Characterization of Products from Chloroform-Extracted Triassic Chang 7 Oil Shale Generated by Semiopen Pyrolysis

Abstract

A semiopen hydrocarbon generation thermal simulation experiment was conducted on the Chang 7 shale after chloroform extraction. The characteristics of expelled, retained, and total generated oils were analyzed. The influence of the free hydrocarbons (S1) was ruled out after extraction, and the kerogen hydrocarbons (S2) dropped nearly 92.5%, which is the main source of oil generation. The oil generation of the Chang 7 shale was divided into three stages: 300–320 °C is the slow hydrocarbon generation stage, which is mainly the initial cracking of kerogen, and the amounts of expelled and retained oils are small; 320–360 °C is the rapid hydrocarbon generation stage, which is mainly the second cracking of kerogen, resulting in a rapid increase in expelled, retained, and total generated oils; and 360–420 °C is mainly the cracking of the retained oil generated in the early stage. The cumulative total generated oil before 360 °C is larger than that generated after 360 °C, which suggests that a large amount of oil was generated before 360 °C; however, this part of the oil is mainly the retained oil. At this stage, as the pyrolysis temperature continues to rise, C14+ in the retained oil generated in the early stage begins to crack. C6–14 and C14+ have a correlation similar to the “zero-sum game”, suggesting that there is an inherent transformational relationship between them. A semiopen hydrocarbon generation thermal simulation experiment was first conducted on the chloroform extraction of the Chang 7 shale. This study confirms that the increase in pyrolysis temperature (>360 °C) is beneficial to generating more expelled oil, which has practical implications for the in situ conversion of shale oil.

Comparison of Geochemical Characteristics of Products from Thermal Simulation Experiments of Hydrocarbon Generation at Different Heating Rates in the Semiclosed System

“…The ratio of expelled oil to total generated oil at 5 °C/h is always larger than that at 20 °C/h after 340 °C (0.68 % R o ), which indicates that a slow heating rate is more conducive to the generation of expelled oil. Total expelled hydrocarbon refers to the sum of expelled hydrocarbon gas (C 1 –C 5 ) and expelled oil. ,,, The ratio of total expelled hydrocarbons to total generated hydrocarbons also has a similar variation law (Figure c). The ratio of the two is the same at 340 °C (0.68 % R o ), both of which are 0.35.…”

“…For 5 °C/h, the net increased yield of total generated oil is positive in each temperature interval before 380 °C (0.96 % R o ), which suggests that the total oil production is generated in each temperature interval before 380 °C (0.96 % R o ). ,, For 20 °C/h, this temperature is delayed to 400 °C (1.19 % R o ). For 5 °C/h, the net increased yield of C 15+ and C 6+ is negative after 380 °C (0.96 % R o ), which indicates that C 15+ undergoes thermal cracking after 380 °C (0.96 % R o ). , For 20 °C/h, the net increased yield of C 15+ and C 6+ is negative after 400 °C (1.19 % R o ), which indicates that there is a retardation phenomenon under the condition of the fast heating rate.…”

“…The yield of hydrocarbon gas gradually decreases with the increase of carbon number (C 1 → C 5 ) (Figure ). This is because the activation energy of hydrocarbon gas with high carbon number is smaller than that of low carbon gas number of hydrocarbon gas. ,, With the increase of temperature, hydrocarbon gas with high carbon number is gradually converted into hydrocarbon gas with low carbon number.…”

“…The total generated oil includes two parts: expelled oil and retained oil. ,, Taking 5 °C/h as an example, the maximum yield of C 6 –C 14 appears at 420 °C (1.47 % R o ), which is 14.92 mg/g. The maximum yield of C 15+ appears at 380 °C (0.96 % R o ), which is 46.73 mg/g.…”

“…The Ordos Basin is the second largest sedimentary basin in China with an area of about 25 × 10 4 km 2 , of which the Chang 7 Member of the Upper Triassic Yanchang Formation has an area of 10 × 10 4 km 2 . − The Ordos Basin is located in the southwest of the North China Craton. During the evolution of the basin, it experienced several major uplifts, such as the late Caledonian tectonic movement, the late Indo-China tectonic movement, and the late Yanshan tectonic movement.…”