(Guangzhou Marine Geological Survey Guangzhou 5 10760)

About the author: Qian Xing (1985-), male, assistant engineer, mainly engaged in scientific research and production in marine petroleum geology. E-mail: made607 @126.com.

Comprehensive application of logging, conventional physical properties, casting thin section, scanning electron microscope, X-ray diffraction, particle size and mercury injection techniques, the characteristics and influencing factors of low porosity and low permeability reservoirs in the third member of Shahejie Formation in an oil field in the eastern South China Sea are analyzed. The analysis results show that the low porosity and permeability reservoirs in the study area are characterized by low lithofacies composition and low structural maturity. The reservoir space includes primary pores, secondary pores, mixed pores and a small amount of cemented pores, and the pore structure type belongs to small pores and micro-throats. The genesis of low porosity and permeability reservoirs in the third member of Shahejie Formation is mainly influenced by sedimentation and diagenesis. The sedimentary facies belt essentially determines the low porosity and permeability characteristics of the reservoir, which is mainly developed in the fan body, fan root and fan front sand body of the inshore underwater fan in the third member of Shahejie Formation. Diagenesis has a decisive influence on the physical properties of reservoirs. Compaction reduces the porosity and permeability of the reservoir, cementation further worsens the porosity and permeability of the reservoir, and dissolution expands the development and communication of the pore system to a certain extent and improves the physical properties of the reservoir.

Keywords Influencing factors of low porosity and low permeability reservoir characteristics

With the continuous growth of energy demand and the continuous development of exploration and development technology, low porosity and low permeability oil and gas reservoirs have become a new direction of oil and gas reserves growth. Compared with conventional reservoirs, low porosity and low permeability reservoirs often have more complex rock structure and genetic mechanism. The dynamic data of oil and gas field development also shows that the development schemes and effects of different types of reservoirs are obviously different [1].

Taking the third member of Shahejie Formation in an oilfield in the eastern South China Sea as an example, the characteristics of low porosity and low permeability reservoirs in this oilfield are studied through experimental analysis data such as logging, casting thin section and physical property analysis, and their causes and influencing factors are analyzed, which has certain guiding significance for further understanding and developing low porosity and low permeability reservoirs.

1 low porosity and permeability reservoir characteristics

1. 1 reservoir petrological characteristics

The lithology of the third member of Shahejie Formation is mainly gray-gray feldspathic lithic sandstone and lithic feldspathic sandstone (Figure 1), and the feldspar content is 19.5% ~ 45.0%, with an average of 36.7 1%. The seasonal content is 65438 08.0% ~ 58.0%, with an average of 465438 0.82%. The content of drilling cuttings is 7% ~ 62.5%, with an average of 265,438 0.74%. The main components are intrusive rocks, extrusive rocks and metamorphic quartzite. The filler content is 65,438+0.5% ~ 35%, with an average of 6.49%. The main components are carbonate cements, such as argillaceous, dolomite and calcite, common secondary siliceous cements and some siderite cements.

Figure 1 Triangle Diagram of Rock Types in the Third Member of Shahejie Formation

Fig. 2 Characteristics of capillary pressure curve in low porosity and low permeability reservoirs

X-ray diffraction analysis shows that illite is the main clay mineral in the reservoir, with an average relative content of 8 1.77%, and the average relative content in the mixed layer of Yimeng is 10.44%, with a small amount of clay minerals such as kaolinite and chlorite.

The grain size of sandstone is generally coarse, mainly coarse sandstone, followed by medium sandstone and fine sandstone; Particle sorting is poor, the standard deviation σ 1 is mainly between 1 ~ 2, and the roundness is low, mostly prismatic and prismatic-subprismatic.

1.2 reservoir pore types and characteristics

Thin section analysis of casting shows that there are primary pores, secondary pores, mixed pores and a small amount of cement pores in the reservoir space of the third member of Shahejie Formation, and the area ratio is 0.5% ~ 10%, with an average of 3. 17%.

The analysis of mercury injection data shows that the pore throats of the reservoir are widely distributed, with the maximum pore throat radius of 73.5 microns, but few pore throats are larger than 10.0μm, and pores smaller than 0.63μm account for 73.3% of the total pore throats, and the pore throats are mainly distributed between 1.0 ~ 2.0 μ m (Figure 2). According to the previous classification standards of pore structure [2 ~ 3], the types of reservoir pore structure mainly belong to small pores and micro-throats.

2 Analysis of influencing factors of low porosity and low permeability reservoirs

2. 1 sedimentation is the basis of determining reservoir physical properties.

