Sequence Stratigraphy— a Global Theory for Local Success

S E I S M I C S

Sequence Stratigraphy— A Global Theory for Local Success

No exploration technique flawlessly locates a potential reservoir, but sequence stratigraphy may come close. By understanding global changes in sea level, the local arrangement of sand, shale and carbonate layers can be interpreted. This enhanced understanding of depositional mechanics steers explorationists toward prospects missed by conventional interpretation.

Jack Neal 01miles Rice University 01miles Houston, Texas, USA

David Risch Houston, Texas, USA 1.0 Peter Vail Rice University Houston, Texas, USA

For help in preparation of this article, thanks to Scott

Bowman, Marco Polo Software, Houston, Texas, USA; Time, sec Carlos Cramez and Bernard Duval, TOTAL Exploration, Paris, France; Andrew Hannan, GECO-PRAKLA, Hous- 2.0 ton, Texas, USA; Ulrich Möller, GECO-PRAKLA, Han- nover, Germany; and John Sneider, Rice University, Houston, Texas, USA.

3.0

nA seismic section interpreted to show the sandy interval (yellow) predicted using sequence stratigraphy. The heavy vertical line shows the well location.

Conventional lithologic correlation maps of reservoirs, source rocks and seals, even if formation tops by interpreting well log data none of them intersect the well (above). alone. It looks at what is there without tak- Sequence stratigraphy is used mainly in ing into account how it got there. Sequence exploration to predict the rock composition stratigraphy combines logs with fossil data of a zone from seismic data plus distant, and seismic reflection patterns to explain sparse well data. It also assists in the search both the arrangement of rocks and the for likely source rocks and seals. Experts depositional environment. Understanding believe that as more people learn the tech- the relationships between rock layers, their nique, it will become an exploitation tool seismic expression and depositional envi- for constraining the shape, extent and conti- ronments allows more accurate prediction nuity of reservoirs. January 1993 51 Sequence Stratigraphy, Seismic Stratig- to basins, causing sea level rise; glacio- Building Blocks raphy—How Many Stratigraphies Can eustasy, controlled by climate, lowering sea The concepts that govern sequence strati- There Be? level during glaciation and raising it during graphic analysis are simple. A depositional Stratigraphy is the science of describing the deglaciation. He recognized that all these sequence comprises sediments deposited vertical and lateral relationships of rocks.1 causes may be partially applicable, and are during one cycle of sea level fluctuation— These relationships may be based on rock not mutually incompatible. He believed by Exxon convention, starting at low sea type, called lithostratigraphy, on age, as in that while eustatic hypotheses apply world- level, going to high and returning to low. chronostratigraphy, on fossil content, wide, tectonic hypotheses do not and vary One cycle may last a few thousands to mil- labeled biostratigraphy, or on magnetic from region to region. Fairbridge summa- lions of years and produce a variety of sedi- properties, named magnetostratigraphy. rized the perceived goal at the time: “We ments, such as beach sands, submarine Stratigraphy in one form or another has need therefore to keep all factors in mind channel and levee deposits, chaotic flows or been around since the 1600s. In 1669, and develop an integrated theory. Such an slumps and deep water shales. Sediment Nicholaus Steno, a Danish geologist work- ideal is not yet achievable and would type may vary gradually or abruptly, or may ing in Italy, recognized that strata are involve studies of geophysics, geochemistry, be uniform and widespread over the entire formed as heavy particles settle out of a stratigraphy, tectonics, and geomorphology, basin. Each rock sequence produced by one fluid. He also recognized that some strata above sea level and below.”3 cycle is bounded by an unconformity at the contain remnants of other strata, and so This brings us nearly to the present. In bottom and top.8 These sequence bound- must be younger. This conflicted with the 1977, Peter Vail at Exxon and several col- aries are the main seismic reflections used widely held view that all sediments were leagues published the first installments of to identify each depositional sequence, and deposited during the flood at the time of such an integrated theory.4 Vail developed a separate younger from older layers every- Noah. Steno developed three principles that new kind of stratigraphy based on ideas pro- where in the basin. form the basis of all stratigraphy—younger posed by L. L. Sloss—the grouping of layers Composition and thickness of a rock layers lie on top of older layers, layers are into unconformity-bound5 sequences based sequence are controlled by space available initially horizontal, and layers continue until on lithology—and by Harry E. Wheeler— for sediments on the shelf, the amount of they run into a barrier. His work was neither the grouping of layers based on what has sediment available and climate. Space widely publicized nor remembered and is become known as chronostratigraphy.6 available on the shelf—which Vail calls often credited to James Hutton (1726-1797) Vail’s approach allowed interpreting uncon- “shelfal accommodation space”—is a func- or Charles Lyell (1797-1875). formities based on tying together global sea tion of tectonic subsidence and uplift and of For 300 years after Steno, stratigraphers level change, local relative sea level change global sea level rise and fall on the shelf. For worked at unraveling the history of the and seismic reflection patterns. This instance, subsidence during rising sea level earth, correlating fossils from one continent methodology, named seismic stratigraphy, will produce a larger basin than uplift dur- to another, assigning names, ages and even- classifies layers between major unconformi- ing rising sea level. The distribution of sedi- tually physical mechanisms to the creation ties based on seismic reflection patterns, ment depends on shelfal accommodation, of rock layers. By 1850, most of the major giving a seismically derived notion of lithol- the shape of the basin margin—called depo- geologic time units had been named. By ogy and depositional setting. sitional profile—sedimentation rate and cli- 1900, most layers had relative ages, and Subsequent seismic stratigraphic studies mate. Climate depends on the amount of rock types had been associated with certain in basins around the world produced a set heat received from the sun. Climate also positions of the shoreline, which was of charts showing the global distribution of influences sediment type, which tends known to move with time. Fine-grained major unconformities interpreted from seis- toward sand and shale in temperate zones rocks such as siltstones and shales were mic discontinuities for the past 250 million and allows the production of carbonates in associated with calm, deep water, and years.7 An understanding emerged that the tropics. coarse-grained, sandy rocks with energetic, these unconformities were controlled by rel- As an exploration tool, sequence stratigra- shallow environments. ative changes in sea level, and that relative phy is used to locate reservoir sands. In At the turn of the century, shoreline move- changes in sea level could be recognized on deep water basins with high sedimentation ment was attributed to tectonic activity—the well logs and outcrops, with or without seis- rates, sands are commonly first laid down as rising and falling of continents. This view mic sections. This led to the interdisci- submarine fans on the basin floor (next was challenged in 1906, when Eduard plinary concept of sequence stratigraphy—a page, “A”) and later as deposits on the con- Suess hypothesized that changes in shore- linkage of seismic, log, fossil and outcrop tinental slope or shelf (next page, “B”). But line position were related to sea level data at local, regional and global scales. as sea level starts slowly rising onto the con- changes, and occurred on a global scale; he The integrated theory sought by Fairbridge tinental shelf, sands are deposited a great called the phenomenon eustasy.2 However, had arrived. lateral distance from earlier slope and basin Suess was not able to refute evidence pre- This article focuses on the subset of deposits.9 Deposits during this time are sented by opponents of his theory—in many sequence stratigraphy that includes seismic deltaic sediments that build into the basin locations there were discrepancies between data, and so falls under the heading of seis- and deep water shales (next page, “C”). If rock types found and types predicted by sea mic sequence stratigraphy. The technique the sediment supply cannot keep pace with level variation. has been shown to work in a variety of set- rising sea level, the shoreline migrates land- In 1961, Rhodes W. Fairbridge summa- tings, in some better than others. Attention ward and sands move progressively higher rized the main mechanisms of sea level here is on an environment where it has up the shelf (next page, “D”). Once sea change: tectono-eustasy, controlled by proved successful—sand and shale deposi- deformation of the ocean basin; sedimento- tion on continental margins. eustasy, controlled by addition of sediments

