Resource Evaluation Of Biogenic Gas From Source Rock And Identification Of Enrichment Layers In The West Of Daqing Placanticline

Biogenic gas from source rock refers to the natural gas formed by the biochemical action of anaerobic bacteria during the diagenesis of sedimentary organic matter in gas source rocks.Songliao Basin is a large continental sedimentary basin with lacustrine shale-dense mudstones mainly composed of the first and second member of Qingshankou formation,first and second member of Nenjiang formation.Since most of the mudstones are in the immature-low maturation stage,they have extremely potential for biogenic gas resource.Nearly all of the 45 known geothermal wells in the west of Daqing Placanticline release large natural gas.Through laboratory tests and comprehensive analysis,it`s judged that the type belongs to the biogenic gas from source rock.Considering that the natural gas in shallow-enriched layers may pose a threat to underground construction,it is necessary to evaluate the biogenic gas from source rock and predict shallow enrichment layers.This paper take the west area of Daqing Placanticline as the research area and the stratum below 800 m as the target layers.By analyzing the genesis mechanism of the biogenic gas from source rock,the calculation model of the gas volume is properly modified.Based on the evaluation of the gas production and release volume of the main gas source rock in the study area,the favorable area is optimized.Then predicting shallow sand bodies and interpreting the gas-bearing properties of logging data,establishing a log interpretation chart for gas-water layer identification.Finally,the actual drilling data is used to verify and reveal the enrichment layers of the biogenic gas from source rock.In the process of gas production calculation,the organic carbon gas production rate is obtained by simulation experiment data of biogenic gas in the Qaidam Basin.Considering the influence of the evolution of geothermal history,redox conditions,organic matter composition conditions,PH,and salinity on the organic carbon production rate.Revise the calculation model of biogenic gas production rate proposed by previous people.The calculation of recovery coefficient of original organic carbon is based on the conservation of carbon element mass.The calculation relationship between recovery coefficient of organic carbon and organic carbon gas yield and carbon dioxide content is established.The source rock density is obtained through statistically recording the relationship between the logging density and the buried depth data.And a calculation model for the density-depth of the source rocks during diagenesis period is preliminarily established.Further,the displacement gas volume and thegas release intensity are calculated,which provided basis for the plane prediction of biogenic gas from source rock.In the favorable areas,the shallow sand bodies are predicted by geo-statistical inversion technique.Using well logging data to establish a discriminating chart of reservoir gas bearing.Finally,through the practical drilling of three shallow pre-exploratory wells,the enrichment layers in shallow of the biogenic gas from source rock are determined.The results show that the C1/(C1~C5)of geothermal associated gas in the study area is above 99%,the value of ?13C in methane is between-72.21‰ and-57.11‰,combined with crude oil distribution and migration conditions,to judge the natural gas type,it belongs to biogenic gas from source rock.The water-gas wellhead tests confirm that the water-soluble gas abundance value is between 0.23 m3/m3 and 0.37m3/m3.Using the modified calculation model of the biogenic gas from source rock,the total gas volume of the four main source rock formations in the study area is about 18.25312?m10.After the further calculation of the content of the remaining gas in the source,the total displacement gas volume is determined to be 6.86312?m10,and the total displacement gas efficiency is 0.38.Choosing the Jiangqiaonan area and the Baiyinnuole area as favorable areas,using the inversion technique of reservoir sand body,the sand body distribution in the reservoir below 800 m can be predicted effectively,and the prediction accuracy of sand body thickness is generally above 90%.The actual drilling results reveal that logging gas-bearing explanatory interpretation plates can effectively identify gas-bearing reservoirs and the accuracy of prediction is above 90%.The source rocks of the study area are mostly rich in water-soluble gas,and the detection means of seismic data are not applicable in this area.In addition to the known Qingshankou Formation,Yaojia Formation and Nenjiang Formation contain rich gas resource,there are shallower Sifangtai Formation and Mingshui Formation in the enrichment zone of the source gas bacteriolysis gas in the study area,while the Da'an Formation and Taikang Formation contain poor conditions.