The intracontinental rift caused by worldwide extension is often a favorable zone for the distribution of source rocks. The Early Cretaceous rift system in northeast China belongs to this kind of basin. However, the early Cretaceous extension intensity in Northeast China was moderately low, but the extension amplitude was relatively large, which was mainly related to low-angle subduction and orogenic belt collapse, so that the intracontinental rift in this area was a lake basin controlled by small and medium-sized semi-graben, and the distribution of source rocks was characterized by a small but widely distributed single fault depression. The distribution of sediments and source rocks in a single faulted lake basin has the following characteristics.
The oblique movement of the (1) fault caused the asymmetric subsidence of the footwall surface. Among the isolated fault basins divided by normal faults, there is a closed continental basin with water system inside.
(2) The slope on one side of the main fault is steep, and the footwall supplies the source of small alluvial fans, which are distributed at the foot of the steep footwall, and alluvial fans and water systems are distributed on the gentle slope.
(3) In the center of the basin, a perennial lake or dry salt lake is formed according to the climatic conditions, and the position of the lake center depends on the alluvial fan, thus forming a trinity system (fan-lake-fan or river).
(4) The fault activity, the subsidence caused by tilting and the abundance of clastic materials have great influence on the distribution of fans, forming a series of overthrust and overthrust strata.
The sedimentary model of Early Cretaceous rift system in Northeast China basically belongs to this category. For example, cliff alluvial fan-perennial lake or lake marsh-alluvial fan or river system is distributed in Xujiaweizi fault depression and Dehui fault depression, and alluvial fan enters the lake to form fan delta. Lacustrine sediments are distributed with dark mudstone and coal source rocks, mainly natural gas, and river fan delta facies is a favorable reservoir facies belt.
The rift range and lake basin area of the early Cretaceous rift system in Northeast China are small, scattered and isolated. However, there are also some large-scale fault depressions, such as Lin Dian-Changjiaweizi fault depression group, Xujiaweizi fault depression group and Dehui-Lishu fault depression group in Songliao Basin, which have relatively high sedimentation rate and are often areas where dark mudstone source rocks are relatively developed.
In the Early Cretaceous rift system in northeastern China, kerogen types of source rocks in different areas are different. Among them, the parent material in the northern Songliao basin and Dehui-Lishu fault depression in the middle belt is mostly type III, followed by type II B. The maceral analysis of the source rocks in Songnan 18 well in Lishu fault depression also shows that the maceral is mainly vitrinite, followed by inert components and crustaceous components, belonging to gas source rocks.
Humic source rocks and sapropelic source rocks are also developed in Boli basin in the eastern belt, and kerogen in Jixi basin is also humic.
In Hailaer and Erlian fault basins in the western belt, and in the southern part of the central belt and Kailu basin, sapropelic kerogen or kerogen mixed with some sapropelic is the main oil source. For example, the analysis and statistics of kerogen elements in Aershan Formation in Abei-Anan sag of Erlian basin show that the organic matter type is mainly mixed, accounting for more than 50%, and the main source rock section in the development period of lake basin is Ⅱ a. The source rocks of other faults, such as Bayindulan-Saihantala, Erlianhao, Jirgalangtu, Baiyinchagan and Wuliyasitai, are sapropelic and metamorphic sapropelic mixed source rocks. The source rocks of faults such as Tabei, Tarnum, Chaokewula, HuGoegler Tu, Yihe Wusu, Butumoji and Sai Han Wuliji are humic kerogen. The kerogen types of dark argillaceous rocks in Wuerxun fault depression, Hongqi fault depression and Beier fault depression are mainly Ⅱ A, with a few Ⅰ and Ⅱ B, and the atomic ratio of kerogen element H/C is the highest in Hongqi fault depression (1. 10~ 1.27). The dark mudstone of Nantun Formation in Hulunhu, Helhongde and Wugunor oil-generating depressions on the east and west sides of Beierhu fault depression is the second (1. 10), which belongs to kerogen type ⅰ-ⅱ, and Wugunor fault depression is superior to Hulunhu and Helhongde fault depression (type ⅱb-ⅲ). The kerogen of dark mudstone in Nantun Formation in Huhehu and Chagannuoer fault depression is mainly Ⅱ B type and Ⅲ type.