52 Oilfield Review

level reaches a maximum for this cycle,

Marsh sands will build basinward as long as sedi- a a a a a

a ment remains available (left, “E”). The aaa a a a a a sequence ends with a fall in relative sea

n level, marked by a break in deposition. The a a aaa a Sequence compo-

a sequence repeats, however, as long as there

0 nents in order of aaa deposition, from is sediment and another cycle of rise and

a a a a

a a bottom to top. The fall in relative sea level that changes the

E aasequence begins shelfal accommodation space (see “A a aa a

a a a when sea level rela- tive to the basin Detailed View of Sequence Stratigraphy,” a a a a a aa floor begins to fall. next page). Reworked aaThe first deposits, (continued on page 56) shore sands a a

sand-rich fans, are

laid down while sea aaa a a level is falling to its 1. For a review:

Schoch RM: Stratigraphy: Principles and Methods. aa aa a a lowest point (A).

Maximum As sea level bot- New York, New York, USA: Van Nostrand Reinhold, sea level 1989. a a a a a toms out and begins

to rise (B), sands 2. Suess E: The Face of the Earth, vol 2. Oxford, England: Clarendon Press, 1906.

aaa a a and shale are

0 deposited in fans on 3. Fairbridge RW: “Eustatic Changes in Sea Level,” in Ahrens LH, Press F, Rankama K and Runcorn SK (eds): a a a a a a a the continental Physics and Chemistry of the Earth, vol. 4. London,

