SEQUENCE STRATIGRAPHY FOR EXPLORATIONISTS

Based on a workshop sponsored by PTTC's Eastern Gulf Region in Jackson, Mississippi on January 12, 2005.

BOTTOM LINE

Sequence stratigraphy shows the stratigraphic succession of genetically related intervals of strata, bounded by discontinuities. Sequence stratigraphic large-scale, three-dimensional analysis of the rock formations combined with seismic interpretations can provide a powerful means of predictive modeling.

PROBLEM ADDRESSED

Sequence stratigraphy has evolved from an academic concept to a valuable tool for oil and gas exploration in the past two decades, but it is still under-utilized. The predictions about reservoir potential based on sequence stratigraphy depend on understanding changes in global sea level, tectonic movements and how they affect lithologic facies and boundaries. Learning how to evaluate and predict the flow-unit boundaries is the key to correct application of sequence stratigraphic models.

KEY WORDS:

Cyclical sedimentation, Eustacy, Experimental flow tanks, Genetic stratigraphic sequences, Maximum flooding surface, Sequence boundaries, Sequence stratigraphy

SPEAKERS:

Sequence Stratigraphy Basics and Seismic Geometries and Patterns of Reflector Terminations,
Bruce Hart, McGill University

Experimental Stratigraphy; Stratal Geometries Associated with Sequences: Delineating Sequence Boundaries,
Nikki Strong, St. Anthony Falls Laboratory, University of
Minnesota

TECHNOLOGY OVERVIEW

Concepts of cyclic sedimentation have been used in geologic interpretation for decades. Cyclical sedimentation is defined by stacked shoaling-upward packages of marine sediment and stacked submarine fans. Sequence stratigraphy refines the old cyclic sedimentation concepts into specific 1st, 2nd, 3rd, 4th and 5th order sequences based on identification of unit boundaries. Identification of the stratigraphic sequences that make up the cycles is based on lithology and boundary changes. Seismic reflections are particularly useful in picking the sequence boundaries. The deposition of sequences is dependent on sea level change, sediment supply and subsidence. Interpretation of sedimentary strata based on sequence stratigraphy using correlated seismic, wireline and core data is important for hydrocarbon exploration.

Seismic Stratigraphy-Basics
Lithologic interpretation is based on physical properties including rock type, color, mineral composition and grain size. Boundaries between stratigraphic intervals are based on clearly identifiable contacts resulting from changes in deposition. Different facies represent different depositional environments. Walther's law expresses the problem with lithostratigraphic interpretation of facies. "As laterally contiguous environments shift with time, facies boundaries shift so that the facies of one environment lie above those of another environment."Modern sequence stratigraphic analysis is a means of interpreting the lateral and vertical patterns on sedimentary deposition.

Illustrations of stratigraphic intervals and boundaries of the Mesaverde Group in Utah have become classic sequence stratigraphy examples. Timelines are used to identify the stratigraphic surfaces that are formed by the interplay of tectonic forces, global sea level changes (eustacy) and sediment supply (volume and type). The timelines allow for correlations across a broad area and relate all factors into predictive models. Unconformities are major sequence boundaries formed by lowstand erosional events. Flooding surfaces are formed by transgressive (landward movement of the shoreline) and regression (basinward movement of the shoreline) movement. The cyclical nature of sedimentation is built by the stacking of prograding outward building shoreline deposition, and retrograde or backstepping shoreline deposition.

Sea Level Change
Transgression and regression depend on the rate of sediment supply and the rate of accommodation space. Accommodation space pertains to the vertical space in which sediment can accumulate and is equal to the water depth in marine or freshwater lacustrine or fluvial settings. Lateral changes in accommodation determine the shape of the prograding or retrograding deposits, such as ramps or shelf breaks.

Sea level changes effect the direction of sediment supply. Base levels fall when the sediment supply halts along the margin of a basin, followed by subaerial erosion or when the inner shelf of the sea floor is eroded in advance of a prograding shoreline. These changes in sea level are read as unconformities in the rock. Unconformities are recognized as evidence of subaerial erosion and truncation of underlying sedimentary intervals. Difference in facies above or below sequence boundaries can form flow unit boundaries reflected in permeability differences.

Sequence stratigraphy concepts were developed for marine deposits and application of them to non-marine stratigraphy presents difficulties in interpretation. Problems result from changes in sediment supply, tectonic uplift, and climate; which can mimic the effects of sea level change. One approach is to limit sequence stratigraphic concepts to a few 10s of miles from a coastline.

