Study on the characteristics of lacustrine dolomite reservoir in the lower member of Shahejie Formation in Qikou Depression

Chen 1 Xiao Dunqing 2 Li Cong 1 Pu Xiugang 2 1

(1. China Youshi University (East China), Qingdao, Shandong 255666; 2. Exploration and Development Research Institute of Dagang Oilfield Company, Tianjin 300280)

Abstract: There are five types of dolomite in the lower member of Shahejie Formation in Qikou Depression: microcrystalline dolomite, micrite dolomite, argillaceous dolomite, sandy dolomite, calcareous dolomite or dolomitic limestone. The dolomite reservoirs are mainly microcrystalline dolomite and micrite dolomite. The reservoir space is dominated by structural fractures and dissolved pores. Dolomite reservoirs are mainly distributed in Bin 1 and Ban 4 oil groups in the lower member of Shahejie Formation. Liujianfang-Lian Meng-Zhouqingzhuang and Zhaojiabao areas, which are dominated by buried dolomite, are the most favorable areas, and Qijiawu area, where quasi-syngenetic dolomite is distributed, is also a favorable exploration zone.

introduce

Lacustrine dolomite occurs in Paleogene Shahejie Formation in Jiyang Depression, Paleogene Hetaoyuan Formation in Biyang Depression, Permian in Junggar Basin and Paleogene in Qaidam Basin. In the past, the research on it mainly focused on the formation conditions and material sources. Tian Jingchun et al. (1998), after studying the dolomite of Paleogene Shahejie Formation in Dongying Depression, think that the occurrence horizon of dolomite has obvious corresponding relationship with transgression period [1]; Sun Yu et al. (2007) believed that transgression, paleoclimatic conditions and volcanic activity all created conditions for the formation of dolomite in the lower member of Shahejie Formation in Huimin Depression [2]; Huang Xingzhen and others (200 1) think that the dolomite of Paleogene Hetaoyuan Formation in Biyang Depression is mainly caused by evaporation [3]; Cai Yi et al. (2005) studied the Paleogene dolomite in Skur, Qaidam Basin, and thought it was mainly caused by evaporation [4]. Dai Chaocheng et al. studied a set of deep lacustrine dolomite of Paleogene Shahejie Formation in Liaodong Bay Basin, and found that its occurrence position was obviously controlled by the NE-trending basement fault at the western boundary of Liaozhong Depression [5]. However, the study on lacustrine dolomite reservoir is relatively weak.

Distribution characteristics of 1 dolomite

Qikou sag is located in the middle of Huanghua depression, with Cangxian uplift in the west and Chengning uplift in the southeast, showing NNE-NE direction as a whole. The lower member of Shahejie Formation can be divided into four oil layers from bottom to top: Bin 1, Plate 4, Plate 3 and Plate 2 (Figure 1). Previous people thought that carbonate rocks in the lower member of Shahejie Formation in this area were mainly limestone, and micrite limestone was poor reservoir [6,7]. With the deepening of exploration, it is gradually recognized that the dolomite of Xiashahejie Formation is widely developed, mainly distributed in Qijiawu, Liujianfang, Wangxuzhuang and Zhaojiabao (Figure 2).

2 dolomite reservoir rock types

According to the actual situation of the study area, the classification of dolomite mainly considers its composition and crystallinity. According to the composition, it can be divided into mixed dolomite and clastic rock (argillaceous dolomite and sandy dolomite) and transitional dolomite and limestone (gray dolomite or dolomite limestone); According to the degree of crystallization, it can be divided into microcrystalline dolomite and micrite dolomite. Micky dolomite refers to dolomite with grain size ranging from aphanitic to microcrystal (Figure 3A), which is mainly developed in the bifurcation area and Chenghai area of Bin 1 and Ban 4 oil groups. The grain size of microcrystalline dolomite is about 2 ~ 5 microns, and its shape is abnormal-semi-self-shaped, mainly abnormal (Figure 3B). It is mainly distributed in Qijiawu, Liujianfang-Zhouqingzhuang-Wangxuzhuang areas of Ban-4 oil formation, with a small amount in Qijiawu and Liujianfang areas of Bin 1 oil formation. The argillaceous dolomite is mainly thin banded dark mudstone and light grayish white dolomite, which are interbedded in a microwave shape (Figure 3C). It is mainly developed in the east and west sides of Kongdian uplift in Ban 4 oil formation and Wangxuzhuang area in Ban 3 oil formation. Sandy dolomite is mainly composed of silty chronological and feldspar particles (Figure 3D). X-ray diffraction analysis also shows that dolomite in this area has a high time content, and feldspar is mainly potassium feldspar, which is mainly distributed in the lower part of Bin 1 and Ban 3 oil groups in Zhaojiabao area. Limestone dolomite is a transitional rock between dolomite and limestone. The grain size of dolomite is obviously larger than that of limestone (Figure 3E), with obvious recrystallization, while micrite limestone generated from dolomite has no obvious recrystallization.

