Eustasy

Key Takeaways

• Sequence stratigraphy, the systematic framework for interpreting the stratigraphic record in terms of relative sea level cycles, was developed by Peter Vail and his colleagues at Exxon Production Research in the 1970s and 1980s, using seismic data tied to well logs and core to document systematic patterns of stratal terminations (onlap, offlap, downlap, truncation) that reflect the geometric response of depositional systems to sea level changes; the sequence stratigraphic model divides the stratigraphic record into sequences (packages of rock bounded by unconformities or their correlative conformities), system tracts (distinct phases of the sea level cycle characterized by different depositional geometries and facies distributions), and parasequences (the basic building blocks of each system tract, representing individual shoaling-upward packages bounded by marine flooding surfaces); for petroleum geologists, sequence stratigraphy provides a powerful predictive framework for anticipating where high-quality reservoir sands will be found (typically in lowstand turbidite systems and transgressive barrier sand systems), where organic-rich source rocks were deposited (typically in transgressive maximum flooding surface shales), and where effective seals cap the reservoir intervals (typically in transgressive marine shales).
• Lowstand systems tracts (LSTs), the intervals deposited when sea level is at its lowest relative to the depositional profile, are among the most important reservoir intervals in many productive petroleum basins because sea level fall exposes the continental shelf and forces river systems to incise valleys and deliver coarse clastic sediment directly to the deep basin as submarine fan deposits; the turbidite sands deposited in lowstand submarine fans have excellent reservoir quality (high porosity and permeability in uncemented systems), are often encased in organic-rich marine shales that serve as both source and seal, and may be stratigraphically trapped by the overlying transgressive shale when sea level subsequently rises; the deep-water turbidite reservoirs of the Gulf of Mexico (Mars, Ursa, Auger), offshore West Africa (Bonga, Girassol, Dalia), and offshore Brazil (pre-salt and post-salt turbidites) are all examples of lowstand systems tract deposits that have become major petroleum production centers.
• The Vail global sea level curve, first published in 1977 using seismic stratigraphic analysis of global offshore seismic data, proposed a framework of global eustatic cycles at multiple scales that could be used to predict the stratigraphic architecture of sedimentary basins worldwide; the curve was controversial because it implied that eustasy (global sea level change) rather than local tectonics dominated the stratigraphic record, a proposition that many geologists disputed; subsequent research has demonstrated that the distinction between eustasy and relative sea level (the combined effect of eustasy and subsidence) is often difficult to make from the stratigraphic record alone, because the same stratal geometries can result from either global sea level change or local subsidence change; modern sequence stratigraphy has evolved beyond the Vail framework to use relative sea level change as the primary controlling variable, agnostic about whether the driving mechanism is eustatic or tectonic, while retaining the systematic framework for interpreting stratal geometries and predicting facies distributions that the original work established.
• Carbonate sequence stratigraphy presents different challenges and opportunities than clastic sequence stratigraphy, because carbonate sediment is produced in place (by biological and chemical precipitation within the marine environment) rather than transported from distant terrestrial sources, making carbonate systems more sensitive to sea level position and less sensitive to sediment supply than clastic systems; during sea level highstands, carbonate production is maximized on flooded shallow shelves where warm, clear, sunlit water supports prolific carbonate-producing organisms; during sea level lowstands, the shelf is exposed and carbonate production largely ceases, while the exposed carbonate undergoes meteoric dissolution (creating secondary vuggy porosity and improving reservoir quality) and dolomitization; understanding the eustatic history of a carbonate basin is therefore directly relevant to predicting where the best reservoir porosity will be found within the carbonate section, and eustasy-driven exposure and dissolution events are frequently the mechanism that creates the exceptional reservoir quality observed in the best Middle East and North African carbonate producing zones.
• The interplay between glacio-eustasy (the dominant eustatic mechanism during glaciated periods of Earth history) and tectonics in controlling the stratigraphy of oil-producing basins requires careful disentanglement in sequence stratigraphic analysis: the large-amplitude (50-120 meter), relatively rapid (100,000-year) glacio-eustatic cycles of the Pleistocene created a very different stratigraphic response than the smaller-amplitude, longer-period eustatic changes of the Cretaceous (when polar ice sheets were absent and eustasy was driven primarily by tectonic processes); petroleum-producing intervals in basins like the Gulf of Mexico, the North Sea, and the Niger Delta span multiple eustatic periods, and the relative importance of eustasy versus local subsidence in controlling stratigraphy varies between stratigraphic intervals within the same basin; identifying the correct eustatic context for a target reservoir interval, using global sea level curves calibrated against independently dated stratigraphic successions, is one of the analytical challenges in applying sequence stratigraphy as a predictive tool in frontier exploration where limited well data constrains the local stratigraphic framework.