On the basis of previous exploration and research, the sedimentary microfacies of a single well are analyzed by studying the lithofacies of each well in this area and combining the characteristics of grain size and logging curve (Figure 3). The results show that the third member of Shahejie Formation in this oilfield is a nearshore subaqueous fan deposit close to provenance, which can be further divided into fan root, fan main body and fan front deposit, in which the fan root is mainly composed of coarse clastic materials, and the monolayer sequence is often composed of reverse and forward progressive sections from bottom to top, and the amplitude of GR curve is not obvious. The fan body is located in front of the fan root, and the glutenite is superimposed. The grain size change of single-layer sequence is conglomerate-glutenite or granular sandstone-sandstone from bottom to top. The grain size of sandstone in front of fan becomes finer, and turbidites in Ma Bao sequence are mainly developed. GR curves are mostly serrated funnel bell-shaped.

Fig. 3 Single well facies analysis

The statistical analysis of core physical property data (Table 1) shows that the porosity of the nearshore subaqueous fan sand body in this section is mainly distributed between 1.7%- 16.90%, with an average of 12.5%, and the permeability is mainly distributed at 0.01/kloc. Further statistical analysis of porosity and permeability of different microfacies sand bodies shows that there are some differences in physical properties of microfacies sand bodies, among which the physical properties of fan root and fan main body sand bodies are relatively good, while the physical properties of fan front sand bodies are relatively poor.

Table 1 Comparison Table of Physical Properties of Sand Bodies and Reservoirs with Different Sedimentary Facies

2.2 Diagenesis has a decisive influence on the physical properties of reservoirs.

The sandstone reservoir performance of the third member of Shahejie Formation in the study area is obviously influenced and reformed by diagenesis, which can improve and destroy the reservoir performance. Among them, diagenesis, which has a great destructive effect on reservoir physical properties, has compaction and cementation, and dissolution has a certain contribution to the improvement of reservoir physical properties

(1) compaction

Compaction is the most common and important diagenesis in the study area, and it is one of the most important factors that cause low porosity and permeability of reservoirs. The third member of Shahejie Formation is buried deeply (3,350 ~ 4,000 m), and the content of soft particles such as feldspar and cuttings in sandstone components is high, which generally undergoes strong compaction, resulting in primary pore loss and poor permeability. Its main manifestations are: (65,438+0) linear and point contact between debris particles (Figure 4a); (2) Plastic particles are bent, deformed and elongated, and some of them squeeze into the pores to form pseudo-impurity clusters (Figure 4b); (3) The enhancement of pressure dissolution makes the particles dissolve at the contact point, which provides a material basis for the timely secondary growth. A large number of studies show that [4 ~ 5], the occurrence of compaction can greatly change the pore structure and distribution of sand bodies, leading to a significant reduction in pore space, and the porosity reduction rate of some sandstones can even be as high as 50%, which will greatly weaken the fluidity of pore fluid in sand bodies, thus affecting the reservoir performance of sand bodies.

(2) Cementation

Cementation destroys the physical properties of reservoirs, which is another main factor that makes the physical properties of reservoirs in this area worse. Clay cementation and carbonate cementation are the main factors, followed by timely secondary cementation.

X-ray diffraction analysis shows that the clay minerals in the third member of Shahejie Formation are mainly illite and illite/montmorillonite mixed layer minerals, with average contents of 865,438+0.77% and 65,438+00.44% respectively, and a small amount of kaolinite, chlorite and montmorillonite mixed layer. Under the scanning electron microscope, feldspar is dissolved to form filiform illite (Figure 4c), while authigenic illite minerals are mostly fibrous or filiform, which are filled in pores in the form of network bridging (Figure 4d), making intergranular pores become intergranular pores, blocking pore throats and increasing intergranular micropores, thus reducing the permeability of sandstone.

Carbonate cement is one of the common cements in this reservoir. The thin section analysis of the casting shows that the carbonate cement is mainly dolomite (iron), with a content of 0.5% ~ 4%, with an average of 65,438 0.28%, and the particles are filled in a sporadic or lumpy way. Occasionally, crystalline granular calcite (iron) is sporadically filled in the particles (fig. 4e). Statistical analysis shows that reservoir porosity and permeability are negatively correlated with carbonate content, and the more carbonate cement content, the worse its physical properties. Carbonate cement fills the pores and occupies part of the detritus, thus forming a dense reservoir that is difficult to transform, resulting in the complete loss of intergranular pores, the difficulty in developing secondary pores and the enhancement of reservoir heterogeneity.

