ANHYDRITE DIAGENESIS IN THE RICHFIELD MEMBER OF THE LUCAS FORMATION, MICHIGAN BASIN, USA

Gypsum and anhydrite are the two most common sedimentary evaporite minerals, particularly in the Michigan Basin. Both minerals are highly soluble, mobile, and have been implicated in the degradation of reservoir quality in many Michigan Basin formations. Despite its potential importance, evaporite mineral diagenesis remains understudied. The current study integrates multiple petrological datasets harvested from the Lucas Formation in the Brown Snowplow #1-5 well (Alpena Co., Michigan) to investigate evaporite mineral diagenesis. Observations include core description, petrography (thin sections and scanning electron microscope imaging), mineralogy (powder x-ray diffraction), and elemental geochemistry (x-ray fluorescence, SEM-energy dispersive spectroscopy).

The studied interval, 2187 - 2189 ft, is characterized by fine crystalline matrix dolomite (5 mm), large spherical nodular anhydrite (2 mm), and elongated anhydrite needles (200-500 mm). The petrographic and SEM analyses showed that the anhydrite needles (laths) extend out from the anhydrite nodules into the fine crystalline dolomite matrix, suggesting that these two anhydrite textures (laths and the nodules) represent two different diagenetic phases. Based on cross-cutting relationships, the nodular anhydrite and dolomite represent the earliest diagenetic phases, whereas the anhydrite laths are interpreted to form later. Anhydrite nodules are interpreted to have formed early as displacive gypsum nodules that were later dehydrated to form anhydrite. The anhydrite laths, in contrast, represent remobilization of calcium-sulfate during progressive burial. Collectively, these observations provide the basis for a diagenetic model of evaporite deposition, dehydration, remobilization, and recrystallization.