Jack E. Neal

Sequence stratigraphy hierarchy and the accommodation succession method

We propose a framework for the hierarchy of sedimentary units observed in stratigraphic data that is based entirely on the geometric relationship of the strata. This framework of geometries is assumed to result from repeated successions of accommodation creation and sediment fill (here named accommodation succession). We have modified existing hierarchal frameworks to describe depositional units resulting from accommodation successions of varying magnitude and duration, across a depositional profile. Each full succession consists of component partial succession sets that are, sequentially, lowstand—progradation to aggradational; transgressive—retrogradation; and highstand—aggradation to progradation to degradation. The terms “highstand” and “lowstand” as originally defined to label systems tracts relative to a shelf edge, and with an implied relationship between sea level and systems tracts, have been the root of confusion. We propose that these terms be used in the strict sense of the original definition, because their meaning has been lost when applied to the many depositional settings and high-resolution data sets to which the concepts of sequence stratigraphy are now applied. We propose that the concept of accommodation succession stacking be used in the interpretation of stratigraphic data within a hierarchal framework of depositional sequences, sequence sets, and composite sequences. This will allow an interpreter to accurately categorize observations, provide a basis for predictions away from control points, and develop a framework that allows revisions as higher-resolution data become available.

Sequence stratigraphy of experimental strata under known conditions of differential subsidence and variable base level

Sequence stratigraphy has been applied from reservoir to continental scales, providing a scale-independent model for predicting the spatial arrangement of depositional elements. We examine experimental strata deposited in the Experimental EarthScape facility at St. Anthony Falls Laboratory, focusing on stratigraphic surfaces defined by discordant contact geometries, surfaces analogous to those delineated in the original work on seismic sequence stratigraphy. In this controlled setting, we directly evaluate critical sequence-stratigraphic issues, such as stratigraphic horizon development and time significance, as well as the internal geometry and migration of the bounded strata against the known boundary conditions and depositional history. Four key stratigraphic disconformities defined by marine downlap, marine onlap, fluvial erosion, and fluvial onlap are mapped and vary greatly in their relative degree of time transgression. Marine onlap and downlap contacts closely parallel topographic surfaces (time surfaces) and, prior to burial, approximate the instantaneous offshore topography. These stratal-bounding surfaces are also robust stratigraphic signals of relative base-level fall and rise, respectively. Marine onlap surfaces are of special interest. They tend to be the best preserved discordance, where widespread, allogenic-based onlap surfaces subdivide otherwise amalgamated depositional cycles amidst cryptic stacks of marine foresets; however, local, autogenic-based marine onlap discordances are present throughout the fill. A critical distinguishing feature of allogenic onlap is the greater lateral persistence of the discordance. Surfaces defined by subaerial erosional truncation and fluvial onlap do not have geomorphic equivalence because channel processes continually modify the surface as the stratigraphic horizons are forming. Hence, they are strongly time transgressive. Last, the stacking arrangement of the preserved bounded strata is found to be a good time-averaged representation of the mass-balance history.

Lessons from large lake systems-Thresholds, nonlinearity, and strange attractors

Lake systems are the largest integrated depositional complexes in the continental realm: modern lakes have areas up to 374,000 km2, and ancient lake strata extend up to 300,000 km2 in the Cretaceous systems of the south Atlantic and eastern China and the Permian system of western China. The largest lakes do not appear to form a significantly different population in many of their attributes. Their area, maximum depth, and volume closely follow power-law distributions with fractional exponents (–1.20, –1.67, –2.37 respectively), with minimal breaks between the largest lakes and the majority of lakes. Controls on lake size and stratigraphic extent are not straightforward and intuitively obvious. For example, there is little relation of modern lake area, depth, and volume, with origin, climatic conditions, mixis, or water chemistry. Indeed, two-thirds of the largest-area lakes occur in relatively dry climates (precipitation-evaporation ratio [P/E] <1.6). In ancient lake strata, deposits of largest areal extent and thickness tended to form mostly under relatively shallow-water, evaporitic conditions in both convergent and divergent tectonic settings. Geometric and dynamical thresholds appear to govern lake systems as complex, sensitive, nonlinear dynamical systems. Phanerozic examples worldwide indicate that the existence, character, and stacking patterns of lake strata are a function of the interaction of rates of supply of sediment + water and potential accommodation change. Lake-system behavior reflects interactions of four main state variables: sediment supply, water supply, sill height, and basin-floor depth. The stratal record ultimately records five main modes of behavior indicating that nonmarine basin dynamical systems are governed by two fundamental bifurcations and five strange attractors in the sediment + water supply – potential accommodation phase plane: fluvial, overfilled lake, balanced filled lake, underfilled lake, and aeolian/playa. Thus, extremely large lakes are highly dependent on intricate convolutions of climatic and tectonic influences and occur in a variety of settings and climates.

Accommodation succession (δA/δS) sequence stratigraphy: observational method, utility and insights into sequence boundary formation

The future of sequence stratigraphy depends on stratigraphers making observations with a common method so that physical frameworks can be clearly separated from interpretations of driving mechanisms. Depositional sequence boundary selection is a well-known controversy that could be resolved with objective recognition criteria. Accommodation succession sequence stratigraphy refines traditional methods, using sedimentary facies, facies associations, vertical stacking, stratal geometries and stratal terminations as the objective record of competing rates of accommodation change and sediment fill through time. Observations are placed in context of lateral (transgression and regression) and vertical (aggradation and degradation) movement of shoreline through time, across multiple timescales in hierarchal stacks. The repeating motif consists of a subaerial unconformity and its correlative subaqueous surface overlain in coastal settings by a basinward shift in coastal onlap and strata with progradational to aggradation stacking, then retrogradation and aggradation–progradation–degradation stacking. These stacking patterns are bounded by key surfaces, recognized by stratal terminations and characteristic vertical successions of facies. This pattern is independent of time duration or position on a sea-level curve, but incorporates data resolution, regional extent and hierarchal stacking. Examples from multiple datasets show the utility and objectivity of the method and provide insights into sequence boundary formation.

Higher Resolution Subsurface Imaging

This is the fifth in a series of articles on the great challenges facing the oil and gas industry as outlined by the SPE Research and Development (R&D) Committee. The R&D challenges comprise broad upstream business needs: increasing recovery factors, in-situ molecular manipulation, carbon capture and sequestration, produced water management, higher resolution subsurface imaging of hydrocarbons, and the environment. The articles in this series examine each of these challenges in depth. White papers covering these challenges are available at www.spe.org/industry/globalchallenges and allow reader comments and open discussion of the topics.

The accumulation of organic-matter-rich rocks within an earth systems framework: the integrated roles of plate tectonics, atmosphere, ocean, and biota through the Phanerozoic

Abstract Fundamental concepts of plate tectonics, palaeogeography, ecology, and atmospheric and oceanic circulation can be used in conjunction with models of source rock deposition to understand the location and character of organic matter–rich rocks (ORRs) at global to regional scales. Ocean and atmosphere circulation patterns directly impact processes governing organic carbon production and preservation. One can convolve these patterns with palaeogeographic reconstructions and make predictions of ORR occurrence and character at global to regional scales. Such convolutions can be made qualitatively or quantitatively in several automated manners. Insights gained from such predictions are also useful for understanding and exploiting mudstone reservoirs and shale-gas and tight-oil plays.