Field Guide to the Minnelusa Formation Ranch A and Newcastle Area, Wyoming and South Dakota

The purpose of this field trip and the lectures is to acquaint participants with the sedimentology of the Middle and Upper Minnelusa Formation, using outcrops in the Black Hills of Wyoming and South Dakota. The trip provides a good opportunity to examine in outcrop many of the sedimentary facies that strongly control oil production in the nearby Minnelusa oil play. Along with study of the outcrops, we review important principles of oil and gas production from eolian reservoirs using modern and ancient oil field and outcrop examples. These ideas may be useful in analyzing eolian reservoirs for secondary or tertiary recovery operations, or understanding past reservoir behavior.

Eolian petroleum reservoirs are found worldwide, many having high-volume production of both oil and gas. As with any geological rock unit, each oil/gas field has production characteristics peculiar to its geological history. However, certain common factors link most eolian reservoirs. Cross-stratification due to bedform migration can influence sweep direction and efficiency. The various kinds of primary eolian strata have different poroperm characteristics. Moreover, stacking of sand seas or bedforms through geological time can create distinctive reservoir flow units in the subsurface. Tectonic activity, especially faults, may create shear zones with reduced poroperm, or partition a reservoir into structurally defined flow units. Faults may also create high-permeability zones that allow water breakthrough. Eolian reservoirs are commonly thought of as clean, and rather simple. However, in some places they are complex in terms grain composition or texture. They are commonly cemented by carbonates, anhydrites or salt, which sets up fabricselective or non-fabric selective patterns of secondary porosity in reservoirs.

We hope you enjoy and learn from this trip. The subject of eolian reservoirs is too extensive to cover in the two days of this excursion, but we will attempt to hit the high points! Meanwhile, always be safe, don’t climb onto exposed places, look out below you for others and use sun protection.

Additional Topics Covered:

Trip Overview

Ranch A

Sand Creek South

Redbird Canyon

Hell Canyon, South Dakota

Minnelusa Regional Geology

Modern and Ancient Analogues

References