Silurian Lockport in Ohio – Basins, Reefs, and “Newburg” Traps
More than thirty-five Newburg gas pools and two oil pools have been discovered in Ohio. Trapping mechanisms include patch reefs, anticlinal structures, structures due to drape over reefs, the carbonate reef bank, and possibly fault traps. In central and eastern Ohio, drillers commonly encounter porosity zones in the Lockport Dolomite and Salina carbonates of the middle Silurian. Although generally water-bearing, these zones have also produced significant oil and gas. Referred to by Ohio drillers and geologists as the Newburg, this porosity zone is typically inconsistent and uncorrelatable, oftentimes even within a single field. The Newburg sometimes correlates with Lockport reefing, but is commonly encountered within the A2 Carbonate or A1 Carbonate above the Lockport, where porosity development is independent of reef growth. This study focuses on correlation and mapping of the formations within the Salina and Lockport Groups of Silurian age in central Ohio. Previous published studies suggest that Lockport deposition (Middle Silurian) was influenced by the development of a deeper water basin in Ohio. This resulted in thick carbonate banks rimming the basin, reef development along basin margins, and thinning carbonate deposition basin-ward. Incorporating an extensive data base of geophysical well logs, detailed correlations of the middle and late Silurian formations present a view of the paleogeography during this time and provide insight into the accumulation of hydrocarbons within Silurian traps in Ohio. The middle Silurian Lockport Group is sub-divided into three formations. The basal unit is a clean dolomite termed the Gasport Formation, which in outcrop, is noted for coral and stromatoporid reef build-up. The middle formation is the Goat Island, a dolomite which generally displays higher radioactivity on the gamma ray log. The upper-most formation is the Guelph dolomite, in which reef development is known to occur. These three units are difficult to discern on geophysical logs, unless clean biogenic carbonates developed in the Gasport and Guelph. Isochore maps that reflect thickening of the Lockport due to extensive biogenic carbonate deposition along a distinct barrier reef bank, reveal not one, but two Lockport basins in central Ohio. The southern basin appears to have had a deeper water depth and reef growth was limited to the basin margins. More shallow water conditions dominated the northern basin resulting in thicker overall carbonate deposition and reef growth scattered throughout the basin.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019