Integrated Approach in Exploring for Dolomite Breccias Along the Cincinnati Arch, Eastern U.S.

Steven A. Tedesco

Dolomite breccias or chimneys occur in Lower to Middle Ordovician rocks from Tennessee to southwestern Ontario along the Cincinnati Arch and also extend into adjacent basins. These breccias are prolific hydrocarbon reservoirs, typically producing 100,000 to 500,000 bbls per well on 20 acres and from less than 5,000 feet. Historically, many of the breccias such as Albion-Scipio (150 + MMbbls BOE), Wood-Hancock Complex (155 + MMbbls BOE) and Dover (1 + MMbbls BOE) fields were discovered by accident. Six characteristics are consistently associated with the chimneys. Breccias are 1) found in synclines, 2) secondary dolomite encased in dense non-porous carbonates 3) associated with basement faulting, 4) related to a thickening of overlying strata, 5) thick reservoirs of 100 to 800 feet and 6) narrow in width and elongated in length. Subsurface mapping provides few clues to the location of these breccias. Based on seismic data, Precambrian unconformities are clearly associated with the breccias. Faulting causes rubblization of the breccias which then provide a migration path for brines. Thus, secondary dolomitization can occur and petroleum can accumulate in breccias.

Since the early 1980s, the use of regional and detail surface geochemical surveys has increased the success of finding breccias by focusing seismic surveys on specific sites to accurately delineate the chimneys. The use of 30 and 60 fold data has consistently shown five characteristics that can be associated with a breccia in seismic profiles. The seismic data acquired across several breccias clearly suggest a non-karst origin for these features along the Cincinnati Arch and adjacent basins.

The association of basement faulting extending into the Middle Ordovician but not penetrating into younger strata provides a tool for exploration. Most breccias are found in close proximity to, or over, Precambrian rifts which focuses exploration efforts in a relatively narrow area. Subsurface geology can provide regional leads in terms of indicating locations of synclines and fault systems, but surface geochemistry allows specific site targeting for seismic. Seismic can delineate the breccia or chimney. However, wells have been known to miss the chimney by 50 feet when located based on seismic. The play is still elusive and requires commitment of more than one well per prospect. Case histories in Ohio, Tennessee, Kentucky and Ontario are presented which show the clear integration of ubsurface geology, surface geochemical methods and seismic that lead to new field discoveries. The integration of the three methods results in a clearer understanding of breccia geometry, the history of formation, and the relationship with Precambrian rifting.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995