Zao-2 and Zao-3 oil groups are the main oil groups in Wang Guan Tun Oilfield, with various pore types, throat shapes and complex physical property changes, which directly affect reservoir fluid flow and oil recovery. In this paper, the pore structure of reservoir is systematically characterized by means of thin section observation, casting thin section and mercury injection curve, and the geological factors affecting pore structure are discussed, and reservoirs are classified and evaluated according to pore structure and other factors.

Pore structure; Pore throat connectivity; Reservoir evaluation

Pore structure refers to the geometry, size, distribution and interrelation of pores and throats in rocks. It plays an important role in reservoir storage and permeability, fluid distribution, oil and gas productivity, oil and water movement in reservoir, water flooding efficiency and crude oil recovery.

I. Pores and throat

1. Pore type

With regard to the classification of pore types, predecessors have put forward many classification schemes from different angles. In this study, the pore types are re-divided by considering the origin, occurrence and geometry of pores.

(1) primary pore

Refers to the pores formed at the same time as sedimentation and reduced by compaction and cementation.

A. Normal intergranular pores

Refers to the porosity reduced by compaction without any filler. This kind of pore content is small, the pore size varies, generally around 50μm, and the maximum can reach 100μm, which is often irregular polygon.

B. Residual intergranular pores

Refers to the primary intergranular pores filled with cement but not completely filled. Such pores are common in reservoirs, and the residual intergranular pores are mainly reduced by secondary expansion, generally triangular and irregular polygons.

C. Micropores in Heterogroups

Refers to micropores in clay matrix. This kind of pore is relatively developed in the film, so the pore size is relatively small (generally < 0.2 micron), and there are clay hetero groups, which makes the color of the impregnating agent here larger than that of the pore without hetero groups.

(2) Secondary porosity

Refers to the pores formed by secondary dissolution and fracture, which are developed in the study interval.

A. intergranular dissolved pores

It refers to the pores distributed among particles, which are formed by the selective dissolution of original cements, heterobases and particles among particles. There are two conditions for its formation. One is that the intergranular carbonate cement and the edge of clastic particles dissolve at the same time; Second, cementing materials such as carbonate fill the particles and explain the edge of the particles at the same time, and then the cementing materials and particles dissolve at the same time. The second situation is more common. Intergranular dissolved pores are characterized by thick throat, good connectivity, irregular pores and serrated edges.

B. Dissolved pores in the assembly

Include intragranular dissolved pore, cemented dissolved pore and metasomatic dissolved pore. Intra-granular dissolved pores are mainly those such as cuttings and feldspar. First of all, they are formed by partial dissolution of particles themselves, which are the main components of dissolved pores in particles. Second, the particles are explained first, and then partially or completely dissolved, forming intragranular dissolved pores. The shape of intragranular dissolved pores is generally irregular, and the edges are often serrated and harbor-like. At the same time, some dolomite with good crystal shape is dissolved and replaced, and regular dolomite crystal mold holes are formed in the particles. The pore size also varies greatly, and the small one can only be distinguished by scanning electron microscope.

C. intergranular pores and intergranular dissolved pores

Refers to the intergranular pores formed by recrystallization or dissolution of crystalline particles.

D. Fracture pores

Refers to fractures formed by tectonism, diagenetic contraction, etc. Due to mechanical compaction or cleavage along cleavage cracks, particles are broken to form common cracks; Cracks formed when rocks are squeezed or stretched; Micro-cracks formed by dehydration and shrinkage of clay in the local enrichment zone of clay matrix during diagenesis. The number of cracks is generally small, but unfilled cracks are of great significance to improve the permeability of rocks.

2. Throat type

Throat is a narrow channel connecting pores, which plays a decisive role in reservoir seepage capacity. The size and shape of throat mainly depend on the contact relationship between rock particles, the type of cementation and the shape and size of particles. According to the observation under the microscope and the image analysis of cast thin slices, there are four common types of throat in the study area.

(1) pore throat shrinkage

Throat is the narrowed part of pore, and there is no obvious boundary with pore. This throat is often developed in sandstone reservoirs dominated by primary or secondary intergranular pores. Rock structures are mostly supported by particles or contacted by floating objects, and there are few cements and clay miscellaneous bases; It belongs to large pore and thick throat, and the diameter ratio of pore to throat is close to 1.

(2) Necked throat

The throat is a constricted part, and the cross section between particles is variable. This kind of throat is common in sandstone with particle point contact, lining cementation or autogenous cementation. This pore structure belongs to the type of large pore and small throat, and the pore throat diameter is relatively large. This kind of reservoir rock may have high porosity, but usually its permeability is low.

