Fluid Histories of Middle Ordovician Fault-Fracture Dolomite Oil Fields of the Southern Michigan Basin

Abstract

The Middle Ordovician Trenton and Black River Formations of the southern Michigan Basin host the type example of fault-fracture “hydrothermal dolomite” reservoirs, the Albion-Scipio Field. Within the basin, a number of parallel trending smaller structures contain the same type of fault-fracture reservoirs including the Freedom, Reading, Northville, Stoney Point, Napoleon, and several other oil fields. These reservoirs share many characteristics in common with Mississippi Valley-type mineral deposits including dolomitized breccias, coarse crystalline saddle dolomite and calcite cements, and sulfide and sulfate mineralization. The fields appear to be structurally related, trending southeast to northwest. However, it is not known if they also share related fluid histories.

Fluid inclusion data in several of the fields display homogenization temperatures (T_h) ranging from 91° C in saddle dolomites to over 250° C in late calcite cements. Last ice melt temperatures (T_m) generally indicate high salinities and range from -10° C to -37.1° C. Infrequently, included daughter halite crystals are observed in the late calcite cements. Some trends exist in these fluid inclusion data. Cross plotted T_h and T_m temperatures for the Reading Field, near the Indiana border, exist in a cluster distinct from the other fields in the study, containing generally cooler and less saline fluids. However, data from the Albion-Scipio trend and the distant Northville display considerable overlap in values with most T_h measurements between 175° C to 250° C and T_m between -19° C and -31° C.

Stable Carbon and Oxygen isotopes for the same fields have been evaluated. Data overlaps well with previous studies with a mean of δ¹⁸O = -8.07 ‰ (VPDB) and mean δ¹³C = +0.47 ‰ (VPDB) for dolomite cement samples. These values are less depleted in both δ¹⁸O and δ¹³C than studies from the early 1980’s, but they are in agreement with more recent studies.

Cathodoluminescence (CL) microstratigraphy observed in dolomite and calcite crystals may allow correlation of diagenetic events among the fields. Multiple distinct CL zones have been observed and are being evaluated. Carbonate cements collected in close geographic proximity show more similar CL cement stratigraphies than cements distal from one another. However, carbonate cements from separate fields display similarities in banding that may indicate related fluid flow events.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018