Sequence

Key Takeaways

• The sequence hierarchy recognizes sequences at multiple scales, from the global first-order sequences (lasting tens to hundreds of millions of years, driven by opening and closing of ocean basins by plate tectonics) to high-frequency fifth- and sixth-order parasequences (lasting 20,000 to 100,000 years, driven by Milankovitch orbital cycles); the standard sequence hierarchy used in petroleum exploration includes first-order (plate tectonic timescale, 50 to 300 Ma), second-order (tectono-eustatic, 3 to 50 Ma), third-order (eustatic and tectonic, 0.5 to 3 Ma), fourth-order (0.1 to 0.5 Ma), and fifth-order (20,000 to 100,000 years) sequences; third-order sequences (0.5 to 3 Ma duration) are the most important in petroleum geology because they correspond to the duration of major transgression-regression cycles that produce sequences thick enough to be resolved by seismic (10 to 300 meters) and to contain economically significant volumes of reservoir and source rock; high-frequency fourth- and fifth-order sequences (parasequences and parasequence sets) are resolved in well logs and core but not in typical exploration seismic data, and their stacking pattern (progradational, retrogradational, or aggradational) provides information about the overall accommodation-supply balance within the third-order sequence; the vertical stacking pattern of parasequences within each systems tract provides the fine-scale prediction of reservoir quality variation (coarsening-upward or fining-upward cycles in shoreface, delta, or fluvial deposits) that controls well-log interpretation and reservoir zonation in development wells.
• Sequence boundaries and their correlative conformities are the most important surfaces in sequence stratigraphy for petroleum exploration because they correspond to major erosional events (during sea level fall and lowstand) that concentrate coarse clastic sediment at and below the shelf break: when sea level falls below the shelf break (a type 1 sequence boundary in the original Vail-Posamentier model), rivers incise into the exposed continental shelf and transport large volumes of coarse sediment directly to the shelf edge, where it cascades down the slope as turbidite fans and channels (the lowstand fan and slope fan systems tracts); the incised valleys on the shelf (preserved as channel-fill sandstones below the sequence boundary) and the lowstand turbidite fans at the base of slope are the primary exploration targets in many passive margin basins (including the Gulf of Mexico, West Africa, and Brazil offshore), where their position below the maximum flooding shale (the primary seal) and above older source rocks creates favorable petroleum system geometry; when sea level falls only within the shelf (a type 2 boundary in the original model, now re-classified within the falling-stage systems tract concept), no shelf-edge erosion or incised valley formation occurs, the basin remains fully marine, and the sequence boundary is more subtle (an increase in progradation rate as accommodation decreases); the distinction between type 1 and type 2 boundaries is important for predicting the presence and quality of lowstand clastic reservoirs.
• Seismic recognition of sequences is based on the reflection termination patterns (onlap, downlap, toplap, and truncation) defined by Mitchum et al. (1977) that distinguish conformable internal reflections from bounding unconformities: onlap (reflections terminating against an underlying inclined surface in a landward direction) indicates the filling of a topographic low during transgression; downlap (reflections terminating in a basinward-dipping direction against an underlying surface) indicates progradation of sediment lobes over a condensed section or distal shale, characteristic of highstand delta progradation onto the deep basin; toplap (reflections terminating against an overlying surface in the updip direction without truncation) indicates progradation without significant depth increase, typical of highstand shelf-margin wedges; erosional truncation (reflections terminating abruptly against an overlying surface due to erosional removal) indicates an unconformity of tectonic or eustatic origin; the combination of these termination patterns in 3D seismic interpretation allows the sequence framework to be constructed from seismic data without well control, providing a predictive geological framework for undrilled areas of a basin and for correlating between widely separated well penetrations using the seismic as a continuous tie.
• Systems tracts within sequences provide the specific predictions of where to find petroleum system components: the lowstand systems tract (deposited when sea level is at or near its lowest position) contains incised valley fill sandstones (potential reservoirs sealed laterally by the valley walls and upward by the overlying transgressive marine shales), lowstand wedge sands (prograding shoreface and deltaic sandstones with good reservoir quality, sealed by the overlying maximum flooding shale), and turbidite fans and channels at the base of slope (the deepwater clastic reservoirs that have become the primary exploration targets in deepwater basins globally); the transgressive systems tract (deposited during sea level rise from lowstand to maximum flooding) contains back-stepping, retrogradational shoreface sandstones (good reservoir quality from reworking by wave energy during transgression) and increasingly organic-rich shales in the upper transgressive to maximum flooding surface (the source rock kitchen in many petroleum systems); the highstand systems tract (deposited from maximum flooding to the next sequence boundary) contains prograding shoreline and deltaic sandstones (the most volumetrically abundant reservoir type in many basins) and is bounded above by the next sequence boundary (which may be an unconformity capable of eroding and concentrating previously deposited sands for trapping).
• Carbonate sequences differ from siliciclastic sequences in the origin of the sediment supply and the response to sea level change: in carbonate systems, sediment is produced in place on the shallow platform by biological and chemical processes (coral reefs, ooid shoals, microbial mats, carbonate mud production) at rates that can exceed siliciclastic sediment supply rates in tropical, clear-water settings; during sea level lowstand, the shallow carbonate platform is exposed and experiences subaerial diagenesis (dolomitization, cavernous porosity development, karstification) that can dramatically enhance reservoir quality -- the unconformity surface at the top of a carbonate sequence often corresponds to a porosity-enhanced karst zone (meteoric dissolution of carbonate during subaerial exposure) that is a primary drilling target in carbonate platform reservoirs (Permian Basin Yates and Grayburg, Williston Basin Ordovician, Middle East Cretaceous carbonates); during sea level rise, the drowned carbonate platform may develop a deep-water condensed section that serves as source rock (organic-rich, slow-accumulation basinal facies) overlying the exposed lowstand carbonate platform, creating the source-reservoir-seal juxtaposition that makes carbonate sequence boundaries some of the most prolific petroleum traps in the world.