Shuvajit Bhattacharya, Bureau of Economic Geology, The University of Texas at Austin; Autumn Haagsma, Battelle; Ron Budros, Jared Bowen, Innova Exploration
This study presents the results from subsurface characterization and explores potential sweetspots for CO₂-enhanced oil recovery from the Trenton-Black River hydrothermal dolomite (HTD) play in the Southern Michigan Basin, integrating geophysical and petrophysical technologies. The study is a part of an ongoing field test, funded by the US Department of Energy (DE-FE00031792). HTD reservoirs present interesting exploration and development challenges and opportunities. There are more than 35 documented HTD plays worldwide. The Trenton and Black River reservoirs are especially challenging for storage and recovery due to heterogeneities, compartmentalization, the presence of thief zones, and lack of conformance. These formations are highly fractured and predominantly contain limestone and dolomite. Basement-rooted faults serve as the conduit for hydrothermal fluid flow, diagenesis, and formation of dolomite, which has higher porosity and permeability than the original, non-reservoir limestone layers. We perform fault interpretation using 3D seismic and high-resolution image logs. Seismic sections and coherence attributes show the presence of riedel shear and negative-flower structures. These wrench faults are mostly en-echelon oriented along NW-SE, punctuated by breaks (for example, Pulaski break) forming en-echelon reservoir compartments with varying fluid contacts. We interpreted several basement-rooted faults that propagate upwards and affect younger sedimentary formations. Some of these basement faults are also reactivated, which result in the development of small-scale faults and develop reservoir compartments. Image log facilitates the identification of natural fractures, most of which are along NW-SE and W-E. These results agree with the 3D seismic analysis. Seismic inversion-based acoustic impedance has been converted to porosity and validated with well data. The acoustic impedance profiles highlight the fractured reservoir sections versus the more consistent non-reservoir limestone sections. The highly variable, porous, reservoir-quality dolomite is created within the wrench fault-related structures in contrast to the non-permeable limestone outside the bounding wrench fault. These porous dolomite sections are reservoirs for carbon storage and enhanced oil recovery. Because fractured carbonate reservoirs show a strong lateral change, seismic inversion is a preferred method to derive reservoir properties, compared to spatially sparse and vintage wireline log-based mapping and geomodeling. In addition, 3D seismic along with image logs and cores prove to be critical in our study area to analyze fracture distributions and their potential impact on CO₂ movement/preferential flow.