Hydrocarbon Generation From Kerogen Pyrolysis And The Constraint On Timing Of Oil And Natural Gas Filling In Pearl River Mouth Basin

Pearl River Mouth Basin is the largest middle Cenozoic basin in the continental margin of south China sea which is abundant in petroleum and natural gas deposits. Natural gas and condensate oil in Panyu low massif and north slope of Baiyun sag were mainly derived from late Oligocene Enping fluvial-lacustrine facies coal measures as source rocks. The multi-stage oil and gas generation and migration in Zhu3depression make oil and gas exploration more difficult. In order to evaluate the Enping source rocks hydrocarbon potential to guide the exploration activities, it is necessary to develop new method and set up a new identification and evaluation criterion for evaluating quantity of the source rocks.In this paper, a series of kerogen pyrolysis in gold tube reactors were conducted with the kerogen (PY) concentrated from Enping Formations, an immature carboniferous mudstone in the Panyu Low Massif and North Slope of Baiyun Sag in Pearl River Mouth Basin, China. For comparison purpose, the isolated kerogens from Green River shale (GR) and Woodford Shale (WF) representing typical type I and II kerogens were employed as well. The organic inclusion of Wengchang A、B sub-depressions were applied to study the hydrocarbon source and determine the timing of oil and natural gas charging in Zhu3#depression. Multiple analyses include type, distribution, fluorescence and homogenization temperature of fluid inclusions were used. The following main achievements were made in this study:1. The total yield of hydrocarbons is low from PY kerogen pyrolysis, and the yield of gaseous hydrocarbons is1.5times to liquid hydrocarbons. In contrast, the total yields of hydrocarbons from GR and WF kerogen pyrolysis are apparently great, and are dominated by liquid hydrocarbons. This indicates PY kerogen thermal decomposition is favorable to natural gas formation.2. The gas to oil ratio (GOR) is controlled by both of organic matter thermal maturity and kerogen type in the thermal conversion of kerogen to oil and gas, yet the ratio of gas and light hydrocarbons is mainly affected by thermal maturation.3. The fraction of liquid hydrocarbon components is related to organic matter type and the levels of thermal stress.The saturate hydrocarbon yield remain essentially constant at low levels of thermal stress (Ro≤1.2%) from NSO’s cracking and decrease at higher levels of thermal stress(Ro>1.2%) corresponding to an increase in the aromatic hydrocarbon yields.4. The carbon isotopic compositions of residual kerogens and alsphalt fraction are almost identical, and less than0.3‰δ13C enrichment to original value of kerogen is observed from0.7%to2.7%Ro during thermal maturation. This suggests that813C of residual kerogens or alsphaltane could be used as an indicator to the reconstruction of organic matter type for high-mature source rock.5. The oil and natural gas bearing hydro-thermal fluids have been charged twice into the sandstone reservoirs in Zhu3" depression in the Neogene reservoir. The first charged stage is characterized by immature oil with yellow-light brown fluorescent light, its hydrocarbon fluid inclusions are dominated by light brown liquid hydrocarbon inclusions. The crude oil was mainly derived from Wenchang source rocks based on the homogenization temperature (Th) of coeval aqueous inclusions ranging from130℃to142℃. The timing of oil and nature gas charging was estimated at early Miocene. The second charged stage is characterized by mature natural gas with blue-white fluorescence, and their hydrocarbon fluid inclusions are dominated by black gas hydrocarbon inclusions and brown liquid hydrocarbon inclusions. The natural gas was mainly derived from Enping source rocks based on the Th, of coeval aqueous inclusions ranging from148℃to155℃, which is about15℃higher than the1st charged stage. And the estimated charging time was late Miocene to Quaternary.