School of Geophysics and Information Technology, China Geo University, Beijing100083; 2. Research Institute of Oriental Geophysical Company of China Petroleum Group, Zhuozhou, Hebei 07275 1)
Author: Wang Zhaofeng, male, postdoctoral, senior engineer, mainly engaged in oil and gas reservoir evaluation and development.
Abstract: Fracture-cave carbonate reservoir is one of the important fields of increasing oil and gas storage and production in the world. However, it is difficult to accurately predict carbonate reservoirs because of their complex shapes and strong heterogeneity. In this paper, the reservoir in Pz block of an oil field in Kazakhstan is taken as the research object, and fine well connection calibration is carried out through well-seismic coordination, which improves the lateral continuity and reliability of the target layer. Modern karst theory is introduced to guide the structural interpretation of basement top surface, and the pinch-off line and scale are implemented to increase the exploration and development area of the study area. Fault modeling technology is used to describe the fault plane in three dimensions to ensure the accuracy of fault interpretation. Using three-dimensional visualization technology, the paleogeomorphology in the study area is divided into three types: peak depression, peak forest valley and ancient erosion gully, and the spatial distribution of favorable lithofacies zones is predicted. According to the characteristics of geology, logging and seismic response, the reservoir can be divided into three types: karst cave pore type, fracture pore type and fracture type. Combined with seismic attributes, seismic inversion and ant tracking modeling technology, the spatial distribution characteristics of reservoirs are described, and the potential areas for further rolling exploration and development are pointed out.
Key words: fractured-vuggy reservoir; Carbonate rock; Reservoir prediction; oil field
Characteristics and prediction of fractured-vuggy carbonate reservoir in pz layer of NWKYZ oilfield in Kazakhstan
Wang Zhaofeng 1, 2, Wang Peng 2, Xin Chen 2, Li Qiang 2.
(1. Institute of Geophysics and Information Technology, China Geo University, Beijing100083; 2. Geophysical Research Institute of China Petroleum and Natural Gas Corporation, Zhuozhou, China 07275 1)
Abstract: Fracture-cave carbonate reservoir is one of the important fields of increasing oil and gas production in the world. It is difficult to predict because of the complex shape and strong heterogeneity of reservoir rocks. Taking the fractured-vuggy carbonate reservoir in Pz layer of tan nwk yz oilfield in Kazakhstan as the target, the well pattern is calibrated by well-seismic integration to improve the consistency and reliability of horizon calibration. We use the latest ka rst theory to guide the structural analysis of the top surface of the basement. Define wedge-out and structural traps to increase the exploration and development area of the study area. The fault plane is displayed intuitively by fault model technology, which ensures the quality of fault interpretation. Using three-dimensional visualization technology, we divide ancient landforms into three categories: peak cluster, peak forest and fossil erosion notch. The distribution of favorable lithofacies is predicted. According to the geological, logging and seismic response characteristics, reservoirs are divided into pore type, fracture type and fracture type. Through seismic attributes, seismic inversion and ant tracking modeling, the distribution of three types of reservoirs is defined, and the potential areas for exploration and development are pointed out.
Key words: fractured-vuggy reservoir; Carbonate; Reservoir prediction; NWKYZ oilfield
introduce
Fracture-cave carbonate reservoir is one of the important fields of increasing oil and gas storage and production in the world [1 ~ 2]. Due to the complex reservoir shape and strong heterogeneity, the success rate of drilling has not been high, which makes the exploration and development of fractured-vuggy carbonate reservoirs a world-class problem [3 ~ 7]. Multi-disciplinary comprehensive application for reservoir prediction is an effective way to solve this problem [8 ~ 9]. Taking the fractured-vuggy carbonate reservoir in Pz layer of an oil field in Kazakhstan as an example, this paper explores the method of comprehensively applying geological, seismic, logging and production performance data to predict the characteristics of fractured-vuggy carbonate reservoir, in order to attract more attention and promote the wide application of multidisciplinary in the prediction of fractured-vuggy carbonate reservoir.
Figure 1 A Oilfield Location (modified according to Hu 20 1 1 [7])
1 regional geological survey
Oilfield A is located on aksay Uplift of Aryskum Depression in the south of South Turgay Basin in Kazakhstan (Figure 1) [1]. Industrial oil and gas flows have been found in M-Ⅱ layer, Jurassic layer and basement Pz layer of an oilfield. The basement Pz layer studied in this study is mainly limestone and dolomitic limestone (Well Kz43 and Well Kz47), and some wells contain a small amount of hard silicate and soft silicate (Kz5 1), which is a typical fractured-vuggy carbonate reservoir.