To interpret stratigraphic sequences it is important to establish a shoreface profile. Progradation during periods of constant sea level generates a gradual coarsening upward shoreline profile in equilibrium. Falling sea levels result in abrupt changes in the profile with erosion seaward of a prograding shoreface generating regressive surfaces of marine erosion with shoreface sediments directly overlying deposits from the outer shelf. Rising sea levels result in accumulation of non-marine sediments in a landward direction on the erosion surface. At the shoreline, deposition ceases and erosion begins, resulting in a change from a transgressive to a regressive trend.

Seismic Geometries and Patterns of Reflector Terminations
Recognition of flooding surfaces is the key to interpreting sequence stratigraphy. Flooding surfaces formed during transgression are evidence of shoreface erosion and abrupt deepening. Erosion events can remove up to 10-20 meters of strata and downcut through previous unconformities. These transgressive lag sediments may cap a flooding surface and diagenetic alternation frequently occurs below the lag, forming permeability flow barriers.

In sequence stratigraphy, parasequences are the main unit of interpretation. Parasequences are restricted to shallow marine sediments and reflect shoaling upward stratigraphic units bounded by flooding surfaces. The stacking patterns formed by parasequences differ for progradational, aggradational and retrogradation system tracts. It is important to understand that in the development of systems all sequences that form may not be preserved. An example from Utah's Cliff House sandstone shows sections of barrier islands formed during transgression, which may later be partly or completely eroded.

Maximum flooding surfaces across a broad area indicates the end of a transgression and the beginning of a regression. Maximum flooding surfaces may represent a period of no or very slow deposition or of marine erosion. They can be recognized as downlapping surfaces on log cross sections or seismic images. Maximum flooding surfaces are used to define sequences.

Theories of Sequence Stratigraphy
Systems tracts or sequences link contemporaneous depositional environments. Three major schools of thought on sequences are based on unconformity (Exxon school, 1977-1988), maximum flooding surfaces (Galloway school, 1989), and subaerial unconformity and transgression surfaces (Embry school, 2002).

The depositional sequence of the Exxon model is based on highstand and lowstand systems tracts. It is bounded by subaerial unconformity, followed by a regressive surface of marine erosion, and then a correlative conformity (which is often difficult to recognize). The sequence begins with each highstand or progradation onto the shelf. The Exxon model is useful as a starting point, but is heavily weighted with terminology and requires flexibility for interpretation of sequences.

The Galloway model of genetic stratigraphic sequences is based on sequences bounded by maximum flooding surfaces often referred to as regressive-transgressive sequences (R-T). The advantage of the genetic stratigraphic model is that only one type of surface is defined, and this maximum flooding surface is easy to identify. However, be aware in the genetic model that unconformities can lay within a sequence.

The Embry and Johannessen model of transgressive-regressive sequences (T-R) is bounded by subaerial unconformities and maximum regressive surfaces. The T-R sequence is divided into two systems tracts. The transgressive tract lies between the sequence boundary at the base and the maximum flooding surface at the top. The regressive system tract lies between the maximum flooding surface at the base and the sequence boundary at the top. The T-R system tract is the newest concept and is simple to apply, but not widely used.

Application of Transgressive-Regressive Sequence Concept
Recent work by Mancini, Parcell and Hart (2004) applies the transgressive-regressive sequence concept using outcrops, logs and seismic images to interpreting Gulf Coast stratigraphy. Examples from Mancini, et al demonstrate how seismic interpretation of sequence stratigraphy can be made. The seismic reflections result from changes in velocity and density in the sedimentary deposits. The seismic reflections form timelines separating older and younger strata. Fluid contacts and barriers formed by diagenesis may alter reflections and the timelines.

As seismic reflections correspond to physical changes in the strata, they can be generated anywhere along a flooding surface and can cross lithologic boundaries. Seismic reflection terminations define the geometric relationship between a reflection and the seismic surface where it terminates. Differences in seismic reflection terminations can be used to identify and define depositional history. The visual truncation, toplap, onlap (marine or non-marine), downlap (marine) reflections can be identified on log cross section or seismic lines. When using reflection terminations for interpretation it is important to flatten the seismic or log sections on a datum to avoid problems, such as changes due to tectonic tilting.