D slope. Submarine England: Pergamon Press Ltd. (1961): 99-185. aaa a aa a a a a channels with lev- 4. Vail PR and Mitchum RM: “Seismic Stratigraphy and ees may meander Global Changes of Sea Level, Part 1: Overview,” in a a a a a aa across the fan. Payton CE (ed): AAPG Memoir 26 Seismic Stratigra- aaSlumps are common. phy—Applications to Hydrocarbon Exploration a a The continuing (1977): 51-52. rise in sea level (C) Mitchum RM, Vail PR and Thompson S: “Seismic aaa a a allows wedges of Stratigraphy and Global Changes of Sea Level, Part 2: a a a Delta sediment to build The Depositional Sequence as a Basic Unit for Strati- Barrier bar graphic Analysis,” in Payton CE (ed): AAPG Memoir a into the basin, with a a a a sands near the 26 Seismic Stratigraphy—Applications to Hydrocar- aaabon Exploration (1977): 53-62. a shore, siltstones and shale basinward. Vail PR, Mitchum RM and Thompson S: “Seismic 0 Stratigraphy and Global Changes of Sea Level, Part 3:

a a a a A rapid rise in sea Relative Changes of Sea Level from Coastal Onlap,” in a a level (D) moves C aaPayton CE (ed): AAPG Memoir 26 Seismic Stratigra- sandiest sediments phy—Applications to Hydrocarbon Exploration a aa a a a a landward as beaches (1977): 63-82. and sandbars. Vail PR, Mitchum RM and Thompson S: “Seismic a a a a a aa Sea level then Stratigraphy and Global Changes of Sea Level, Part 4: rises at a lower rate Global Cycles of Relative Changes of Sea Level,” in a a aa (E), allowing sedi- Payton CE (ed): AAPG Memoir 26 Seismic Stratigra- phy—Applications to Hydrocarbon Exploration aaa a a ments to build basin- (1977): 83-98. a a a ward again. Sandy Slump sediments are usu- Vail P, Todd RG and Sangree JB: “Seismic Stratigraphy and Global Changes of Sea Level, part 5: Chronos- a a a ally restricted to the Fan lobe tratigraphic Significance of Seismic Reflections,” in aanearshore margin. Payton CE (ed): AAPG Memoir 26 Seismic Stratigra- a Colors follow a phy—Applications to Hydrocarbon Exploration 0 convention used by (1977): 99-116. a a a a a Vail. In order of a a a Sangree JB and Widmier JM: “Interpretation of Depo- B aadeposition, basin sitional Facies From Seismic Data,” Geophysics 44 a aa a a aa a (February 1979): 131-160. a floor fans are yel- low, slope fans are 5. An unconformity is a surface separating younger from a a a a a Channel aa brown and subma- older layers, along which there is evidence of erosion and levee aarine slumps are pur- or a significant break in deposition. a a ple. River deltas 6. Sloss LL: “Sequences in the Cratonic Interior of North that build out dur- America,” Geological Society of America Bulletin 74 aaa a a ing low relative sea (1963): 93-113. a a a level are pink. Rocks Wheeler HE: “Time Stratigraphy,” American Associa- associated with tion of Petroleum Geologists Bulletin 42, no. 5 (May a a a 1958): 1047-1063. Minimum higher relative sea aa7. Haq BU, Hardenbol J and Vail PR: “Chronology of a sea level level—and usually less likely to be Fluctuating Sea Levels Since the Triassic,” Science 235 0 (1987): 1156-1166.

a a a sandy—are green a a a a a and orange. Within 8. In some cases the unconformity may correlate later- A aaally with a conformity. A conformity is a surface that aaa a a every part of a a aa a a a a conforms to those above and below it, with no evi- sequence, sand- dence of erosion or nondeposition. a a a a prone zones are a a a a a a 9. In the case of the Gulf of Mexico, a 100-m rise in rel- yellow. Submarine ative sea level would cause a 150-km landward shift aaaaa canyon a a of the shoreline: Matthews RK: Dynamic Stratigraphy. Englewood Basin aaa a a Cliffs, New Jersey, USA: Prentice-Hall, Inc. (1984): floor fan 394. January 1993 53 A Detailed View of Sequence Stratigraphy