The basement of South Turgay Basin was consolidated at the end of Early Paleozoic. According to the different basement composition and metamorphic degree, it can be further divided into two sets of structural layers, namely, the Proterozoic Guyu-Lower Paleozoic deep metamorphic folded basement, which is the real basement of the basin, and the other set is Devonian-Carboniferous carbonate-basement Pz, which is the transitional basement between the basin and the land. The basement in the study area belongs to carbonate transitional basement [1]. Jurassic, Cretaceous, Tertiary (Paleogene-Neogene) and Quaternary are mainly developed on the basement, and the overlying strata are in large-angle unconformity contact with the basement (table 1).
The South Turgay Basin is located in the shear zone at the juncture of the Urals-Tianshan suture in the south-central part of Kazakhstan, and it is a Mesozoic rift basin developed on the Hercynian basement uplift [10]. According to the sequence of stratigraphic structural markers, Cenozoic can be divided into five stages reflecting the characteristics of regional tectonic evolution, namely, initial extension stage, fault depression development stage, fault depression transformation stage, depression development stage and late uplift stage [10]. Pz is the basement of the target layer in the study area, which was consolidated at the end of Paleozoic and suffered from uplift and strong erosion. The lithology of oilfield bedrock is complex. According to the analysis of core, logging and microscopic data, the main lithology of reservoir can be divided into four categories: limestone, dolomite, breccia and siliceous rocks. Logging curve has the characteristics of high resistivity, high velocity, low neutron and high density.
Table 1 Strata Summary of South Turgay Basin
2 Fine structural interpretation
2. Fine calibration of1well-seismic combination well
Accurate seismic geological horizon calibration is the basis of seismic structure interpretation. During calibration, ensure that each geological interface corresponds to a seismic event axis, and well match each event axis of reservoir profile, so that seismic data in time domain and logging data in depth domain can be correctly combined [1 1]. The method of "well-seismic fine calibration combined with well connection" is adopted in this horizon calibration, that is, on the basis of accurately dividing the correlation strata, the drilling, logging and mud logging data of 29 completed wells in the research area are comprehensively used for horizon calibration and comparison. Through the production of multi-well synthetic seismic records and the comparative verification of vertical and horizontal cross-well profiles in the study area, the lateral continuity and reliability of horizon calibration are ensured (Figure 2). In the calibration process, the standard layer is determined according to the longitudinal variation law of logging curve. Among them, the mudstone section of Cretaceous ares Qom Formation is relatively stable in the work area and can be used as a standard layer.
Fig. 2 calibration profile of nwkyzyjia 50-58-54-48-57-32-51-31
2.2 Introduce modern karst theory to guide the interpretation of basement top structure.
By comparing the characteristics of karst landform formed by modern karst (Figure 3-A) with the seismic profile of the study area (Figure 3-B) to guide seismic interpretation, the contact relationship between overlying strata and basement with complex paleogeomorphology is divided into three types: U-shaped, V-shaped and wedge-shaped, and the study area with complex paleogeomorphology in the study area is reinterpreted. Redefine MII and J3ak pinch-out lines and structures by 26. 1km2, and define carbonate buried hill structure by 52.7km2
Fig. 3 Structural interpretation of the top surface of fractured-vuggy carbonate basement guided by modern karst.
2.3 Fault model ensures the accuracy of fault interpretation.
On the basis of fault identification by using coherent volume, dip angle, time slice, three-dimensional visualization and other methods, fault modeling is carried out, and fault interpretation accuracy is ensured by using fault model (Figure 4). There are 50 * * interpretation faults in the whole area, and 30 pass through basement faults, of which 10 extends over 1.5km (Figure 5).
Fig. 4 Oilfield Profile Model
Fig. 5 Plane distribution of fractures at the top of Pz layer in the oilfield.
2.4 structural realization and three-dimensional visualization of ancient landforms
On the basis of fine interpretation of the reflection layer at the top of Pz, a three-dimensional velocity field is established by using the time-depth relationship of 29 wells in the study area, and the time-depth conversion of horizons is carried out, and then the well correction is carried out to obtain the structural map at the top of the target layer (Figure 6). The top surface of Pz basement is mainly divided into two uplifts, east and west, and some small anticline traps are developed locally. A total of 6 traps/kloc-0 were identified in this study, with an area of17.88km2.
Fig. 6 Top Structure Diagram of Pz Layer in Oilfield
On the basis of the implementation of the structure, the paleogeomorphology is restored, and the paleogeomorphology characteristics of the study area are displayed by using three-dimensional visualization technology (Figure 7). The paleogeomorphology in the study area can be divided into three types: peak depression, peak forest valley and ancient erosion gully.
