(China Petroleum Exploration and Development Research Institute, Beijing 100083)

The Devonian system in Tahe area is distributed in Toputai work area, and the sand bodies in the study area are generally distributed in the north-south direction, with thick northwest and thin southeast. The reservoir space of the reservoir is mainly secondary pores such as primary intergranular pores, intergranular dissolved pores and intragranular dissolved pores, with few other types of pores. There is a moderate deviation in the sequencing of reservoir throats, and most of them are thin throats. The Donghe sandstone reservoir in this area mainly develops diagenetic types such as compaction, cementation and dissolution. According to reservoir physical properties and reservoir evaluation criteria, the reservoirs in this area are divided into three categories and classified and evaluated.

Devonian reservoir in Tuoputai, Tahe

Study on Reservoir Characteristics of Devonian Donghetang Formation in Tahe Area

Feng xingqiang

(China Petrochemical Exploration and Development Research Institute, Beijing 100083)

Devonian strata are distributed in Toputai area. The sand body direction is S-N, and the reservoir pore types are mainly residual intergranular pores, intergranular dissolved pores and intragranular dissolved pores. The classification of the throat is a classification. Sandstone reservoir is a fine throat. The results show that Donghe sandstone in this area experienced compaction, clay cementation, Shi Ying amplification, carbonate cementation and dissolution during diagenesis. According to the physical properties and evaluation criteria of reservoirs, the reservoirs are divided into three categories and evaluated.

Keywords Devonian stratigraphic reservoir in Tuoputai, Tahe

Devonian in Tahe area is distributed in Toputai work area, which is located in Kuqa County and Shaya County of Xinjiang Uygur Autonomous Region. This structure is located at the southwest end of Akkule Uplift of Shaya Uplift in the northeast depression area of Tarim Basin, adjacent to the main area of Tahe Oilfield in the northeast, adjacent to Halahatang sag in the northwest, and with Shuntuogole low uplift in the south.

Only Donghetang Formation is developed in Devonian in this area, and the residual thickness of strata is thick in the west and thin in the east. The deposit at the thickest part of the stratum is 180m, which is mainly distributed in TP4 well area, and the stratum thickness in TP5 well area is only 20m. In recent years, with the continuous expansion of exploration field in Tahe area, a number of exploration wells have been drilled into Devonian system, and good oil and gas shows have been obtained one after another, indicating that Donghetang Formation has great oil and gas exploration potential.

1 Basic characteristics of oil reservoir

1. 1 reservoir petrological characteristics

According to the detailed observation of more than 20 drilled cores and the careful analysis of more than 540 rock slices, it is found that the rock type of Donghetang Formation reservoir is mainly timely sandstone, followed by lithic timely sandstone and a small amount of feldspar lithic sandstone. Gray, gray-white fine-grained timely sandstone interbedded with green-gray, brown, dark gray mudstone, argillaceous siltstone, silty mudstone and calcareous mudstone with unequal thickness. South sandstone increases. The timely content of rocks is above 65% ~ 90%, and the average content of cuttings and feldspar is below 10% (Figure 1). The matrix is basically argillaceous, and its content is generally 1% ~ 20%. The cementation type is mostly porous, with a small amount of crystal base cementation and particle support, and the cementation is gray. The roundness of grinding is mainly sub-circle-sub-angle, followed by angle, which is a little round.

Figure 1 Classification Map of Devonian Sandstone in Topitai Work Area

1.2 sand body distribution characteristics

According to the statistical data of sandstone thickness of nearly 40 wells in Toputai work area, the sandstone isothickness map of Devonian Donghetang Formation is made (Figure 2). As can be seen from the figure, the sandstone of Donghetang Formation is basically distributed in the Toputai research area, and it is generally distributed in the north-south direction. Sandstone is thick in the northwest and thin in the southeast. The sandstone thickness in TP4 well area is close to160m, while the sandstone thickness in TP5 and TP9 well areas is only18m.

Fig. 2 Distribution map of sandstone thickness of Donghetang Formation in Toputai area

1.3 reservoir space type

According to the observation and data statistics of ordinary thin slices and casting thin slices, the primary intergranular pores and secondary dissolution pores are relatively developed in the reservoir space of Devonian reservoir in the work area, which can be divided into the following four types according to the pore fabric characteristics and causes (Figure 3).

Fig. 3 Photo of Donghetang Formation Reservoir Castings in Toptaigong Area

(1) Primary intergranular pores: Primary intergranular pores are unfilled or semi-filled pores between particles, mostly residual intergranular pores formed by compaction and cementation. This kind of pore is rare, and the pore size is not large, and the pore shape is often irregular.

