The Sediment Controls Of Coalbed Methane System About The Longtan Formation Of The Panguan Area In Western Guizhou

In this work,the sequence stratigraphic framework in Longtan coal-bearing strata?Upper Permian?of the Panguan area was established according to geological data,including outcrop,coal cores,logging response and laboratory test results.The sedimentary environment and its distribution characteristics was investigated,and the coal accumulation law under the control of sequence stratigraphic framework was also determined.Finally,the vertical variation of gas-bearing system controlled by the sequence framework was analyzed,and it is also verified by measured gas content of different coal seams of parameter wells.Four three-order sequences?SQ1,SQ2,SQ3,SQ4?and 8 system domains of Longtan Formation were divided based on the identification of the key stratum,the stratigraphic superposition and sedimentary cycle characteristics,which show that the low-stand system tract is not developed in Panguan area.Moreover,the sedimentary evolution model and coal accumulation characteristics in each sequence are clarified,and the main two depositional systems are found,including the Delta and the tidal flat-lagoon facies.In the SQ1 period,the main depositional system was the tidal flat-lagoon depositional system,which mainly developed in the northern and central regions of the west wing of the syncline;Regional marine regression occurred during the SQ2 period,and the delta deposits expanded to the eastern and southern part of the syncline;For the SQ3,the delta sedimentary system was dominant,while the tidal flat lagoon depositional system only developed in the southern part of the east wing of the syncline;During the SQ4 period,the major marine transgression occurred,and the delta deposits began to shrink to the northwest with a limited narrowing range.The coal seam in the study area developed in all sequences in the lagoon swamps,tidal plain swamps,underwater distributary bay swamps and delta plain swamps.Generally,it is characterized by“The north part and the east wing is thin in thickness,the south part and the west wing is thick”.Also,the development of coal seams in different regions and different sequences is different due to the influence of the sedimentary environment.The coal seam is the thickest in the SQ3 period,due to that the delta sedimentary system was dominant in this period with stable water body and weak tide action,and two coal gathering centers of Huopu-Jinjia and Laowuji-Yueliangtian area can be found.There are 5 depositional sequence assemblages?including coal seams,roof and floor?,including the lagoon-tidal flat transition deposit sequence,tidal flat subtidal sedimentary sequence,complete tidal flat sedimentary sequence,delta front depositional sequence and delta plain sedimentary sequence.According the sequence framework,J1 well was taken as an example to analyze the vertical distribution of the gas-bearing systems,which shown that there are 3independent gas bearing systems,e.g.coal seam No.29#-coal seam No.24₁#?I?,coal seam No.18#-coal seam No.18₁#?II?,and coal seam No.17#-coal seam No.1#?III?.The closed layers between I,II,III systems are SQ1 transgressive system tract,SQ2transgression system tract,and SQ3 transgressive system tract,respectively.