(1. China Petroleum Exploration and Development Research Institute, Beijing100083; 2. Youshi University, China, Beijing 102249)
The logging analysis method based on rock electrical experiment mainly applies the geophysical properties of logging technology. With the increasing complexity of oil and gas exploration and development objectives, the limitations of this method are increasingly apparent. This paper puts forward the concept of "geological attribute of logging information" and discusses its three types: correspondence, specificity and unity. Among them, studying the corresponding relationship between logging information and geological background is helpful to restore some geological events by using logging information; Studying the specificity of logging response and some geological events is helpful to infer the important geological phenomena hidden in logging information. Studying the unity of macroscopic geological action and microscopic rock structure between logging information and its geological background is helpful to the study of logging technical geological prediction. Through the analysis of geological attributes of logging information, the productivity of lower Paleozoic reservoirs in an exploration area in Saudi Arabia was successfully predicted, and the age attribution of 1000m mudstone stratum accidentally drilled by an exploration well in Australia was determined.
Logging analysis method; Geophysical attributes; Geological attribute
Study on geological characteristics of logging information
Li Hao 1, 2
(1. Sinopec Petroleum Exploration and Development Research Institute, Beijing100083; 2. China Shiyou University, Beijing 102249)
On the basis of rock electrical test, geophysical characteristics are mainly used for logging evaluation. However, the complexity and concealment of oil and gas exploration and development goals make logging evaluation technology face challenges. This paper puts forward the concept of geological characteristics of logging information, and discusses three types of geological characteristics: correspondence, specificity and unity. Studying the correspondence between logging information and geological background is helpful to describe or invert some geological events by using the shape of logging curve. Studying logging response and the particularity of some geological events is helpful to infer important hidden geological phenomena by using logging information. Studying the unification of macroscopic geological process and microscopic reservoir structure in logging information and geological background is helpful to geological prediction.
During the evolution of China Petrochemical's overseas oil and gas resources, the exploration areas with reservoir productivity in Lower Paleozoic in Saudi Arabia were successfully predicted, and the time attribution of clay rocks accidentally encountered by Australian exploration wells was determined.
Keywords logging evaluation of geophysical characteristics and geological characteristics
At present, the main problem faced by logging evaluation technology is the contradiction between the fuzziness of its measurement method itself and the increasing complexity and concealment of oil and gas exploration and development goals [1 ~ 4]. Facts have proved that using a single technology and method for logging evaluation has exposed many disadvantages. Combining logging evaluation technology with macro-geological background, reducing the fuzziness of logging evaluation knowledge and improving the geological application of logging information has become the knowledge of logging technology development. Deepening the research method of logging geology can not only reduce the ambiguity of evaluation, but also provide some key evidence for geological research.
1 The difference of geological background determines the difference of logging response.
Geological internal factors fundamentally determine the response characteristics of logging information under different geological conditions. With a deep understanding of this point, it is possible to use logging technology to identify geological events or reveal hidden important geological phenomena, provide geologists with the basis for research and reference, and provide guidance for the prediction of special oil and gas reservoirs.
Figure 1 and Figure 2 show the logging response characteristics of Cretaceous and Lower Cretaceous strata in Iran K- 1 oilfield, respectively. Although the reservoir lithology is carbonate rocks, the logging curves show great differences in resistivity and porosity response characteristics. The reason is analyzed, and it is speculated that it is caused by sedimentary background factors. Among them, the upper Cretaceous reservoir in Figure 1 has seen a lot of sand debris in the core photos, which is probably related to the fact that a large number of terrigenous materials entered the seawater during the transgression, resulting in the mixing of terrigenous materials with carbonate rocks. The resistivity and porosity of this kind of reservoir change stably on the logging curve. When the fracture factor has little influence, Archie formula is used to explain the reservoir water saturation, which is highly consistent with underground geology. Fig. 2 The reservoirs in the Middle and Lower Cretaceous are pure carbonate rocks, which may be related to regression factors. The lack of land supply during the regression period made the rhythm of carbonate rocks constitute the main body of sedimentary strata [5]. The rhythm of this reservoir makes the change of resistivity and porosity on logging curve unstable. Interpretation of reservoir water saturation by Archie formula is quite different from logging oil-bearing display. As can be seen from the figure, non-reservoir sections often deal with high oil saturation (Sw is low), which is difficult to be explained by mathematical analysis model.
Fig. 1 logging response characteristics of carbonate rocks in hydraulic period
2 Geological attributes of logging information and its research purpose
The above two examples show that logging information is closely related to geological evolution, including geological attributes. Studying the internal relationship between logging information and geological background evolution is of exploration value for transforming logging information into geological analysis model.
