Pesky hydrocarbons just want out

Is the Trap Half-Full or Half-Empty?

Dick Bishop
Dick Bishop

A number of years ago, past AAPG president Dick Bishop was asked to investigate the challenging topic of percent trap fill.

Bishop, now executive director and chief geologist at U.S.-based consulting and investment firm RSK, noted there has been a long-standing effort in the energy field to predict both trap content, i.e. oil, gas or both, and volume, which is commonly dependent on an estimate of column height.

“I looked at hundreds of fields in different types of basins and never found a trap I could demonstrate was underfilled but always filled to a spill point or leak point,” he said. “I found a lot of traps that were indeterminate because they were so complex – but only a couple of cases where they might be underfilled.”

“People were so used to looking at traps in map view that when I said traps were full, they thought of it as down to synclinal spill point,” he said, “and not to a leak point.

“There was, and continues to be, misunderstanding and resistance to the idea – until folks take the time to look at some well documented fields themselves.”

Bishop, who presented his thoughts at the recent AAPG Annual Convention in Long Beach, Calif., says field observations and geochemical studies make the case that the source rock commonly generates more than traps can hold. Many examples of full to a leak point come from the literature and include the Allan diagram and publications on fault seal.

“The difference between the trap volume and the volume generated by the source determines what can be in there,” he noted. “If a source generates X volume and the trap has Y volume available, the smaller of the two will determine the prospect size.

“You have to look at traps in three dimensions in order to find what controls the contact (i.e. where is the leak point), and that affects how one assesses trap volume and risk,” Bishop continued. “The key is to ask what controls the contact.”

The significance of the observation lies in its implications to understanding the control of contacts and then interpreting fluid movement in the subsurface. This includes such things as gas displacement or predicting oil formation volume factors.

“A distracting question becomes whether this is a worldwide phenomenon or if it happens in only a few well known prolific basins,” he said. “But full traps do happen – and after all the basins I’ve looked at, I think it happens most of the time.”

The implications are, he suggests, if full traps happened once, then it could happen twice, and then three times ...

Oil vs. Gas

Bishop stated that charge overwhelming trap capacity applies to both oil and gas fields. However, if both oil and gas are in the system, it implies that gas should displace all the oil.

The question thus becomes one of explaining how oil and gas occur in the same reservoir.

“Explaining how traps have both oil and gas in them probably is not possible to prove, but preferential leakage of free gas is what I have seen,” he said.

In other words, the gas-oil contact occurs at a leak such as a fault intersection, across a fault, sometimes through a top seal as a gas chimney or up the fault.

A similar question comes with how oil fields occur in systems with free gas.

“There are a lot of oils in the world saturated with natural gas,” he said. “Mother Nature didn’t send up just enough gas to saturate that oil field and then stop sending the gas.”

The reason the gas didn’t displace the oil is interpretive.

“Again, the reason is the top seal or a fault, enabled preferential leakage of the gas,” Bishop noted. “The trap is configured such that it leaks free gas, leaving behind saturated oil fields.

“In terms of predicting trap content in plays with both oil and gas available, I don’t think there’s a consistently good technology to determine pre-drill whether a prospect will be oil, gas or both other than local knowledge,” he continued. “We have first principles to make these predictions but we do not have the subsurface resolution required to predict trap content.”

You can’t address these questions just with basin modeling, he cautioned.

“Forecasting trap content requires consideration of what the hydrocarbons might do in the subsurface that you otherwise wouldn’t imagine,” he said. “In other words, what are the possibilities?

“When oil and gas are both available to a trap, the best way I know to predict what will be in the trap is to consider the role of leakage,” Bishop continued. “It helps to appreciate the no-see-ums in the subsurface – the hydrocarbons that have migrated through these systems and are gone now.

“There’s no flag that tells you ‘Leroy was here,’” he quipped.

Full to Spill

Traps filled to spill commonly are associated with thick regional seals, because much of the time the faults die into that seal – and the fault then becomes bed seal.

“If the hydrocarbons can’t leak up, they will fill to a spill point,” he said. “For example, in and around the Gulf Coast, especially south Texas, many four-way anticlines and high side closures are filled to spill point because the faults die in the overlying Vicksburg shale.

“It’s the same around here (Houston area) with the Frio sands under the Anahuac shale.”

Bishop also cited a number of thick regional seals worldwide particularly in failed rifts and some margin sag basins.

The thick Cretaceous section in the North Sea is such an example.

A Valuable Tool?

The real argument here is not about how common this is globally, but how to use the concept to evaluate prospects if charge overwhelms trap capacity, Bishop said, “and just what are the implications of gas displacement, leak points, things like that.

“Regardless of how one feels about the commonality of ‘overcharge,’ I do think it’s sufficiently common to warrant it becoming a part of everyone’s toolkit,” he added.

For example, some of the implications include:

  • Spill from downdip traps may reduce risk of charge in updip traps.
  • Trap risking can be based on number and type of hydrocarbon boundaries as well as top seal quality.
  • Given similar stratigraphy, traps with more potential leak points have greater risk than those with fewer leak points.

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