FEATURE ARTICLE

SEQUENCE STRATIGRAPHY: A PRACTITIONER'S POINT OF VIEW

LOUIS M. LIRO Texaco Exploration Bellaire, Texas 77401

Abstract Sequence stratigraphy has become the pre-eminent method for describing the cyclicity and architecture in sedimen- tary basins. In addition, sequence stratigraphy is now routinely used to evaluate exploration opportunities and to facilitate exploitation and develop- ment projects. The rapid development of sequence stratigraphic concepts and models, however, requires a careful consideration of their applicability.

The greatest challenge is the need to address stratigraphy from multiple perspectives (autocyclic to basin- wide), often incorporating multiple data sets (thin section to seismic sec- tion). This challenge requires a multi- disciplinary approach, tuned to the specific questions at hand.

Key to the application of sequence stratigraphy at any scale is the identi- fication of a vertical fill profile. This is the best "value added" method in exploration, allowing the explo- rationist to rapidly characterize the stratal section and identify key strati- graphic elements (or establish their absence).

The use of scaleless, generalized mod- els in stratigraphic evaluations should be discouraged. These models create unrealistic images of vertical fill, as well as present overly simplified depositional processes. It is more rational to identify the fill profile, and then draw on depositional models spe- cific to the environment interpreted or anticipated.

There has been much discussion of the relative merits of regional erosion sur- faces ("sequence boundaries") and regional flooding surfaces as key cor- relation surfaces. In practice, the spe- cific conditions in each study area (geologic setting, data types and avail- ability, etc.) dictate which (or both) correlation surfaces are useful.

Introduction Sequence stratigraphy has become the pre-eminent method for describing the cyclicity and architecture in sedimen-

3rd dimension effects? /

tary basins. The precursor of sequence ties. (Table 1). In fact, sequence stratigra- stratigraphy, seismic stratigraphy, was phy is now not only an essential part of the first used some twenty Years ago evaluation of exploration opportunities at (Payton, 1977) to define large-scale, any scale, but also an internal part of all regional stratigraphic patterns on seis- field development efforts, whether in ini- mic data. Prior to this development. tial development of a discovery or re-eval- seismic data had been used almost uation of existing assets. exclusively for structural interpreta- tion. With increasing seismic resolu- Our expectations from sequence strati- tion owing to improvements in seis- graphic evaluations, however, require that

NDENSED SECTION u . 0 o , - , , - - 9 . . . . . . . . . HIGHSTAND

SCALE ? Figure 1 Texas Gulf Coast Uano Uplift to abyssal plain, - 500 Krn

the methodologies be applied intelligently. Gone are the days when sequence stratigra- phy could be applied to single data sets (such as seismic, well log, or outcrop); it is now fully expected that the geosci- entist will integrate not on]y these data sets but also allied approaches such as paleontol- ogy, geochemistry, reservoir performance, seal c0mpe- tence, etc. Indeed, the full benefit of Sequence Stratigra- phy comes from its applica- tion and integration across a

'pectrum of a n a l ~ now literally "from thin sec- tion to seismic section." In this paper, I examine some of the strengths and shortcom-

Stratigraphic / Evaluation Perspective

Frontier exploration

Mature basin exploration

Development (initial I reevaluation)

Exploitation

Increasing stratigraphic resolution This usage often ignored that the basins or allows the integrated seismic and well sections being evaluated had fundamental- log examination to approach not only ly different bounding tectonics and defin- the exploration effort, but also devel- ing stratigraphic processes. The prolifera- OPment and exploitation opportuni- tion of this admittedly generalized model,

ings of current application of sequence mic acquisition and processing, as stratigraphy and offer suggestions from well as the advent of 3-D seismic vol- my experience. umes, more stratigraphic detail became evident, and the onset of sub- Generalized Depositional Models sequence or systems tract interpreta- For many years, it was "standard practice" tion was reached. In many studies, the to present the generalized Exxon model resolution of seismic data can be for depositional patterns in a stable, pas- directly related t~ the resolution of sive margin basin (the "sea slug"; Figure well log data, truly integrating the I ) , and use it as the explanation for strati- exploration effort. graphic patterns observed in many basins.

