Diagenetic factor

Diagenetic environment determines the nature of diagenesis. Ordovician carbonate rocks mainly experience three environments: seawater (bottom) diagenesis, buried diagenesis and atmospheric light water diagenesis. Caledonian "plane" ups and downs and subsequent tectonic movements led to the cross process of three diagenetic environments, belonging to an open and unsustainable buried diagenetic system. The main diagenesis includes dehydration and compaction, new mineral transformation, authigenic mineral precipitation, recrystallization, replacement, dissolution, filling and compaction. Among them, dolomite and dissolution are the factors that build pores, and sedimentation and filling are the factors that seal and destroy pores.

(a) marine diagenetic environment

The diagenetic environment of seawater is closely related to sedimentary environment. The difference of sedimentary development in different sea areas or periods of North China platform makes the sediments have two different submarine diagenetic environments, normal seawater and salinized seawater, which directly affects diagenetic properties and pore evolution.

1. Normal marine diagenetic environment

It belongs to normal marine diagenetic environment. Its open marine stucco deposits were transformed from aragonite and compacted, dehydrated and densified to form rocks. Shoal facies particles are cemented into rocks by one or two generations of dentate, fibrous or crystalline calcite precipitation. Their primary intergranular pores gradually disappeared. If the top of the shoal is exposed, it will be dissolved by atmospheric precipitation to form intergranular pores or dolomite mixed with water to further form intergranular pores.

2. Hydrodiagenetic environment of saline sea

The sediments in the extremely shallow water environment of the evaporation tidal flat and the upper tidal flat belong to the saline water diagenetic environment, and are concentrated into salt water under the action of the quasi-contemporaneous "evaporation pump" With the increase of Mg/Ca ratio in the process of gypsum crystallization, gypsum was replaced by magnesium-rich brine, forming quasi-contemporaneous dolomite. These gypsum-bearing dolomites are located in the regressive sequence and are often leached by atmospheric fresh water during frequent exposure. Gypsum is dissolved to form mold holes, and then the micropore dissolution of quasi-contemporaneous dolomite expands to intergranular dissolution holes and intergranular dissolution holes, which match the dry fractures of interlayer fracture diagenesis and form the bedding distribution of microfracture-micropore dolomite reservoir. When the covering layer continues to deposit gypsum salt, the pore layer is protected. The existence of these pores has been confirmed by the natural gas reservoir found in the gypsum salt layer of Majiagou Formation in eastern Ordos.

(2) Buried diagenetic environment

In the middle and late Caledonian movement, the North China block was uplifted in an all-round way and buried intermittently in Ordovician. Before uplift, it was in shallow burial environment; After Silurian-Early Carboniferous erosion and late Carboniferous-Triassic subsidence, the burial depth increased to below 3000 meters, belonging to deep burial environment. The two environments have different diagenetic evolution.

1. Shallow buried diagenetic environment

At the initial stage of sediment burial, the diagenetic environment and sedimentary environment are still internally related: for example, the sediments in the lower part of Majiagou Formation and the middle and upper part of Shangmajiagou Formation were in normal marine diagenetic environment in the early stage, but soon after burial, they were infiltrated downward by the overlying salt water in the lower part of Majiagou Formation and the lower part of Fengfeng Formation, respectively, forming dolomite; At this stage, Zhuozishan Formation or Majiagou Formation in Tianchi-Huanxian-Weibei area in the Qin-Qi transitional sea area was also laterally infiltrated by salt water from the North Sea, resulting in dolomitization of salt water or mixed water. Layered or lenticular fine-grained dolomite is generated, and dolomite-gray dolomite-dolomite-dolomite (leopard) porphyry is generated in turn with the downward or outward penetration of saline water weakening. Dolomitization metasomatism gradually disappeared to the deep or far end. This set of dolomite has good crystal shape, large particle size, skeleton pores supported by rhombic crystals and good compression resistance. Other strata in this diagenetic environment are mainly represented by pressure-soluble micro-suture lines, spherical yellow iron formation, granular limestone filled with the third generation cement and general silicification of Liangjiashan Formation.

2. Deep burial diagenetic environment

The late Carboniferous North China block gradually subsided and the Ordovician was buried again. When the burial depth is below 1500m, with the increase of temperature and pressure, the organic components in the rock are transformed and decarboxylated to release CO2, forming acidic water, which re-dissolves the large pressure-soluble suture along the previous fracture. When the overlying Carboniferous-Permian coal measures entered the diagenetic compaction stage, acid water was continuously discharged and entered the carbonate rocks along the Ordovician weathering crust or cracks, which aggravated the early interlayer dissolution and the Caledonian weathering crust karst transformation, leading to the development of fracture-cave karst. In the later stage of geochemical environment evolution, it turned into alkaline water, and its dissolution was replaced by mineral precipitation, and ankerite iron-containing calcite and insoluble clay residues harmful to pores were precipitated in cracks and caves. The Tarim block continued to subside, forming a deep-shallow water environment, further eroding the Ordovician.

During Indosinian-Yanshan movement, there were obvious differences in Paleozoic diagenetic evolution between North China and Yangtze region. Ordos basin is dominated by subsidence. At the end of the early Cretaceous, the Ordovician was buried below 3300 meters ... carbonate rocks were mainly replaced by heavy brine (barite, celestite, arsenolite, anhydrite, etc.). ) and porphyritic pyrite. In this depth range, dolomite can still maintain certain pores because of its strong compressive strength. If the pores are full of oil and gas, the precipitation of authigenic minerals can be inhibited. The east of the platform is dominated by backflow, uplift and erosion.

