Petroleum Generation Kinetics Of Jurassic Coaly Source Rocks In Junggar Basin

Junggar basin is one of the main oil and gas generation basins in China.The southern margin of the basin is rich in oil and gas,and many oil and gas fields and structures have been discovered in this area.In 2019,the high-yield oil and gas flow was obtained in GT1 in Sikeshu depression in the west of the southern margin,which set a record for the daily production of single well in Junggar basin.The oil and gas of GT1 derived from Jurassic coal source rocks.The southern margin of Junggar basin has become a hot area of oil and gas exploration.In this paper,the coal source rocks in the southern margin of Junggar basin are selected for hydrocarbon generation dynamics experiment,combined with vitrinite reflectance and gas carbon isotope,and the main stages of oil and gas generation as well as the corresponding maturity range are predicted.Mean random vitrinite reflectance?%Ro?is a maturity indicator widely used by petroleum geochemists.Vitrinite releases volatile moieties?e.g.,H₂O,CO₂,oil components,and gaseous hydrocarbons?and subsequently becomes more aromatized and reflective with thermal stress level increasing.Based on this feature,vitrinite reflectance can be used as an indicator of organic matter thermal maturity.A number of vitrinite maturation models based on the thermal histories of sedimentary strata have been presented by previous researchers,such as TTI and EASY%Ro.Due to the differences of kerogen types,compositions and structures in different source rocks,the maturity ranges of the main oil generation stage?oil generation window?and gas generation stage are also different.It is important for oil and gas exploration to determine the maturity range of the oil generation window and main gas generation stages of source rocks in the petroliferous basin.In this study,confined pyrolysis experiments?gold capsules?were performed on six coals from Jurassic strata in the southern margin of Junggar Basin.Vitrinite reflectances?%Ro?were measured on coal samples prior to and after heating.On the basis of the measured%Ro and calculated EASY%Ro values,the following observations can be obtained:?1?in addition to temperature and heating time,Rock-Eval hydrogen index?HI?and heating rate significantly influenced the measured vitrinite reflectance.Coals with relatively higher initial hydrogen indices have relatively lower measured%Ro at same experimental conditions than those with relatively lower initial hydrogen indices,and hydrogen-rich vitrinites demonstrated a retardation effect in measured%Ro;?2?for all six coals,the measured%Ro values are lower than the calculated EASY%Ro values at EASY%Ro>1.5.At the same maturity?EASY%Ro values?,the measured%Ro values from experiments at higher heating rate?20°C/h?are lower than at lower heating rate?2°C/h?,and the difference between the calculated EASY%Ro and measured%Ro is greater for experiments at higher heating rate?20°C/h?than at lower heating rate?2°C/h?,The increase of vitrinite reflectance for experiments at higher heating rate was delayed;?3?Relationships between the transformation ratios for oil and gas generation from source rocks?kerogens?and EASY%Ro values can be well established using confined pyrolysis method,which can be applied to constrain the maturity range for the main oil and gas generation stages of source rocks under the geological conditions.Hydrocarbon gases in most giant and medium gas fields discovered in China are dominantly derived from coal source rocks.However,the potentials for oil and gas generations and the maturity range for major gas generation stage remain poorly understood.In the present study,confined pyrolysis experiments were performed on ten coals and five source rocks?kerogens?within the Jurassic strata from the Junggar Basin.The main results are listed as follows:?1?for these coaly source rocks,there is no clear relationship between Rock-Eval HI values and the maximum oil yields?oil potentials?in confined pyrolysis experiments;?2?under the geological condition at heating rate of5°C/My,the transformation ratios for gas generation for J1bA?J1bB?J2xB coal samples and JSB source rocks range from 21%to 29%at EASY%Ro 1.50,indicating that the main gas generation stage for these Jurassic coaly source rocks occur at post mature stage?EASY%Ro>1.50?;?3?oil yields were mainly controlled by initial oil generation potentials while gas yields were controlled by maturities for these coaly source rocks.Oil potentials of coal beds in Sikeshu sag in the western part of the southern margin are higher than those in the other areas of the south margin.The coal beds have higher maturities and gas potentials at Huo-Ma-Tu anticline belt in the middle part than those in the other areas of the southern margin.The carbon isotopic compositions of gases from coals are closely related to their precursor compositions and maturities.Carbon isotopes were measured on gas products from pyrolysis experiments on four coals and five kerogens.The main observations are listed as follows:?1?From diagrams of ln?C₂/C₃?versus ln?C₁/C₂?and?¹³C₁–?¹³C₂versus ln?C₁/C₂?,gas generation from coaly source rock can be divided into four stages.At the first stage from EASY%Ro 0.66–0.95 and temperature 323–372°C at heating rate 2°C/h,gaseous hydrocarbons generated mainly from primary cracking of kerogen and their yields decrease in the sequence C₃>C₂>C₁;At the second stage from EASY%Ro 1.06–1.67 and temperature 383–431°C at heating rate 2°C/h,gaseous hydrocarbons generated from both kerogen primary cracking and oil secondary cracking;At the third stage from EASY%Ro 1.86–3.20 and temperature 443–515°C at heating rate 2°C/h,wet gases cracked and the yields of methane and ethane increase significantly;At the fourth stage from EASY%Ro 3.44–4.44 and temperature 527°C–600°C at heating rate 2°C/h,wet gas finally cracked to methane and methane also partly generated from residual solid kerogen.?2?It needs to consider both thermal maturity and kerogen type for determining the origin of natural gas by using diagrams of?¹³C₂–?¹³C₃ versus C₂/C₃ and?¹³C₂–?¹³C₃ versus?¹³C₁.