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Method of the sequence stratigraphic analysis.pdf
- 1. Method of the Sequence Stratigraphic Analysis By Group A Roll No.: 1 to 7 M.Sc. 1st Semester, 2078 Central Department of Geology Kritipur, Nepal 1. Nagesh Rijal 2. Krishna Pd. Niraula 3. Binod Dhakal 4. Pradip Devkota 5. Sujan Raj Pandey 6. Upendra Poudel 7. Arun Shrestha
- 2. Sequence Stratigraphy • Sequence stratigraphy (Posamentier and Allen, 1999): the analysis of cyclic sedimentation patterns that are present in stratigraphic successions, as they develop in response to variations in sediment supply and space available for sediment to accumulate. • Sequence stratigraphy (Embry, 2001a): the recognition and correlation of stratigraphic surfaces which represent changes in depositional trends in sedimentary rocks. Such changes were generated by the interplay of sedimentation, erosion and oscillating base level and are now determined by sedimentological analysis and geometric relationships. • Simplest, and that preferred by the authors, is 'the subdivision of sedimentary basin fills into genetic packages bounded by unconformities and their correlative conformities‘. • Sequence stratigraphy is generally used to resolve and explain issues of facies cyclicity, facies associations and relationships, and reservoir compartmentalization, without necessarily applying this information for larger- scale correlations. • Sequence stratigraphy is commonly regarded as only one other type of stratigraphy, which focuses on changes in depositional trends and their correlation across a basin.
- 3. • The sequence stratigraphic approach has led to improved understanding of how stratigraphic units, facies tracts, and depositional elements relate to each other in time and space within sedimentary basins. • The process of interpreting the origins of sequence stratigraphic surfaces, the depositional setting and gross sedimentary geometry of the rocks enclosed within the sequence stratigraphic framework involves Niels Steno‘s Laws of Superposition and Walther's Law.
- 5. Methods of Sequence Stratigraphic Analysis • Facies and facies model • Facies analysis • Concept of depositional system • Classification of depositional environment • Walther‘s law • Paleocurrent direction • Pedology • Iconology • Well log • Seismic data • Age of determination techniques • Workflow of sequence stratigraphy analysis
- 6. Facies, Facies Association & Facies model Facies: • It is a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain condition of sedimentation, reflecting a particular process or environment. • It is sum total characteristics of a rock including its chemical, physical and biological features that distinguishes it from adjacent rock. • Facies (Bates and Jackson, 1987): the aspect, appearance, and characteristics of a rock unit, usually reflecting the conditions of its origin; esp. as differentiating the unit from adjacent or associated units. •Lithology, grain size, sedimentary structure, color, composition, biogenic content are the fundamental of facies studies. • Generally facies are distinguished by what aspect of the rock or sediment is being studied; • Lithofacies: facies based on petrological characteristics such as grain size and mineralogy. • Bio facies: facies based on fossil content (flora and fauna) • Ichnology: facies based on trace fossil.
- 7. Facies • Facies are controlled by sedimentary processes that operate in particular areas of the depositional environments. Hence, the observation of facies helps with the interpretation of syn-depositional processes. • Sedimentary Facies are bodies of sediment that are recognizable distinct from adjacent sediments that result from different depositional environment. They are either terrigenous, biogenic or chemical. • As condition change with time, so different depositional site may change their shape and characteristics. • The deposition environment includes: a) Fluvial environment – Siwalik b) Oceanic environment- different facies within the ocean (Continental shelf, submarine fan-turbidite, deep marine deposits) c) Glacial environment. d) Lacustrine environment. Figure 2.1: Showing Sedimentary Facies in different depositional basin and environment
- 8. Facies Association & Facies Model • Constitute several facies that occur in combination and typically represent one depositional environment. • Facies Association (Collinson, 1969): groups of facies genetically related to one another and which have some environmental significance. • The understanding of facies associations is a critical element for the reconstruction of paleo-depositional environments. In turn, such reconstructions are one of the keys for the interpretation of sequence stratigraphic surfaces. • Facies succession: A progressive change in certain facies properties in a specific direction (vertical or horizontal). • Facies model (Walker, 1992): a general summary of a particular depositional system, involving many individual examples from recent sediments and ancient rocks. • A facies model assumes predictability in the morphology and evolution of a depositional environment, inferring ―standard‖ vertical profiles and lateral changes of facies.