The components of depositional sequences are eral darcies.2 It may be overlain by a thin clay- ing shoreline (next page, “C”). Log response called systems tracts. Systems tracts are divided rich layer that can act as a seal, but more often it shows more sand higher in the section and less into three groups according to relative sea level is overlain directly by the next depositional unit. sand basinward, indicating a coarsening upward. at the time of deposition—lowstand at low rela- In these cases, the basin floor fan acts as a The seismic signature shows moderate- to high- tive sea level, transgressive as the shoreline hydrocarbon migration pathway. Fossil content is amplitude continuous reflectors that downlap moves landward, and highstand at high relative minimal, since deposition rates are often very onto the basin floor. This depositional unit often sea level. Systems tracts are depositional groups high. Basin floor fans derive their hydrocarbon contains ample sand, especially near the sedi- that have a predictable stratigraphic order and from previous sequences. ment source. Updip seals are typically poor, how- predictable shapes and contents. A close look at In areas of high deposition rate, the major ever, and structural trapping is required for hydro- systems tracts, their geometries and lithologies, component of the lower lowstand systems tract is carbon accumulation. shows how sequence stratigraphy can be used to the slope fan complex. Slope fans can be exten- The transgressive systems tract represents foretell reservoir location and quality. sive and can exhibit several depositional styles, sedimentation during a rapid rise in sea level Each systems tract exhibits a characteristic log depending on the vertical gradient of the slope (next page, “D”). The shoreline retreats landward, response, seismic signature and paleontologic face and on the sediment source (next page, “B”). depriving the basin of sediment. SP and gamma fingerprint, and performs a predictable role in the The complex may include submarine channels ray logs show a fining upward. Retreat of the oil and gas play—reservoir rock, source rock or with levees, overbank deposits, slumps and shoreline gives rise to seismic patterns that seal. Gamma ray (GR) and spontaneous potential chaotic flows. Log responses commonly are cres- appear to truncate basinward. In practice, this (SP) logs are expected to read low in sands and cent shaped. A sharp base within the crescent systems tract is commonly thin, and such pat- high in shales. Resistivity logs show the reverse, commonly indicates sand in a channel, with a terns are usually imperceptible on typical seis- reading high in hydrocarbon-filled sands and low bell shape indicating fining upward as the chan- mic sections. Basal transgressive sands derived in shales. nel is abandoned. On the other hand, channels from reworked lowstand sands can be excellent Apparent layering interpreted on seismic sec- may fill with mud. On seismic sections, leveed reservoirs, except where shell fragments may tions—called stratal patterns—is determined by channels in the fan show a characteristic mound later cement the sands. Shoreface sands will fol- tracing seismic reflections to their terminations. with a slight depression in the top. Sand-filled low strike-oriented trends. The termination is categorized by its geometry channels make excellent exploration targets, but The top of the transgressive systems tract is and associated with a depositional style. Fossils may be difficult to track.3 Sands flowing over the limit of marine invasion, and is called the are described by their abundance, diversity and channel levees may be deposited as overbank maximum flooding surface. Widespread shale first or last occurrence, allowing dates to be deter- sheets and alternate with shales, creating sub- deposition results in a condensed section. Abun- mined based on correlation with global conditions. parallel reflectors. Such sands can provide dant fossils provide ages and well ties across the Starting with the lower lowstand systems tract stacked reservoirs with porosities of 10 to 30%, seismic section. This clay-rich layer shows low at the bottom of a sequence, basin floor fans are but are usually very thin. Slumps from shelf edge resistivity and high gamma ray readings. The typically isolated massive mounds of well-sorted deltas create a chaotic or jumbled pattern— seismic pattern of this surface is downlap, which grain flows or turbidite sands1 derived from allu- “hummocky” in interpreter’s vernacular—easily becomes conformal—parallels adjacent reflec- vial valleys or nearshore sands (next page, “A”). identifiable on seismic data. Hydrocarbon tors—basinward, and disappears above the shelf. Log responses are blocky, with a sharp top and sources for channel and overbank reservoirs are This surface is usually a very continuous reflec- bottom bracketing clean sand. Seismic reflec- deeper sequences. Seals are provided by a tor. At the shelf edge, it can commonly be identi- tions curve down and terminate on the underlying widespread “condensed” section of shale, a thin fied by changes in reflection patterns above and sequence boundary—a feature called down- layer representing prolonged deposition at very below. lap—while the top may form a mound. The low- low rates that comes with the rise in sea level. stand facies makes an excellent reservoir, with The sealing shale also contains abundant marine porosity often over 30% and permeability of sev- fossils used for dating. Part of the upper lowstand, the prograding wedge complex derives its name from shallow- ing-upward deltas that build basinward from the shelf edge and pinch out landward at the preced-

54 Oilfield Review

Deposition GR Seismic cross section or SP Resistivity reflection patterns Seismic example a a a aa a a a aa a a a aa a a aa E a a a a a a aaa aaa a a aa a a a a a a aa D a a a a a a a aaa a a a aa a a a a a a aa C a a a a a a a aaa a a aa a a a a a B aaa a a a a a a aaa a aa a a a a a A aa aa a a a a a aa a aaa

nComponents of sequences, their log responses, and predicted and observed seismic reflection patterns.

Layers deposited during highest relative sea moidal—S-shaped—stratal patterns, similar to 1. A turbidite is a rock deposited from sediment-laden water level are known as the highstand systems tract prograding wedge reflections. There may be moving swiftly down a subaqueous slope. (above, “E”). Early highstand sediments are usu- deltaic and shoreface sands at the top of the sec- 2. Sangree JB, Vail PR and Sneider RM: “Evolution of Facies Interpretation of the Shelf-Slope: Application of the New ally shaly. The late highstand complex, deposited tion, but in general, this systems tract has poor Eustatic Framework to the Gulf of Mexico,” paper OTC as the rise in sea level slows, contains silts and reservoir sands, and updip seals are uncommon. 5695, presented at the 20th Annual Offshore Technology Conference Proceedings, Houston, Texas, USA, May 2-5, sands. Some late highstand sediments are Fossil abundances diminish as the marine envi- 1988. deposited in the open air as fluvial deposits. ronment becomes restricted to the deeper parts 3. Sometimes channels can be seen in slices of 3D seismic Gamma ray and SP responses show a gradual of the shelf. volumes, but sequence stratigraphic studies are not com- monly done on 3D data. An exception is the study decrease in gamma ray, indicating coarsening described on page 62. upward associated with decreasing water depth. Seismic reflections are characterized by sig-

