The characteristics of tectonic movement in rift basin are different from those in geosyncline basin, and folding movement is the main one with strong tectonic movement. It is also different from platform basin (craton basin), which is dominated by oscillation and moderate tectonic movement; Rift basin is dominated by differential uplift and strong tectonic movement. Generally speaking, there are three main tectonic movements in the development of rift basin: one is between rift basin and underlying basement. Before the rift basin was formed, the crust was uplifted and eroded. Rift basin is an unconformity on the basement of geosyncline or platform. For example, Erlian rift basin unconformity covers Paleozoic geosyncline basement, while Bohai Bay basin unconformity covers platform basement. Secondly, during the development of rift basin, when a rift period ends, the crust is squeezed and uplifted, and the rift disappears. Such as Bohai Bay Basin and Erlian Basin in the rift system in eastern China, occurred in the second act of Yanshan Movement, that is, between the middle and late Jurassic. This tectonic movement is mainly horizontal movement, which produces strong folds, reverse faults and volcanic eruption activities, and transforms the previous fault depression and produces new fault depression. Thirdly, it occurred in the late stage of rift basin development, that is, from synrift period to post-rift period, from fault depression period to depression period. This tectonic movement is a major tectonic movement in the rift basin, which is mainly compressive uplift and structural inversion. This tectonic movement is the main period of structural formation and finalization in rift basin, and also the main period of oil and gas migration. Such as Bohai Bay Basin, Jianghan Basin, Subei Basin and Pearl River Mouth Basin in eastern China, this tectonic movement occurred during Himalayan movement, that is, between Neogene and Paleogene. The tectonic movement of Erlian basin occurred in the late Yanshanian and early Cretaceous, that is, between Saihantala period and Tengger Singh period.
In addition to the above three major tectonic movements, there have been many tectonic movements in the basin, most of which are characterized by differential lifting movements or fault block tilting movements.
For example, Jizhong Depression, influenced by Yanshan and Himalayan movements, has formed many sedimentary discontinuities and angular unconformity after many uplifts, denudation and subsidence, mainly including three regional sedimentary discontinuities and three local sedimentary discontinuities. Among them, the regional scale sedimentary discontinuities are located between Paleogene and Neogene, between the third member of Shahejie Formation and the fourth member of Shahejie Formation, and between Paleogene and Pre-Paleogene, which mostly form regional angle unconformity surfaces. The local sedimentary discontinuities are between Dongying Formation and Shahejie Formation (Chaheji), between Shahejie Formation and Shahejie Formation (south-central Hebei), and between Shahejie Formation and Shahejie Formation (Langgu Depression and Yanling Area). The existence of the above six unconformities affects the development of stratigraphic deposits and the formation and distribution of stratigraphic lithologic reservoirs.
There are many regional unconformities between Aershan Formation and Tengger Singh Formation, Tengger Singh Formation and Saihantala Formation, Upper Cretaceous and Lower Cretaceous, and Neogene and Quaternary in Erlian Basin.
(2) Tectonic movement and stratigraphic lithologic reservoirs
1. Tectonic movement and formation of stratigraphic traps
Tectonic movement causes the strata below the unconformity surface to be eroded, weathered and leached, which improves the reservoir performance and is covered by caprocks such as argillaceous rocks covering the unconformity surface, forming unconformity traps. For example, in the west of Bieguzhuang, Langgu Depression, Jizhong Depression, due to Himalayan movement between Paleogene and Neogene, the Paleogene strata suffered denudation, and the sand layer of the upper member of Shahejie Formation was covered by mudstone unconformity of Neogene Minghuazhen Formation, forming an unconformity reservoir in Well 27 (Figure 2- 15). Another example is the unconformity reservoir of Well Ba 48 in Bayin Dulan Depression of Erlian Basin. During the Yanshan Movement, Tengger-Singh Formation and Aershan Formation were unconformity contacts. Below the unconformity surface, the upper sand layer of Member A 4 is covered by the argillaceous caprock of Member A 1, forming a stratigraphic unconformity trap (Figure 2- 16).