- 9. Fig.3: Generalized facies model and architecture for a typical meandering river system. • Facies models are based on facies association and are designed to show the 3D relationship (architecture between individual facies ( architectural elements for a depositional system. • Models can be tailored to specific stratigraphic unit or can be generalized to show an average characteristics for a typical depositional system.
- 10. Facies Analysis • Facies analysis is a fundamental sedimentological method of characterizing bodies of rocks with unique lithological, physical, and biological attributes relative to all adjacent deposits • Facies analysis is of paramount importance for any sequence stratigraphic study, as it provides critical clues for paleogeographic and paleoenvironmental reconstructions, as well as for the definition of sequence stratigraphic surfaces. • As such that, facies analysis is an integral part of both sedimentology and sequence stratigraphy. • In the context of sequence stratigraphy, facies analysis is particularly relevant to the study of cyclic changes in the processes that form individual depositional systems in response to base level shifts. • The facies concept refers to the sum of characteristics of a sedimentary units, commonly at a fairly small scale(cm-m) • It includes the analysis of parameter such as the configuration continuity, amplitude, phase, frequency, interval velocity. These variable give an indication of the lithology and sedimentary environment of the facies.
- 11. Importance of Facies Analysis • Used for hydrocarbon exploration. • Used for determining paleography. • Useful for groundwater exploration. • Understanding climate changes and subsidence history of depositional basin. • The reconstruction of paleo depositional environments via facies analysis is an important pre-requisite for sequence stratigraphic interpretations. • Gradational (vertical) transitions from one facies to another indicate original adjacency and genetic relationship during formation. • Sharp/erosional (vertical) contacts between the facies provides no evidences of contemporaneous genetic relationship of depositional environments.
- 12. Concept of depositional system • Depositional basin are the region of the earth long term subsidence creates accommodation space for accumulation of sediments. • A depositional system is the product of sedimentation in a particular depositional environment;. hence, it includes the three-dimensional assemblage of strata whose geometry and facies lead to the interpretation of a specific paleo depositional environment. • Convergent boundary creates accretionary wedge and foreland basins through tectonics compression of oceanic and continental crust during lithospheric flexure. • Tectonic extension at divergent boundaries where continental rifting is occurring can create a nascent ocean basin leading to either an ocean or the failure of the rift zone. • In strike- slip settings, accommodation spaces occur as transgressional, transitional basins according to the motion of the plates along the fault zone and local topography.
- 13. Types of Depositional Basin
- 14. Classification of depositional environment • A depositional environment is a specific type of place in which sediments are deposited, such as a stream channel, a lake, or the bottom of the deep ocean. • Depositional settings may be classified into three broad categories as shown below: • In coastal areas, the river-mouth environments (i.e., sediment entry points to the marine basin) are separated by stretches of open shoreline where the beach environment develops. • Coastal environments are critical for sequence stratigraphy, as they record the history of shoreline shifts and are most sensitive in providing the clues for the reconstruction of the cyclic changes in depositional trends.
- 15. Fig 5: Depositional environments and sedimentary basin
- 16. Figure 6: Transition from marine to nonmarine environments. The large arrows indicate the direction of shoreline shift in the two river-mouth environments (R—regressive; T— transgressive). Between the river-mouth environments, the coastline is an open shoreline. Note that the character of the shoreline (transgressive vs. regressive) may change along strike due to variations in subsidence and sedimentation rates.
- 17. Figure 7: Dip-oriented profile illustrating the main geomorphic and depositional settings of a continental shelf: alluvial plain, coastal plain, coastline (including the intertidal and supratidal environments;), and shallow-marine (shoreface and shelf) environments (modified from Posamentier et al., 1992b
- 18. Walther‘s law • Walther‘s law(1860-1937) states that, the vertical succession of facies reflect lateral change in environment. • Conversely, it states that when a depositional environment ―migrates‖ laterally, sediment of one depositional environment come to lie on top of another. • Walther‘s Law (Posamentier and Allen, 1999): the same succession that is present vertically also is present horizontally unless there is a break in sedimentation. • This law is applicable only to situations where there is no break in sedimentary sequences. • The rock succession that is present vertically is also present horizontally unless there is a break in the sedimentation. • The concept was first stated by German geologists Johannes Walther in 1984. • Golovkinsky (1834-1897). A classic example of this law is a vertical stratigraphic succession that typifies marine transgressions and regression.
