Keith W. Shanley

Perspectives on the Sequence Stratigraphy of Continental Strata

This report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 NUNA conference in Banff, Canada. To date, sequence stratigraphic concepts have been applied mainly to the marine realm, but unconformity-bounded units have long been recognized in nonmarine strata. Successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply. As with shallow marine environments, relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings. Farther inland, stratigraphic base level, which determines accommodation space, is more complex and takes various forms, such as the graded profile for fluvial strata, groundwater tables for some eolian strata, and lake level for some intermontane sediments. Sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area. The influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments. The fact that the major controls of climate, tectonism, and eustasy are somewhat interdependent, and that a change in one parameter will most likely be reflected in others, is also more readily apparent in continental strata. Although in its infancy, sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture. Of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata, and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits. The application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs.

Factors controlling prolific gas production from low-permeability sandstone reservoirs: Implications for resource assessment, prospect development, and risk analysis

Low-permeability reservoirs from the Greater Green River basin of southwest Wyoming are not part of a continuous-type gas accumulation or a basin-center gas system in which productivity is dependent on the development of enigmatic sweet spots. Instead, gas fields in this basin occur in low-permeability, poor-quality reservoir rocks in conventional traps. We examined all significant gas fields in the Greater Green River basin and conclude that they all occur in conventional structural, stratigraphic, or combination traps. We illustrate this by examining several large gas fields in the Greater Green River basin and suggest that observations derived from the Greater Green River basin provide insight to low-permeability, gas-charged sandstones in other basins. We present evidence that the basin is neither regionally gas saturated, nor is it near irreducible water saturation; water production is both common and widespread. Low-permeability reservoirs have unique petrophysical properties, and failure to fully understand these attributes has led to a misunderstanding of fluid distributions in the subsurface. An understanding of multiphase, effective permeability to gas as a function of both varying water saturation and overburden stress is required to fully appreciate the controls on gas-field distribution as well as the controls on individual well and reservoir performance. Low-permeability gas systems such as those found in the Greater Green River basin do not require a paradigm shift in terms of hydrocarbon systems as some have advocated. We conclude that low-permeability gas systems similar to those found in the Greater Green River basin should be evaluated in a manner similar to and consistent with conventional hydrocarbon systems.To date, resource assessments in the Greater Green River basin have assumed a widespread, continuous-type resource distribution. Failure to recognize some of the fundamental elements of low-permeability reservoirs has led to an underappreciation of the risks associated with exploration and development investment decisions in these settings and likely a significant overestimation of available resource levels.

Predicting facies architecture through sequence stratigraphy—An example from the Kaiparowits Plateau, Utah

Recognition of depositional sequences in Upper Cretaceous strata in the Kaiparowits Plateau allows examination of facies-tract geometries within coeval shoreface, alluvial, and coal-bearing strata. Changes in depositional architecture, sandstone-connectedness, sand-shale ratios, coal-bed geometry, and degree of shoreface and foreshore preservation are related to position within a depositional sequence. Base-level falls produced regionally extensive sequence boundary unconformities and a basinward shift in facies tracts. Slow rates of base-level rise resulted in amalgamated fluvial channel complexes, thin discontinuous coal beds, and progradational shoreface parasequence sets. More rapid rates of base-level rise produced vertically isolated meander-belt sandstones, thick extensive coal beds, and aggradational shoreface parasequence sets. The highest rates of base-level rise resulted in retrogradational shoreface parasequences and tidally influenced fluvial systems at least 60 km inland of coeval shoreface deposits.

Organic control on shoreface stacking patterns: Bogged down in the mire

In ever-wet climates, raised mires that are elevated several metres above flood levels can cover significant portions of coastal plains. Because peat accumulation may keep pace with moderate rates of base-level rise, the development of raised mires may reduced the areal extent of marine transgressions. Thick, low-ash coals are present immediately landward of many vertically stacked shoreface parasequences in Cretaceous state of the Western Interior of North America. We suggest that these coals formed in raised mires that stabilized shorelines for long periods of time. In such settings, the rate of sediment supply (including peat accumulation) to the coastal environment is partly a function of the rate of change in base level.

Sequence-stratigraphic relationships and facies, architecture of Turonian-Campanian strata, Kaiparowits Plateau, south-central Utah

Outcrop study of the Straight Cliffs Formation in the Kaiparowits Plateau allows examination of facies tract development within a sequence-stratigraphic framework. This approach provides a new perspective to regional chronostratigraphic and lithostratigraphic relationships of Turonian through Campanian strata. These strata are arranged in geometric patterns that can be related to lowstand, transgressive, and highstand system tracts.