Figure 1 Comprehensive Stratigraphic Histogram of Lower Member of Shahejie Formation in Qikou Depression

Fig. 2 Dolomite Distribution Map of Lower Member of Shahejie Formation in Qikou Depression

Three types of reservoir space

There are various types of dolomite reservoir spaces in the lower member of Shahejie Formation in Qikou Depression, including intragranular dissolved pores, mold pores, extra-large dissolved pores, intergranular dissolved pores and structural fractures.

3. 1 internal solution hole and die hole

Intragranular dissolved pores mainly exist in bioclastic dolomite, oolitic dolomite or bioclastic limestone. Bioclastic or oolitic particles with intragranular dissolved pores are mainly limestone with a small amount of dolomite. When some particles are strongly dissolved and only retain the original particle shape, they become mold holes (Figure 4A, B), and the size of the mold holes varies from tens of microns to several millimeters.

Fig. 3 Dolomite types of the lower member of Shahejie Formation in Qikou Depression

3.2 Super-large dissolution hole

Extra-large dissolved pores are mainly developed in microcrystalline dolomite and bioclastic dolomite, and the volume of dissolved pores is twice or more than the largest clastic particles or particles around. When extra-large dissolved pores develop in granular carbonate rocks, it is actually the expansion of intergranular dissolved pores, that is, dissolution not only erodes cement, but also erodes particles (Figures 4C and d).

3.3 Intergranular Dissolved Pore

Intergranular dissolved pores are mainly developed in crystalline dolomite. Because the dolomite in the study area is mainly micrite-microcrystalline dolomite, such pores are very small and difficult to observe under the microscope. However, it can be found by scanning electron microscope that intergranular pores are more developed, especially in fine-grained dolomite with a high degree of autocrystallization (Figures 4E and F). Some microcrystal pores are larger, and the micropores can reach 5 ~ 10μ m, which can be turned into extra-large dissolution pores when dissolution is strong.

3.4 Structural fracture

Structural cracks refer to cracks formed when the structural stress exceeds the elastic limit of rocks under the action of structural stress. The structural fractures in the study area are mostly flat, extending far away, appearing in groups, with obvious directionality, which can be divided into macro fractures and micro fractures (Figure 5). Macro cracks are obviously visible on the core, generally around 65438±0mm, and micro cracks can be observed on the microscope line, generally less than 65438±0mm. ..

Fig. 4 Spatial types of dolomite reservoirs in the lower member of Shahejie Formation in Qikou Depression.

Fig. 5 Characteristics of dolomite fractures in the lower member of Shahejie Formation in Qikou Depression

According to the cathodoluminescence characteristics of dolomite, the fracture filling in the study area can be divided into three stages. In the early stage, iron-bearing dolomite was dominant, and iron-bearing dolomite basically did not emit light or gave off dark brown light. In the late stage, it was filled with iron calcite, partly with argillaceous filling, followed by argillaceous and iron calcite. Iron calcite emits brown light, while the mud in the vein emits indigo light, and the pores do not emit light (Figure 6).

Fracture is the most important type of reservoir space in the study area. Although it is found in core observation that dolomite fractures are highly filled in the later period, it is found in casting thin section observation that many filled fractures were partially dissolved in the later diagenetic changes, forming effective reservoir space. The width of corrosion cracks in some areas is 0.64mm, which is a beaded corrosion hole (Figure 7).

Fig. 6 Cathodoluminescence characteristics of dolomite fracture filling in the lower member of Shahejie Formation in Qikou Depression (10× 10)

Late dissolution characteristics of dolomite fracture fillings in the lower member of Shahejie Formation in Qikou Depression.

4 Physical characteristics of reservoir

4. 1 physical characteristics

Through the statistical analysis of the distribution histogram of porosity and permeability of dolomite reservoir in the lower member of Shahejie Formation in the study area, it can be seen that the porosity distribution of dolomite reservoir is relatively scattered, mainly between 2% and 9%, with samples with porosity less than 5% accounting for 60% of the total samples, 5%-8% accounting for 10% of the total samples, and samples with porosity greater than 8% accounting for 30% of the total samples. However, the permeability distribution is concentrated, and the samples between 20× 10-3 μ m2 and 50× 10-3 μ m2 account for 70% of the total samples. Samples larger than 90× 10-3μm2 account for about 20% of the total number of samples. The overall performance is low porosity and low permeability reservoir.

