The study and mapping of sedimentary facies palaeogeography should not only absorb the achievements of predecessors, but also carry out necessary field work. In the working method, it emphasizes the combination of macro and micro, and the combination of outdoor and indoor; In terms of work steps, it generally includes design preparation, field work, indoor analysis and appraisal, and surveying and mapping. These different stages of work are often organically linked and intertwined. In the design preparation stage, the following work should be done:
(1) Comprehensive data collection
On the basis of stratigraphic division and comparison, the outcrop profile, core logging (including coring and sidewall coring), cuttings logging, paleontology and paleoecology identification, analysis and testing (including thin slices, heavy minerals, grain size analysis, geochemical indicators, oil, gas and water analysis, etc.), electrical logging and geophysics are systematically collected and collated, and carefully checked, paying attention to accuracy and representativeness, so as to ensure the solid and reliable mapping basic data.
To sort out the original data, generally, the facies analysis profile and lithofacies paleogeographic card are established first, and then statistics are made by wells, such as sandstone type, heavy minerals, grain size parameters, bedding characteristics, paleontology, mudstone color, geochemical indicators, etc.
The collection of formation data includes: ① measured profile data in regional maps of1∶ 50,000 and1∶ 200,000, as well as formation profile, core data of various boreholes and deep oil wells (including electric logging, radioactive logging and cuttings logging data, etc. ) petroleum and geological survey and exploration; ② Various geophysical data used for analyzing and interpreting strata and structural interfaces; ③ Special research data of geotectonics; (4) Research results of lithofacies palaeogeography and stratigraphic palaeontology in the working area and its adjacent areas; ⑤ Data of sedimentary and stratabound deposits.
(2) unify the stratigraphic division and comparison scheme and determine the mapping unit.
The compilation of middle and small scale sedimentary facies palaeogeographic maps is usually based on chronostratigraphy, that is, to study the changes of sedimentary environment in the same time range. This requires that all stratigraphic sections are approximately isochronous. Therefore, at the beginning of the work, we should pay attention to the isochronous, diachronic and phase change problems of strata, correctly select the time limit and stratification marks of strata correlation, and put forward a reasonable stratigraphic division and correlation scheme as the basis for measuring profiles and data collation.
When determining the stratigraphic division and correlation scheme, the rationality of mapping unit should also be considered. Because the current mapping method still uses two-dimensional space to express the change of three-dimensional space, the time limit of mapping unit directly affects the quality of the map. Generally speaking, the shorter the mapping time unit is, the more accurately it can reflect the sedimentary environment and its changes in the work area, but the higher the accuracy of stratigraphic correlation and sedimentary data collection is required.
According to the statistical analysis method, the study of lithofacies palaeogeography is finally completed by compiling lithofacies palaeogeography map. In other words, the compilation of lithofacies palaeogeographic map is the summary of lithofacies analysis and palaeogeographic research.
How to compile lithofacies palaeogeographic map, what data to collect and sort out, what basic map to make first and how to analyze it are different for different regions, different intervals and different facies.
There are three basic stages in the compilation of lithofacies palaeogeographic map of terrigenous clastic sedimentary basin, namely, the collection and arrangement of basic data, the compilation and analysis of main basic maps, and the compilation and use of lithofacies palaeogeographic map.
fieldwork
Field work is the basis of sedimentary facies palaeogeography research and mapping. By measuring the sedimentary facies profile, we systematically study the rock composition, structure, structure, paleontological assemblage, ecological characteristics and trace fossils of the profile, collect all kinds of paleocurrent data, then divide the genetic units of rocks, comprehensively analyze the sequence structure characteristics of the profile, and preliminarily determine the sedimentary facies types. At the same time, necessary samples should be collected for indoor analysis and research, which will lay a foundation for correctly dividing sedimentary facies types, establishing regional sedimentary facies models and comprehensive mapping.
8.2.2. 1 Sedimentary facies profile measurement
(1) site survey
During the design period, the work area and main sections should be investigated to understand the basic characteristics, stratum outcrop and sedimentary facies of the whole area, initially draw up a set of legends and rock classification naming schemes, and determine the type, quantity and steps of section research.
(2) The type, deployment principle and accuracy requirements of the profile.
The field study of sedimentary facies is mainly carried out on different types of profiles. Therefore, the rationality of profile deployment and research level directly affect the final drawing quality.
Section types include control section, auxiliary section and routing section. Generally, it is required that the main section of each sedimentary unit displayed separately on the plan should be no less than 1 and the auxiliary sections should be no less than 2 ~ 3. The contour accuracy is suitable for planning. The profile scale of large-scale maps is generally not less than1:500, and the medium and small scale is generally1:1000 ~1:2000.
(3) Profile measurement procedure and sample collection
The measurement degree and method of phase profile are the same as those of stratigraphic profile, but its observation focus should be on the study of phase markers and the analysis of sedimentary environment. Generally, it includes three steps: reconnaissance, actual measurement and phase diagram. In the process of profile measurement, necessary test and analysis samples should be collected for indoor research.
