80% of Shaanxi's territory is located in the Ordos Basin, in fact, in the 450 million years of the Ordovician Period, most of Shaanxi was a vast ocean. Did you find limestone in your hometown? Is there natural gas in your hometown?
First, the Ordos Basin
The Ordos Basin is a vast area, spanning the five provinces (regions) of Shaanxi, Gansu, Ningxia, Mongolia and Jin, and is the second largest sedimentary basin in China, with an area of about 37×104km2. The basin ranges from YinShan in the north, Qinling Mountains in the south, LiupanShan in the west, and Lüliang Mountain in the east, with a rectangular outline. According to the current tectonic form, it can be divided into six first-class structural units, including the Yimeng uplift, the Weibei uplift, the Tianhuan Depression, the western edge of the thrust zone, the Jinxi flexion zone and the northern Shaanxi slope (Figure 1).
Figure 1, Shaanxi's location in the Ordos Basin (Lu Ping, 2021)
The Ordos Basin is not only famous at home and abroad for its rich mineral resources such as oil, natural gas and coal, but also has a stable internal geological structure, the development of peripheral outcrops, and the complete outcrop of strata.
Figure 2, Field profile of the Ordos Basin
II. The Ordovician
The Ordovician Period, the name of the geological chronology, is the second paleozoic era (the emergence of primitive vertebrates), which began 480 million to 440 million years ago and lasted for 44 million years.
Figure 3, Fossil starfish from the Ordovician
The Ordovician climate was mild, shallow seas were widespread, many parts of the world (including most of China) were covered by shallow seawater, and marine organisms developed unprecedentedly and flourished more than the Cambrian. Fossils are most commonly found in trilobites, plitodycetes, brachiopods, echinoderms, and nautilus in molluscs.
Figure 4, Fossil Ordovician trilobites
What is Dental Stone? Scientists have been asking this question for nearly 140 years. Fossils of tartar are small and can only be studied under a microscope. Most of them are shaped like elongated cones, some look like pointed rakes or combs, others like jagged rods or uneven blades, and some even like leaves. Are they tiny animal shells or slightly larger animal teeth?
Figure 5, Ordovician tooth-shaped stone in the section of Baoji Qishan Mountain (Dai Feng, 2020)
Iii. The paleoenvironment of the Ordovician period in Shaanxi
The Ordovicians in North China are mainly carbonate deposits, the Lower Ordovician is fully developed and the lithofacy is stable; the Middle and Upper Ordovicians are incompletely developed, and there are only deposits in a few areas. The Ordovician ordovician basin has three carbonate terrace sedimentary patterns: the western part of the basin is the narrow landshed carbonate terrace sedimentary pattern, the southern part of the basin is the wide landshed carbonate terrace sedimentary pattern, and the central and eastern part of the basin is a closed-semi-closed lagoon sedimentary pattern (Figure 6).
Figure 6, Ordovician paleoenvironment map of Shaanxi
The Ordos Basin is located in the southwest of the North China Plot, which has long been at the intersection of the North China Sea and the Qilian Sea, so the evolutionary relationship between the two seas has been a major problem in the study for many years. The Ordos block was once again in an uplift state in the Early Ordovician period, and the narrow Liangjia Mountain Formation distributed on the eastern and southern edges indicates that the entire Ordos mass has undergone a northwest-tonal uplift and southeast-like tilting movement at the same time as the overall uplift.
Figure 7, Ordovician Paleoenvironment in Shaanxi (Zheng Xuping, 2016)
In the early Ordovician period, the Ordos paleolude ran through the north and south of the basin, and only the Yeli and Liangjia mountain period sediments developed in the southeast edge of the paleolust and the western Helan Mountain area.
Figure 8: 460 million years of Enshi limestone in Hubei Province
In the Middle Ordovician period, in the Ordos region, due to the multi-stage sea intrusion and the change of paleoclimate, the paleogeographic features of the lithofacy underwent 6 large-scale evolutions. Among them, Ma I, Ma II, Ma IV and Ma Vi are sea advances, Ma San and Ma V are sea retreats, the western part of the basin is dominated by open seas, the central and eastern parts are dominated by Yunping, and the ointment deposits are mainly located in northern Shaanxi.
Figure 9, Ordovician Majiagou Formation limestone in Sulige gas field (Xie Kang, 2020)
In the early Majiagou period of the Middle Ordovician period, seawater once again invaded the Ordos region. The central and eastern parts of the Majiagou Phase II seawater rise basin caused the first strong sea level rise in the Lower Majiagou Period. In the third phase of Majiagou, the general trend of sea level decreased slightly, but a vast land shed evaporation basin environment developed on the basis of the lateral depression of the central and eastern sides of the basin.
Figure 10, Sedimentary pattern of The Ordovician Majiagou period in Yulin Daniudi gas field (Fu Jinhua, 2017)
In the middle and late Period of Majiagou of the Middle Ordovician Period, it was the largest sea invasion period in the Ordos Basin. The sharp rise in sea level has caused the ordos ancient land to retreat to the northern Islamic League, and a large area of the area has been submerged by seawater, and the central paleo-uplift area has been reduced to an underwater uplift, forming a vast land shed basin environment. A large number of natural gas resources have also developed.
Figure 11, Daniudi gas field in Yulin, Shaanxi
4. Primary evidence
As can be seen from Figure 6, Figure 7, there was a bulge in the middle of the ocean at that time. In the central paleouple site, there is a thickness of up to 1000m carbonate rock, when the ordos basin base of the overall uplift, due to the difference in specific gravity, under the adjustment of the crust, mantle equilibrium, this part of the integrity of the carbonate sediment will naturally be higher than other parts of the basin, forming the so-called central paleo-uplift.
Figure 12, Ordovician limestone
This carbonate sediment, when formed, was overly covered from southwest to northeast, and with the superposition of carbonate sediments from the late Qingyang-Xifeng Qingbaikou period, this paleo-uplift formed the so-called "L" shape with north-south as the main body.
Figure 13, The seafloor environment of the Ordovician ocean
In summary, Ordovician Shaanxi was a vast ocean, deposited with thick layers of limestone and rich fossils. Among them, the carbonate reservoir is the main reservoir of natural gas in the Ordos Basin, and natural gas has been produced in Xiaotutu and Wushenqi Surige in northern Shaanxi.