The source rocks of Jiufotang Formation are mixed with sapropelic in Kailu and Lujiapu fault basins in the south of the central belt. Kerogen of Dalazi Formation in eastern Yanji Basin basically belongs to type II mixed type (according to Shen Yan 1 well data).
To sum up, kerogen types and hydrocarbon generation potential vary greatly in different regions. Because the information obtained is not systematic enough, it is difficult to determine its laws and the main reasons that affect the differences. However, according to the analysis of some well-studied basin data, faulted lake basins with high extension, relatively large area, relatively large settlement rate and higher settlement rate than accumulation rate are relatively beneficial to well-developed source rocks, especially ⅰ-ⅱ A kerogen source rocks. Therefore, this kind of basin should give priority to oil exploration, and other basins with type ⅲ kerogen should give priority to natural gas exploration.
2. Faulted basins control the formation and characteristics of petroleum systems.
Early Cretaceous rift basins are composed of several independent semi-graben and graben fault basins with similar sedimentary development history and structural evolution history, and they often form independent oil and gas systems with fault basins as units. Due to the different development degree of geological elements, geological processes and the time-space allocation relationship between geological elements and geological processes, the oil and gas enrichment degree of each petroleum-bearing system is very different. The distribution range and subsidence amplitude of rift lake basin in the same rift period directly control the development degree, organic matter type and abundance of source rocks, while the structural uplift amplitude and continuous subsidence amplitude after rift period directly affect the maturity of source rocks and oil and gas preservation conditions. The development degree and hydrocarbon generation ability of mature source rocks in the fault depression are the key to determine the formation of petroleum system and the degree of oil and gas enrichment. Therefore, the faulted basin petroleum system can be classified according to the subsidence amplitude, the development and maturity of source rocks and the abundance of organic matter. Taking Erlian fault basin group as an example, it can be roughly divided into three types of petroleum systems (Figure 6- 1).
(1) Fault depression with long-lasting fault depression time, large fault depression amplitude and deep lake facies development: The fault depression amplitude in the same rift period is large, generally above 2,500 m ~ 3,000 m, and the structural sequence is fully developed, which is larger than that in the deep lake area. The mature source rocks have a large area, ranging from 300 km2 to 700 km2, and the thickness is 650m to 1300m. The source rocks are relatively mature and abundant, with the average content of organic carbon of 1.3%~2.8% and the average content of chloroform asphalt "a" of 1.20 ‰ ~ 2.35. Generally speaking, there are two sets of source rocks: Aershan Formation and Ito Formation. Such as Anan (see Figure 6- 1a), Abei, Jirgalangtu, Erlianhaoer, Ulyastai, Honghaoershute, Baiyinchagan and Bayin Dulan, constitute the main oil and gas systems in Erlianpen area.
(2) Faults with long-lasting rifting time, moderate rifting amplitude and relatively developed lacustrine facies: in the same rifting period, the rifting amplitude is generally1500m ~ 2,500m, the structural sequence is well developed, and semi-deep lacustrine source rocks are developed, but the sedimentary environment is unstable. The buried depth of source rocks is only 1300m~2000m, and the maturity is low. The source rocks are rich in organic matter, with organic carbon content of 0.62%~2.36%, chloroform asphalt "A" content of 0. 134‰~0.7 10‰ and total hydrocarbon content of 0.27 ‰. Such as Hugue Gilletu (see Figure 6- 1b), Zhunpeng, Naomugen, Baoerguoji, Sai Han Wuliji, Gaolihan, Aqituula and Butumoji. This kind of fault depression shows oil and gas, but the exploration degree is low.
(3) Faults with short sustained subsidence time, small subsidence amplitude and strong uplift in the later period: in the same rift period, the subsidence amplitude is small, generally less than 1500m, the structural sequence is not fully developed, shallow lake facies and swamp facies are developed, and dark mudstone is poorly developed. Because the source rocks are shallow and immature, the abundance of organic matter is low, the organic carbon content is less than 0.5%, and the hydrocarbon generation potential is less than 2 kg/t. At present, no oil and gas is found in drilling, such as Habang (see Figure 6- 1c), Yihe and Wusu faults.