(2) intergranular dissolved pores: intergranular dissolved pores are pores formed by the dissolution of interstitial substances between groundwater dissolved particles or the edge of debris particles. Devonian in this area mainly has two types of intergranular dissolved pores and intergranular dissolved pores, which often coexist, with irregular shape and large pore size, and are the main pore types of Devonian in this area.

(3) Intra-granular dissolved pores: Intra-granular dissolved pores are intra-component pores formed by internal dissolution of rock particle components. It is mainly formed in feldspar, unstable cuttings and other particles, and it is distributed in isolation, often accompanied by other types of dissolved holes, and its distribution is also relatively common.

(4) Micro-fracture: Micro-fracture is a fracture caused by tectonic movement, with a flat interface, which is a bridge connecting pores, and its distribution is very limited, only found in individual wells.

1.4 reservoir pore structure characteristics

The pore structure of reservoirs in the study area is analyzed by using capillary pressure curves of more than 30 wells. The results show that the throat sorting coefficient is mostly between 2 and 3.5, and the variation coefficient is mostly between 0.19 and 0.5, which indicates that the throat sorting of Devonian reservoirs in this area is moderately deviated, with an average value of 7 ~ 12 and a displacement pressure of 0. 1 ~. It reflects that the reservoir performance is medium, and the median radius is mostly below1μ m. From these pore structure parameters, it can be seen that except for a few samples, the throat is mostly thin.

According to the statistical analysis of the capillary pressure curve of the sample, it can be seen that the slope of the curve in this area is relatively large, and there is basically no platform, indicating that the degree of pore-throat separation is lower, and the curve gradually transitions from slightly coarse deviation in the lower left corner to fine deviation in the upper right corner (Figure 4), and the displacement pressure gradually increases, reflecting the gradual deterioration of pore structure.

Fig. 4 Capillary pressure curve in the study area

By analyzing pore structure parameters and capillary pressure curves, Devonian reservoirs in the study area are divided into three types of pore structure.

1.4.1Ⅰ pore structure

This type of reservoir with pore structure has high porosity and permeability, with porosity generally exceeding 15% and permeability generally above 100× 10-3μm2. The displacement pressure is less than 0. 1MPa, and the average throat is generally greater than 7 μ m. The capillary pressure curve is distributed at the lower left (Figure 4), and the skewness belongs to a slightly coarse skewness type. The observation of casting and electron microscope shows that intergranular pores or (and) dissolved intergranular pores are generally developed in this kind of reservoir. The lithology is mainly medium-fine sandstone, with good sorting and low interstitial content, mainly argillaceous.

1.4.2 Ⅱ pore structure

This kind of reservoir generally has medium-low permeability, with porosity ranging from10% to15%, permeability ranging from tens to tens of×10-3 μ m2, displacement pressure ranging from 0. 1 ~ 1 MPa, and capillary pressure curve. Lithology is generally dominated by fine sandstone, with more interstitial materials, mainly calcium.

1.4.3 Ⅲ pore structure

Reservoirs with this pore structure type are generally low or ultra-low reservoirs and permeability, with porosity generally less than 10% and permeability generally less than several millidarcy, with high displacement pressure, capillary pressure curve distributed in the upper right, and skewness belonging to fine deviation-fine deviation. The lithology is fine, generally dominated by silty sandstone.

1.5 reservoir physical characteristics

According to the statistical results of porosity and permeability of Donghe sandstone (Figure 5), the average porosity is generally 6% ~ 20%, the minimum is 1. 1%, the maximum is 33.55%, and the average is1.25%. The main distribution range of permeability is (0.1~100 )×13 μ m2, with the minimum value of 0.02× 10-3μm2 and the maximum value of 388× 10-3μm2. The above statistical data of porosity and permeability show that the sandstone of Donghetang Formation belongs to medium-low porosity and medium-low permeability reservoir, although its physical properties are relatively good. The relationship between porosity and permeability given in Figure 6 shows that there is a good positive correlation between porosity and permeability of sandstone in Donghetang Formation. When the porosity is less than 12%, the permeability is very small, mostly less than 5× 10-3μm2. When the porosity is greater than 12%, the permeability increases rapidly with the increase of porosity. The fracture signs are not obvious, and the physical parameters reflect the characteristics of porous reservoirs.

Fig. 5 Histogram of porosity and permeability of Donghe sandstone

Fig. 6 Comparison of porosity and permeability of sandstone in Donghetang Formation

1.6 reservoir diagenesis

According to the observation of diagenetic characteristics of a large number of rock slices, scanning electron microscopy and cathodoluminescence of reservoir samples, Donghe sandstone reservoir in this area has mainly experienced the following diagenesis.

