The Gas Generation Potential And Processes Of High Mature Hydrocarbon Source In Deep Basin

The oil/gas generation and accumulation in deep and ultra-deep reservoir have attracted geologists’attention, in either theoretical investigation or oil/gas exploration. Two questions need to be answered before deep oil/gas exploration:(a) What is the generation source of oil/gas reservoir in deep basin (b) How to evaluate the heat stability of oil reservoir? In order to answer these questions, this research investigated the gas generation potential and processes of deep kerogen and liquid hydrocarbon (e.g., residual bitumen and crude oil). Firstly, the artificially matured kerogen structures were represented by different degradation methods to study the interaction between bitumen and kerogen and to evaluate its effects on the hydrocarbon generation processes of kerogen in high to over mature stage. Secondly, the gas generation contribution of kerogen and residual bitumen was quantitatively evaluated to reveal the hydrocarbon generation properties and potential. Finally, we compared the oil-cracking products under low, medium and high pressure to reveal the effects of pressure on the heat stability of deep oil/gas resource. We have achieved the following conclusions:(1) Even-odd carbon predominance of n-alkanes in the medium carbon number range was presented in the chloroform bitumen’A’and HyPy products of medium-high mature lacustrine/marine source rocks. The magnitude of even predominance increases with increasing maturity. The mono-saturated fatty acid in ruthenium ion oxidation degradation products also presented even predominance (n-C16and n-C18) in high mature stage. It is speculated that this abnormal even predominance can be attributed to the interaction between bitumen and kerogen (the regroup of aromatization and polycondensation). The interaction controls the evolution of kerogen in high to over mature stage, which makes the kerogen hydrogenated and activated again to improve the gas generation potential of kerogen in high mature stage. (2) The pyrolysis simulation demonstrated that the residual bitumen in high mature stage can be effective gas generation source for organic-rich shales. The gas generation of bitumen cracking is characterized by high moist and12C-rich methane. The mass balance calculation of hydrogen indicated that the residual bitumen in3.29%TOC lacustrine shale and6.27%TOC marine shale in high to over mature stage presents gas generation contribution of34-79m3/t TOC and16-32m3/t TOC, accounting for30%and20%of the total gas generation, respectively.(3) Compared with low pressure, medium-high pressure suppresses the oil-cracking gas generation; but the suppression shows variation in stages with different maturity. In low mature stage, the degree of suppression increases with increasing pressure; in high mature stage, the increasing pressure suppresses gas generation in low pressure domain, promotes gas generation in medium pressure domain, and slightly suppresses gas generation again in high pressure domain; in medium mature stage, the effect of pressure on gas generation is in between. With increasing pressure, methane carbon isotope presents abundance of13C, with fractionation up to2-3%o, whereas the carbon isotope fractionation of ethane and propane is smaller than1‰.(4) Compared with low pressure, the generation of light-hydrocarbon has been suppressed under medium-high pressure. However, the suppression is uneven through the medium-high pressure range; light-hydrocarbon with different structures present different behaviors reacting to the pressure changes. The increase of pressure also affects the indices of light-hydrocarbon (e.g., paraffin and aromaticity) to some extent, but does not present obvious effects on Mango index. Compared with low pressure, the isoparaffin and aromatic hydrocarbon in medium-high pressure show obvious abundance of13C, whereas cycloparaffin shows no carbon isotope fractionation and therefore can be used as indicatives for the oil/gas generation of deep oil/gas resource in high-pressure to overpressure basins.In summary, the interaction between bitumen and kerogen plays an important control on the hydrocarbon generation processes, which indicates that there is still hydrocarbon generation potential for high-over mature source rock. The geochemical experiments in this research demonstrated that the contribution of bitumen can be up to30%of total gas generation. Therefore, the gas generation potential of residual bitumen should be paid enough attention in the hydrocarbon exploration in high-over mature shales.