Abstract
The potential of microbial carbonate rocks as hydrocarbon source rocks still remains uncertain. To evaluate the hydrocarbon generation and expulsion capability of microbial carbonate rocks, stromatolite obtained from the Xiaganchaigou formation of the western Qaidam Basin has been used for thermocompression experiments to simulate hydrocarbon generation and expulsion at temperature of 280 to 380°C. The results show that the generation potentials of total oil, residual oil, expelled oil, and hydrocarbon gases for stromatolite are 143.39 to 379.99 mg/g total organic carbon (TOC), 47.08 to 223.92 mg/gTOC, 71.12 to 170.81 mg/gTOC, and 6.60 to 101.74 mg/gTOC, respectively. A three-stage model of hydrocarbon generation and expulsion evolution has been constructed for stromatolite. The results show that the first stage is characterized by relatively slow oil generation with associated hydrocarbon gases, having limited capability of oil expulsion in the temperature range of 280 to 360°C (0.50% ≤ Ro ≤ 0.70%). The second stage is characterized by rapid oil generation with associated hydrocarbon gases, having incremental capability of oil expulsion in the temperature range of 320 to 360°C (0.70% ≤ Ro ≤ 1.15%). The third stage is characterized by hydrocarbon gas generation with associated oil, having efficient oil expulsion capability in the temperature range of 360 to 380°C (1.15% ≤ Ro ≤ 1.45%). The experimental data of argillaceous dolostone, gray mudstone, and marlstone, which were reported with the same hydrocarbon generation experimental method, was compared with the data for stromatolite. Compared with traditional source rocks, the oil generation potential of stromatolite is less than that of argillaceous dolostone and gray mudstone but greater than that that of marlstone in the same amount of organic carbon. Especially, the hydrocarbon gas generation potential of stromatolite is approximately double than that of traditional source rocks. Therefore, the experiments indicate that the microbial carbonate rocks in the western Qaidam Basin could be effective source rocks.
Highlights
The Qaidam Basin is a large continental Meso-Cenozoic intermountain petroliferous basin in the northeastern QinghaiTibetan Plateau
The second stage is characterized by rapid oil generation with associated hydrocarbon gases, having incremental capability of oil expulsion when temperature is in the range of 320–360°C (0.70% ≤ Ro ≤ 1.15%)
The third stage is characterized by hydrocarbon gas generation with associated oil, having efficient oil expulsion capability when the temperature is in the range of 360–380°C (1.15% ≤ Ro ≤ 1.45%)
Summary
The Qaidam Basin is a large continental Meso-Cenozoic intermountain petroliferous basin in the northeastern QinghaiTibetan Plateau. A few samples with high organic matter abundance have been found (Zhang et al, 2017) This is not consistent with the discovered source rocks and high abundance of oil and gas reserves in the exploration of the western Qaidam Basin. With the same simulation experiment method, the hydrocarbon generation potential (GP) and expulsion efficiency of lacustrine argillaceous dolostone (Li et al, 2018), lacustrine gray mudstone (Ma et al, 2012), and marine marlstone (Fu et al, 2017) were previously simulated, and those results were compared with the results of stromatolite An understanding about these issues is of greatcommercial interest and can lead to improvement in prediction of petroleum reservoirs in microbial carbonate rocks
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