Geochemical Characteristics,Hydrocarbon Potential,and Depositional Environment Evolution of Fine-Grained Mixed Source Rocks in the Permian Lucaogou Formation,Jimusaer Sag,Junggar Basin

摘要

The Permian Lucaogou Formation (P(2)l) in the Jimusaer Sag, Junggar Basin is an essential source rock and a tight oil-producing layer in China. The source rock of the Lucaogou Formation is characterized by mixed sedimentation of siliciclastics, carbonates, and tuffaceous materials, and the formation of mixed source rocks is different from that of conventional source rocks (e.g., mudstones or carbonates). Our petrological observation and X-ray diffraction mineralogical analysis suggest that the mixed source rocks are divided into three types: terrigenous-dominated mixed rocks (Td-mixed rocks), carbonate-dominated mixed rocks (Cd-mixed rocks), and tuffaceous-dominated mixed rocks (Tuf-mixed rocks). Compared with the upper section, more Td- and Tuf-mixed rocks, as well as less Cd-mixed rocks, developed in the lower section. Integrated organic geochemical and molecular geochemical analyses indicate that the Lucaogou mixed source rock is abundant in organic matter (OM) (TOC = 3.12 wt %), which is at the thermally low mature to mature stage. The sources of OM mainly originate from the mixture of planktonic organisms, bacteria, and terrigenous higher plants. Comparatively, some geochemical parameters (e.g., TOC, S-1 + S-2, and hydrogen index) of the upper section of P(2)l show slightly higher values than those of the lower section both before and after extraction, indicating a higher hydrocarbon generation potential of the source rocks in the upper section. Besides, more finely disseminated bacterial and planktonic organics are present in the upper section, whereas the lower section is more enriched in terrestrial OM. The mixed source rocks of the lower section were deposited in a deeper, anoxic-dysoxic, and brackish environment, whereas those of the upper section were deposited in a shallow, suboxic, saline, and evaporative environment. Moreover, we found that the lacustrine basin evolved from a more balanced-filled scenario with moderate productivity during the lower section deposition to an under-filled lake basin with higher productivity during the upper section deposition, in response to the climatic change from warm humid to hot semiarid conditions. Accordingly, the corresponding depositional models of mixed source rocks in the upper and lower sections of the Lucaogou Formation are proposed. Overall, the mixed rocks of the Lucaogou Formation can be regarded as good source rocks, which could offer favorable targets and resource potential for tight oil exploration.