Fig. 7 Paleogeomorphology Analysis Diagram of Pz Layer in Oilfield
3 Reservoir characteristics and prediction
3. 1 reservoir lithofacies characteristics
Core, thin section and logging data show that the main lithology of Pz basement is limestone, dolomite limestone, siliceous rock and breccia. It can be seen from the lithofacies analysis diagram of single well (Figure 8) that the electrical logging characteristics of basement lithology are mainly divided into two types: the first type of limestone and dolomite limestone are low gamma, medium-high resistivity, low acoustic wave time difference and high density; Another kind of siliceous rocks is just the opposite of breccia, with medium-high gamma, low resistivity, high acoustic time difference and low density. The curve shapes of similar lithology are basically the same, mostly linear. From the contact relationship, the contact relationship between limestone and dolomitic limestone and overlying clastic rocks is abrupt, while the contact relationship between siliceous rocks and breccia and overlying clastic rocks is gradual. Reservoir lithofacies has strong heterogeneity in both horizontal and vertical directions, and breccia, siliceous rocks and dolomitic limestone are distributed in blocks. The spatial distribution pattern of lithofacies can be displayed intuitively by using attribute modeling technology (Figure 9).
3.2 Reservoir classification characteristics
The fractured-vuggy carbonate reservoir in Pz block of an oilfield has developed secondary pores, strong heterogeneity and good reservoir physical properties, and is the main pay zone in this area. According to the core, logging and seismic response characteristics, the reservoirs in the study area can be mainly divided into three types: cave pore type, fracture pore type and fracture type (Table 2).
(1) Karst cave porous reservoir. Karst caves are completely filled with siliceous rocks and breccia, and the reservoir space is dominated by pores between karst cave fillings. Generally, it has a certain tectonic background, and the seismic response is characterized by lenticular abnormal strong reflection, and the lower part is concave discontinuous strong reflection. The logging response is box-shaped or funnel-shaped, with low GR, high DT and low density.
Fig. 8 Comprehensive histogram of lithofacies analysis of Pz section of Well NWKYZYJIA49 in Oilfield
Fig. 9 Lithofacies Model of Pz Profile in Oilfield
Table 2 A Reservoir Classification Characteristics of Pz Member in Oilfield
(2) Fractured and porous reservoirs. Fractures and matrix pores are relatively developed, which is a typical dual-medium reservoir. Seismic response is often discontinuous reflection, and its characteristics are not obvious, and it is mostly adjacent to cracks and large faults. The logging curve has little change, with low GR, low DT and high density.
(3) Fractured reservoirs. The reservoir space is mainly microfractures. The seismic response is mainly characterized by continuous strong amplitude interface, small change of logging curve, low GR, medium-high DT and medium-high density.
3.3 Reservoir prediction based on seismic attributes
Seismic attribute analysis is an important technical means to predict the distribution of holes and fractures in carbonate rocks. Different scales and filling degrees of karst caves and fracture systems will cause subtle changes in seismic response, and it is difficult for naked eyes to identify such changes from the changes of seismic events [12]. However, this change may be implied in the difference of seismic attributes. Each seismic attribute reflects the underground changes from different aspects, and different genera have different sensitivities to cracks and caves. The reflection amplitude contains the velocity, density and thickness information of a single interface, which can be used to predict the lateral rock change and the possibility of oil and gas existence, and the distribution of fractured-vuggy reservoirs can be identified by using the properties of amplitude class [13]. Frequency is the characteristic of seismic pulse, which is related to geological factors, such as the lateral change of thickness or velocity of reflection layer and the existence of gas. Usually, low frequency reflects the thick characteristic more, while high frequency is sensitive to the thin characteristic, and the change of oil and gas and reservoir will cause the absorption and attenuation of high frequency. Because the fractured-vuggy carbonate reservoir is relatively microscopic in a large set of carbonate strata, the frequency division information is very helpful to describe the heterogeneity of the reservoir in the prediction of fractured-vuggy carbonate reservoir [14]. Reflection continuity is closely related to formation continuity, and it is a physical parameter to evaluate the lateral extension ability of seismic event axis, which is usually described by the nature of facies.
(1) frequency division property. Frequency-division interpretation technology is a new seismic data interpretation method, which is a spectrum decomposition technology based on Fourier transform, maximum entropy method and wavelet transform [14- 15]. Frequency division attribute combined with three-dimensional visualization is a powerful means to describe heterogeneous reservoirs in detail. This method can analyze the amplitude corresponding to each frequency in frequency domain when imaging and interpreting the time thickness and geological discontinuity of three-dimensional seismic data. This analysis method eliminates the mutual interference of different frequency components in time domain and makes the interpretation result higher than the traditional resolution. Through the analysis of cross-well profile of frequency-divided data volume, it is concluded that the frequency-divided response of the reservoir in the study area has the following laws: the favorable reservoir frequency-divided response is relatively high (warm color) tuning amplitude, and the poor reservoir frequency-divided attribute response is often low (cold color) tuning resonance amplitude (figure 10). Through the study of this method, it is considered that the favorable reservoirs of basement carbonate rocks are mainly distributed in the middle of the study area, with the erosion ditch as the boundary, and the area around the two ancient rock ridges distributed east and west is about 20km2.