Seismic Techniques
In seismic stratigraphy unconformities can be recognized on seismic lines as erosional truncation of underlying stratigraphic intervals. The unconformities result from subaerial erosion related to relative sea level dropping. Internal convergence occurs when sediment packages thin to a reflection termination. Recognition of downlap surfaces at the base of prograding sequences is associated with maximum flooding surfaces and frequently are associated with shale intervals. In deltaic settings downlap is formed by lobe switching. Downlap surfaces in shale may be vertical barriers to fluid flow.

As a caution when interpreting reflection terminations, not all seismic lines are dip sections, and stratigraphic and structural features are often difficult to recognize. Seismic resolution decreases with depth as velocity increases and wavelengths become longer and higher frequencies are attenuated. Frequency changes and content in seismic data may affect resolution and apparent seismic stratigraphic relationships. Due to frequency content, reflection geometries visible on seismic lines may not truly represent stratigraphic relationships.

When using seismic stratigraphy without log or core data, it is essential to establish a workflow based on available data and scale. Use reflection terminations to define the flooding surfaces to tentatively define the depositional environments. Develop a relative sea level history and integrate depositional environments to make predictions on lithology. Define system tracts using seismically determined surfaces. Identify drilling targets and if possible correlate with outcrop analogs or field data.

If well data is available seismic interpretation can be correlated with log or core data to increase the value of predictions about lithology, continuity and depositional systems. One of the most useful techniques is to overlay wireline logs (particularly gamma ray logs) over seismic data. Advanced seismic techniques such as inversions and seismic attribute analysis can be used for predictions on porosity, lithology, seals, and to identify drilling targets.

Challenges with Sequence Stratigraphy
There may be confusion on whose theories to apply. The concepts have changed and evolved over time. Each theory has a multitude of terms that may be contradictory. The difficulties in interpreting cycles, parasequences and sequences can be solved by focusing on the basic principles. Using both logs and seismic data allows for better correlation of intervals and evaluation of the data. Analysis of global sea level changes and determining the order of sequences relies on interpretations of biostratagraphic and subaerial unconformities.

Summary
The object of the one-day short course was to acquaint the audience with the basic principles of sequence stratigraphy, and mention some of the shortcomings or problems with interpretations. The concepts of cycles and sequences are a method that incorporates global sea level changes, local and regional subsidence and uplift, and changes in sediment supply. Sequence stratigraphy depends on interpreting surfaces generated during sea level changes including subaerial unconformities, regressive and transgressive events, and maximum flooding surfaces. The stratigraphic surfaces are used to define the sequences and systems tracts and depositional history. Sequence stratigraphy can be a valuable tool for evaluation of nearshore sediments and prediction of drilling locations in hydrocarbon exploration.

Experimental Stratigraphy: Stratal Geometries Associated with Sequences, Delineating Sequence Boundaries
Experimental research on sequence stratigraphy at the National Center for Earth-Surface Dynamics at the University of Minnesota relies on detailed tank studies and sophisticated computer analysis. The tank studies allow the research team to precisely measure all the potential changes in water levels, sediment supply, subsidence and uplift. This data can be used to reconstruct flooding surfaces and sequences.

Images created by superimposing, in chronological order, surfaces in the experiment tank are captured with laser and sonar topographic measurements. The research helps to analyze the processes that form the surfaces and boundaries, which form the sequences we see in the sedimentary strata. The genetically related facies formed within a framework of chronostratigraphically significant surfaces defines sequence stratigraphy. Laboratory research on the processes and patterns developed during deposition adds significantly to the body of knowledge of sequence stratigraphy.
 

CONNECTIONS:

Bruce Hart, Associate Professor
McGill Seismic Research Group
Earth and Planetary Sciences
McGill University
3450 University St.
Montreal, Quebec, Canada H3A 2A7
Ph: (514) 398-3677 Fax (514) 398-4680
Email: hart@eps.mcgill.ca

Nikki Strong
St. Anthony Falls Laboratory
University of Minnesota
Mississippi River at 3rd Ave SE
Minneapolis, MN 55414
Ph: (612) 250-1763
Email: stro0068@umn.edu

For information on PTTC’s Eastern Gulf Region and its activities contact:

Ernest A. Mancini, Professor of Geology, University of Alabama,
Box 870338, 202 Bevill Building, Tuscaloosa, Alabama 35487
Phone 205-348-4319, Fax 205-348-0818, Email emancini@wgs.geo.ua.edu

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