Black diamonds are conventional vertical wells and red diamonds are horizontal wells.
Just out this afternoon from the EIA:
"Between 2009 and 2011, Pennsylvania's natural gas production more than quadrupled due to expanded horizontal drilling combined with hydraulic fracturing (see chart below). This drilling activity, which is concentrated in shale formations that cover a broad swath of the state, mirrors trends seen in the Barnett shale formation in Texas.
The animation above illustrates Pennsylvania's relatively recent transition from conventional vertical wells (black diamonds) to horizontal wells (red diamonds), drilled mostly in sections of the Marcellus, Utica, and Geneseo/Burket shale formations located in the northeast and southwest portions of the state. The animation also shows that as horizontal drilling increased, the number of vertical wells—which are typically less productive—fell, resulting in an overall decline in the state's new well count. Historically, natural gas exploration and development activity in Pennsylvania was relatively steady, with operators drilling a few thousand conventional (vertical) wells annually. Prior to 2009, these wells produced about 400 to 500 million cubic feet per day of natural gas. With the shift to and increase in horizontal wells, however, Pennsylvania's natural gas production more than quadrupled since 2009, averaging nearly 3.5 billion cubic feet per day in 2011 (see chart above)."
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