Stratigraphic Principles

1. Stratigraphic Principles • Types of Stratigraphy • Focus on Modern Stratigraphy is Sequence-based • History of Sequence Stratigraphy • Terminology of Sequence Stratigraphy • Examples from Permian Basin

2. It was recognized in the 1600's that in a sedimentary sequence, the older beds are on the bottom, and the younger beds are on the top. This has come to be called the Principle of Superposition. You can visualize how this occurs if you imagine a stack of newspapers in the corner of a room. Every day you put another newspaper on the pile. After several weeks have passed, you have a considerable stack of newspapers, and the oldest ones will be on the bottom of the pile and the most recent ones will be on the top. This fairly obvious, but very important fact about layering was first noted by Nicholaus Steno, and is the first of three principles which have come to be known as Steno's Laws. Steno's second law is the Principle of Original Horizontality, which states that sediments are deposited in flat, horizontal layers. We can recognize this easily if we consider a sedimentary environment such as the sea floor or the bottom of a lake. Any storm or flood bringing sediment to these environments will deposit it in a flat layer on the bottom because of the sedimentary particles settling under the influence of gravity. As a result, a flat, horizontal layer of sediment will be deposited. Steno's third law is the Principle of Original Lateral Continuity. If we consider again the sediment being deposited on the seafloor, the sediment will not only be deposited in a flat layer, it will be a layer that extends for a considerable distance in all directions. In other words, the layer is laterally continuous. Steno’s Laws

3. Types of Stratigraphy Classical branches • Lithostratigraphy • Chronostratigraphy • Biostratigraphy Newer subdisciplines • Seismic stratigraphy • Cyclostratigraphy • Magnetostratigraphy • Chemostratigraphy • Integration of disciplines in Sequence Stratigraphy

4. Terminology of Lithostratigraphic Units • Supergroup • Group • Formation • Member • Bedset • Bed • Types of unconformities • Angular unconformity • Disconformity • Paraconformity • Nonconformity • hiatus

5. History of Sequence Stratigraphy: Unconformities • James Hutton (1726-1797): Father of Modern Geology • First described unconformity at Siccar Point in northern Scotland • In a paper entitled “Theory of The Earth” (1785) Hutton recognized the importance of unconformities • Also recognized stratigraphic nature of igneous rocks and believed came from molten material within Earth

6. History of Sequence Stratigraphy: Uniformitarianism Principles of Geology (1833)

7. History of Sequence Stratigraphy William Smith measured and described outcrop stratigraphy during excavation of transport canals in England beginning in 1791

8. History of Sequence Stratigraphy: Facies Johannes Walther (1892) proposed what is now known as “Walther’s Law of Facies Succession” "Facies adjacent to one another in a continuous vertical sequence also accumulated adjacent to one another laterally".

9. History of Sequence Stratigraphy: Time-significant surfaces 1909: Eliot Blackwelder published use of unconformities as time markers; introduced concept of time represented by surfaces (erosional removal and sedimentary hiatus)

10. History of Sequence Stratigraphy: Cyclicity 1916: Amadeus Grabau proposed the pulsation theory.

11. History of Sequence Stratigraphy: Base Level 1917: Joseph Barrell stated the most fundamental events in geologic history--the time-space distribution of deposition and non-deposition: the alternating rise and fall of Base-level.

12. What Really Happened: New Concepts from New Technology 1951 John L. Rich proposes the concept of clinoforms… …recognition of seismic reflection geometries

13. History of Sequence Stratigraphy: Time Stratigraphy 1958 Harry Wheeler produced first chronostratigraphic chart

14. History of Sequence Stratigraphy: Time Stratigraphy 1963 Lawrence Sloss recognized 6 major sequences in North America controlled by eustatic sea level changes

15. Official Birth of Sequence Stratigraphy 1977 Peter Vail and Robert Mitchum co-ordinated the publishing of AAPG Memoir #26 based on the assumption that a seismic relection surface represents a time line

16. A Definition of Sequence Stratigraphy • A framework of genetically related stratigraphic facies geometries and their bounding surfaces used to determine depositional setting • Subdivision & interpretation of sedimentary record using a framework surfaces seen in outcrops, well logs, & 2-D and 3-D seismic. • Include: Surfaces of erosion & non-deposition (sequence boundaries), systems tracts • Flooding (trangressive surfaces [TS] &/or maximum flooding surfaces [mfs]); require that relative sea-level is known • This framework used to predict the extent of sedimentary facies geometry, lithologic character, grain size, sorting & reservoir quality

17. A Simpler, More Useful Definition of Sequence Stratigraphy A method to impose the dimension of time on the relationships of rock units in space (area and depth) To do this we first describe facies… …Then we observe the geometric relationships among facies By understanding how rock units are related in time and space, we can better interpret how they are connected as oil and gas reservoirs

18. How is “Facies” Used In Sequence Stratigraphy • Facies: the total textural, compositional and structural characteristics of a sedimentary deposit resulting from accumulation and modification in a particular environment: • Grain size, sorting, rounding • Lithology • Sedimentary structures • Bedding type • In other words, the simplest description of the genetically related vertical and lateral succession

19. The Problem of Preservation Potential Sedimentary facies models are useful to explain spatial relationships of co-eval facies in the modern But what will be preserved in the stratigraphic record?

20. Original Sequence Stratigraphic Approach (seismic stratigraphy) was based on recognition of unconformity-bound sequences using geometry and termination patterns of seismic reflectors.

21. A critical assumption of the seismic stratigraphic approach, illustrated in this diagram from Vail et al (1977), is that seismic reflectors follow time surfaces rather than facies impedance boundaries. Note the regional scale of this illustration.

22. The Classic Slug Exxon Model SB – sequence boundary LST – lowstand systems tract TST – transgressive systems tract HST – highstand systems tract SMW – shelf-margin wedge Mfs – maximum flooding surface Lsw – lowstand wedge Sf – slope fan Bf – basin-floor fan

23. Sequence Boundary • The unconformity or correlative conformity that bounds a sequence • Not always a major physical feature • Not ever exposure surface is a sequence boundary! • Commonly (but not always) represents a significant change in stratal arrangements and therefore reservoir properites

24. Maximum Flooding Surface • Surface that marks the turn-around from landward-stepping to seaward stepping strata • Farther out on platform coincides with the downlap surface (depending on the degree of condensation of clinoform toes) • Recognition of the MFS is important for separating TST and HST, which in turn is important for other stratigraphic analysis, but on the platform top (where 99% of carbonate reservoir facies occur) this can be difficult to pin down precisely. Don’t get hung up on this. Try to pick it as closely as possible, knowing that your colleague will disagree in order to appear enlightened.

25. Transgressive Systems Tracts • Bounded below by underlying sequence boundary and above by maximum flooding surface • Generally more mounded in geometry • Sets of high-frequency cycles show upward thickening and upward deepening trends • Typically less grainstone prone, more diverse skeletal assemblages

26. Highstand Systems Tracts • Bounded below by maximum flooding surface and above by overlying sequence boundary • Generally shingled or offlapping (clinoformal) stratal geometry • Sets of high-frequency cycles show upward thinning and upward shallowing trends • Typically grainstone prone, less diverse skeletal assemblages

29. Terminology of Stratigraphic Hierarchies Composite sequence High-frequency sequence High-frequency cycle

30. Transgressive sequence set = retrogradational sequence set Highstand sequence set = progradational sequence set Lowstand sequence set

31. Can we do sequence stratigraphy in carbonates with just logs? Does anyone here work for Schlumberger?

32. Indicator facies analysis Walther’s Law Model development