Petroleum Formation and Tracing in the Paleozoic Marine strata-Isotope System
Wenhui Liu, Jie Wang, Tenger, and Cheng Tao
Sinopec Petroleum Exploration & Production Research Institute, Beijing, China
In recent years, Paleozoic marine petroleum exploration has made great progress in China, which shows a huge exploration potential. But there are multiple source rocks developed in marine strata, which are old, big span of time and space, complex original organic matter and accumulation mechanism, and such these characters of source rocks make it difficult to trace the marine petroleum formation and accumulation and to research petroleum and source rocks correlation. Besides the normal hydrocarbon source rocks, many types of hydrocarbon source including collective (e.g. paleo-reservoir, bitumen) and dissipated soluble organic matter can make the gas-producing process extend to higher evolution phase and prove to be important in the high-over mature stage. So it is key point to identify the marine material characteristics and the time nodes evolution. In the tracing system of the petroleum formation and accumulation, dating technology mainly solves source rock deposition, petroleum formation and accumulation time. For defining the key time, besides the geologic method, the radioactive isotope characteristics possessing the dating effect is a main way. By means of the containing potassium clay minerals growth-inhibited and the K-40Ar age clock starting caused by petroleum charging in the clastic reservoir, the filling time of petroleum can be determined. The geological time clock begins to start after the helium four entering into closure system in minerals with uranium and thorium, indicating the uplift process of petroliferous basin and limit the petroleum accumulation time. The composition characteristics of helium and argon isotopes are common tracing and dating indicators. According to the formation means and the effect of age accumulation of argon isotopes in sedimentary source rocks, the dating model equation for the gas-source rock ages based on mathematical statistics methods is presented and is suitable for the age estimation of the clastic source rocks. Because the potassium contents are low in carbonate strata and regenerative hydrocarbon sources in marine strata, the 40Ar dating equation of mathematical model with multi-parameter for the marine source rocks by means of the relationship among the 40Ar content in the gas, the radioactive 40K content in source rocks and the physical parameters of the reservoirs are initially established and used well in the age estimation of the gas source rocks in Weiyuan Sinian gas filed. At the same time, on the basis of the effect of age accumulation on noble gas and the gas reservoir preservation mechanism, the 4He age estimation model of the accumulation of marine gas reservoir being preliminarily established provides a new indicator to identify the dating of gas accumulation. The isotope dating systems of tracing metals (e.g. U-Pb, Rb-Sr, Sm-Nd and Re-Os) in bitumen, kerogen and oil are also important methods to obtain indirectly the time of petroleum formation, injection and reformation. The material compositions in the oil-source correlation being the component, type and state of the direct hydrocarbon material are mainly used to trace and correlate the oil and sources. Isotope composition is a principal way to identify the material composition, besides the isotopes of main organic elements (e.g. C, H, O, N, S) and there are noble gas isotopes coexistent with petroleum. According to the effect of age accumulation on radioactive isotope and the inheritance effect on deep mantle noble gas, helium and argon isotope composition characteristics in the gas reservoir can trace the crust derived and mantle derived matter, and estimate the mantle-crust mix ratio in the natural gas. It plays an important role in the gas genetic identification, gas-source correlation and oil-source correlation of oil reservoirs in ancient hidden mountains. Carbon isotope compositions of the hydrocarbon materials usually appear a certain inheritance of parent material and the thermodynamics fractionation effect for different kinds of hydrocarbon source. The authors started with material source, key time and accumulation process, and have systematically studied the material change, process tracing and time estimation during the period of hydrocarbon formation and accumulation for the different hydrocarbon sources with the help of the composition characters of the stable isotope and the noble gas isotope in different hydrocarbon sources and petroleum. So the isotope tracing systems for petroleum formation and accumulation in marine strata have been developed in China.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013