Study On The Geochemical Character And Hydrocarbon Generation Potential Of Shanxi Formation In Xiasiwan Area,Ordos Basin

As transitional coal- bearing strata in Ordos Basin, Shanxi formation is one of the shale gas exploring layers and its shale gas penitential has attracted much attention.In this paper, the study of muddy rocks geologic feature was made on the basis of drilling, logging and other geological data. The source rocks were evaluated taking the measure of organic geochemical analysis. Hydrocarbon-generation process and model of the source rocks were analysed according to the pyrolysis results and geological hydrocarbon-generation history was recovered by the means of combining kinetics and thermal evolution. Ultimately, evaluation on gas-generating potential was conducted.Shanxi formation is the coal measure strata of transitional facies with an average layer thickness and muddy rocks thickness of 113.20 meters and 74.61 m in the study area, respectively. It appears to be bulk muddy stones and sand interlayers.The average TOC value of Shanxi formation muddy stones is 1.01%, showing the good quality of organic matter. The composition of microscopic organic macerals is: a large number of vitrinite and inertinite, while a small number of exinite. The average value of IH and D are 101.26mgHC/gTOC and 9.99%, respectively. Evidence discussed above points to III type organic matter. The vitrinite reflectance（Ro）, ranging from 1.76% to 3.72%, with an average value of 2.63%, proves that the dry gas generation is predominant during the high mature and post mature phases.The results of pyrolysis experiment indicates that the product yields are affected by heating rates and they appear similar variation tendency in both heating rates. The yields of gaseous products（hydrocarbon and non-hydrocarbon） increase with temperature: non-hydrocarbon gas is dominant during low temperature while relatively decrease with temperature compared with hydrocarbon gas, in line with natural hydrocarbon generation process.The process of gas generation could be divided into three stages: stage of primary oil production with small amount of shale gas, stage of oil and gas co-existent with wet gas and stage of dry gas formation. At the early episode of early Triassic, Shanxi formation muddy stones were immature. From then on, they began to generate liquid hydrocarbon with a bit of gas. When it reached to the late episode of late Jurassic, gaseous hydrocarbon became majority and peaked in early episode of late Cretaceous. Until now, the cumulative yield of gaseous hydrocarbon could reach 237.9 ml/gTOC.Five indexs synthetical assessment was carried out to evaluate the gas generation potential of Shanxi formation muddy stones. The results reflect that there are 8 and 8 qualified layers in Well YY1 and Well YY3, separately. Compared with Barnet shale, muddy stones of Shanxi formation have lower TOC, higher Ro and equal hydrocarbon generation potential. There are two gas-generating center, of which the maxium of gas-generating intensity could reach 35.7×10⁸m³/km². The genetic method was adopt to estimate the geological resource quantity of study area. By calculation, the hydrocarbon-generation yield of Shanxi formation is about 1.48×10¹²m³.