Siliceous cementation is mainly manifested by the secondary increase of the timely detrital particles, and some siliceous substances often grow in the form of spontaneous reaction in the pore walls or dissolved pores of feldspar and other particles (Figure 4f), and its mass fraction in reservoir sandstone is generally 0.4% ~ 0.6%. The timely secondary expansion makes the pores smaller, the throat narrower, and the severely developed section even blocks the pores, which seriously damages the reservoir performance and reduces the reservoir quality.

Fig. 4 Various diagenesis of the third member of Shahejie Formation.

(3) dissolution

Sandstone reservoirs often experience different degrees of dissolution, forming various types of secondary pores. Dissolved secondary pores are formed by mineral dissolution caused by carbonic acid or organic acid, which has a positive effect on improving the reservoir performance of sandstone reservoirs [6]. The thin section analysis of castings shows that the main dissolved pores in the study area are enlarged intergranular dissolved pores, intragranular dissolved pores, mold pores and cement internal pores, among which the enlarged intergranular pores are the main ones, accounting for 60% ~ 82% of the surface area of dissolved pores. The widespread dissolution in this area further expands the development and communication of pore system, and the secondary pores produced can improve the reservoir physical properties to some extent.

3 Conclusion

1) The reservoir sand body of the third member of Shahejie Formation is dominated by feldspathic lithic sandstone, with poor particle sorting, mostly subangular and mainly linear contact. Sandstone particles are generally characterized by low maturity in composition and structure.

2) The reservoir space of the reservoir is mainly intergranular pores with a certain amount of dissolved pores. Pore radius is distributed in a wide range, but mainly in the range of 1.0 ~ 2.0μ m, and the pore structure belongs to the type of small pores and micro-throats.

3) Sedimentary environment is the geological basis that affects the reservoir performance in the study area. The third member of Shahejie Formation is a nearshore subaqueous fan deposit, and the fan root, fan main body and fan front sand body have poor reservoir properties, all of which are low porosity and permeability reservoirs.

4) Diagenesis has a decisive influence on the physical properties of the third member of Shahejie Formation. The compaction and cementation in this area have obvious influence on the physical properties of the reservoir. Dissolution further expands the development and communication of pore system, and the secondary pores produced can improve the physical properties of reservoir to some extent.

refer to

[1] Li Daopin. Development of low permeability sandstone oilfield [M]. Beijing: Petroleum Industry Press, 1997.

Luo Zhetan, Wang Yuncheng. Pore structure of oil and gas reservoirs [M]. Beijing: Science Press.

Wang Yuncheng. Oil and gas reservoir evaluation [M]. Beijing: Petroleum Industry Press, 1999.

Liu, Jin Zhijun, Zhang. Experimental study on diagenetic compaction of clastic rocks [J]. Acta Sedimenta Sinica, 2006, (3).

Liu, Liu Yang and Zhang. Influence of compaction on sandstone reservoir properties [J]. Journal of Youshi University (Natural Science Edition), 2006, (4).

Zheng Junmao, Pang Ming. Diagenesis of clastic reservoir rocks [M]. Wuhan: China Geo University Press,1989,82 ~156.

Characteristics of low porosity and permeability reservoirs and their influencing factors: A case study of the third member of Shahejie Formation in an oilfield in the eastern South China Sea

—— Take the third member of Shahejie Formation in an oilfield in the eastern South China Sea as an example.

Qian xing

(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

Abstract: Based on logging data, physical property analysis, casting section, scanning electron microscope, diffraction, particle size analysis and mercury injection, the characteristics and influencing factors of low porosity and low permeability reservoirs in the third member of Shahejie Formation in an oil field in the eastern South China Sea are comprehensively studied. The results show that low porosity and low permeability reservoirs have the characteristics of low composition maturity and low structure maturity. The reservoir types include intergranular pores, secondary pores, mixed pores and a small amount of cement dissolved pores, and the pore structure is mainly fine pores and micro-throat. Sedimentation and diagenesis act together on low porosity and low permeability reservoirs, and the sedimentary environment of the third member of Shahejie Formation determines the nature of the reservoirs, and the low porosity and low permeability reservoirs are mainly distributed in the sand bodies of offshore underwater fan system. Diagenesis has obviously changed the physical properties of sand bodies, and compaction and cementation are the main factors leading to the deterioration of porosity and permeability of turbidite sandstone. On the other hand, dissolution promotes the development and communication of pore system and improves the reservoir performance to some extent.

Keywords: factors affecting the reservoir characteristics of low porosity and low permeability reservoirs