Biogenic Composition And Organic Matter Enrichment Mechanism Of Shallow Water Platform Source Rocks

The industrial gas flow of 28.34×10~4 m~3/d was first discovered from Well Shancan1 in 1989,which marked the discovery of Jingbian gas field and confirmed the great exploration potential of natural gas in Majiagou Formation.However,the effectiveness of source rocks of Majiagou Formation and the main controlling factors of organic matter enrichment are still unclear.This paper takes the source rocks of the Ordovician Majiagou Formation in the mid-eastern part of Ordos Basin as the research object.Based on the multidisciplinary theoretical methods such as cyclostratigraphy,organic geochemistry,elemental geochemistry and mineral petrology,the composition of source rocks of Majiagou Formation,the formation,environment and the main controlling factors of organic matter enrichment are comprehensively analyzed,and then a new organic matter enrichment model of source rocks in the shallow water platform facies was established.The pre-salt gas in Majiagou Formation is mainly oil-type gas,mainly oil cracking gas and wet gas secondary cracking gas,and the carbon isotope reversal of the pre-salt gas is controlled by maturity and thermochemical sulfate reduction(TSR).Eighteen microfacies(MF)and four microfacies series(MFS)are developed in the Majiagou Formation.Mudstone/muddy dolomite microfacies series(MFS1)contain the quartz-bearing mudstone(MF1),dolomite-bearing mudstone(MF2),horizontally muddy dolomite(MF3)and storm brecciated muddy dolomite(MF4).The average TOC value of MFS1 is 0.27%,which can be used as effective source rocks.The results of carbon isotope of kerogen and bitumen,scanning electron microscope observation,biomarkers show that the organic matter of source rocks in the Majiagou Formation are originated from phytoplankton,followed by bacteria and fungi.Based on the cyclostratigraphy theory,combined with conodonts and carbon/strontium isotope,geological age and deposition time of the Majiagou Formation are clarified.Astronomical chronological scale is established according to the GR logging data of Well Shan473.The deposition time of the Om1-Om3,Om4 and Om5 are 470.0Ma-477.7Ma、466.92Ma-470.0Ma and 458.41Ma-466.92Ma,corresponding to the Floian(7.70Ma)、Dapingian-Upper Darriwilian(3.08Ma)and Lower Darriwilian(8.51Ma),respectively.The average sedimentation rates are 3.24cm/kyr,4.70cm/kyr and3.25cm/kyr for Om1-Om3,Om4 and Om5,respectively.The result of correlation coefficient analysis of sliding window is consistent with high-resolution sedimentation rate.Based on the dynamic sedimentation noise model,the relative sea level change is obtained.Based on organic geochemical and inorganic geochemical theories,the effects of terrigenous detrital input,paleoclimatic conditions,redox conditions,paleosalinity and paleoproductivity of the Majiagou Formation on organic matter enrichment are discussed.Correlation analysis results show that terrigenous detrital input and paleoproductivity are the main factors,followed by paleoclimate,paleosalinity and redox conditions.Moreover,a new model of organic matter enrichment of source rocks in shallow water platform is established.