January 1993 55 Components of a sequence may be which was the direction of the sediment diversity and abundance are measured ver- repeated or missing, depending on local source. Layers dip and thicken to the south. sus depth, which is converted to seismic conditions and the rate of sea level change, Initially, seismic data and logs were inter- travel time for easy comparison with the but the basic sequence structure is pre- preted independently to identify sequences seismic section. Fossils of planktonic (float- dictable. Computer-generated models of and their bounding unconformities. Log- ing) organisms are more widespread than sequence cycles are used to show the derived boundaries were compared with those of benthic (bottom-dwelling) organ- effects of sea level change, sediment supply those from seismic data and the interpreta- isms and are therefore more useful in estab- and depositional profile (below).10 These tion refined iteratively. Detailed seismic lishing regional time correlations. However, inputs can be varied to test their relative interpretation began with the most easily in shallow-water environments, benthic fos- importance, or to produce stacks of interpreted reflection patterns, and was sils are used because nearshore conditions sequences in an attempt to match real data. pieced together—working upward, down- may be too variable for planktonic fossils. ward and back toward the wells—respect- Fossils are also indicators of relative sea Searching for Sand—A Case Study ing the stratigraphy suggested by the level. High fossil counts, or peaks, are asso- Sequence stratigraphy was applied in 1992 sequence model. ciated with shales deposited during low sed- in the East Breaks area, offshore Texas (next Logs from wells on the seismic lines were imentation. Such conditions occur in the page, top). Data included a two-dimen- converted from depth to time using the basin during time of high relative sea level, sional (2D) seismic line and logs from seven nearest check shot—here, 3 miles [4.8 km] but also in deep water between fan deposits wells. The seismic line was processed for away.11 Sands interpreted on spontaneous and outbuilding delta deposits (page 58, structural imaging (see “Structural Imaging: potential, gamma ray and resistivity logs top). Two shale sections are expected within Toward a Sharper Subsurface View,” page were associated with seismic reflections at each sequence, one at the top of the slope 28) and the structural interpretation used the well and tracked along the seismic sec- fan and the other at the maximum flooding four other lines to view the basin as a whole tion. Shales indicated by logs were noted for surface, associated with the furthest land- (see “Going for the Play: Structural Interpre- correlation with fossil data from cuttings. ward position of the shoreline. Biostratigra- tation in Offshore Congo,” page 14). In this Next was integration of biostratigraphy.12 phy also holds the key to paleo- case, the big picture shows a basin con- Fossils from cuttings help identify and date bathymetry—a measure of topography of trolled by normal faulting to the north, boundaries of each rock sequence. Fossil the ancient ocean floor—needed to interpret

Distance, km 01530 nSequences simu- 0 35 0lated using Scott Bowman’s tech- 30 40 nique, showing all Relative sea level the components on 80 3210page 53. Inputs are 100 initial basin shape, 20 Age, Ma sedimentation rate 0 5 10 15 20 25 30 35 15 25 and relative sea Layer number level. Geologic time 200 lines are numbered from 5 to 35, oldest to youngest. The sequence begins at Lithologies indicated by logs Expected 5. Expected log 300 10 responses are plot- Depth, m gamma ray, SP Gamma ray ted at ten locations. or SP Resistivity and resistivity responses The left curve is SP or gamma ray, the Sand 400 right curve resistivity.

Shale

500

5

56 Oilfield Review

a a a a

a01miles a aa a aa aa a aaa C SP Fossils a a a aaa a a aaD aa 1.0 3000 a 4000 a a aa Time, sec B 5000 Depth, ft