Products of investigation

Stratigraphic framework - basic stratigraphic profile

Refined stratigraphic framework - identification of new targets '

and reevaluation of existing targets

Flow units, fluid contacts. intra-reservoir flow barrlers

Untapped compartments, stratigraphlc opportunities, "mlssed play"

Houston Geoloaical Societv Bulletin Se~tem ber 1 997

Extensions of existing productive basins (such as extending plays into deep water) would require both 'Frontier'' and "Mature Basin" exploration perspectives, depending on available data and understanding of the relevant petroleum systems.

Table 1

derived from or aimed at the seismic inter- preter, oversimplifies depositional condi- tions and hence is typically found unuse- able at increasingly finer levels of investi- gation.

In most cases. the use of a generalized model as a forward model for stratigraph- ic interpretation runs into difficulty when the factors controlling depositional processes of that model do not match the factors of the stratigraphic interval being interpreted. In plain terms, this is a case of '-comparing apples and oranges." Unfortunately. this has often resulted in yet further proliferation of "new" strati- graphic terminology in the literature that serves to "patch" the generalized model rather than describe discrete depositional processes. The message here is that gen- eralized models are just that- general- ized. They make reasonable teaching tools, but are uwally not very applicable as predictive tools beyond the initial strati- graphic overview of a study area.

Environment-Specific Models The implication of scale independence for eustatic control on depositional systems allows the generalized model to be pre- sented without reasonable or realistic dimensions. Applying reasonable scales, it is clear to see that the generalized model is unlikely to be observed in its entirety (or even in its majority) across any modem data set, with the possible exception of reconnaissance 2-D seismic data in fron- tier basins.

In addition, in as much of the original sequence stratiyraphic concepts has been applied to deepwater regions in both fron- tier and mature basin exploration, one can be lulled into a "deepwater perspective," where volumetrically most of the deposi- tion is occurrin_r during relative lowstands of sea level. Combined with the use of generalized well log patterns and absence (or lack of integration) of cuttings, core, or paleo data that more correctly frame the paleoenvironment, this perspective is often incorrectly applied to neritic, lacus- trine, and non-marine settings. Only in the last five years or so have models and stud- ies appeared in the literature that correctly consider the interaction of relative sea level (or more correctly, base level) with the dominant shallow water or fluvial depositional processes.

1 believe that i t is critical to the success of a sequence stratigraphic examination to determine, early in the life of a project, both the anticipated vertical profile (suc- cession of depositional environments and facies that will he encountered) and the

depositional models that best address the specific environments that will be encountered in this profile. Only then can critical evaluation of such issues as reservoir extent. connectivity. rela- tive quality, etc. be made. For detailed reservoir evaluation, I advise the use of the "architectural element" (Miall, 1985) approach to facies description.

Vertical Profiles In practice, different data sets within a given area display differing resolution and clarity of the anticipated vertical profile. Often, one data set displays the variation in vertical profile so well, that it drives the interpretation and integration of the remaining data and may serve as the correlation focus or defining point for the entire study area.

The biostratigraphic information from this well (Figure 2) clearly records a second-order clastic regressive wedge deposited in reaction to a regional tec- tonic downwarping. The vertical pro- file is easily interpreted, and deposi- tional models for each bathymetric zone can be used to compare to facies interpretations made from seismic, well log and cuttings data.

In the lower portion of this column. paleobathymetric interpretations indi- cate non-marine to inner neritic water depths. To study this portion of the vertical succession, non-marine to transitional, dominantly fluvial mod- els were used. The choice of models was in part due to the paleobathymet- ric interpretation and in part due to our experience with the time-equivalent interval elsewhere in the basin.

At approximately one-quarter of the way up section, a pronounced deepen- ing of the paleobathymetry is consis- tent with a major regional downwarp- ing in the area, with abrupt transgres- sion.

Above this pronounced break, the sec- tion displays several thousand feet of rather monotonous but successively shallowing paleobathymetry. This observation is consistent with the well log interpretation, which displays suc- cessively shallower water para- sequence sets.

