Controls of Amorphous Organic Matter on the Hydrocarbon Generation Potential of Lacustrine Shales: A Case Study on the Chang 7 Member of Yanchang Formation, Ordos Basin, North China

摘要

Organic matter in black shales is composed of oil-prone macerals, such as alginite and amorphous organic matter (AOM), and non-oil-prone macerals, including vitrinite and inertinite. Organic matter composition plays an important role in controlling the hydrocarbon generation potential of black shales. In this study, a total of 17 shales of the Chang 7 Member of Yanchang Formation, Ordos Basin, were selected from 11 drill cores to investigate the petrographic characteristics of organic matter and their influences on the hydrocarbon generation potential of shales. Point-counting analysis indicates that AOM is the dominant organic matter in the Chang 7 Member shales and two types of AOM were observed: sapropelic AOM and humic AOM. Organic matter in shales from the southeast region of the basin is dominated by sapropelic AOM, whereas that from the southwest region is mainly composed of humic AOM. Sapropelic AOM appears to be yellow to orange-brownish color under an optical microscope and has a small size (1–50 μm), whereas humic AOM is brown to black and has a relatively larger size, ranging from 20 to 200 μm. Rock-Eval pyrolysis reveals that the studied shales are at the peak oil-window maturity, with vitrinite reflectance (iR _(o)) equivalent ranging from 0.78 to 1.03%. Rock-Eval pyrolysis shows that organic matter in the studied samples is mainly type II kerogen, with some type III kerogen. Kerogen types can also be correlated to AOM content: high sapropelic AOM content is associated with type II kerogen, and high humic AOM content is associated with type III kerogen. S2 shows non-monotonic relationships with AOM content, of which S2 increases slightly with increasing sapropelic AOM content and decreases with increasing humic AOM content. In this study, a hypothetical evolutionary pathway of AOM formation from lacustrine phytoplankton and terrestrial higher plants to amorphous kerogen during early diagenesis was proposed, which has three zones from bottom-water zone to fermentation zone, with each zone having different microbial activities, depending on dissolved oxygen content. Sapropelic and humic AOM are suggested to be microbial degradation products of lacustrine phytoplankton and terrestrial higher plants by anaerobic bacteria in the fermentation zone, respectively. Significant differences in AOM type and content between shales from the southeast and southwest areas were noticed in this study, which could result from differences in organic matter input (phytoplankton vs terrestrial) and deposition environment between these two areas.