(3) Flake throat

The throat is flaky or curved, which is a long channel between particles. It often appears in sandstone with strong mechanical compaction or large autogenous expansion. The ratio of pore throat diameter is generally large.

(4) Throat of tube bundle

It is a micro-pore between heterogeneous and authigenic cement crystals, and its pore size is generally less than 0.5μm, which is both a pore and a throat. If the rock is basically micro-porous, it belongs to micro-pore and micro-throat type, and the ratio of pore throat diameter is 1. Permeability of rocks is extremely low.

3. Pore combination type

Although the reservoir space of sandstone reservoir is composed of many types of pores, it is often dominated by one or more types of pores. Different pore combination types have different effects on reservoir physical properties and pore structure. The pore types in the study interval are mainly intergranular dissolved pores, primary intergranular pores and micropores in the matrix, while other types of pores are undeveloped. According to thin section identification and mercury injection data, the pore combination types of sandstone reservoirs in the studied interval can be roughly divided into the following four categories.

(1) Primary intergranular pore type

Normal intergranular pores and residual intergranular pores are dominant, secondary intergranular dissolved pores and intragranular dissolved pores are not developed, and micropores in primary matrix are not developed due to low clay matrix content. The throat is mainly of shrinkage cavity type and necking type. Pore sorting is good, and the percentage of secondary pores is less than 25%. The content of argillaceous matrix and cement is small, and the particles are loosely arranged. Generally, the porosity is greater than 15% and the permeability is greater than 100× 10-3μm2, which is common in alluvial fan channel sedimentary sand bodies with a buried depth of less than 2000 m. ..

(2) Type of secondary intergranular dissolved pores

Its pores are mainly secondary intergranular dissolved pores, and there are few primary intergranular pores. In addition, there are a few other dissolved pores, such as intragranular dissolved pores, mold pores and super-large pores. The content of clay heterobase is low, generally less than 10%, so the micropores in the heterobase are not developed. The throat types are mainly sheet throat and constricted throat, with poor pore selectivity and point contact of particle arrangement. Generally, the porosity is more than 20% and the permeability is more than 500× 10-3μm2. It is a medium-high porosity and high-high permeability reservoir with good reservoir performance. It is commonly found in alluvial fan channel sedimentary sand bodies with a buried depth of more than 2000 meters, strong dissolution and low clay matrix content.

(3) Intragranular and intergranular micropore type

Pore types are mainly clay matrix, authigenic clay mineral intergranular micropores and carbonate cement intergranular micropores, while other types of pores are undeveloped, and throat types are mainly tube bundle throat and sheet throat. Clay content is generally greater than 10% or carbonate cement content is greater than 15%, porosity is generally less than 20%, and permeability is less than 10× 10-3μm2. Rock types are mainly argillaceous sandstone or siltstone, poorly sorted unequal-grained sandstone, carbonate and other closely cemented sandstone, which are commonly found in alluvial fan channel sediments and other sand bodies. The burial depth is less than 2000 meters, and the dissolution is weak or deep, but the clay matrix content is high. It belongs to medium-low porosity and low permeability reservoir or impermeable layer.

(4) intergranular dissolved pores are combined with micropores.

Pore types are mainly intergranular dissolved pores and intergranular micropores of heteropoly and cement, and other pore types are also common, but the content is less. According to the relative content of intergranular dissolved pores and micropores, it can be further divided into two sub-primary combinations: intergranular dissolved pores-micropore type and micropore-intergranular dissolved pores. The former is mainly intergranular dissolved pores, with a content of more than 50%, which is common in sandstone with high clay matrix content (generally 10% ~ 15%) and incomplete dissolution of carbonate cement; The latter is mainly micropores, the content of which is above 50%, and it is common in sandstone with high argillaceous content, weak dissolution or undeveloped. In a word, the common throat types of this kind of pore combination are sheet throat, tube bundle throat and bent throat, with porosity generally less than 20% and permeability between 100× 10-3 and 500× 10-3 μ m2. It belongs to a medium porosity and medium permeability reservoir, and is commonly found in sand bodies such as alluvial fan channel deposits with a buried depth of more than 2000 meters and medium clay matrix content.

Secondly, the pore structure characteristics of interval clastic reservoir are studied.