In a second example (Figure 3), three scales of cyclicity are recorded in the upper Fort Union Formation, a Paleocene non-marine interval in the Wind River Basin of Wyoming:

Successive lnflll 01 accommodation with a regressive wedge (increasing gross sand content upwards)

Major tectonic ' .

downwarping of basin

Figure 2

(1) abrupt creation of accommodation space and lake formation, followed by overall regressive deposition until accommodation space was filled. This is a second-order tectonically forced depositional cycle, likely related to the uplift of the Owl Creek thrust front immediately adjacent to the study area.

(2) forestepping and backstepping (systems tracts) of facies patterns within individual third-order sequences in an overall regressive episode

(3) asymmetric shoaling-upward parasequences. The high number of individual parasequences suggests that autocyclicty may be a significant factor in this depositional episode.

In this study, identification of the cycle hierarchy, as well as the vertical profile on well log and seismic data, allows detailed facies correlation and prediction away from the well control.

Sequence Boundaries Perhaps one of the most hotly debated "issues" in sequence stratigraphy is the near-dogmatic rallying around the

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Iacustrinr (tectonic) cycle

Figure 3 sequence bounding surface, whether it be the regional erosion surface associ- ated with relative regression ("sequence boundary" of the Exxon model) or the regional flooding surface ("genetic stratigraphic sequence boundary" of Galloway [1989], or the apparently similar "maximum flooding surface" of the Exxon model). Although much literature has debated the relative merits of each correlative surface, in practice the geoscientist must determine, by observation or stratigraphic reasoning, which surface allows the best correlation in a given study area. Of course, it is possible that both of these surfaces may be suffl- ciently well expressed to be used in the same study, although it has been our experience that this is typically not the case.

Part of the difficulty in using a single boundary criteria for sequences is that most data sets (particularly 3-D seis- mic grids) sample a relatively small portion of the relevant depositional environment, making it difficult to unambiguously determine a single dominant style for the sequence bound- ing surface.

It has been our experience that each of these surfaces may be used for regional correlation. Regional erosion surfaces are quite evident in non-marine fluvial settings, but often in neritic settings shifting sand deposysterns make identi- fying a time-specific and correlative erosion surface problematic or ambigu- ous. Regional flooding surfaces tend to be well expressed on seismic data. In certain situations, we have even found that transgressive surfaces (fmt tran- gression over lowstand deposits) may serve as a good local correlation sur- face. In any case, it is far more impor- tant to identify a correlation surface of regional extent than to worry which correlation surface it is. As a study pro-

gresses. by stratigraphic context it will become obvious which term needs to be applied. It is clear that in a given study, one must use the correlation sur- face most evident from the data at hand. At all times, stratigaphers must address the data at hand rather than the depostional models being invoked.

Order of Cycles One of the more difficult aspects of sequence stratigraphy (in part due to confusing nomenclature and general- ized 2-D depositional models) is the identification of precisely which "order" of sequence one is interpreting. Much of the original literature implies sequences are largely due to eustatic changes; that is, that depositional pat- terns directly follow the eustatic cycle. This effect is essentially true in older passive margins, where tectonic influ- ence and thermal subsidence consider- ations have little impact (such as the Gulf of Mexico and the Niger delta). In younger and/or tectonically active basins (California margin, Southeast Asia, U.S. Western Interior Seaway), such factors must be considered; often their effects mimic eustatic cyclicity, but their effects on sediment prove- nance and flux can be quite different. In such cases, particularly in the absence of adequate absolute age con- trol, determining the "order" of cyclic- ity, and hence the extent of related stratigraphy, can be difficult.

It is particularly important (and a good practice in all sequence stratigraphy) to fmt identrfy the relative cycles and their hierarchy, then correlate the sig- nificant surfaces defining this hierar- chy through the data grid, and then (and only then) apply an interpretation of the relevant sequence stratigraphic nomenclature. Attempting to immedi- ately interpret sequence boundaries or systems tracts, commonly by identify- ing "distinctive" well log patterns

(which are typically as generalized and dimensionless as the 'sea slug' model) often causes the interpreter to 'force' the interpretation to fit the generalized models. Most generalized vertical suc- cessions do not account for stratigraph- ic autocyclicity or structural complexi- ty (uplift, downwarp, faults, missing section, etc.).