- 19. • Walther‘s Law is an important principle upon which the origin of vertical rock successions is explained. • Sediments are deposited in environments that change over time as a result of relative sea-level fluctuations. • As the environments change, so does the nature of the sediments deposited at any one location. The vertical succession thus records the lateral changes in environments over time. Figure 8: Walther's law model of facies succession 1894.
- 20. Applications of Walther‘s Law • This is a fundamental principle of stratigraphy, which allows the geologist to visualize predictable lateral changes of facies based on the vertical profiles observed in 1D sections such as small outcrops, core or well-logs. • Vertical changes in litho and bio facies have long been used to reconstruct paleogeography and temporal changes in depositional environments and, with the aid of Walther‘s Law, to interpret lateral shifts of these environments. • Walther‘s law is a powerful tool for facies modeling. • The transgression and regression of sea-level causes the sediments in depositional basin migrate seaward and landward. This causes the sediment deposition to shift laterally and vertically over a period of time so we can actually see the horizontal sequence changes in vertical rock sequence record. Thus, Walther's law is a useful method for sequence stratigraphic analysis. • Walther‘s law is equally valuable when applied to system tracts, as the internal architecture of each tract involves progradational or retrogradational shifts of the faces which translate into corresponding facies changes along vertical profile.
- 21. Paleocurrent direction • Paleocurrent analysis is performed in order to determine the orientation of the current (river, stream) responsible for dispersing and depositing a sedimentary rock unit in a basin. • In the case of tectonically active basins (graben, rift, foreland), the paleocurrent direction data may provide strong evidence for sequence delineation, paleogeographic reconstructions, and stratigraphic correlations. • The change of paleocurrent direction may be due to tectonic reorganization of sedimentary basins which changes its tilt direction and can cause major breaks in stratigraphic records. • In some past historical cases, in a region where vertical profile of sequences are ambiguous and do not show any stratigraphic breaks, paleocurrent data have shown multiple second order depositional sequences separated by unconformities.
- 22. • The orientation of the long axes of cobbles, the orientation of cross-bedding, ripple crests, groove and flute casts can be analyzed for paleocurrent direction • But, these kind of analysis can be done only for sedimentary rocks that are at least in part clastic in origin and coarse grained enough so that they exhibit noticeable directional structures or easily analyzed textural trends. • Paleocurrent direction can also be analyzed from the rose diagram as shown in figure. • It is constructed by plotting directional data and the frequency as a circular histogram through 360˚ Figure 9: Rose diagram
- 23. Pedology Pedology refers to scientific discipline concerned with all aspects of soils, including their physical and chemical properties, the role of organisms in soil production and in relation to soil character, the description and mapping of soil units, and the origin and formation of soils. Paleosols also called fossil soils are buried soil from geological past. Pedological studies have various geological applications: • Interpretations of ancient landscapes, from local to basin scales. • Interpretations of ancient surface processes such as sedimentation, nondeposition, erosion. • Interpretations of paleoclimates. • Stratigraphic correlations, and the cyclic change in soil characteristics in relation to base-level changes. All of these applications are related to sequence stratigraphy.
- 24. Formation of soil/paleosols • The soil/paleosol form in varieties of different conditions. • Paleosols form mostly on non marine settings such as alluvial, palustrine, eolian and also costal and marginal to shallow marine environment. • Soils also form in conjunction with different surface process such as sediment aggradation, sediment bypass (nondeposition), sediment reworking and others. • Soils formed during sediment aggradation occur within conformable successions, whereas soils formed during nondeposition or erosion are associated with stratigraphic hiatus, marking diastems or unconformities in the stratigraphic record. • These issues are important in sequence stratigraphy to distinguish between paleosols with significance of sequence boundaries, playing the role of subaerial unconformities and paleosols that occur within sequences and system tracts. • From a sequence stratigraphic perspective, paleosols may provide key evidence for reconstructing the syndepositional conditions (e.g., high vs. low water table, accommodation, and sedimentation rates, paleoclimate) during the accumulation of systems tracts, or about the temporal significance of stratigraphic hiatuses associated with sequence-bounding unconformities.
- 25. • Compound, composite and cumulative paleosols occur within conformable successions, hence within depositional sequences. • ‗Truncated‘ paleosols are associated with stratigraphic hiatuses, and therefore mark diastems or unconformities. Figure: 11: Interplay of pedogenesis and surface processes (modified from Morrison, 1978; Bown and Kraus, 1981; Marriott and Wright, 1993; Kraus, 1999).
