Numerical stratigraphic models are capable of accurately simulating stratigraphic geometries and facies attributes of petroleum reservoirs. Correctly constructed stratigraphic models may be converted into a petrophysical model for reservoir simulation. Accurate simulations which match the stratigraphy of a reservoir are generated by models which have: (1) geologically reasonable algorithms which adequately simulate the stratigraphic process-response system; (2) a set of correctly acquired observations; and (3) accurately determined values of model process parameters. The first condition is accomplished by translating correct knowledge about the process-response system into computer code. The second is accomplished through careful measurement of properties which are comparable to attributes of the simulation. The third may be accomplished by trial-and-error simulations combined with subjective judgment of comparability, but this approach produces nonunique and nonrobust answers. A better approach is through simultaneous, multi-parameter stratigraphic inversion which acquires estimates of process parameter values directly from the observations.

A new forward model of fluvial stratigraphy was developed using this philosophy of stratigraphic modeling. The Multi-Scale Fluvial Model (MFM) consists of nested, deterministic models that simulate sediment accumulation in fluvial/floodplain environments as a function of processes operating at variable spatial and temporal scales. The forward model algorithm reproduces the self-organized criticality characteristics observed in natural fluvial systems by incorporating nonlinear dynamics, feedback, buffers, thresholds, and memory. Sedimentary facies of variable grain-size distributions accumulate by geomorphic processes, but are variably preserved as a function of stratigraphic base level. Model output is comparable to easily collected observations of multiple sedimentologic (e.g., grain size, environment) and stratigraphic (e.g., thickness, geometry, facies distributions) attributes. Model simulations of sedimentary facies are convertible to a 3-D petrophysical model to be used in reservoir fluid-flow simulations.

In the next phase, model inversion, simulations should correctly describe 3-D reservoir properties beyond well bores because the simulation will have matched observations at the well bores. Inversion produces the best estimates of the forward model parameters, an estimate of the uncertainty in these parameters, an estimate of the confidence interval of the best fit forward model, and suggests sources of errors and shortcoming in the observations.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas