Organic Matter Enrichment Of Fine-grained Source Rock In Shollow Lake Facies

The Ordos Basin is one of the most important petroliferous sedimentary basins in China,and the Chang 7 unit sediments deposited during the Late Triassic has been recognized as the primary hydrocarbon source rock for the Mesozoic petroleum system.Many studies about the Chang 7 unit source rock have been conducted in the central basin,but lack studies on the distribution and genetic mechanism of the hydrocarbon source rock in the periphery area of the Ordos Basin,which limited the oil and gas exploration and development in the periphery area of the basin.Based on the logging data in the Yanchi-Dingbian area and the geochemical data from 156 source rock samples in the study area,the organic geochemical characteristics,sedimentary environment and the main controlling factor of the organic matter enrichment in the Chang 7 unit source rock have been made clear with the synthetically application of geology,geochemistry and geophysics,and then establish an organic matter enrichment model for the fine-grained sediments in the periphery area of the Ordos Basin.The mean total organic carbon content and hydrocarbon generation potential are2.68%and 15.81 mg/g,respectively,representing a high organic matter abundance of the Chang 7 unit source rock in study area.According to the organic petrology,rock-eval and elemental analysis of kerogen,the organic matter type in the Chang 7 unit source rock in study area is dominated by type II,which indicates the origin of organic matter is a mixture of lower aquatic organisms and terrestrial higher plants.Based on the Ro,Tmax and biomarkers from the Chang 7 unit source rock,the thermal evolution degree of the source rock in study area is low maturation to maturation.Overall,the hydrocarbon generating potential from the Chang 7 source rock is good to well.In addition,the analysis of biomarkers,redox-sensitive elements?RSEs?and pyrite framboids suggest the sedimentary environment is oxic-dyoxic with fresh water during the Chang 7 stage.According to the gammy-ray and density logging data,the astronomical time scale?ATS?of the Chang 7 unit has been established on the basis of cyclostratigraphy.Based on the new ATS,the duration,starting/ending time and high-resolution sediment accumulation rate?SAR?of the Chang 7 sediments can be acquired.Meanwhile,the high-resolution SARs from ATS coincide exactly with the results of evolutionary correlation coefficient?e COCO?of the GR series.In addition,the crystal size distribution theory?CSDT?of pyrite framboids also can be used to acquire the variation of relative SAR.A new parameter(TOC_pup,total organic carbon per unit paleoproductivity)has been established to investigate the impact of the SAR on organic matter enrichment in order to eliminate the effect of paleoproductivity on organic matter enrichment.The relationship between SARs and organic matter abundance suggests that a suitable SAR can be benefit to organic matter enrichment,but an inaptitude?too fast or too slow?SAR can increase the oxidation consumption of organic matter or dilute the organic matter abudance.The paleoproductivity parameters(including P_org,Ba_bio,TOC and Mo)can be used to qualitative evaluate the paleoproductivity of the Chang 7 stage in study area.Moreover,considering terrestrial organic matter input,organic matter oxidation consumption and terrestrial debris dilution during the sedimentation,the paleoproductivity of the Chang 7 stage can be quantitative evaluated.The results of paleoproductivity suggest that the sedimentary basin is eutrophic-extremely eutrophic during the Chang 7 stage in study area.In addition,on the basis of the effects of productivity model and preservation model on paleoproductivity parameters(including P_org?Ba_bio and TOC),paleoproductivity has been proved to be the mainly controlling factor of organic matter enrichment in the Chang 7 unit source rock.Combined with the paleoclimate,sedimentary water conditions?including water salinity and paleo-water-depth?,chemical properties of provenance,volcanic and hydrothermal activities,an organic matter enrichment model has been established:under the background of warm and humid paleoclimate,high terrestrial debris input and abundant nutrients derived from volcanic and hydrothermal activities are benefit to the formation of the Chang 7 unit excellent hydrocarbon source rock.