- 26. Ichnology • Ichnology is the study of traces made by organisms, including their description, classification and interpretation (pembertonetal.,2001). • Ichnofossils are the geological records of biological activity. • Such traces may be paleoichnology or neoichnology and generally reflects basic behavior patterns.(e.g., resting, locomotion, dwelling or feeding). Importance of ichnology in sequence stratigraphy: a) Represents both sedimentological and paleontological entities. b) Helps in recognition of various types of discontinuities and their genetic interpretation. c) Assists on the identification and interpretation of bounding surface of stratal units. d) Ichnology may be employed to resolve surfaces of sequence stratigraphy mainly; through the recognition of substrate-controlled ichnofacies and through careful analysis of vertical ichnological successions. Figure 12:Trace fossils of annelid.
- 27. Classification of Ichnofacies Figure 13: Classification of ichnofacies based on substrate type and consistency, as well as depositional environment (modified from Bromley et al., 1984, and Pemberton et al., 2001).
- 28. Well Log • Well logs represent geophysical recording of various rock properties in bore holes. • The process of extracting the detailed records of geologic formations made by a well or a borehole is called well logging. • One major advantage of geophysical logs over outcrops is that they provide continuous information from relatively thick successions, often in a range of kilometers. This type of profile (log curves) allows one to see trends at various scales, from the size of individual depositional elements within a depositional system, to entire basin fills. For this reason, data provided by well logs may be considered more complete relative to the study of outcrops. Types of well logging and geological form ( modified from Cant, 1992)
- 29. Figure 14: Types of well logs, properties they measure, and their use for geological interpretations (modified from Cant, 1992).
- 30. Seismic Data • Seismic data are collected along a grid of linear profiles, that results 2D seismic lines(two way travel time vs. horizontal distance). • Then raw seismic information are processed further before using it for geological interpretation. • Then the seismic lines provide continuous subsurface information over distance of 10s of km and depth in range of kms. • The continuous character of seismic data shows major advantages over other methods of stratigraphic analysis. • Seismic data provide the fundamental means for the preliminary evaluation of a basin fill in the subsurface, usually prior to drilling, in terms of overall structure, stratigraphic architecture, and fluid content. • The seismic data analysis are analysed in a workflow of ; Reconnaissance Studies, Interval Attribute Maps, Horizon Attribute Maps and 3D Perspective Visualization
- 31. • Seismic surveys are an integral part of hydrocarbon exploration, as they allow; (1) assess the tectonic setting and the paleo depositional environments; (2) identify potential hydrocarbon traps (structural, stratigraphic, or combined); (3) evaluate potential reservoirs and seals; (4) evaluate source rocks and estimate petroleum charge in the basin; (5) Evaluate the amount and the nature of fluids in individual reservoirs; (6) develop a strategy for borehole planning based on all of the above; and (7) significantly improve the risk management in petroleum exploration. Reconnaissance Studies: initial scrolling through the data (side to side ,front to back, top to bottom) in order to assess the overall structural and stratigraphic styles (Hart,2000). Interval Attribute Maps: then, the intervals bracketing sections of geologic interest can be evaluated in more detail by constructing interval attribute maps. Horizon Attribute Maps: Enhances the visualization of geomorphologic and depositional elements of specific paleodepositional surfaces by picking the geological horizon of interest within the seismic window. 3D Perspective Visualization: illustrates surfaces extracted from 3D seismic data.
