"It's always fascinating to me to see old oil- and gas-producing areas get reborn with the application of new technology," says Dan Morrison, managing director and senior research analyst for Global Hunter Securities LLC. "We see it in the Permian, in the Bakken…

"The one trend in horizontal technology that seems to be emerging is more of a focus on carbonates than on true shales."

For example, the Niobrara shale play in the Rockies is really a complex mix of shale and carbonates, as is the Texas Eagle Ford as well as the western Louisiana version of the formation, he says. The hot new Midcontinent horizontal oil play, the Mississippi Lime, from which thousands of vertical wells have produced in the past, is a carbonate system, he adds.

"And, if you look up into the Hugoton Basin in Kansas…, there are a lot of shallow, carbonate fields that have been fairly marginal vertically that are all getting a fresh look these days. What's been a 'shale revolution' is rapidly turning into a 'horizontal carbonate oil revolution.'"

The more prolific part of the Texas Eagle Ford is more than 70% calcite, and the primary source of the calcite is the same micro-fossils that comprise chalk formations, he adds. He likes to call it "dirty chalk"—a highly calcareous shale.

"The composition of the Louisiana (side of the Eagle Ford) play hasn't been fully revealed, but a core I've seen from the western end of the Louisiana play looks just like a core from Lavaca County (Texas) to the naked eye."

The Monterey shale play in southern California is also complicated, he adds. Instead of a carbonate-rich shale, like the Eagle Ford, it is a highly siliceous shale.

Meanwhile, the Bakken—at least the Bakken member from which operators are making a world-class new play—isn't a shale either. "It is sort of a misnomer," says Chuck Stanley, president and chief executive of QEP Resources Inc. The Middle Bakken is a complex carbonate and clastic—or sand—interval, he says.

Ben Dell, co-head, energy investing, for AllianceBernstein, says, "Of five unconventional-oil plays––the Bakken, Tuscaloosa, Waskada, Barnett and Niobrara––only the Bakken is currently producing meaningful volumes of oil economically. More importantly, while operators have garnered economic flow rates from the Bakken, the play is essentially not shale; it is a carbonate sandwiched between two shales. As such, it shares few geologic characteristics with the other plays."

Shale or not, Rehan Rashid, managing director and group head, energy and natural resources research, for FBR Capital Markets, estimates production from just four current horizontally targeted oil plays—the Bakken, Texas Eagle Ford, Permian and Niobrara—will grow from slightly under 500,000 barrels a day in 2010 to more than 2.2 million in 2015, with roughly half of that coming from the Bakken alone.

Production of natural gas liquids from just five current plays—the Barnett, Cana/ Woodford, Texas Eagle Ford, Marcellus and Granite Wash—will grow from about 150,000 barrels a day in 2010 to more than 550,000 in 2015, he adds, with roughly half of that coming from the Eagle Ford.

Rashid estimates the Eagle Ford play alone might be worth between $85- and $200 billion to producers. The low-case number is based on current initial-production (IP) rates and estimated ultimate recovery; the high case is based on improvements in tapping the resource. In the Barnett, with each doubling of wells drilled, the IP has grown an average of 17.5%, he says; in the Fayetteville, 23% each time; the Bakken, 16%; and the Haynesville, 15%.

Using the low case—15%—at the current pace of drilling in the Eagle Ford, where some 140 rigs are at work, the IP may double every 12 to 15 months, he estimates. Under that scenario, the Eagle Ford will be making 800,000 barrels of oil and condensate per day by 2015; in the high-case scenario, 1.5 million a day.

Missed out on any of the current horizontally targeted plays? Rashid says 350 billion barrels of oil remains onshore the Lower 48 in conventional—that is, non-shale—formations. He calls the horizontal revolution an "energy super-cycle."

"Technology is redefining exploration parameters, driving a once-in-a-half-century reserve-growth cycle."