A sequence stratigraphic model for the Silurian A-1 Carbonate of the Michigan Basin

Abstract

The Lower Salina A-1 Carbonate is the primary source rock for hydrocarbons in the Silurian (Niagaran) Reef Complex reservoirs, which are some of the most prolific reservoirs in the Michigan Basin. To date, State records from the Michigan Office of Oil, Gas and Minerals report that A-1 Carbonate hydrocarbon production occurs in 31 fields as sole producing unit in that field or in combination with Niagaran or other Salina units. Nearly 3.3 million barrels of oil and 37.5 BCF of natural gas has been produced from the A-1 Carbonate throughout the MI Basin. Previous studies interpret the A-1 Carbonate as a shallow subtidal to supratidal deposit that sits on top of the reefs. Despite their close proximity and strong association in the hydrocarbon system, the sequence stratigraphic relationship between source and reservoir rock are poorly constrained.

To establish the timing and environments across the basin, fundamental sedimentological observations from cores are coupled with geochemical data collected with a handheld X-ray fluorescence spectrometer. One of the principle observations comes from an anhydrite unit called the “Rabbit-Ear Anhydrite (REA),” which has been previously interpreted as a localized supratidal deposit that formed only around the periphery of paleo-topographic highs of the older Niagaran reefs. New observations from cores and wireline logs from deeper in the basin show that the REA is instead a basin-wide depositional unit which ranges from sabkha to deep-water settings and separates the A-1 Carbonate into an upper and lower unit. This suggests that REA is a time-correlative unit across the basin and can be used to separate the A-1 Carbonate into upper and lower units.

Sequence stratigraphic correlations are further aided by lithofacies relationships and elemental trends (Mg, Ca, S, Fe, etc.), which point to four regionally-extensive, fourth-order shallowing upward cycles. Each shallowing upward cycle exhibits intervals of dolomite corresponding to the most regressive depositional facies. These dolomitic prone intervals are correlated across the basin and should allow for more informed exploitation of resources with the A-1 Carbonate. In contrast, elements such as S, Fe, Si, Sr, and K are more prominent within maximum flooding zones.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016