1.6 1 compaction

With the increase of buried depth, compaction is gradually strengthened. The contact relationship between particles has also changed from point contact to line contact, even convex-concave contact. Through thin section observation, it can be seen that the particles in the study area are in point contact-line contact, indicating that compaction has little influence on the reservoir. This is mainly because Donghe sandstone in the study area has been in the shallow burial stage after deposition, and only Neogene Donghe sandstone is in the deep burial stage.

1.6.2 cementation

The study area mainly includes argillaceous cementation, carbonate cementation and timely cementation.

(1) Iron mud cementation: During compaction, the iron mud matrix binds the surrounding particles together. This clay mineral is generally iron chlorite and montmorillonite, which are deposited at the same time with the particles and then attached to the surface of the particles, which is the earliest cementation of rocks and reduces the porosity by 2% ~ 10%.

(2) Carbonate cementation: Carbonate is the most developed cementation mineral in this area, and the cementation of carbonate in some intervals can reach 40%, mainly calcite, iron-bearing calcite and dolomite, mainly with pore cementation, including basement cementation and basement-pore cementation.

(3) Increase cementation in time: This cementation is common, with a general content of 5% ~ 7%, and a few reach 10%. It is mainly from narrow to large, and it is filled in compacted residual holes and gypsum dissolved holes, forming quartzite sandstone locally. Siliceous debris is mostly timely and has comb-like particles. This is related to many factors, such as less heterobase, undeveloped clay ring edge, high timely content, high structural maturity, weak plastic compaction and so on. In addition, some timely particles are cemented, and some feldspar is greatly increased.

1.6.3 dissolution

The dissolution of Donghe sandstone in this area is not obvious. After carbonation and kaolinization, the major phases are mostly along the kaolinite site, and authigenic minerals and particles, such as carbonate, cuttings, feldspar, kaolinite, etc., are dissolved, forming dispersed super-large dissolved pores and pores, with the maximum of 1.4mm and the average of 0.65,438+0 ~ 0.75 mm, with more kaolinite remaining, and then filled with a small amount of asphalt.

2 reservoir evaluation

Through the above analysis and research, according to the reservoir development in this area and combined with reservoir physical properties, Devonian reservoirs in the study area are divided into the following three types.

2.1Ⅰ reservoir

This kind of reservoir has high permeability, with porosity greater than 15% and permeability greater than 100× 10-3 μm 2, and is a medium porosity-medium permeability reservoir. The average radius of pore throat is more than 9μm, the displacement pressure is less than 0. 1MPa, the mercury injection curve is rough or slightly rough, and the platform is very short. The lithology is mainly medium-fine grained timely sandstone, with a small amount of interstitial materials, mainly argillaceous.

2.2 Secondary reservoir

This kind of reservoir has a porosity of 9% ~ 15% and a permeability of (10 ~100) ×10-3 μ m2, which is a medium-low porosity-medium-low permeability reservoir. Pore space is dominated by various dissolved pores and residual primary intergranular pores. The average radius of pore throat is 2 ~ 9 microns, which is relatively small, so the displacement pressure is distributed between 0. 1 ~ 0.3 MPa, the mercury injection curve is not curved, and the intermediate platform is not obvious or very short. Lithology is mainly fine-grained lithic sandstone and feldspar sandstone. There are few muddy bases and weak carbonate cementation.

2.3 tertiary reservoir

This kind of reservoir has small median pore throat radius and high displacement pressure, mainly low porosity and permeability. Capillary pressure curve is characterized by fine skewness, and there is almost no middle platform of skewness. The porosity of the reservoir is 6% ~ 9%, which is mainly intragranular dissolved pores, intergranular dissolved pores and micropores, and the permeability is generally between (1~10) ×10-3 μ m2. The lithology is some very fine-grained lithic feldspathic sandstone and very fine-grained feldspathic lithic sandstone with high calcium content. And there is a certain muddy matrix.

3 Conclusion

(1) The rock types of Donghetang Formation in the study area are mainly timely sandstone, followed by lithic timely sandstone and a small amount of feldspathic lithic sandstone. Sand bodies are generally distributed in the north-south direction, thick in the northwest and thin in the southeast.

(2) The reservoir space is mainly composed of primary intergranular pores and secondary dissolution pores, with few other types of pores. There is a moderate deviation in the classification of reservoir throat. Except for a few samples with large pore throats, most of the throats are thin throats with general reservoir characteristics.

(3) Donghe sandstone reservoir in this area mainly experienced diagenesis such as compaction, cementation and dissolution. According to the reservoir physical properties and development status, the reservoirs in the study area are divided into three categories.

I would like to express my heartfelt thanks to Yang Suju and Ding Yong, senior engineers from Geological Exploration Institute of China Petrochemical Northwest Branch, for their help.

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