aa a a aaH. sellii C. macintyrei 6000 2.0

G. miocenica a a aa a aa a aa a aa a aa D. tamalis a aa a A a a aa a a aa a a E aa aaNanno Foram

3.0 nEast Breaks, offshore Texas, seismic line with sequence components interpreted in color, SP log and fossil abundance curves. Block diagrams from page 53 point to representative examples on the section. This seismic section has nine sequences. Sequence components shown are not necessarily from the same sequence, which is why D appears stratigraphically above E. the depositional environment. Paleodepth is Overcoming Limitations of Sequence T E X A S L O U I S I A N A derived from benthic fossils with known Stratigraphy depth habitats (next page top, right curve). Sequence stratigraphy has proven useful for Knowing water depth helps to interpret deep petroleum exploration, but it is commonly 13 or shallow water rock types and expected misapplied. There is controversy over G U L F O F M E X I layer thicknesses. whether the technique can be applied to C O Once seismic, log and biostratigraphic carbonate systems since it was designed to East Breaks data are combined, a final, color-coded explain sand-shale systems. Some experts Green Canyon interpreted section is made. Very high maintain that sequence stratigraphy is easier 0miles 100 amplitude reflections may be highlighted in carbonates because carbonates are 0km 161 with hatching. These so-called bright spots extremely sensitive to sea level change.14 are analyzed for anomalies in amplitude There is unanimous agreement, however, variation with offset associated with hydro- that low sedimentation rates often pose spe- when sedimentation rate is low, several carbons (see “Hydrocarbon Detection With cial problems. When sedimentation rate is sequences might fit within a seismic wave- AVO,” page 42). In the East Breaks exam- moderate to high, layers within a sequence length. Sequence stratigraphy cannot be ple, the most promising prospect is a large, are tens to hundreds of meters thick, com- confidently applied here, but it has been sandy basin floor fan. Shales interpreted fortably within the resolving power of a typ- done countless times. A useful interpretation above and below could provide seal and ical seismic wave (next page, bottom). But in thinly-bedded regions requires abandon- source rock, respectively. 10. Bowman SA and Vail PR: “Computer Simulation of 12. For an integrated biostratigraphic study: 14. Vail P, Audemard F, Bowman SA, Eisner PN and Stratigraphy,” American Association of Petroleum Bell DG, Selnes H, Bjorøy M, Grogan P, Kilenyi T Perez-Cruz C: “The Stratigraphic Signatures of Tec- Geologists Bulletin (1993): in preparation. and Trayner P: “Better Prospect Evaluation with tonics, Eustasy and Sedimentology—An Overview,” 11. A check shot is a wireline survey that checks the Organic Geochemistry, Biostratigraphy and Seismics,” in Einsele G et al. (eds): Cycles and Events in Stratig- seismic travel time from the surface to a chosen Oilfield Review 2, no. 1 (January 1990): 24-42. raphy. Berlin, Germany: Springer-Verlag (1991): 617-659. depth in a well. Depths are chosen from logs. A geo- 13. Posamentier HW and James DP: “An Overview of phone is conveyed by wireline to the desired depth Sequence Stratigraphic Concepts: Uses and Abuses,” and a seismic source is set off at the surface. The in Posamentier HW, Summerhayes CP, Haq BU and travel time is recorded and doubled to compare with Allen GP (eds): Stratigraphy and Facies Associations the surface seismic travel time. in a Sequence Stratigraphic Framework, Interna- tional Association of Sedimentologists Special Publi- cation, 1992.

January 1993 57 n Plankton Typical Gulf of ing small-scale features and concentrating Mexico fossil abun- on larger scale, longer term processes that Foram Nannofossil Paleodepth dance peaks and control the generation of sequences.

Depth, ft abundance abundance ft x100 paleodepth curve. 1630 Fossil abundance With this in mind, Vail and coworkers 3000 curves based on proposed a hierarchy of stratigraphic cycles H. sellii analysis of cuttings based on duration and amount of sea level for foraminifera— change.14 Duval and Cramez at TOTAL protozoa with cal- C. macintyrei careous external Exploration worked with Vail to provide 5000 skeletons—and subsurface examples and to expand the D. brouweri nannofossils, a application to hydrocarbon exploration.15 broad category of The hierarchy assigns frequencies to the D. brouweri “A” extremely small, mechanisms of eustasy enumerated by Fair- usually algal fossils. 7000 G. miocenica Peaks indicate the bridge (page 52), viewed in light of plate D. pentaradiatus presence of shales tectonics (next page, top). The first-order at the top of the cycle, which is the longest, tracks creation slope fan complex of new shorelines resulting from the (brown) and at 9000 maximum sea breakup of the continents. Although this breakup does not follow a cycle, it has hap- H. emaciatum D. tamalis level (green). The right curve indi- pened twice, with a duration of over 50 mil- cates fossil habitat lion years. The second-order cycle is land- depth, in which ward and basinward oscillation of the 11000 dark blue is deep water and light shoreline that lasts 3 to 50 million years. blue is shallower. This oscillation is produced by changes in the rate of tectonic subsidence and uplift, caused by changes in rates of plate motion. 13000 (?) R. pseudoumbilica Both first- and second-order cycles may cause changes in the volume of the ocean basins resulting in long-term variations in global sea level. The third-order cycle is the High sedimentation rate Distance, km sequence cycle, lasting 0.5 to 3 million 03060 years. Fourth- and higher order cycles may 0 be correlated with periodic climatic changes. The following example, with its low deposition rate, approaches the limit of interpretation in terms of third-order cycles. Depth, m It comes from the Outer Moray Firth basin 600 in the UK sector of the North Sea, where the initial basin shape, tectonic activity and variation in the rate of deposition add a Low sedimentation rate 030 60 twist to the interpretation (pages 60-61). 0 Stratigraphic interpretation of the last 65 million years of sediments in the Outer Moray Firth is more difficult than in the Gulf of Mexico because slower deposition in the Central North Sea resulted in thinner units, Depth, m many of which cannot be resolved by seis- 600 mic waves. During this period, the Outer Moray Firth has 17 sequences totaling 5000 nHigh (top) and low (bottom) sedimentation rates, shown on synthetic sequences. The feet [1524 meters] of sediments (pages 60- basic shape of each layer follows the initial basin shape, but layer thickness varies 61, middle and bottom), compared to the with sedimentation rate. Only thick layers can be resolved with seismic methods. Gulf Coast, with 10 sequences totaling 9000 ft [2750 m]. In the North Sea, however, depositional processes juxtaposed a variety of lithologies, providing reliable calibration points for accurate conversion of logs from