Conclusions Although sequence stratigraphy has been shown to be a valid concept for the evaluation of the stratigraphic development within a basin, care should be taken when applying the methodologies to basins dissimilar to the stable passive margins where the concepts were originally developed, or to depositional environment where eustatic cycles are not likely the domi- nant control on depositional patterns. In these situations, the stratigraphic record may not match the generalized models.

We suggest the following general strat- egy for effective application of sequence stratigraphic concepts:

(1) Always fmt ask, "What question are we attempting to answer?" There is no greater efficiency gain and cycle time reduction in a project than direct- ed and focused analysis. A detailed workflow plan, along with anticipated products, streamlines the process. An associated question should also be, "How will this analysis allow better understanding of the overall explo- ration (or development) risk?"

( 2 ) Examine the available data set to iden@ the basic cycle hierarchy (tec- tonic scales, eustatic scales, and high- er-ordered scales, although initially these may not be identified as such). This is particularly important in explo- ration evaluations.

(3) Identify the probable vertical pro- file of the basin or study area. Understanding the vertical profile becomes increasingly important as the window of investigation becomes smaller (development and exploitation evaluations).

(4) Identify and initially correlate key regional surfaces throughout the study area, developing an understanding of the probable depositional environ- ments. Typically, the number of corre- lation surfaces will approximate the cyclicity identified in step (2), above.

( 5 ) With these efforts done, identify

Houston Geological Society Bulletin September

depositional models for the specific environments anticipated. Ii the study at hand involves 3-D seismic data, time- or horizon-specific slices (along with attribute analysis) should be taken through the data volume for compari- son.

(6) Refine the initial interpretation, including re-evaluation of correlation surfaces if necessary. Depending on the scope of the given project (and the time available). additional, more localized correlation surfaces may be identified to help refine and identify particular reservoir or trapping limits.

(7) Apply relevant sequence strati- graphic terminology.

Although we have not discussed any of the other components of the petroleum system (hydrocarbon charge, develop- ment of trapping mechanism, seal com- petency, etc.) in this paper, their studies parallel the interpretation schemes dis- cussed in this paper.

Acknowledgements Over the last 18 years the author has had many opportunities to test and evaluate the concepts and practicality of seismic and sequence stratigraphy. I have had the benefit of working with a number of co-workers in various geo- logic disciplines whose perspectives and insights have contributed greatly to our collective understanding of physi- cal and qequence stratigraphy. These individuals include my sequence stratigraphy team members: Barbara Radovich. Khib Kugler, and Sally Zellers. as well as my fellow former members of Texaco's Petroleum Systems Team: Barry Katz, Bill Dawson. and Vaughn Robison. On var- ious specific projects I have greatly benefited from discussions with Bill Almon, Steve Johansen, Terry O'Hearn, Paul Lawless, Janet Thornburg, and John Russo.

I thank Texaco for permission to pre- sent this paper and for giving me the uncommon opportunity to have worked with core, well log, biostratigraphic, and seismic data on a worldwide basis to help formulate these ideas, for which, naturally, I take full responsibil- ity.

Finally, I thank the Houston Geological Society for the invitation to present this paper, both in written form and on the HGS Web site.

References Note: View this article at the HGS Web Galloway. W. E.. 1989. Genetic strati- Pugr http://www.hougeo.org and graphic sequences in basin analysis I: http://members.aol,com/LMLiro/HGS architecture and genesis of ~ . ~ , t ~ l flooding-surface bounded depositional units: Bull American Association of Petroleum Geologists v. 73, p. 125-142.

Liro, L. M., 1993. Sequence stratigra- phy of a lacustrine system: upper Fort Union Formation (Paleocene), Wind River Basin, Wyoming, U.S.A., in, P. Weimer and H. W. Posamentier, eds, Siliciclastic sequence stratigraphy: recent developments and application: American Association of Petroleum Geologists Memoir 58, p. 317-333.

Miall, A. D., 1985. Architectural-ele- ment analysis: a new method of facies analysis applied to fluvial deposits: Earth-Science Reviews v. 22, p. 261-308.

Payton, C. E., ed., 1977. Seismic stratigraphy applications to hydrocar- bon exploration: American Association of Petroleum Geologists Memoir 26, 5 16pp.U

September 1997 Houston Geological Society Bulletin