1. Size and distribution of pores and throats

According to the image analysis data of casting thin slices, the maximum pore size of samples in the study area is between 160 ~ 553 microns, with an average of 272.5 microns. The minimum value is between 54 ~ 165 microns, and the average value is 89.92 microns ... See Table 1 for the pore throat size and distribution characteristic parameters of the research interval in Wang Guan Tun Oilfield. It can be seen from the table that the sandstone reservoirs in the study interval are generally dominated by medium-thin throats, and the content of capillary throats with diameters less than 0. 1μm is relatively high, ranging from 4.29% to 90.52%, with an average of 35.93%. Pore throat size and distribution characteristics play an important role in controlling reservoir physical properties. According to the study, the porosity (Por) of sandstone reservoir in the target interval is inversely proportional to the average pore throat (X); It is positively correlated with the average throat radius (R). It is negatively correlated with logarithm of displacement pressure (Pd). It is inversely proportional to the minimum unsaturated porosity (Smin). Pore volume percentage (Vr) controlled by different throats has a direct impact on reservoir storage and permeability, fluid distribution and productivity. With the increase of pore volume percentage controlled by throat radius (r) less than 0. 1μm, porosity decreases (Figure1); Porosity and pore volume also decrease; Oil saturation decreases and water saturation increases; X has a good linear positive correlation with it; When it is less than 25%, Sp and D are negatively correlated with it; When it is more than 25%, Sp and D are positively correlated with it. It is inversely proportional to r; There is a good linear positive correlation with P50. Positive correlation with Pd; There is a good linear positive correlation with Smin.

Table 1 Statistical Table of Pore and Throat Size and Distribution Characteristic Parameters in Wang Guan Tun Oilfield

With the increase of pore volume percentage controlled by throat (R) greater than 10μm, the porosity increases exponentially (Figure 2). The logarithmic values of porosity and pore volume also increase exponentially; Oil saturation tends to increase, while water saturation tends to decrease. X is inversely proportional to it. When x < 10, the change of x will cause its rapid change. It is proportional to r; It has a good exponential inverse relationship with P50. It has a good exponential inverse relationship with Pd; Is inversely proportional to Smin.

Figure 1 less than 0. 1 μ m Throat Controlled Porosity and Pore Volume Percentage Scatter Diagram

Fig. 2 scatter plot of throat-controlled porosity and pore volume percentage greater than10 μ m.

2. Pore connectivity

Pore connectivity can be characterized by pore throat diameter ratio, mercury extraction rate and coordination number. According to the image analysis of the cast thin section, the ratio of pore throat diameter varies from1.412 to 5.288 with an average of 4.4. Coordination number is a direct sign to measure pore connectivity. The higher the coordination number, the better the connectivity of holes. The maximum coordination number of sandstone reservoir pores is generally between 2 and 7, with an average of 3.2. The coordination number is directly related to the contact relationship of particles, the content of cement and the development degree of secondary pores. Generally speaking, the weaker the mechanical compaction degree, the lower the cement content, the more developed the secondary pores and the higher the pore coordination number.

To sum up, the sandstone reservoirs in the study interval are mainly small and medium-sized pores and small and medium-sized throats, with complex pore shapes, various types and poor connectivity. The microscopic heterogeneity is moderately strong, and the storage and permeability conditions are moderately deviated. Of course, it is not excluded that individual intervals have good reservoir performance in local areas.

Three. Geological factors affecting pore structure

The pore structure of reservoir is controlled by many factors such as sedimentary environment, diagenesis and tectonism.

1. Influence of sedimentary environment

The sandstone reservoir in the study interval is a set of sandstone bodies formed in alluvial fan environment. Generally speaking, rocks have poor sorting, low maturity of composition and structure, and poor original pore structure conditions. In contrast, the pore structure of sand bodies deposited in alluvial fan channel is better than that deposited between channels.

2. The influence of particle size

The effect of particle size on pore structure is that porosity increases with the increase of median particle size (Figure 3); Permeability also tends to increase with the increase of particle size.

Fig. 3 Porosity and particle size median scatter plot

3. The influence of particle classification

The effect of particle sorting on pore structure is that porosity decreases with the increase of sorting coefficient (Figure 4); The logarithm of permeability decreases with the increase of separation coefficient.

4. The influence of cement composition and content

Common cements include carbonate, authigenic clay minerals and siliceous substances. The higher the content of clay heterobase, the more micropores and the worse the pore structure. Authigenic clay minerals form a thin film on the particle surface or fill the pores in the form of pore lining or pore filling, which narrows the throat, reduces the pore size and deteriorates the pore structure. Carbonate cement is soluble, so in the early stage of diagenesis, if carbonate cement develops, porosity and connectivity will be partially or completely lost; In the late diagenesis stage, the pore structure became better due to the partial or total dissolution of carbonate minerals; Autogenous enlargement will fill pores and throat and make pore structure worse. The influence of cement content on pore structure is that porosity decreases with the increase of carbonate content (Figure 5); With the increase of carbonate content, the permeability tends to decrease; With the increase of carbonate content, displacement pressure tends to increase; With the increase of carbonate content, R tends to decrease. The minimum unsaturated pore volume percentage increases with the increase of carbonate content.

