Suping Yao

Episodic petroleum fluid migration in fault zones of the northwestern Junggar Basin (northwest China): Evidence from hydrocarbon-bearing zoned calcite cement

Hydrocarbon-bearing zoned calcite cements occur widely in Jurassic–Cretaceous fault-zone cores and sandstone outcrops of the northwestern Junggar Basin (northwest China). Hydrocarbon-bearing bands alternate with nearly hydrocarbon-free bands at a micron scale. Analytical results from biomarker organic geochemistry, Fourier transform infrared microspectroscopy, and trace-element geochemistry on these zoned cements suggest that at least three different types of fluids have participated in their formation. The first fluid type is probably primary, unmodified lacustrine formation water, from which the hydrocarbon-poor bands are formed and are characterized by Mg-rich calcite. The other two types of fluids include basinal fluids (e.g., hot hydrocarbon-bearing fluids) and meteoric water. The hydrocarbon-rich bands in which the hydrocarbons have been biodegraded and the Mn content is relatively high suggest a mixture of hydrocarbon-bearing basinal fluid and meteoric water. The alternating growth of hydrocarbon-bearing and hydrocarbon-free bands of calcite cements implies that the cement formation is episodic; it is related to alternating episodes of mixed petroleum-bearing fluid and unmodified primary formation waters, respectively. The fault appears to have been a mixing zone where seismic pumping during the movement of associated regional faults occurred. Thus, in the northwestern Junggar Basin, the micron-scale hydrocarbon-bearing zoned structure of the calcite cements is likely a reflection of episodic petroleum fluid migration in fault zones.

Evolution of coal structures: FTIR analyses of experimental simulations and naturally matured coals in the Ordos Basin, China

A series of coals of varying rank, from brown coal to anthracite, were studied by Fourier transform infrared spectroscopy (FTIR). Curve-fitting analysis was employed to characterize coal structural evolution during the coalification process. The study was carried out on samples of a natural evolutionary series as well as experimental simulation coals dispersed on KBr pellets throughout the Ordos Basin, China. The results showed that the infrared spectrum of coal allowed quality and even quantity identification of the degree of coalification. Oxygen-containing groups and alkyl side chains of coal cracked at different rates with increasing degree of coalification. The cracking rates were divided into three stages according to the main changes in coal structure. These stages were carboxyl groups, fat groups and aromatic rings. Carboxyl groups decreased when Ro was less than 0.5%, and these groups were maintained until the fat coal stage began. Fat groups mainly cracked at the asphaltization stage and formed abundant hydrocarbons. These groups were the main sources for the formation of immature and low maturity, coal-generating oil before asphaltization. In the high evolutionary stage, after asphaltization, all alkyl side chains in the coal had cracked, while the degree of condensed aromatic rings had increased greatly. The ratios of aromatic hydrocarbons (3000 to 3100 cm−1) to aliphatic carbons (2800 to 3000 cm−1), CH2/CH3 (2920/2950 cm−1) and carboxyl groups to aromatic carbons (1705/1620 cm−1 + 1600cm−1 + 1580 cm−1) appeared to be suitable parameters for assessing the natural maturation of coal.

Hydrocarbon potential of the Lower Cretaceous mudstones in coastal southeastern China

The Cretaceous hosts the highest hydrocarbon potential among all geological stratigraphic sequences worldwide, and the Lower Cretaceous in coastal southeastern China, with the development of black mudstones and shales, has potential for hydrocarbon generation. This potential, therefore, is of significance to regional hydrocarbon exploration both onshore and offshore; however, it has not been investigated in detail. Here, we address this issue based on the results of integrated organic geochemistry and petrology of four representative outcrop sections in the region. Results show that there are two possible source rock sequences in the Lower Cretaceous of coastal southeastern China, that is, a lower sequence represented by the Bantou Formation black mudstones in the Fujian Province and an upper interval represented by the Guantou Formation black mudstones or calcareous mudstones in the Zhejiang Province. The organic matter abundance of these two formations reaches levels of poor to good source rocks. The kerogen is dominated by type III, whereas type II can also be found locally in the northwestern Fujian Province. Most samples are highly mature to overmature, except for the samples with less influence of volcanism in the northern Guangdong Province. Comparatively, the organic matter abundance of the early-stage source rocks is higher than for the late-stage rocks, and the kerogen type is less favorable; however, the maturity shows little correlation with age and is largely related to volcanism. Therefore, the Lower Cretaceous in coastal southeastern China seems to have hydrocarbon potential (gas in particular) in local areas, including the northern Guangdong Province and its analog in offshore basins for the early stage and the northwestern Fujian Province and its analog in offshore basins for the late stage. These results and understanding provide insights for expanding new hydrocarbon exploration targets in southeastern China both onshore and offshore and may also have implications for understanding the regional Cretaceous geology.