4.2 pore structure characteristics

The results of mercury injection test and analysis show that when the pressure is lower than 2.56MPa, there is basically no mercury injection. When the pressure reached 5. 12MPa, the mercury intake began to increase greatly, and when the pressure exceeded 20.48MPa, the mercury intake decreased obviously. The pore throat radius is mainly distributed between 0.0038 ~ 0.5859 micron, and the main mercury inlet throat radius is distributed between 0. 1.465 ~ 0.0366 micron, indicating that the pore structure in the study area is mainly micro-throat (Figure 9).

Fig. 8 Distribution histogram of porosity (left) and permeability (right) of dolomite reservoir in the lower member of Shahejie Formation.

Fig. 9 Capillary pressure curve and pore distribution characteristics of microcrystalline dolomite

Figure 10 Dolomite Porosity-Depth Relationship Diagram

4.3 Relationship between Dolomite Porosity and Depth

Generally speaking, with the increase of buried depth, reservoir porosity and permeability gradually decrease under the compaction of overlying strata. However, the porosity of dolomite reservoir in the lower member of Shahejie Formation in the study area does not decrease obviously with the increase of depth, but has an unusually high value between 2500 and 2700 m (Figure 10). According to the analysis of the origin of dolomite in the study area, the dolomite in the limited depression in Qijiawu and Huanghua shallow lake area is mainly formed by quasi-contemporaneous evaporation and concentration, and the dolomite in the semi-deep lake area in Liujianfang-Zhouqingzhuang-Zhaojiabao area is mainly formed by buried dolomite. The crystallinity of buried dolomite is higher than that of quasi-contemporaneous dolomite. Through the analysis of the controlling factors of fracture development, it is found that fractures are developed in microcrystalline dolomite and micrite dolomite with high crystallinity. Therefore, although the development depth of buried dolomite in the study area is greater than that of quasi-contemporaneous dolomite, its porosity does not decrease with the increase of depth.

4.4 Plane Distribution Characteristics of Dolomite Reservoir

Based on the analysis of the distribution characteristics of plane porosity and permeability in the lower member of Shahejie Formation in the study area, it is found that four dolomite reservoirs are the best. This is because there are a large number of microcrystalline dolomite and micrite dolomite in Ban 4 oil formation, which have high crystallinity and pure composition, which are beneficial to the development of fractures and dissolution pores.

The porosity of dolomite reservoir in Ban-4 oil formation is between 0 ~ 18%, and the high porosity areas are distributed in Zhouqingzhuang-Wangxuzhuang area, Yangsanmu area, Zhaojiabao area and near the wellhead 10 well area (Figure 1 1). Among them, the development area of microcrystalline dolomite in Zhouqingzhuang-Wangxuzhuang area is the largest, and the cumulative thickness of reservoir is generally 2 ~ 6m, with the maximum thickness of10m; . The porosity is generally 0 ~ 10%, the high-value area is generally greater than 10%, and the maximum value can reach 18.5%. The cumulative thickness of reservoirs in Yangsanmu area is generally 2 ~ 5m, the thickest part can reach 8.6m, the porosity is generally 0 ~ 12%, the high value area is generally greater than 10%, and the largest part can reach 17.28%. The cumulative thickness of reservoirs in Zhaojiabao area is generally 4 ~ 8m, the thickest part can reach 1 1.8m, the porosity is generally between 0 ~ 10%, the high value area is generally greater than 10%, and the largest part can be seen as 15.89%. The cumulative thickness of the reservoir near the mouth depth 10 well area is generally 1 ~ 4m, the maximum thickness can reach 6m, the porosity is generally between 0% and 8%, the high value area is generally greater than 10%, and the maximum value is 13.6%.

Figure 1 1 Plane Distribution of Dolomite Porosity in Ban 4 Oil Group

The high permeability areas of dolomite reservoir in Ban4 oil formation are mainly in Zhouqingzhuang-Yangsanmu-Wangxuzhuang area and Zhaojiabao area (attached figure 12), in which the permeability values of Zhouqingzhuang and Yangsanmu areas are generally between (0 ~ 30) × 10-3 μ m2, and the high permeability areas are generally greater than 30×12. However, the permeability in Zhaojiabao and Wangxuzhuang areas is generally between (0 ~ 20) × 10-3μ m2, and the high-value areas are generally above 30× 10-3μm2, among which the local permeability in Zhaojiabao and Wangxuzhuang areas is greater than100×1.