Fig. 4 Scatter diagram of porosity and particle size sorting coefficient

5. The influence of diagenesis

In the early diagenetic stage, due to mechanical compaction and cementation of authigenic minerals, the primary pores and throats were destroyed to a great extent, and the pore structure became worse. In the late diagenetic stage, due to the development of dissolution, a large number of secondary dissolution pores were produced, and the cement was partially or completely dissolved, which made the pore structure better.

Fig. 5 scatter plot of porosity and carbonate content

6. The influence of tectonic action

Tectonic movement breaks the rock, produces a lot of cracks, connects the pores of the reservoir, and enhances the permeability. Throughout this area, structural fractures are extremely undeveloped, so it is of little significance to improve the pore structure of reservoirs. It is speculated that the sandstone reservoir located at the fault may form some structural fractures due to fault activity, thus improving the pore structure.

Capillary pressure curve and gradient radius characteristic map of different types of reservoirs.

Table 2 Types and characteristics of clastic reservoirs in Kong Yi section of Wang Guan Tun Oilfield

Four. Reservoir classification and evaluation

Reservoir classification and evaluation is an important work in reservoir research. Different reservoir types have different reservoir conditions and micro-pore structures, and the seepage mechanism of fluid in them is also different, so the effects of water flooding and oil recovery are also different. According to the pore structure characteristics of the study area, combined with the macroscopic physical parameters and other characteristics, the sandstone reservoirs in the study interval are divided into four categories. The classification and evaluation of various reservoirs are shown in Table 2, and the typical capillary pressure curve shape and pore throat distribution are shown in Figure 6. Among them, Class I reservoir is a high porosity and high permeability reservoir, with a permeability greater than 500× 10-3μm2, accounting for about 19.05% by sample percentage. The permeability100×10-3 ~ 500×10-3 μ m2 and porosity 17.5% ~ 26.6%, with an average of 22.0 1%, are good reservoirs in this area. The permeability of Class III reservoir is between10×10-3 ~100×10-3 μ m2, and the porosity is generally 14. 1% ~ 24.3%, with throat type. The permeability of Class IV reservoir is less than 10× 10-3μm2, and the porosity is between 12.3%-22. 1%. Although the porosity changes greatly, the measured permeability is very small. The throat is mainly tube bundle or sheet throat, and this kind of reservoir has poor storage performance, mainly sand mud flat deposition, accounting for about 15.87% of the total number of samples. To sum up, as far as sample statistics are concerned, the studied intervals are mainly Class I, II and III reservoirs, accounting for about 85%, of which Class I and II reservoirs have good reservoir performance, accounting for about 33.34%.

Verb (abbreviation of verb) conclusion

(1) There are 7 pore types, 4 throat types and 4 pore-throat combination types in Zao Ⅱ and Zao Ⅲ oil formations.

(2) Pore structure is influenced by sedimentary environment, cement content and diagenesis.

(3) According to the quantitative indicators such as permeability and pore structure characteristics, combined with other indicators, the reservoirs of Zao-2 and Zao-3 oil groups are divided into four categories, of which three types of reservoirs account for 50.79% of the total reservoirs.

refer to

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Qiu Yinan, Xue Shuhao, et al. Oil and gas reservoir evaluation technology. Beijing: Petroleum Industry Press, 1994.

Qiu Yinan, Xue Shuhao, Ying Fengxiang. Essays on oil and gas reservoirs in China (continued 1). Beijing: Petroleum Industry Press, 1993.

(4) Characteristics and influencing factors of Paleogene reservoirs in Nanpu Depression, Xu Long, Journal of Jianghan Petroleum Institute, 1994, 16(2).

Ying Fengxiang. Diagenetic stage code of clastic rocks. Beijing: Petroleum Industry Press, 1993.

Zheng Junmao, Pang Ming. Diagenesis of clastic reservoir rocks. Wuhan: China Geo University Press, 1989.

(7) Science and Technology Development Department of China Petroleum and Natural Gas Corporation. Essays on oil and gas reservoirs in China. Beijing: Petroleum Industry Press, 1993.