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.