The Cenozoic Ordovician is a negative structural unit with a buried depth of 2500 ~ 6000 m, and the diagenetic changes are mainly heavy brine metasomatism and pyritization. At this stage, pores are only formed along the solution near the deep fault zone.

(3) Atmospheric light water diagenetic environment and evolution

The atmospheric light water diagenetic environment in the Paleozoic surface outcropping zone has three stages: early diagenetic stage, Caledonian uplift stage and Yanshan Himalayan stage. They are an important period for the development of reservoir pores.

1. Atmospheric and fresh water environment in the early diagenetic stage

During the CAMBRIAN-Ordovician period, the shallow water facies areas of evaporation tidal flat and granular beach, especially the newly consolidated sediments of the former, were frequently affected by atmospheric fresh water in the process of regression, resulting in interlayer dissolution phenomena such as stratification and clouding, forming massive, needle-like and nodular gypsum mold holes and dolomite intergranular and intragranular dissolved holes, which are dolomite reservoirs that can form dissolution.

2. Atmospheric and freshwater environment of Caledonian uplift

Caledonian uplifted in the middle and late period, and successively entered the stage of long-term weathering and erosion. Carbonate rocks about 200m below the denudation surface, especially in the depth range of 0 ~ 100 m, are eroded by atmospheric precipitation, forming fresh water permeable zone, latent zone and karst phase. Its development degree is controlled by lithology. Limestone dissolves quickly, resulting in a large area of caves and cracks; Dolomite is mainly composed of intergranular pores and early interlayer dissolution, which has high solubility, so large, medium and small dissolution holes are developed. According to the statistics of core samples, the porosity of dolomite is 2.5 times higher than that of limestone on average. The upper parts of Kerimori Formation, Pingliang Formation and Beiguoshan Formation in the southwest margin of the block are argillaceous carbonate rocks, which are rich in clay and hinder the development of karst, and it is difficult to form dissolution pore zones, which are restricted by paleostructure, paleogeomorphology and water flow velocity. For example, the seismic reflection T9 wave group in Ordos area is obviously a low-speed and low-resistance layer with discontinuous amplitude. There are underwater platforms and residual hills with a height difference of 30 ~ 100 m below the erosion surface, and pore dolomite, gypsum dolomite and breccia layers formed by karst collapse are all target layers for natural gas exploration. In addition, the uplift in Huai-yuan period erodes the exposed surface of Liangjiashan Formation or Yeli Formation, and there are also eroded karst zones, common isolated small crystal caves and dissolved holes under the weathered surface. According to the research, the medium-fine dolomite developed under the erosion surface is affected by multi-stage dolomitization, especially in the early stage, the overlying Majiagou Formation salt water permeates downward and generally fills intergranular pores and dissolved pores, resulting in the secondary enhancement of dolomitization, leading to the densification of rock structure and affecting its reservoir performance. There is a lack of multi-stage strata in Tabei area of Tarim basin, and Ordovician is a variety of surface karst formed by atmospheric fresh water environment.

3. Atmosphere and fresh water environment in Yanshan-Himalayan period.

During this tectonic period, except Ordos basin, the Ordovician system was well preserved, and the southwest margin of the platform and Lvliang and Taihang areas were exposed and denuded due to large-scale uplift, while the uplift and westward uplift of the North China fault block area experienced hundreds of millions of years of surface exposed transformation. During this period, the eroded karst was rugged due to strong tectonic activities, and the lush higher plants in Mesozoic and Cenozoic increased their solubility in atmospheric water by adding a large amount of biogenic CO2. The scattered cutting of rivers and lakes with negative topography leads to the vertical and lateral seepage erosion of carbonate bedrock by surface runoff, which has the dual function of superimposing authigenic karst on the basis of authigenic karst, so it has unprecedented dissolution strength and scale. According to the statistics of the opportunity rate of more than 260 boreholes (Table 5-65438), karst facies mainly develops at the depth of 0-200m below the erosion surface, and the most developed part is 0-50m. Because the surface soil facies is very thin, it can form a network system of holes-holes-fractures and a pore zone of karst breccia layer with extremely developed fractures. At the same time, the combination of dissolution and tectonic action has formed various types of carbonate buried hill oil and gas reservoirs. According to the research in central Hebei, there are not only weathering zones with developed pores at the top of the buried hill, but also multi-layer horizontal dissolution zones and layered dissolution zones due to the fluctuation of groundwater level in the deep and everywhere. For example, Renqiu buried hill only developed three horizontal dissolution zones between the weathered surface and the crude oil-water interface during the deposition period of Kongdian, Shasan and Shasi in Himalayan movement. Buried hill is a kind of oil and gas reservoir with considerable productivity, such as Renqiu, which has been developed.

Table 5- 1 Statistical Table of Opportunity Rate of Ordovician Dissolved Fissures and Caves Below Erosion Surface in Different Periods in North China

(According to Chen Xia 1994)

Near the fault zone in this period, fresh water on the surface or underground seeped down thousands of meters along the fault system, causing deep carbonate rocks to dissolve and form small holes, cracks and breccia. This kind of karst is quite prominent in the southwest margin, such as breccia limestone in the middle and upper part of field 1 well gypsum layer in the west margin, fine-grained dolomite in Table Mountain Formation of Well Yi 25 of 4200-4520m, and 1 well122 ~ in the south margin. In addition, the dolomite of the South 1 1 Jingmajiagou Formation in Huaibei area is 4500m long, and all of them are developed with deep-seated cavities and fractures extending longitudinally along the fault zone, which better improves the porosity and permeability of deeply buried carbonate rocks. Similar karstification occurred in Tarim and Yangtze areas.