- 32. Figure 15 : Sample of a three-dimensional seismic volume showing a prograding Permian shelf margin from the Delaware Basin (from Hart, 2000
- 33. Age Determination Techniques • Evaluation of geological age Involves relative abundance of radioactive isotopes. • Age Determination Technique is one of the important methods of sequence stratigraphic analysis. • Age data are useful to have, and particularly useful to correlate at larger scales which are controlled by; Biostratigraphy (on the basis of available fossils and biological succession). Magneto stratigraphy(based on the polarity of remanent magnetism of rock strata ). Isotope Geochemistry (based upon the study of natural variations in the relative abundances of isotopes of various elements. ) Chrono stratigraphy (deals with the relative time relations of rocks) Marker Horizons (Oolitic Hematite of Bhainskati Fm)
- 34. Workflow Of Sequence Stratigraphy Analysis • The purpose of stratigraphic analysis of an area is to determine lithostratigraphy, chronostratigraphy, possible reservoir and depositional environment studies. • Integrate & Interpret as many types of data as possible., derived from the study of outcrop , core, well logs, and seismic volume. • The workflow suggests the basic building of systematic sequence stratigraphic analysis to approach. • Here, the data interpreter should have a board flexibility in adapting different geological circumstances (e.g., type of basin, subsidence, and sedimentation history) and different data. • An example on how the data are analyzed is discussed here on the basis of principle that understanding of the larger-scale tectonic and depositional setting must be achieved first, before the smaller-scale details can be tackled. • The accuracy of sequence stratigraphic analysis, as with any geological interpretation, is proportional to the amount and quality of the available data.
- 35. • Workflow indicates the steps that need to be carried out in order to analyze the sequence stratigraphic. • Following approach are taken for sequence stratigraphic analysis as shown in figure below. Stage -1 Stage -2 Stage -3 Figure: Workflow of sequence stratigraphic analysis • Here, workflow progresses at a gradually decreasing scale of observation and increasing level of detail.
- 36. Stage 1. Tectonic Setting • First stage, the type of basin hosts the sedimentary succession of different fundamental variables are to be constrained. • Tectonic setting is unique in terms of subsidence patterns stratigraphic architecture, nature of depositional systems such that it controls the formation of basin. • These pattern represent primary control on overall geometry and internal architecture. • The reconstruction of tectonic setting must be based on regional data (obtained from well log, seismic lines, fossil content, etc.) and the analysis of these data proceeds. Figure 16: Generalized dip-oriented cross section through divergent continental margin, illustrating overall subsidence pattern and stratigraphic architecture • Example: graben, rift valley, etc having high subsidence rate in distal direction and foreland basin in proximal direction.
- 37. Stage- 2. Paleo-depositional Environment • Interpretation of paleo depositional environment is another key step in sequence stratigraphic workflow. • Each depositional environment has its own geometry and morphology. • Identification of specific depositional elements evaluate stratigraphic unit. • Paleoenvironmental reconstruction are important for both inside and outside the scope of sequence stratigraphy. • For the workflow 3D seismic data are more useful than 2D seismic data. • Interpretations depends on integration of multiple data set of seismic, well-log, core, outcrop. • Finally, the result of paleo environment is presented in the form of paleogeographic map.
- 38. Figure 17 :Azimuth map (top) and structure map (bottom) of the seafloor relief, offshore east Java, Indonesia, showing the tectonic and depositional settings during the Late Pleistocene relative sea-level lowstand (images courtesy of H.W. Posamentier).. • The detached shorelines (sediment ridges) on the continental shelf formed during relative sea-level fall, prior to the shoreline reaching its lowstand position. • The slump scars indicate instability at the shelf edge, a situation that is common during times of relative fall.
- 39. Stage - 3 Sequence Stratigraphic Framework • It explains the origin of enveloped erosional and depositional surface boundaries. • It provides the genetic context in which separated strata and surfaces are placed into a coherent model for all temporal and spatial relationship of the facies filling a sedimentary basin. • Most efficient exploration approach for natural resources. • It proceeds with observation of stratal termination, identification of sequence stratigraphic surfaces and labeling of strata in terms of system tracts and sequences. Figure 18: A sequence stratigraphic framework leads to interpretations of depositional setting and predictions of lithofacies geometries.
- 40. 3.1 Stratal Termination • Refers to the geometric relationships between strata and stratigraphic surfaces. • Stratal terminations may also be inferred on well- log cross-sections of correlation. • It provides critical information regarding the direction and Syn- depositional shoreline shift. 3.2 Stratigraphic Surface • It helps to build the chronostratigraphic framework for the sedimentary succession under analysis. • Can be identified on the basis of nature of contact, the nature of depositional system, types of stratal termination and depositional trends. 3.3 Systems Tracts and Sequences • Last stage of sequence stratigraphic workflow. • Represent the fundamental mapping until that contains depositional systems for paleogeographic map can be drawn. • Once the position and type of stratigraphic surfaces are at hand, identification of systems tracts on cross-sections is a straight forward procedure.
- 41. Types of data sets to the sequence stratigraphic interpretation.
- 42. Thank You