Figure 12 Plane Distribution Map of Dolomite Permeability of Ban 4 Oil Group

Figure 13 Relationship between dolomite physical properties and oil and gas in the lower member of Shahejie Formation

5 Prediction of favorable reservoir zones

5. 1 reservoir classification evaluation

According to the relationship between dolomite rock types and porosity, permeability, sedimentation-diagenesis and fracture distribution in the study area, and the relationship between dolomite reservoir physical properties and oil-gas bearing capacity in the lower member of Shahejie Formation (attached figure 13), when the porosity is less than 5%, it is mainly a dense layer with no oil-water display; The porosity of oil layer, water layer and oil-water layer is mainly distributed in the interval with porosity greater than 5%. Because of the sensitivity of reservoir permeability change, even if the porosity is 0.5%, the permeability may change by several millidarcies, dozens of millidarcies or even hundreds of millidarcies. Therefore, the lower limit of reservoir physical properties is mainly determined according to the relationship between porosity change characteristics and oil-bearing grade (Table 1). On this basis, dolomite reservoirs are divided into three categories.

Class I reservoir: it is the best dolomite reservoir in the lower member of Shahejie Formation, and the reservoir space is dominated by structural fractures, followed by intergranular dissolved pores and cement dissolved pores. The representative rocks are mainly microcrystalline dolomite, followed by micrite dolomite; This kind of reservoirs are mainly distributed in Qijiawu area, Liujianfang-Zhouqingzhuang area and Zhaojiabao area in the west, mostly related to faults and limited depressions, with high industrial productivity.

Table 1 Classification and Evaluation Table of Dolomite Reservoir in Lower Member of Shahejie Formation in Qikou Depression

Class Ⅱ medium reservoir: The reservoir space is mainly composed of various dissolved pores, dolomite intergranular pores and a few micro-fractures. , the representative lithology is mainly micrite dolomite and calcareous dolomite. The distribution range is similar to that of Class I reservoir, mainly distributed in Yunping and Huiyunping environments between limited depressions and shoals. For example, in recent two years, wells Wang 32 and Fang 30 in Qijiawu and Liujianfang areas all showed good oil and gas.

Class III non-reservoir: mainly argillaceous dolomite, sandy dolomite and part of calcareous dolomite, which are mostly found in lakes, bays and Yunhuiping far away from faults and bulges.

5.2 Prediction of favorable reservoir zones

The distribution of favorable reservoir facies belts of dolomite reservoir in the lower member of Shahejie Formation in the study area is quite different in vertical and horizontal directions. Class I reservoirs are mainly distributed in Bin I oil group and Ban 4 oil group, in which Ban 4 oil group is relatively developed, and the favorable zone of dolomite reservoir in Ban 2+3 oil group is relatively small and isolated, and the reservoir performance is mainly medium-poor class II reservoirs. Next, the oil group of the disc 4 will be described as an example.

Dolomite reservoirs in Ban 4 oil formation are widely distributed, and the first-class reservoirs are mainly in Wangxuzhuang-Yangsanmu to Zhaojiabao (Figure 14), in which the main reservoir rocks in Wangxuzhuang-Yangsanmu area are microcrystalline dolomite and micrite dolomite, and the oil test shows that most of them are oil layers and oil-water layers, while the main reservoir rocks in Zhaojiabao area are microcrystalline dolomite, and the oil test shows that most of them are oil layers.

6 conclusion

Five types of dolomite are developed in the lower member of Shahejie Formation in Qikou Depression: microcrystalline dolomite, micrite dolomite, argillaceous dolomite, sandy dolomite, calcareous dolomite or dolomite limestone. The reservoir rocks are mainly microcrystalline dolomite, with a particle size of about 2 ~ 5 microns, which is heteromorphic-semi-self-shaped, mainly heteromorphic, and mainly developed in Qijiawu and Liujianfang of Ban 4 oil formation.

(2) There are various types of dolomite reservoir spaces in the lower member of Shahejie Formation in Qikou Depression, including intragranular dissolved pores, mold pores, oversize dissolved pores, intergranular dissolved pores and structural fractures, and fractures are the most important types of reservoir spaces in the study area.

(3) The porosity of dolomite reservoir in the lower member of Shahejie Formation in Qikou Depression is mainly distributed between 2% and 9%, while the permeability distribution is relatively concentrated. The samples between 20× 10-3μ m2 and 50× 10-3μ m2 account for 70% of the total samples, and generally represent low porosity and low permeability reservoirs. Because the Liujianfang-Zhouqingzhuang-Zhaojiabao area is dominated by buried dolomitization, the porosity between 2500 and 2700 m shows an abnormally high value development zone.

(4) The distribution of favorable reservoir facies zones of dolomite reservoir in the lower member of Shahejie Formation in Qikou Depression is quite different in vertical and horizontal directions. The first-class reservoir is mainly distributed in Binyi oil group and Bansi oil group, and the first-class reservoir of Bansi oil group is mainly distributed in Wangxuzhuang-Yangsanmu-Zhaojiabao area.

Figure 14 Prediction of favorable zones under the fourth member of Shahejie Formation in Qikou Depression

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