58 Oilfield Review 1 order Continental breakup n depth to time using synthetic seismograms Hierarchy of cycles, decreasing (right, bottom). In the Gulf of Mexico, this downward in the conversion is typically done with only duration of sea nearby checkshots; sands and shales com- level change and monly show periodic alternation with depth in area of influence. at wavelengths that make comparisons At the top, the first- > 50 Ma order cycle lasts at between seismic sections and synthetic seis- least 50 million mograms nonunique. years and is Stratigraphic study is always preceded by caused by major structural interpretation. In addition, a pale- changes in the con- figuration of tec- ogeographic interpretation of the Outer 2nd order— Movement of shoreline tonic plates. The Moray Firth shows that late in the Creta- second-order cycle, ceous period—when the sequences under Basinward lasting 3 to 50 mil- study began to be deposited—a smooth lion years, is also basin floor sloped gently from northwest to time Increasing controlled by plate motion. It involves southeast. During a relative fall in sea level, Landward movement of the sediments were deposited as slope fans. shoreline landward Their seismic expressions indicate lobes and basinward, on the scale of conti- with channels and some chaotic flows— 3-50 Ma nents. The white large-scale slumps with jumbled seismic area shows the character. As sea level rose, a wedge of out- time in which there building deltas was deposited. Sea level is no rock record, maximum is associated with a depositional usually due to lack hiatus, shown only as a thin line. Deposits of depostition. The third-order cycle, of synchronous with this surface may be found 3rd order— Sequence cycle 500,000 to 3 million on what is now land in Europe, but in the years, is the basin, sediments that correspond to periods sequence cycle of high relative sea level are rare. described in the main text. It is Why are elements of the classic Vail caused by long- model missing from this sequences in this term tectonic activ- 0.5-3 Ma basin? One explanation is the competing ity and short-term influences of tectonic uplift and sea level global sea level change. As global sea level rose and fell, changes. Fourth- and higher order continual regional uplift kept the sea from 4th+ order— Parasequence cycle (periodic) cycles, of 10,000 to reaching levels high enough to allow forma- 500,000 years, are

tion of units typical of high relative sea thickness Increasing of shortest duration. level. Only once, at the top of the third They are driven by sea level changes sequence, does a thin layer of high relative caused by periodic sea level sediments appear (orange). Ma 0.01-0.5 climatic variation. Another interpretation is that thin, high rela- Distance Cycles of this order tive sea level sediments were deposited, but are called parase- eroded, and so are not preserved in the sec- quence cycles. tion (pages 60-61, bottom). AB WE 0 nSynthetic seismo- 15. Duval B, Cramez C and Vail P: “Types & Hierarchy of Stratigraphic Cycles,” presented at the Interna- grams and gamma tional Symposium on Mesozoic and Cenozoic ray logs from two Sequence Stratigraphy of European Basins, Dijon, wells tying with the France, May 18-20, 1992. Outer Moray Firth seismic line. Syn- thetics, based on sonic and density logs, provide a depth-to-time corre- lation for integra- 1.0 tion of log, paleo and seismic data. (Courtesy of Amoco UK.) Time, sec

2.0 January 1993 59

Synthetic Gamma 05km ray 60 nel willterminateinasand-richfan. horizon slice,geologypredictsthatthechan- channel depositsisdifficulttoidentifyinthe exploration targets. Althoughlithologyofthe is visiblewithintheslopefaninterval. stacked channels,possiblyfilledwithsand, discovered inthenextblockofseismicdata. channel wastrackedsouth,andafan terms ofsecond-ordercycles( miles [138km interpreted overasix-blockarea(54square panels for3Dinterpretation. data, interpretersassembledaseriesof2D nearby wells.Zoominginonasubsetofthis mic dataandpaleontologiccontrolfromsix stratigraphy wasestablishedusing2Dseis- million yearsago.Regionalsequence in asynclineonthecontinentalslope1to2 interpreters concentratedonafandeposited Green CanyonareaoftheGulfMexico, dimension tosequencestratigraphy. Inthe small enough,workstationscanaddanew If thevolumeofearthinastudyareais Studies in3D style ofbasinfill. represent majorchangesinthedepositional cycle, andtheboundariescanbeseento exist attheboundariesofeachsecond-order biostratigraphy. Majorbiostratigraphicgaps cycles, basedonphysicalstratigraphyand can bebracketedbyfivesecond-order The entiresetof17depositionalsequences 200 m[656ft]( delta sandsintodeepwater, greaterthan nels thatcarriedshallow-watershelfand slope fancoincideswiththestackedchan- 16. Ahorizonisthesurfaceseparatingtworocklayers. The goalofthisinterpretationistoidentify This sectioncanalsobeinterpretedin The topandbaseoftheslopefanwere color-coded. Depths ortimestothereflectionarecontoured and displayedinplanviewiscalledahorizonslice. A reflectiontrackedina3Dcubeofseismicdata In seismicdata,ahorizonshowsupasreflection. 2 page 62, middle 62, page ]). Thethickestpartofthe ]). page 62, top 62, page ). Aseriesof 16 The ).

