Organic Geochemical Characteristics Of Carboniferous-Permian Coaly Rocks In The Dagang Oilfield And Evidences For Their Contribution To Oil Reservoirs

Since the discovery of the Carboniferous-Permian coal-derived oil and gas fields in the Suqiao area of North China in the early 1980s,there has been controversy over the hydrocarbon generation and accumulation of the Carboniferous-Permian coaly rocks in the Bohai Bay Basin.Since 2015,major breakthroughs have been made in oil and gas exploration around the Carboniferous-Permian in the Dagang oilfield of the Bohai Bay Basin,and high-yield oil and gas flows have been obtained in Wumaying buried hill and Qikou sag.However,there is still no conclusion on whether the discovered oil and gas belong to the hydrocarbon generation and accumulation of the Carboniferous-Permian coaly rocks.In this paper,the organic geochemical analysis of the Carboniferous-Permian coaly rocks in the Dagang oilfield is carried out in detail,including the composition of biomarker compounds in the organic matter of the source rocks,the distribution characteristics of aromatic hydrocarbon compounds,and the stable carbon isotope composition of single hydrocarbons.Based on this,the causes of condensate oil and gas in Wumaying buried hill were dissected,and the main conclusions are as follows:（1）The sources of organic matter in the Carboniferous-Permian coaly rocks in the Dagang oilfield are mainly terrestrial higher plants,and the input of algae increased significantly during the deposition of dark mudstones,but the overall organic matter belongs to humic type.The combination of biomarker compounds is:high pristane/phytane value;high hopanes/steranes value,characterized by relatively low gammacerane and low Ts content in pentacyclic triterpenoids;C₂₉ regular steranes were dominant（>50%）in the distribution of C₂₇-C₂₈-C₂₉ regular steranes.;rich in diterpenes compounds derived from coniferous resins and the corresponding aromatization compounds like retene,1,2,5-trimethyl-naphthalene and 1,2,5,6-tetramethyl-naphthalene,etc.（2）The Carboniferous-Permian coaly rocks in the Dagang oilfield are mainly formed in the delta-swamp facies sedimentary environment with strong oxidation.The biomarker compounds were characterized by a significant predominance of pristane,and high levels of dibenzofurans were detected.The ratios of ADBTs/ADBFs were all below 1.0,indicating that they developed in a relatively oxidative sedimentary environment.In contrast,coal and carbonaceous mudstones are deposited in strong oxidizing environments,and the Pr/Ph values are mostly in the range of 3.5 to 7.5;dark mudstones are deposited in weakly oxidative environments,and the Pr/Ph values are mostly in the range of 1.0 to 3.0.（3）The carbon isotope composition of n-alkane monomer hydrocarbons in the organic matter of coaly rocks reveal that the carbon isotope of n-alkane in coal is heavier than that in mudstone in Shanxi Formation.For the Taiyuan Formation coaly rocks,the carbon isotope of medium-chain n-alkane monomers in the coal is significantly enriched in ¹²C,and the ship-forming distribution pattern is different from the mudstones,which is the potential indicator for regional identification of the contribution of coal and mudstone to oil generation.（4）Wumaying buried hill condensate oil belong to low-wax and low-sulfur condensate oil.The oil-source comparison results show that the condensate oil has not undergone secondary alterations such as thermal cracking,gas invasion fractionation,and biodegradation.The high pristane/phytane value and high content of benzene series have good comparability with the coaly rocks.The carbon isotopic composition of n-alkanes strongly indicated that the Wumaying buried hill condensate oil were mainly contributed by the Carboniferous-Permian mudstones.According to the characteristics of the fluid system of the Wumaying buried hill and combined with the regional sedimentary structural profile,this paper proposes that its accumulation process has the characteristics of"differential hydrocarbon generation,fault-controlled accumulation".