Depositional Topography: Key Element of Stratigraphic Interpretation and Panacea for Log Correlation: Part 1: Concepts and Transitional Icehouse-Greenhouse Systems

Scott W. Tinker and Charles Kerans: Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas

Abstract

In any modern or ancient carbonate setting, one depositional environment eventually transitions into another, and facies change. Topography is a primary driver of lateral facies change. In a given shelf or ramp profile, carbonate facies tend to be more continuous along strike and more apt to change along dip. If topographic dip is steep, facies change in shorter distances than if topographic dip is gradual. Because facies (1) dictate original petrophysical properties and strongly influence diagenetic alteration, (2) strongly influence petrophysical properties and associated wireline log and seismic responses, and most importantly (3) change within a time-bound package of rock, it is highly unlikely that the correlation of similar wireline log signatures or the tracing of a continuous seismic reflector for great distances will result in an accurate chronostratigraphic interpretation.

Although these concepts are widely understood and accepted today, and in spite of ever-improved seismic imaging technologies, accomplishing the feat of building realistic depositional topography into subsurface stratigraphic interpretation remains a difficult task. Even though sequence stratigraphy has revolutionized both exploration and exploitation in the oil industry by showing that chronostratigraphy improves the ability to predict the 3D distribution of reservoir, source, and seal strata, stratigraphers continue to hedge bets towards flat correlation by correlating similar log signatures and carrying continuous seismic reflectors.

The only way to avoid this tendency toward the horizontal is to impose a facies-driven model of depositional topography onto the stratigraphic interpretation. An accurate assessment of depositional topography requires a sedimentologic analysis of core and outcrop analog data to determine reasonable water-depth ranges for component facies, and the definition of a hierarchy of stratigraphic cyclicity to determine longer term water-depth variation represented by component facies. Concepts of depositional topography are applied differently for each high-frequency cycle (cycle) as follows:

• Constructional cycle - Individual cycles should illustrate depositional topography that represents the water depth of the included facies (i.e., if a cycle contains a range of facies from tidal flat through 100-ft-water-depth outer ramp facies, then the bounding surfaces should illustrate that topography). Because of the relatively short time duration represented by an individual cycle, each cycle should allow reconstruction of paleobathymetry once compaction/subsidence are removed.
• Draped cycle - High-frequency cycles that drape preexisting topography will not necessarily show a simple facies to predicted water-depth correlation.
• Conformable high-frequency sequence (HFS) boundaries - are a composite stratigraphic record of short-term and long-term eustasy, subsidence, sedimentation rate, and compaction, and therefore may not show a direct relationship to facies that are found directly below them but will nonetheless control depositional topography of the facies in the overlying sequence.
• Disconformable (erosional) HFS boundaries - are not candidates for reconstruction of depositional topography of the underlying facies, but like conformable HFS boundaries, they will control depositional topography of the facies in the overlying sequence.

Permian-age outcrops and subsurface datasets from West Texas and New Mexico, representing transitional icehouse-greenhouse systems, provide an excellent starting point to illustrate the importance of depositional topography on stratigraphic interpretation. Eustatic amplitude in transitional icehouse-greenhouse systems was neither too great nor too small, but just right to record a relatively complete stratigraphic signal along the depositional profile. Examples include ramp to steep-rimmed profiles.