7000 Depth, m Time, sec Time, sec 0 4 0 4 0 W Geologic crosssection Interpreted seismic Raw seismic Oilfield Review Oilfield E E

010km

nComparison of seismic and log data from the Moray Firth, North Sea, including the original seismic line (top), the interpreted line (mid- dle) and the geologic cross section with gamma ray logs. Permian and Carboniferous basement (green) are followed by Jurassic to Lower Cretaceous sediments (purple). Both show evidence of rifting during and after deposition. In the Upper Cretaceous, chalk (blue) filled the earlier rifts. The chalk was deposited in open marine conditions, with no land exposed nearby. On the left (west) edge, a Pale- ocene (Danian) chalk debris flow appears as a chaotic zone on top of the earlier chalk. Basin relief was minimal and sea level was high at the onset of the first sequence in the Tertiary. Low relative sea level deposits—slope fans and chaotic flows—are brown (color conven- tion is the same as in the Gulf of Mexico example). River deltas building out during low relative sea level are pink. Sea level maxima appear as thin green lines.

January 1993 61 W E Lower Eocene Middle Eocene Lower Oligocene

Lower Paleocene Upper Paleocene

nOuter Moray Firth data reinterpreted in terms of second-order cycles. Basinward movement of the shoreline is orange and landward movement is green. Colors correspond to second-order cycles (page 59, top).

Cross section T E X A S L O U I S I A N A Frontiers Sequence stratigraphy continues to evolve. One area of investigation is high-resolution sequence stratigraphy, which is performed at a higher resolution than seismic wave- G U L F O F M E X I C O lengths, usually with log and outcrop stud- ies. ARCO, TOTAL and Esso scientists per- East Breaks formed a very high-resolution sequence Green Canyon 0miles 100 stratigraphy study on a roadside ditch, 0km 161 which served as an analog of an incised val- ley and delta system. The system measured nSeismic line and horizon slice from a only 50 cm [20 in.] from bottom to top but cube of 3D data in Green Canyon, offshore obeyed the same physical laws as systems Louisiana. The top (yellow line) and bottom 17 (red) of a slope fan were interpreted using hundreds of meters thick. sequence stratigraphy. A stack of subma- Sequence stratigraphy is achieving some rine channels can be seen in the seismic success in areas where it was not designed line (oval). Normally, channels like these to work. Studies in carbonates show that would be difficult to track, but in 3D the task is simple. The 3D data can be flat- although the depositional layering patterns tened at the top of the fan, and sliced hori- are different from sand-shale systems, the zontally to reveal a map (bottom). Here, the technique has the power to explain and pre- channel can be seen to meander from Horizon slice dict sediment distribution and lithologic north to south around an emerging salt content.18 Sequence stratigraphy has also dome. A slump off the flank of the salt (cir- cle) has fallen into the channel. This chan- been applied with success to nonmarine nel was tracked to the next block south, deposits in continental basins and in marine where it terminated in a slope fan lobe, basins isolated from continental sediments. predicted to be sandy. Sequence stratigraphy, as proposed by the Exxon group, is not without controversy. Salt Some alternative schemes to explain sequences place more emphasis on sedi- ment supply19 or tectonic activity.20 What- ever their area of expertise, stratigraphers agree on the main problem in sequence stratigraphy—overextending the model to fit every study area. Bilal Haq, while at the National Science Foundation in the USA, 17. Posamentier HW, Allen GP and James DP: “High 19. Galloway WE: “Genetic Stratigraphic Sequences in compiled ten commandments for sequence Resolution Sequence Stratigraphy—The East Coulee Basin Analysis I: Architecture and Genesis of Flood- stratigraphers21 and Henry Posamentier of Delta, Alberta,” Journal of Sedimentary Petrology 62 ing-Surface Bounded Depositional Units,” American (1992): 310-317. Association of Petroleum Geologists Bulletin 73, no. ARCO Oil and Gas Company has written 13 18. Jacquin T, Garcia J-P, Ponsot C, Thierry J and Vail PR: 2 (February 1989): 125-142. on the uses and abuses of the technique. “Séquences de Dépôt et Cycles Régressif/Transgres- 20. Sloss LL: “Tectonics—The Primary Control on They approach the subject from different sifs en Domaine Marin Carbonaté: Exemple du Dog- Sequence Stratigraphy: A Countervailing View,” Dis- perspectives, but their message is the ger du Bassin de Paris,” Contes Rendues de tinguished Lecture Tours Abstracts, American Asso- l’Académie des Sciences de Paris 315 (1992): 353- ciation of Petroleum Geologists Bulletin 74, no. 11 same—where sequence stratigraphy works, 362. (November 1990): 1774. use it; if it doesn’t work, the problem is with 21. Hag BU: “Ten Commandments for Sequence Stratig- the application, not the theory. —LS raphers,” presented at the International Symposium on Mesozoic and Cenozoic Sequence Stratigraphy of European Basins, Dijon, France, May 18-20, 1992.

62 Oilfield Review