Fig. 2 logging response characteristics of carbonate rocks in water regression period
The definition of attribute refers to the nature and characteristics of things. As far as the formation of logging information is concerned, it has both geophysical and geological properties. The former comes from the geophysical response of logging instruments from launch to reception, and different instruments measure different geophysical data results; The latter comes from the information expression of logging data on reservoir geological background, and different curve characteristics are measured under different geological background. These two attributes of logging information are indirect expressions of the real underground situation. At present, logging evaluation is mainly based on its geophysical properties, and the understanding and application of its geological properties are relatively insufficient.
The specific manifestation of the geological attribute of logging information is that the characteristic phenomena in the process of geological evolution must have the characteristic response of logging records. As long as the correct logging-geological conversion relationship [6 ~ 7] is established, it is possible to restore some or main reservoir geological features with logging information, so logging geological attributes may be the theoretical basis for deepening logging geology.
According to the application, logging information should have three types of geological attributes: ① correspondence, that is, logging response has a corresponding relationship with its geological evolution background. According to this attribute, some geological events can be described or recovered by logging curve shape, such as logging facies modeling, describing faults and unconformity dip angles, etc. (2) Specificity, some special responses of logging information often belong to a specific geological phenomenon, such as the increase of acoustic time difference between abnormal high pressure and mudstone, strong geostress and high mudstone resistivity; (3) Unity. Geological problem is the unity of macroscopic geological process and microscopic reservoir structure. In addition to the construction factors, the special changes of local logging information must be the response of internal factors of macro-geology. Therefore, the unity of macro and micro is helpful to accurate geological prediction.
The purpose of studying the geological attributes of logging information is to recover and infer the essential characteristics of some geological evolution processes by using logging information, and to establish the conversion mode between logging information and geological background through forward or inverse analysis, so as to improve the application efficiency of logging information and give play to the prediction function of logging information.
Application of geological attributes of logging information in foreign oil and gas resources evaluation
3. 1 Identify the unknown stratum encountered in the accident.
Well V 1 is located at the high point of anticline in Vulcan sub-basin in the west of Bonaparte basin on the northwest shelf of Australia. From Lower-Middle Jurassic to Lower Cretaceous, drilling in the study area revealed several strata, including Plover stratum, Montara stratum, Lower Vulcan stratum, Upper Vulcan stratum, Echuca shoals stratum and Jamieson stratum. Nearly 20 wells were drilled in the high steps in the east of the sub-basin, and all of them encountered the Plover formation of the Lower Jurassic (Figure 3). The design target layer of V 1 well is also Plover formation, but unexpectedly, at the predicted depth of 3,400 m, mudstone with 1 000 m is drilled, and the stratum attribution becomes the focus.
Fig. 3v 1 well structural background.
3. 1. 1 stratigraphic correlation and sedimentary facies study
Stratigraphic correlation identified three sets of correlation marker beds: unconformity at the bottom of Jemieson Formation, limestone mudstone at the top of Lower Vulcan Formation exceeding 100 m, and unconformity at the top of Plover Formation in the eastern platform area. The above-mentioned marker beds are in good agreement with well profile and seismic profile tracking, indicating that the stratigraphic correlation conclusion is correct and reliable.
The study of sedimentary facies for many years shows that the Plover Formation of Lower Jurassic is a channel-delta sedimentary background with typical shallow water sedimentary characteristics. Marine shale and local underwater fan developed in Lower Vulcan Formation of Upper Jurassic, characterized by deep-water deposition.
The study of logging information shows that Plover Formation and Lower Vulcan Formation have different logging geological attributes. First, the logging facies is different, Plover formation is thick box sandstone, with low natural gamma value, stability and smoothness; Thick mudstone is developed in the Lower Vulcan Formation, with high and stable natural gamma value. Thin sandstone is often difficult to see in thick mudstone. Second, the material composition is different. On the logging curve, it is difficult to see the development of calcareous thin layers in the sedimentary strata of Plover Formation. In the sedimentary strata of the Lower Vulcan Formation, a thin calcareous layer is often developed on the logging curve, indicating that the provenance between the two strata may change (Figure 4).
The above is the obvious logging evidence to distinguish two sets of strata by using logging information.
3. 1.2 newly drilled 1000m mudstone formation attribute analysis
Based on the study of geological attributes of logging information, it is considered that the 1000m mudstone drilled below 3400m in V 1 well belongs to the Lower Vulcan Formation of Upper Jurassic.
(1)V 1 well 1000m mudstone logging facies indicates a deep-water sedimentary environment, which is consistent with the marine shale developed in the Lower Vulcan Formation.
