Friday, December 29, 2017

Sequence Stratigraphy

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.

 Conventional lithologic correlation maps formation tops by interpreting well log data alone. It looks at what is there without taking into account how it got there. Sequence stratigraphy combines logs with fossil data and seismic reflection patterns to explain both the arrangement of rocks and the depositional environment. Understanding the relationships between rock layers, their seismic expression and depositional environments allows more accurate prediction of reservoirs, source rocks and seals, even if none of them intersect the well. 

Sequence stratigraphy is used mainly in exploration to predict the rock composition of a zone from seismic data plus distant, sparse well data. It also assist in the search for likely source rocks and seals. Experts believe that as more people learn the technique, it will become an exploitation tool for constraining the shape, extent and continuity of reservoirs.

 Sequence stratigraphy, seismic stratigraphy - How many stratigraphies can there be?

Stratigraphy is the science of describing the vertical and lateral relationships of rocks. These relationships may be based on rock type, called lithostratigraphy, on age, as in chronostratigraphy, on fossil content, labeled biostratigraphy, or on magnetic properties, named magnetostratigraphy.

At the turn of the century, shoreline movement was attributed to tectonic activity - the rising and falling of continents. This view was challenged in 1906, when Eduard Suess hypothesized that changes in shoreline position were related to sea level changes, and occured on a global scale; he called the phenomenon eustasy. However, Suess was not able o refute evidence presented by opponents of his theory - in many locations there were discrepancies between rock types found and types predicted by sea level variation. 

In 1961 , Rhodes W. Fairbridge summarized the main mechanisms of sea level change : tectono-eustasy, controoled by deformation of the ocean basin; sedimento-eustasy, controlled by addition of sediments  to basins, causing sea level rise ; glacio-eustasy, controlled by climate , lowering sea level during glaciation and raising it during deglaciation. He recognized that all these causes may be partially applicable, and are not mutually incompatible. He believed that while eustatic hypotheses apply worldwide , tectonic hypotheses do not and vary from region to region. Fairbridge summarized the perceived goal at the time: "We need therefore to keep all factors in mind and develop an integrated theory. Such an ideal is not yet achieveable and would involve studies of geophysics, geochemistry, stratigraphy, tectonics, and geomorphology, above sea level and below.

This brings us nearly to the present. In 1977, Peter Vail at Exxon and and several colleagues published the first installments of such an integrated theory. Vail developed a new kind of stratigraphy based on ideas proposed by L.L Sloss - the grouping of layers into unconformity-bound sequences based on lithology  - and by Harry E. Wheeler - the grouping of layers based on what has become known as chronostratigraphy. Vail's approach allowed interpreting unconformities based on tying together global sea level change, local relative sea level change and seismic reflection patterns. This methodology, named seismic stratigraphy, classifies layers between major unconformities based on seismic reflection patterns, giving a seismically derived notion of lithology and depositional setting.

Subsequent seismic stratigraphic studies in basins around the world produced a set of charts showing the global distribution of major unconformities interpreted from seismic discontinuities for the pas 250 million years. An understanding emerged that these unconformities were controlled by relative changes in sea level, and that relative changes in sea level could be recognized on well logs and outcrops, with or without seismic sections. This led to the interdisciplinary concept of sequence stratigraphy- a linkage of seismic, log, fossil and outcrop data at local, regional and global scales. The integrated theory sought by Fairbridge had arrived. 

The concepts that govern sequence stratigraphic analysis are simple. A depositional sequence comprises sediments deposited during one cycle of sea level fluctuation - by Exxon convention, starting at low sea level, going to high and returning to low. 
One cycle may last a few thousands to millions of years and produce a variety of sediments, such as beach sands, submarine channel and levee deposits, chaotic flows or slumps and deep water shales. Sediment type may vary gradually or abruptly, or may be uniform and widespread over the entire basin. Each rock sequence produced by one cycle is bounded by an unconformity at the bottom and top. These sequence boundaries are the main seismic reflections used to identify each depositional sequence, and separate younger from older layers everywhere in the basin.

Composition and thickness of a rock sequence are controlled by space available for sediments on the shelf, the amount of sediment available and climate. Space available on the shelf -which Vail calls "shelfal accomodation space" is a function of tectonic subsidence and uplift and of global sea level rise and fall on the shelf. For instance, subsidence during rising sea level will produce a larger basin than uplift during rising sea level. The distribution of sediments depends on shelfal accomodation, the shape of the basin margin-called depositional profile -sedimentation rate and climate. Climate depends on the amount of heat received from the sun. Climate also influences sediment type, which tends toward sand and shale in temperate zones and allows the production of carbonates in the tropics. 

As an exploration tool, sequence stratigraphy is used to locate reservoir sands. In deep water basins with high sedimentation rates, sands are commonly first laid down as submarine fans on the basin floor and later as deposits on the continental slope or shelf. But as sea level starts slowly rising onto the continental shelf, sands are deposited a great lateral distance from earlier slope and basin deposits. Deposits during this time are deltaic sediments that build into the basin and deep water shales. If the sediment supply cannot keep pace with rising sea level, the shoreline migrates landward and sands move progressively higher up the shelf.  Once sea level reaches a maximum for this cycle, sands will build basinward as long as sediments remains available. The sequence ends with a fall in relative sea level, marked by break in deposition. The sequence repeats, however, as long as there is sediment and another cycle of rise and fall in relative sea level that changes the shelfal accomodation space.

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