Figure/10 NWKYZYJIA Area Basement 50Hz Frequency Division Attribute Visualization Rendering
(2) Amplitude class attribute. Amplitude is the response of impedance difference of lithologic interface. The greater the impedance difference between the upper and lower strata, the stronger the reflection amplitude [16]. Basement carbonate rocks in the study area are characterized by weak amplitude. When there are holes, holes and cracks, it is equivalent to the emergence of a new reflection interface, which is prone to abnormal amplitude and local strong reflection.
On the plan of AC component of basement reflection intensity in NWKYZYJIA area (Figure 1 1), the area with strong middle reflection intensity (warm colors such as orange and yellow) represents the area where Class I reservoirs are developed, while the area with weak surrounding reflection intensity (cool colors such as blue and green) represents the area where holes are not developed. It can be seen that the strong reflection region can be roughly divided into two parts, which are basically consistent with the prediction results of frequency division technology. On this basis, each part can be divided into several bands distributed along NW-SE direction, which is basically consistent with the distribution direction of main faults in the study area, indicating that the development of holes is affected by regional stress and faults.
Figure11plan of AC component of basement reflection intensity in nwkyzjia area
3.4 Seismic Inversion Reservoir Prediction
Seismic inversion technology is to make full use of abundant information such as structure, horizon and lithology provided by logging, drilling and geological data to infer information such as wave impedance, density, velocity, porosity, permeability, percentage of sandstone and mudstone, pressure, etc. Underground stratum of conventional seismic profile [17]. This inversion is accomplished by constrained sparse spike inversion in Jason software.
According to the development law of Ⅰ and Ⅱ reservoirs in the basement of the study area, the Ⅰ and Ⅱ reservoirs in the thickness range of 120m at the top of the buried hill are classified by using the volume view of Jason software (Figure12, Figure 13), and the wave impedance of the Ⅰ reservoir is defined as 5000 ~10000. Near Well NWKYZYJIA56, the thickness of Class I reservoir reaches 70m. Class Ⅱ reservoirs are developed on structural slopes, but scattered in other places.
3.5 Using ant tracking modeling technology to predict reservoir fractures.
Fracture prediction has always been a difficult point in the study of fractured-vuggy reservoirs. Ant tracking technology is adopted in this crack prediction. The principle of this technology is that a large number of ants are sown in the seismic data volume, and ants that meet the preset crack conditions in the seismic attribute volume will "release" some signals, and ants in other areas will be gathered at the cracks for tracking, and other cracks that do not meet the crack conditions will not be marked [18]. Finally, a data volume with low noise and clear fracture trace is obtained. According to the fracture model traced by ants at 0 ~ 120 m below the top surface of Pz in the study area (Figure 14), it can be seen that Class III fractured reservoirs are obviously affected by faults and develop near them.
Fig.12 0-120m first-class reservoir thickness map below the top surface of Pz in nwkyzjia work area.
Fig.13 0-120m secondary reservoir thickness map below the top surface of Pz in nwkyzjia work area.
Figure14 distribution characteristics of 0 ~ 0 ~120m Ⅲ fractured reservoirs below the top surface of Pz in nwkyzyjia work area.
4 conclusion
(1) Well-seismic combination technology can enhance the continuity and reliability of horizontal horizon calibration.
(2) Modern karst theory is introduced to guide the structural interpretation of the basement top surface, and pinch-out lines and structural traps are implemented. In the study area, the pinch-out line and pinch-out structure of MII and J3ak are reconstructed by 26. 1km2, and the buried hill structure of carbonate rock is reconstructed by 52.7km2, which increases the exploration and development area.
(3) Fault modeling technology can display fault plane intuitively, which is helpful to ensure the quality of fault interpretation.
(4) Using three-dimensional visualization technology to display the characteristics of ancient landforms is helpful to the analysis of ancient landforms. The paleogeomorphology in the study area can be divided into three types: peak cluster depression, peak forest valley and ancient erosion gully.
(5) According to the geological, logging and seismic response characteristics, the reservoirs in the study area are divided into three types: karst cave pore type, fracture pore type and fracture type.
(6) Synthesizing seismic attributes, seismic inversion and ant tracking modeling technology, the spatial distribution characteristics of three types of reservoirs in the study area are defined. It is considered that type I karst cave porous reservoir mainly develops along the high part of paleostructure, and the higher the position, the greater the reservoir thickness. Class Ⅱ fracture-pore reservoirs are developed on structural slopes and scattered in other places. Class ⅲ fractured reservoirs are obviously affected by faults and developed near them.
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