KINETICS OF HYDROCARBON GAS GENERATION FROM MARINE KEROGEN AND OIL: IMPLICATIONS FOR THE ORIGIN OF NATURAL GASES IN THE HETIANHE GASFIELD, TARIM BASIN, NW CHINA

Yunpeng Wang*+, Zhaoyun Wang**, Changyi Zhao**, Hongjun Wang**, Jinzhong Liu*, Jialan Lu* and Dehan Liu*

*State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China.

**Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China.

+Corresponding author, email: wangyp@gig.ac.cn

In this paper we derive kinetic parameters for the generation of gaseous hydrocarbons (C1-5) and methane (C1) from closed-system laboratory pyrolysis of selected samples of marine kerogen and oil from the SW Tarim Basin. The activation energy distributions for the generation of both C1-5 (Ea = 59-72 kcal, A = 1.01014 s-1) and C1 (Ea = 61-78 kcal, A = 6.061014 s-1) hydrocarbons from the marine oil are narrower than those for the generation of these hydrocarbons from marine kerogen (Ea = 50-74kcal, A = 1.01014 s-1 for C1-5; and Ea = 48-72kcal, A=3.91013 s-1 for C1, respectively). Using these kinetic parameters, both the yields and timings of C1-5 and C1 hydrocarbons generated from Cambrian source rocks and from in-reservoir cracking of oil in Ordovician strata were predicted for selected wells along a north-south profile in the SW of the basin. Thermodynamic conditions for the cracking of oil and kerogen were modelled within the context of the geological framework. It is suggested that marine kerogen began to crack at temperatures of around 120oC (or 0.8 %Ro) and entered the gas window at 138oC (or 1.05 %Ro); whereas the marine oil began to crack at about 140 oC (or 1.1 %Ro) and entered the gas window at 158 oC (or 1.6%Ro).

The main geological controls identified for gas accumulations in the Bachu Arch (Southwest Depression, SW Tarim Basin) include the remaining gas potential following Caledonian uplift; oil trapping and preservation in basal Ordovician strata; the extent of breaching of Ordovician reservoirs; and whether reservoir burial depths are sufficiently deep for oil cracking to have occurred.

In the Maigaiti Slope and Southwest Depression, the timing of gas generation was later than that in the Bachu Arch, with much higher yields and generation rates, and hence better prospects for gas exploration. It appears from the gas generation kinetics that the primary source for the gases in the Hetianhe gasfield was the Southwest Depression.

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