The DL deposit, located in the southwestern Songliao Basin, has a length of 9 km and a width of 200–1200 m. The deposit is hosted by the lower member of the Yaojia Formation. Late Paleozoic granitoids comprise the basement rocks in the study area. The basin is cut by NE–SW-trending faults. The primary reduced sandstones of the lower member of the Yaojia Formation are dominantly grayish in color, fine- to medium-grained, and contain Fe–Ti oxide, pyrite, and organic matter (OM). Most of these sandstones in the DL deposit experienced secondary oxidization, whereby pyrites and Fe–Ti oxides were modified to hematite and Ti oxides by oxidizing groundwaters, respectively, leading to a reddish and light yellow coloration. These oxidized sandstones developed mainly along NE–SW-striking zones that are >30 km in length and 1–30 km in width. The primary reduced sandstones also exhibit secondary reduced green and white zones. The secondary green zones may be related to reduction by oil, oilfield brines, and hydrothermal fluids derived from diabasic magmatism, which formed kaolinite and chlorite (Figure 5b). These zones are locally restricted to the diabase dikes and deep faults. The secondary white zones may be related to acidic fluids produced by anaerobic bacteria, which caused the development of kaolinite and illite. X-ray diffraction results show that the clay minerals in the sandstones of the white zones are mainly composed of kaolinite and illite, comprising 45–81 wt.% and 19–30 wt.%, respectively.

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References