Field study on sedimentary facies in 8.2.2.2
Field phasing is very important in the study of sedimentary facies palaeogeography, and indoor work is only a supplement, revision and deepening of field work. The determination of sedimentary facies types mainly depends on the comprehensive analysis of sedimentary facies indicators such as structure, structure, biological assemblage and ecological characteristics, as well as the study of the structural characteristics of sections and the temporal and spatial relationship of facies associations. The identification of these marks and characteristics is mainly done in the field or in the observation and research of cores. Quasi-phasing in the field requires mastering the main characteristics of different sedimentary facies types and models in different environments, and being good at finding and identifying various facies marks; It is required to grasp the field key points according to different profile characteristics, observe carefully, comprehensively analyze according to various signs, profile structure and phase sequence change characteristics, and then make a judgment.
(1) sedimentary facies mark observation
The fifth chapter systematically introduces the identification marks of sedimentary facies. Different rock types have different profiles and phase mark observation points. For the section dominated by terrigenous clastic rocks, besides the above signs, we should also pay attention to the maturity of sandstone and the morphological characteristics of sand bodies. Maturity of sandstone includes composition maturity and structure maturity, both of which are closely related to sedimentary environment (Figure 8. 1), and are important indicators for determining sedimentary facies of terrigenous clastic rocks.
Fig. 8. 1 Relationship between structural maturity of sandstone and sedimentary facies (according to M.K., Tucker, 1980, cited by Liu Baojun and others, 1990).
The geometric shape of sand bodies refers to the distribution, extension direction and morphological characteristics of sedimentary sand bodies with the same characteristics. The sand bodies deposited in different sedimentary environments have different shapes. For example, the sand bodies in the river-controlled delta are finger-shaped and perpendicular to the coast; The delta sand body controlled by waves is parallel to the coast; Tidal channel or channel sand body has flat bottom and flat top, while offshore sand dams often have flat bottom and convex top. The study of sand body shape is not only beneficial to staging, but also meaningful to the general survey and evaluation of oil and gas resources and sedimentary minerals.
The observation of carbonate profile should focus on the observation of structure, bedding type and thickness, and biological combination. The structural types of carbonate rocks are complex and closely related to sedimentary environment. The presence or absence of particles, the ratio of particles to mud, the type of particles and the presence or absence of bright crystals need to be carefully observed on site. The color and bedding characteristics of carbonate rocks are also closely related to sedimentary environment, and the light-colored thick layers are mostly shallow water environment; Dark thin layers are usually associated with lagoons, continental shelves or deep-water basins. The bedding and bedding structure of carbonate rocks mainly appear in shallow water and high-energy granular limestone. The formation of carbonate rocks is closely related to biological action. Therefore, it is very important to study its biological combination and ecological characteristics. According to the analysis of biological assemblage and ecological type, it can be determined whether the biological assemblage is wide salt type or narrow salt type during sedimentation, and whether the biological burial is in situ or in different places.
(2) Research on cross-section structure
Profile structure refers to the relevant vertical sequence displayed by specific lithology, structure, structure and biology. Or facies sequence or lithofacies combination. These sequences are descriptive, and their scale can be large or small. In most cases, repeated rhythmic layers or obvious sedimentary cycles are caused by specific hydrodynamic conditions and evolution laws of different sedimentary environments. The geological interpretation of these profile structures with different characteristics is called facies model or sedimentary model. It is of great significance to master the characteristics and formation mechanism of these phase diagrams in the field. In terrigenous clastic rocks, the upward thickening sequence includes progradation alluvial fan, delta, fort island and submarine fan model (Figure 8.2). Although they are all thickened upwards, they have their own characteristics. The upward thinning sequence includes channel deposition of sandy braided river and sedimentary facies model of meandering river such as beach, tidal flat and tidal channel (Figure 8.3). In carbonate rocks, the upward shallow sequence often appears repeatedly as subtidal → intertidal zone → supratidal environment, but it shows different combination characteristics under different conditions, such as gypsum sequence, grain sequence, stromatolite sequence, reef sequence and coastal Sabha sequence (Figure 8.4). As for Ma Bao sequence and tempestite sequence in turbidite, they are more familiar. In a word, mastering and using these known sequences and models skillfully is very helpful to determine sedimentary facies. Some simple profile structures can be explained by field observation and summary, and then compared with typical models. For the profile with complex structure and unclear cyclicity or regularity, after detailed measurement, the model profile can be made by using mathematical Markov chain analysis method, and then compared with the typical facies model to determine the sedimentary facies.
Fig. 8.2 upward thickening cross-sectional structure (according to A.D. 1984)
Fig. 8.3 upward thinning profile structure (according to AD 1984)
Fig. 8.4 Common profile structures in carbonate rocks (according to N.P.James, 1979, quoted from Liu Baojun and others, 1990).