Figure 2- 15 Unconformity Reservoir Profile of Jing 27 Formation in Langgu Depression, Jizhong Depression
Figure 2- 16 Reservoir Profile of Stratigraphic Unconformity and Stratigraphic Overlay of Ba 48 Aershan Formation in Erlian Basin
When the new strata overlap with the unconformity surface, the top and bottom of the reservoir are blocked by the sealing layer, forming a stratigraphic overlap trap. For example, the strata and reservoirs of well Ba 48 in Bayindulan sag of Erlian basin overlap. The sand layer in the lower part of the 4th member of Aer overlaps with the unconformity surface of the 3rd member of Aer or Jurassic, and is blocked by the cover of the roof and floor, forming a stratigraphic overlapping reservoir (Figure 2- 16). Another example is the stratigraphic overlap reservoir of the third member of Shahejie Formation in well Ma 97 in Nanmazhuang area of Raoyang sag in Jizhong depression. The unconformity of fine sandstone in the third member of Shahejie Formation overlaps the Ordovician buried hill surface, which is a stratigraphic overlapping reservoir covered by mudstone and unconformity surface. In addition, the conglomerate reservoir of the first member of Shahejie Formation in Well Jin 7 on the west slope of Shulu Depression also belongs to stratigraphic overlap reservoir.
2. Unconformity surface and reservoir reconstruction.
The unconformity surface is the recording surface of one or more tectonic movements and the sedimentary discontinuity surface. Its representative time is long or short, up to several generations. During this period, the underlying strata under the unconformity surface suffered from different degrees of erosion, weathering and leaching, which greatly improved the reservoir performance, provided superior storage space for oil and gas accumulation and created conditions for high oil and gas production.
Take the unconformity reservoir of Subu Formation on the east slope of Wuliyasitai sag in Erlian basin as an example. The oil reservoir is located in the oil-bearing strata below the unconformity between Tengger Singh Formation and Aershan Formation of Lower Cretaceous. The rock types of Aershan Formation are mainly lithic sandstone, with timely content of 20% ~ 30%, feldspar content of 20% ~ 30% and lithic content of 45% ~ 60%. Sandstone is currently in the late diagenetic stage A and B, and the reservoir space is dominated by intergranular dissolved pores, followed by intragranular dissolved pores and die pores, with porosity of 5.6% ~ 16. 1% and permeability of (2 ~ 8) × 10-3 μ m2, with the maximum1. It can be seen that secondary pores are developed and reservoir physical properties are improved due to weathering and leaching.
Therefore, the tectonic movement makes the reservoir below the unconformity surface suffer from different degrees of erosion, weathering and leaching, which improves the reservoir performance. In the early stage of rift basin formation, the tectonic movement was relatively strong and experienced many tectonic movements. The underlying stratum under the unconformity surface has been eroded, weathered and leached for a long time, and the maximum depth of influence can reach more than 600 meters, generally within 150 meters. Dissolved holes, caves and fractures are developed, which is the main producing stratum. However, in the oil-bearing strata, the general tectonic movement is relatively gentle, the sedimentary discontinuity time is short, and the underlying strata under the unconformity surface are eroded, weathered and leached, but the reservoir performance is still obviously improved.
3. Unconformity surface is an important channel for oil and gas migration.
Because the unconformity surface is weathered and denuded, the strata below the unconformity surface have been weathered, denuded and leached to varying degrees, which improves the porosity and permeability conditions, not only forms a good reservoir space, but also provides an important channel for oil and gas migration. The unconformity surface is the convergence surface of oil and gas. The hydrocarbon source layer and sand layer covered on the unconformity surface overlap with the unconformity surface, and the oil and gas generated by the hydrocarbon source layer directly migrate to the unconformity surface or migrate to the unconformity surface through permeable sand layer, forming the convergence surface of oil and gas migration. At the same time, the unconformity surface is widely distributed, which makes the oil and gas in the hydrocarbon-generating area migrate to the non-hydrocarbon-generating area, thus making the hydrocarbon-generating layer continuously expel hydrocarbons, so the unconformity surface is an important channel for oil and gas migration. It is particularly emphasized here that the unconformity surface in mature source rocks is the main channel for lateral migration of oil and gas, and oil and gas are preferentially accumulated in traps above and below the unconformity surface to form reservoirs. For example, in the mature source rocks of Erlian Basin, that is, the unconformity between Tengger Singer Formation and Aershan Formation of Lower Cretaceous, the proven oil reservoirs are mainly located in the lower part of Tengger Singer Formation and the upper part of Aershan Formation, accounting for 70% of the proven oil reservoirs and 80% of the proven reserves as of 1998.