gaoshiti-Moxi structure in the middle of Sichuan Basin.The Cambrian Longwangmiao Formation in Moxi area has discovered the largest carbonate gas field in China so far,with an additional proven natural gas reserves of4403.8×10~8m~3,moreover,the Sinian and Cambrian gas reservoirs in Gaoshiti-Moxi area have been preliminarily confirmed to be over trillion cubic meters in size.Why can gas reservoirs be well preserved in the ancient carbonate strata which have undergone long-term evolution and multiple tectonic movements?Based on drilling and logging data,through the combination of macro and micro,geological anatomy and experimental analysis,the development characteristics of mudstone caprock in gas reservoir are studied in detail,and its sealing ability is evaluated comprehensively.The recovery method of displacement pressure evolution of mudstone caprock under the background of multi-stage uplift is established,and the time effectiveness of sealing natural gas by mudstone caprock of Qiongzhusi Formation and Longtan Formation in Anyue area is evaluated.The results show that:1.There are two sets of main regional mudstone caprocks in Anyue gas field,namely,the lower Cambrian Qiongzhusi Formation mudstone caprocks and the upper Permian Longtan Formation mudstone caprocks.The mudstone caprock thickness of Lower Cambrian Qiongzhusi Formation is 50 m~450 m,which is characterized by"high in West and low in east";the mudstone caprock thick ness of Upper Permian Longtan Formation is 70 m~150 m,which is characterized by"center thickness and thin around".2.The displacement pressure of the lower Cambrian Qiongzhusi Formation mudstone caprock in Anyue gas field is relatively high,ranging from 16Mpa to 50Mpa,which is gradually decreasing in the southeast direction in plane.The displacement pressure of the upper Permian Longtan Formation mudstone caprock now ranges from 8Mpa to 42Mpa,and there are three high displacement pressure areas near GS1 well,GS23 well and MX11 well.3.The sealing mechanism of mudstone caprock of Lower Cambrian Qiongzhusi Formation to lower Dengying Formation gas reservoir in Anyue gas field is capillary sealing(strong capping and normal reservoir type);the sealing mechanism of upper Permian Longtan Formation mudstone caprock to lower Cambrian Longwangmiao Formation gas reservoir is capillary blocking residual pressure sealing(strong capping and strong reservoir type).4.The comprehensive evaluation value of the mudstone caprock of the Lower Cambrian Qiongzhusi Formation in Anyue Gas Field ranges from-0.85 to 0.06.The overall distribution characteristics of the mudstone caprock of the Qiongzhusi Formation are"high in the West and low in the east,high in the South and low in the north".This indicates that the sealing capacity of the mudstone caprock of the Qiongzhusi Formation is gradually increasing from west to East and from north to south.The comprehensive evaluation value of the mudstone caprock of the Upper Permian Longtan Formation ranges from-0.12 to 0.16,showing the distribution characteristics of"high middle,low northeast and southwest",which indicates that the sealing capacity of the mudstone caprock of the Longtan Formation increases gradually from the central area to the northeast and southwest direction.5.The formation time of effective sealing natural gas ability of Qiongzhusi Formation and Longtan Formation mudstone caprocks in Anyue Gas Field is earlier than that of large amount of hydrocarbon expulsion from lower source rocks.The sealing effect of two sets of mudstone caprocks on natural gas is effective in time.These understandings can reasonably explain the reasons why natural gas can still be preserved after long-term evolution and multiple tectonic movements in the study area,and also have important guiding significance for effectively predicting favorable distribution areas of gas fields. |
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