Indoor work
The indoor work of sedimentary facies palaeogeography research includes the analysis and identification of various samples, comprehensive arrangement, various basic maps, comprehensive maps and the preparation of the final research report. The former includes the identification of rock slices and the analysis and statistics of their results, the granularity analysis of clastic rocks, the identification and statistical analysis of biological detritus, species and ecological characteristics in rock slices, the qualitative and quantitative research of some sedimentary diagenetic minerals, and the comprehensive arrangement and interpretation of various test data (including spectrum, chemical analysis and isotope analysis). This paper mainly introduces the main points of thin section identification of sedimentary rocks, the compilation of some main maps and several problems in compilation.
8.2.3. 1 Identification of sedimentary rock slices
Thin section identification of sedimentary rocks is the most basic and important indoor identification work, which can provide necessary microscopic evidence for the determination of sedimentary facies and provide a lot of qualitative and quantitative information for the final mapping work. In order to combine field and indoor work, macro and micro research closely, field personnel are encouraged to identify thin slices themselves, and indoor specialized appraisers should also participate in the measurement of main slices.
Identification of (1) sandstone slices
Sandstone is mainly composed of clastic particles, cement, matrix and pores. The identification of sandstone thin sections includes four parts: composition, structure, microstructure and epigenetic changes of diagenesis. The percentage contents of chronological (Q), feldspar (F) and debris (R) should be counted, and the ratio of stable and unstable debris components should be calculated accordingly, that is, Q (chronological+siliceous debris) /(F+R). This ratio reflects the composition maturity of sandstone.
Heteromatrix refers to fine-grained materials which are mechanically deposited by clastic materials, mainly clay materials, but also fine sand and carbonate mortar. The rocks with high heterobase content have poor sorting and low structural maturity.
The structure of sandstone includes particle structure characteristics (particle size, morphology, surface characteristics, etc. ) and matrix structure (contact relationship between particles, support type and cementation type). The data of particle size analysis can be obtained by sieve analysis, thin section method or water separation method. Data collation includes various parameter calculations (including average, standard deviation, skewness, kurtosis and sharpness) of various granularity analysis graphs (commonly used frequency curves, cumulative curves and cumulative curves of probability values), as well as granularity or granularity parameter images (such as C-M graphs and discrete graphs).
The study of diagenetic epigenesis includes the types, stages and evolution of diagenesis. These have a great influence on the porosity and permeability of sandstone, and are of great significance to oil and gas exploration or the formation of some epigenetic metal deposits.
(2) Identification of carbonate slices
Attention should be paid to the study of mineral composition, structure and diagenetic characteristics of carbonate rocks. The common mineral components in carbonate rocks are calcite, dolomite, terrigenous detritus, clay and organic matter.
The study of carbonate rock structure includes four parts: particles, mortar matrix, cement and pores. Attention should be paid to the identification: ① determine the type of particles and the integrity of biological bones. Abiotic bone particles include oolites, pellets (or pellets), nucleated stones, lumps and internal debris; ② Content changes, such as the relative percentage of different types of particles, the content of mortar matrix, the content of bright crystal cement, the ratio of total particles to matrix content, the ratio of matrix to bright crystal cement, etc. (3) the study of biological debris, according to the microstructure characteristics of biological bones to determine its category and relative content.
In the study of diagenetic epigenesis of carbonate rocks, it should be determined whether dolomite is quasi-contemporaneous, diagenetic or epigenetic. It is usually determined by its particle size and crystal shape characteristics, combined with on-site occurrence and other factors. Except for quasi-contemporaneous dolomitization, the rest have no environmental significance. Therefore, for diagenetic or epigenetic dolomite, various means should be used to restore its original rock characteristics to determine its sedimentary facies type.
Study on argillaceous rocks in 8.2.3.2
The argillaceous rock particles are very small and must be studied by other methods and means. The study of argillaceous rocks is generally to observe its color, primary sedimentary structure (structure, bedding, biological disturbance structure), mineral composition and content, organic matter content, biological fossils and so on.
Acquisition and Analysis of Experimental Test Data in 8.2.3.3
Including a variety of laboratory test analysis (such as spectral analysis, chemical analysis, isotope analysis, micro-ancient identification, X-ray diffraction analysis, cathodoluminescence analysis, electron scanning and probe analysis, etc.). ).
Comprehensive Indoor Cartography and Analysis in 8.2.3.4
The final result map of lithofacies palaeogeography analysis should be completed indoors, including various single factor analysis maps (such as sand body map, sandstone percentage map, a rock thickness map, etc.). ) and various comprehensive analysis maps (such as sedimentary system map, lithofacies zoning map, comprehensive paleogeographic map, mineral formation and distribution map, etc.). ).
The techniques and methods of compiling lithofacies palaeogeographic map will be introduced in the following chapters.