Stratigraphy

Key Takeaways

• Lithostratigraphy is the practical, rock-based classification of sedimentary successions into formations, members, and groups defined by their distinctive physical characteristics (lithology, color, texture, sedimentary structures, and mineral content) that can be recognized and mapped across geographic areas: a formation is the fundamental lithostratigraphic unit, defined as a mappable body of rock with distinctive lithological character that can be recognized and correlated from place to place, named after a geographic locality where it is well exposed or first described (the Wolfcamp Formation, Permian Basin; the Niobrara Formation, Western Interior Seaway; the Kimmeridge Clay Formation, North Sea); formations are subdivided into members where internal lithological variations are significant enough to warrant separate description and mapping, and grouped into groups when several formations share a common depositional history; lithostratigraphic correlation between wells using wireline log signatures (gamma ray, resistivity, sonic) is the most common well correlation method in petroleum exploration, with log cross-plots and log template correlations identifying the same lithological unit across a field or basin; lithostratigraphic correlation assumes that similar lithological signatures represent the same time interval (time-transgressive lithostratigraphy, where the same facies may be diachronous — deposited at different times in different locations as a shoreline migrates — is a common source of erroneous correlation in clastic petroleum systems where the same sandstone facies represents a different time interval at different geographic positions along the depositional dip).
• Biostratigraphy uses the first and last occurrences of fossil taxa in a stratigraphic section to define biozones and to assign relative or absolute ages to rock intervals, providing the primary tool for age determination in marine sedimentary sequences and the basis for global geological time scale calibration: foraminifera (calcareous tests of single-celled marine organisms) provide the highest-resolution biostratigraphy for Mesozoic and Cenozoic marine sequences, with planktonic foraminifera (floating in the water column) providing global correlations and benthic foraminifera (living on the seafloor) indicating water depth and environment; calcareous nannofossils (coccoliths and related microscopic carbonate plates from marine algae) provide an equally precise biostratigraphic framework for the same time interval; dinoflagellate cysts (palynomorphs from the organic-walled resting stage of marine dinoflagellates) provide biostratigraphy for the marine Mesozoic and Cenozoic in wells where calcareous microfossils are absent or poorly preserved; spores and pollen provide biostratigraphy in non-marine and terrestrial sequences; the resolution achievable by biostratigraphy (typically 0.5-2 million years for the best-calibrated taxa in favorable preservation conditions) determines the level of stratigraphic detail achievable in petroleum well correlation and source rock age determination, and the absence of index fossils in poorly preserved or atypical facies is a significant challenge in petroleum biostratigraphic programs in frontier basins.
• Sequence stratigraphy interprets the geometric packaging of sedimentary rock units in terms of cycles of relative sea level change, providing a predictive framework for locating reservoir, source, and seal facies in specific systems tract positions within a sequence: the Vail-Mitchum sequence stratigraphic model (developed at Exxon Production Research Company in the 1970s) defines sequences as the fundamental units bounded by unconformities (sequence boundaries formed during relative sea level fall when the exposed shelf is eroded), with each sequence divided internally into lowstand systems tract (LST, deposited when relative sea level is at its lowest, including basin floor fans and slope fans in deep water, and incised valley fills in shallower water), transgressive systems tract (TST, deposited during relative sea level rise, characterized by retrogradational stacking and coastal onlap), and highstand systems tract (HST, deposited during relative sea level highstand, characterized by progradational stacking and downlap); each systems tract has characteristic facies associations that determine reservoir presence and quality: LST fans in deep water are the primary reservoir target in many deep-water petroleum systems (Gulf of Mexico, offshore West Africa, offshore Brazil), TST transgressive sandstones form stratigraphic traps against sequence boundaries, and HST deltaic and platform carbonates form the reservoir targets in shallower-water plays; the sequence stratigraphic framework thus provides a predictive map for petroleum exploration that guides well location before drilling and improves the probability of reservoir discovery relative to purely structural exploration.
• Well log correlation and cross-section construction are the primary practical applications of stratigraphy in petroleum exploration and development, using the wireline log signatures of formations to match stratigraphic units between wells and to construct the structural and stratigraphic cross-sections used for reservoir geometry interpretation: the gamma ray log (which measures the natural radioactivity of formations, primarily from uranium, thorium, and potassium in clay minerals) is the most widely used correlation log because shales (high gamma ray, clay-rich) and sands (low gamma ray, quartz-rich) produce distinctive log signatures that can be matched between wells even at moderate correlation distances; the identification of marker beds (distinctive, regionally extensive beds with a recognizable log signature — high resistivity carbonate, low gamma ray clean sand, or high gamma ray radioactive shale) provides anchor points for log correlations that constrain the relative positions of the correlating formations between the marker beds; stratigraphic cross-sections constructed from correlated well logs display the thickness variations, facies changes, and structural relationships between formations in two dimensions, and their interpretation provides the basis for reservoir volume estimation, fault and structural trap mapping, and identification of reservoir pinchouts and stratigraphic traps that are the primary exploration targets in mature basins; 3D seismic reflection data extends well log stratigraphy into the inter-well space by providing reflection images of the same stratigraphic surfaces identified in wells, enabling the construction of three-dimensional stratigraphic maps that guide development drilling in complex reservoirs.
• Chronostratigraphy and the geological time scale provide the absolute time framework for petroleum generation modeling and source rock characterization: the geological time scale assigns numerical ages (in millions of years before present) to the boundaries between stratigraphic periods, epochs, and stages, derived from radiometric age dating (principally U-Pb dating of zircon crystals in volcanic ash beds intercalated with fossiliferous marine sediments) and calibrated by global correlation of biostratigraphic and chemostratigraphic events; the absolute ages assigned to source rock deposition, burial, and maturation determine the timing of petroleum generation and the window of opportunity for trap formation before petroleum migrated — a source rock that generated petroleum before the overlying structural trap was formed will have expelled its petroleum into water-saturated formations without accumulating in a viable trap; the calibration of source rock ages by biostratigraphy and the conversion to absolute ages by the geological time scale are therefore key steps in petroleum system modeling that determines the prospectivity of a petroleum play; the ongoing refinement of the geological time scale by the International Commission on Stratigraphy (ICS) through improved radiometric age dating of key stratigraphic boundaries revises the absolute ages of stratigraphic events used in petroleum system modeling, and differences between the time scale used in an older petroleum system model and the current standard time scale can introduce small but potentially significant age errors into paleo-reconstruction and generation timing calculations.