Research On The Hydrocarbon Generation Characteristics,Mechanism And Deadline Of Coaly Source Rocks

Coaly source rocks have widely distributions in the sedimentary basins in China, and Coal-derived gas occupies an important position in the natural gas resources. Currently, different researchers had studied the characters of hydrocarbon generation by detailed geological statistics and simulation experiments, and established hydrocarbon generation models of various organic matters. However, due to the limitations of the samples and experimental technologies in early studies, it remains controversial about the gas generation potential and deadline of coaly source rocks, meanwhile, the gas generation mechanism is still uncertain. Hence, this paper collected numerous samples of different types and with various maturity, and studied the hydrocarbon generation characteristics and deadline of coaly source rocks by gold-tube pyrolysis experiments. By detailed quantification of yields of the liquid and gas products generated during the pyrolysis experiments, this paper addressed the difference of the oil and gas generation characteristics between coal, coaly mudstone and carbon mudstone as well as lacustrine organic matter. Meanwhile, this article discussed the mechanism and deadline of gas generation from coal by elemental and Nuclear Magnetic Resonance (NMR) analysis of geological samples with various maturity and solid residues during pyrolysis involving low mature coal. Moreover, the maximum gas yield of coal was reached according to stepwise-pyrolysis method and theoretical calculation collectively. Based on these works, this paper proposed a "double-increasing" hydrocarbon generation model of coal in geological condition, which can provide a credible foundation for the evaluation of gas potential in typical coal-bearing basins. The major conclusions of this paper include following three aspects:(1) The results of the simulated experiments on the whole process of hydrocarbon generation from three coaly source rocks (coal, coaly mudstone, and carbon mudstone) in Jurassic formation of the same region in Ordos Basin indicated the oil generation characters of different coaly source rocks during pyrolysis differs from each other; The oil generation potential was evidently lower relative to typical lacustrine or marine source rocks. The maximum yields of oil derived from three coaly source rocks all did not exceed20mg/g.TOC, wherein that of coaly mudstone was19.69mg/g.TOC, which was almost two times of coal and carbon mudstone. This result is consistent with the result from elemental analysis that coaly mudstone has a higher H content. Distinctions exist in the pyrolysis temperatures corresponding to the oil generation summit of coaly mudstone, coal rock, and carbon mudstone, and that of the latter one is lower which is about350-380℃. The simulated results above indicate that the most important contributor of coal-derived oil is coaly mudstone. There is little differences in the maximum gas volume quantities of coaly mudstone, carbon mudstone and coal, which are300.5,297.2,304.1ml/g.TOC respectively. Since the original TOCs of the first two ones are usually much lower than that of coal rock, the quantity of the gas generated by per unit quantity of coal rock could be dozens of times of those of the first two source rocks. The quantities of the total gases generated by the heating up pyrolysis of these three coaly source rocks are184.6,211.9and195.9ml/g.TOC respectively. The foregoing results have proved that coaly source rocks mainly produce gas and coal is the major contributor for coal derived gases, while the gas generation potential of carbon mudstones should not be neglected, either. Although these three different coaly source rocks are originated from the same region and their organic matter has similar depositing environments, there are significant distinctions in the features of the stable C isotope of the hydrocarbon gases generated by these three kinds of coaly source rocks. The stable C isotopes of the methane yielded by the pyrolysis of carbon mudstone and coal are obviously heavier than that of coaly mudstone, while the carbon isotope of the methane generated by coaly mudstone is3-5%o lighter than that by carbon mudstone. Identically, the distribution of the carbon isotope of ethane has the following characters: δ13C2(carbon mudstone)> δ13C2(coal rock)> δ13C2(coaly source rock).(2) The results of the simulated experiments on the whole process of hydrocarbon generation of the heating-up pyrolysis based on geographical samples (coal samples) of different mature degrees have found that the highly mature coal (Ro>2.5%) still has some gas generation potential, which is not the same as our common knowledge about the death boundary of gas generated by coals. The quantity of the gas generated when Ro is more than2.5%is about45ml/g. coal. It could be about20%of the total amount of the gas generated by coal. Some amount of natural gas (5.32ml/g.TOC) could still be generated when Ro is5.32%, which indicates that the mature boundary of the ending of coal derived gas generation is at least a Ro of5.32%.The gas generation time limit of coaly source rocks is significantly longer than that of lacustrine organic matter, and the quantity of the gas generated by the former one is also higher than that of the latter one which is only about100mg/g.TOC.(3) It could be seen through stepwise thermostatic pyrolysis and the theoretical calculation of lowly matured coal sample that the maximum quantity of the gas generated by coal is300-350ml/g.TOC, which is clearly higher than that generated through traditional heating-up pyrolysis. On this basis, we proposed a "double-increasing" model for gas generation from coal in geological conditions, in which the maturity boundary of deadline of the gas generation from coal could reach Ro of5.5%. Compared with former studies, this model have a significant implication for the evaluation of natural gas resources.