Sedimentary Characteristics Of Black Shale And Enrichment And Preservation Conditions For Shale Gas Of Lower Devonian In Yanyuan Basin

In recent years,the Devonian Lower Devonian in Yanyuan Basin has been considered as a potential shale gas exploration zone,but up to now,little petroleum exploration work has been carried out.Therefore,it is urgent to combine the theoretical study of sedimentary characteristics of black shale with the clarification of shale gas enrichment conditions so as to provide a strong preparation for unconventional oil and gas exploration and development in Yanyuan Basin.Based on some methods such as the field stratigraphic profile,drilling cores,lithological characteristics,paleontological characteristics,microscopic characteristics,sedimentary structure and characteristic element analysis,this paper studies the sedimentary characteristics and sedimentary model of the Lower Devonian black shale in Yanyuan Basin.The organic geochemistry and mineral composition,pore size and gas content characteristics of the lower Devonian black shale in Yanyuan Basin are tested and analyzed to study the enrichment conditions of shale gas in this area.And the preservation conditions of shale gas in Yanyuan Basin are evaluated by dividing structural units,fault structural profiles,occurrence and indoor analysis of magmatic rocks,shale burial depth,alongside the study of main gas generation period and tectonic activity and magmatic thermal event activity time.The study shows that the main forming environments of the lower Devonian black shale in the study area are littoral facies and shallow shelf facies which include the following characteristics:the lower Devonian paleontology in there is developed,with biobeds and fossil belts;the sedimentary structure is generally developed,mainly with bedding structure;and the micro-characteristics are typical of Strawberry pyrite.The organic matter enrichment conditions of black shale are judged synthetically.It is concluded that the organic matter enrichment conditions of black shale in the upper part of the lower Devonian Suotoushan Formation in Yanyuan Basin are the best,followed by the lower Suotoushan Formation and the lower Pojiao Formation,and last the Posongchong Formation.The Devonian crustal depression and passive continental margin environment in the western part of the Upper Yangtze Plate provided a stable sedimentary environment in the Devonian extensional environment,which led to large-scale transgression.The paleostructure inherits the trend of High East and low West and the distribution pattern of uplift and depression which has been formed since Silurian.The distribution of sedimentary system is obviously controlled by fault structure.The analysis shows that the sedimentary model of continental black shale in the Devonian Lower Devonian in Yanyuan Basin is formed under the comprehensive control of passive continental margin-paleo-sea level rise-paleostructure-anoxic environment and other factors.The study based on the division of tectonic units in Yanyuan Basin,field fault structural profiles,field occurrence and indoor analysis of magmatic rocks,shale burial depth,alongside the study of main gas generation period and tectonic activity and magmatic thermal event activity time shows that the research results are satisfactory.show that the lower Devonian shale gas enrichment conditions in Yanyuan Basin are best in the suotoushan formation,followed by the posongchong formation and the pojiao formation.The average organic carbon content and Ro are1.31%and 1.13%respectively.The main types of organic matter are II 1 and II 2.The cumulative effective thickness of organic shale is over 100 m.The comprehensive evaluation shows that the source rocks of the Lower Devonian Youtoushan Formation in Yanyuan Basin are the best,followed by the Pojiao Formation and the Posongchong Formation.Mineral composition analysis indicates that the average content of brittle minerals in organic-rich shale is much higher than 40%,the average brittleness of minerals is 59.30%,and the average content of non-expansive clay minerals is 97.89%.The results of pore experiment show that the average porosity and permeability of shale are 2.51%and 0.053 mD,which have typical characteristics of low porosity and low permeability.The maximum amount of adsorbed gas in the study area is 3.02m~3/t,which has a high amount of adsorbed gas.Through the field fault structural profile,magmatic rock occurrence and indoor analysis,shale burial depth,and combined with the study of main gas generation period and tectonic activity time,the study shows that the preservation conditions of lower Devonian shale gas there are generally relatively good.Lower Devonian source rocks in the study area began to generate and expel oil in the middle Triassic,and entered a large number of generation and exhaust stages from the late Middle Triassic to the early Late Triassic.Meanwhile,the boundary faults of Yanyuan basin have certain destructive effects on the preservation of shale gas,but the internal structure of the basin is dominated by folds,and the faults are relatively undeveloped which have little destructive effect on shale gas.Magma-thermal events that may have an impact on shale gas evaluation are only exposed in part of the basin at present,but because of their small scale,they will not have an overall influence on the formation and preservation of shale gas there.And what's more,the burial depth of the study area is deep in the north-west and shallow in the south-east,with a maximum depth of5150m and a wide range of areas with favorable burial depth of 1500-4000m,which is a favorable shale gas burial depth area.