(2) Thin calcareous layers are often developed in large mudstone, indicating that their provenance is close to the Lower Vulcan Formation.
Fig. 4 Logging Geological Analysis Diagram of Unknown Drilling Formation
(3) It is extremely difficult to see sandstone or thin sandstone deposits in mudstone of 3) 1000m, which is also the most important evidence. Because Plover formation in the eastern platform area is a typical shallow-water sedimentary feature, sandstone transported by strong hydrodynamic force should be seen more or less in Plover formation in deeper water area, even if there is a great sedimentary phase transition with this well.
The sporopollen analysis provided by foreign cooperative companies shows that the mudstone in this section belongs to late Jurassic strata, which also supports the results of this study and provides decision-making for the next exploration in this block.
3.2 Analysis of Abnormal Formation Pressure of Lower Paleozoic Reservoir in an Exploration Area of Saudi Arabia
A block in Saudi Arabia is the natural gas exploration area of Sinopec. It is a depression basin developed on the basis of salt graben formed by rifting at the end of Precambrian. The whole Phanerozoic has been sinking and the Cambrian-Neogene strata have been deposited. The accumulated thickness of sedimentary rocks in some areas exceeds 9000m. Clastic rocks dominated in Paleozoic, while carbonate rocks dominated in Mesozoic and Cenozoic. In the research before 2005, Devonian J Group, Permian K Group and U Group have always been considered as the main reservoirs. Therefore, in the initial research and evaluation, only U group was evaluated, and the lower Paleozoic reservoir failed to get due attention. For example, the drilling depth of MK- 1 well reaches 55 10m, but the logging interpretation depth only reaches 4580m.
Fig. 5 Formation pressure analysis diagram of target layer of MK-1well
When well MK- 1 was reinterpreted in 2005, it was found that there was abnormal high pressure in the lower Paleozoic strata, which was likely to have productivity (Figure 5), which provided an important direction for oil and gas exploration. In 2006, the newly drilled S2 well failed in Permian, and oil and natural gas were tested in Lower Paleozoic, which confirmed the above speculation.
Fig. 6 shows the logging curves of Paleozoic strata in well MK- 1 and S2, where the thickness of well S2 1 is14.7m, the resistivity is 99.22 Ω m, the porosity is 3.95%, the permeability is 0.02× 10-3μm2, and the water content is high. The thickness of Layer 2 is 3 1.7m, the resistivity is 55.68 Ω m, the porosity is 4.5%, the permeability is 0.03md, and the water saturation (SW) is 34.97% (according to the undergraduate application). During drilling, it was found that the total hydrocarbon content at the top of Ordovician Sala Formation began to increase, reaching 3.02% at 5703.4mTG. The Palaeozoic of this well was tested twice, and the second test was successful, with a daily gas production of 400 ~ 8000 m3/d, which confirmed that this layer was a gas-bearing reservoir and provided a basis for the next oil and gas exploration.
Fig. 6 Logging curve of Lower Paleozoic strata
4 conclusion
(1) The characteristic phenomena in the process of geological evolution must have the characteristic response of logging records, so logging information has geological properties.
(2) Studying the geological background and evolution characteristics of logging information can reduce the ambiguity of logging evaluation and improve the geological application level of logging information.
(3) The key to the further development of logging geology is to deeply study the corresponding relationship between logging information and its geological background, and establish a conversion analysis model between logging information and geological information.
refer to
Li Guoxin, Liu Guoqiang, Zhao Peihua. Orientation, demand and development of logging technology in China Oil and Gas Co., Ltd. [J]. Logging Technology, 2004,28 (1):1~ 6.
Zhao Jiangqing, Kuang Lichun, Liu Ying. Logging evaluation method of pore throat structure in heterogeneous reservoirs [J]. Logging technology,1998,22 (increase): 60 ~ 63.
Ouyang Jian. Strengthening petrophysical research of target block and improving reservoir logging identification and evaluation ability [J]. China Petroleum Exploration, 200 1, 6 (1): 24 ~ 30.
Ouyang Jian. Strengthening petrophysical research to improve the benefit of oil and gas exploration [J]. Petroleum Exploration and Development, 200 1: 28 (2): 1 ~ 5.
Mei Mingxiang. Carbonate cycle and sequence. Guiyang: Guizhou Science and Technology Press [M], 1993.
Wang Guiwen, Guo Rongkun. Logging geology. Beijing: Petroleum Industry Press, 2000.
Wang Guiwen, Guo Rongkun. Logging geology. Beijing: Petroleum Industry Press, 2000.