Technology unlocking Inlet riches

Aurora Gas' Clifford explains how techniques such as 3-D seismic combined with an understanding of the Cook Inlet petroleum reservoirs help locate hydrocarbons in the Southcentral Alaska basin

By Alan Bailey

Petroleum News

Alaska’s Cook Inlet isn’t an easy place in which to explore for oil and gas. High quality seismic data are difficult to obtain. And the inlet is notorious for the complexity of its reservoirs, which often consist of multiple, thin sandstone layers that are difficult to find and trace.

At the joint meeting of the Cordilleran Section of the Geological Society of America, the Pacific Section of the American Society of Petroleum Geologists and the Western Region of the Society of Petroleum Engineers in May in Anchorage, Andy Clifford, vice president for exploration for Aurora Gas, talked about evolving approaches to exploring the Cook Inlet basin.

The basin, a deep section of sedimentary strata under the Cook Inlet and surrounding lowlands, has yielded a cumulative production of about 8.4 trillion cubic feet of gas and 1.3 billion barrels of oil, Clifford said. But exploration in the area dropped abruptly in the late 1960s, when interest switched to the North Slope after the discovery of the Prudhoe Bay and Kuparuk River fields. However, the need for new natural gas reserves to supply Southcentral Alaska has triggered an increase in exploration in the Cook Inlet area in the past few years.

And companies such as Aurora are finding ways to meet the exploration challenges of the area.

Forearc basin

The relative motion of the plates that form the Earth’s crust along the south of Alaska has wrenched the crust in the area of the Cook Inlet, caused the Cook Inlet basin to sink and formed a distinctive pattern of faults. The wrenching has also caused crumpling of the Tertiary strata into a series of major folds aligned in a north-northeast/south-southwest direction.

And a brief examination of a map of Cook Inlet oil and gas fields shows that the major petroleum discoveries line up along that north-northeast/south-southwest trend — much of the discovered oil and gas in the Cook Inlet area has been reservoired in the crests of major folds in the Tertiary.

Major oil and gas fields in the basin include the Swanson River, Kenai, Cannery Loop and Ninilchik fields on the Kenai Peninsula; the McArthur River, Trading Bay, Middle Ground Shoal and Granite Point fields along the west side of the Cook Inlet; and the Redoubt Shoal, Middle Ground Shoal and North Cook Inlet fields towards the middle of the inlet.

The wrenching of the basin has caused particularly intense faulting along its western side, Clifford said.

“This might be one of the reasons why many of the pool sizes on the west side are smaller than the bigger fields that have been found to date,” Clifford said.

However, Aurora has been working the west side of the basin for several years and now operates five onshore gas fields in that area: the Moquawkie, Lone Creek, Three Mile Creek, Nikolai Creek and Kaloa fields. The gas in these fields often comes from the upper parts of the Tertiary Beluga formation. The reservoirs consist of discontinuous, thin shaly sands that are very hard to delineate, Clifford said.

Most of the gas discovered in the Cook Inlet is biogenic in origin, formed by the action of bacteria on organic material in the Tertiary sediments. The Cook Inlet oil also lies in Tertiary reservoirs but has a different origin.

“A lot of the oil is reservoired in the Hemlock and Tyonek formations and sourced from the Jurassic, and maybe the Triassic also,” Clifford said.

The main oil source rock, the Jurassic Tuxedni formation, lies below a major break in the rock sequence at the base of the Tertiary strata.

Petroleum systems

Tuxedni oil migrating into the Hemlock and Tyonek constitutes the first of these systems and accounts for the oil accumulations in fields such as Swanson River and McArthur River, Clifford said. But a Triassic source may account for the oil in the Cosmopolitan and Redoubt Shoal fields, he said (Cosmopolitan is a known oil accumulation off the southwest side of the Kenai Peninsula and is currently being evaluated by a group headed by Pioneer Natural Resources).

The biogenic gas in the Tertiary constitutes a third petroleum system.

Mesozoic oil shows in the lower Cook Inlet suggest the existence of another system. And there is a Jurassic producing reservoir in the McArthur River field, Clifford said. In fact there is huge potential in the Mesozoic sands near existing fields, he said.

“Very little exploration’s been done for that,” he said.

Coalbed methane in the Tertiary constitutes another as yet unproven system.

There is also evidence, including geologic structural histories inferred from seismic cross-sections, that oil migrated at different times into different oil fields — light oils in the Middle Ground Shoals and Granite Point fields appear to have migrated later than the oils from a less mature source that filled the Swanson River, Trading Bay and Redoubt Shoal fields, Clifford said.

Although all of the existing fields involve major folds in the Tertiary strata, often combined with faulting, the detailed structures of fields vary quite a bit. And the structures that trap the oil and gas in the fields can be classified into several “trapping styles.” Some trapping styles, such as the anticline and fault combination at Swanson River have been extensively explored, while others present significant opportunities for further exploration, Clifford said.

And many of the fields in the Cook Inlet basin involve some element of stratigraphic trapping.

But people have rarely explored for pure stratigraphic traps, Clifford said, commenting that the Hanna prospect on the west side of the Cook Inlet is a possible example of a stratigraphic trap (Aurora plans to drill Hanna in partnership with Trading Bay Oil and Gas).

Seismic and log data

However, the company has progressed to good quality 3-D data — Clifford showed a seismic section from the company’s Nicolai Creek gas field. The section clearly depicts the fold structure of the field and also shows a gas chimney, in which gas is percolating upwards through the strata. The field is productive from five intervals in the Tyonek formation, Clifford said.

Clifford commented on the value of 3-D data in tracing the subsurface stratigraphy.

“The good thing about 3D is that you can actually do amplitude work — you can actually derive stratigraphic information from that data,” Clifford said.

For example, Aurora was able to map an underground Tyonek sand and show that it formed an ancient northwest trending channel. Transforming the seismic data to flatten the top of the Tyonek formation showed thickening of the sands to the east, probably as a result of faulting.

And by transforming offshore seismic data, geologic features such as potential gas sands that were previously invisible start to pop out of the seismic image.

Good quality seismic data in the area of Aurora onshore gas fields also depict what appear to be sand channels on the sides of the folds in the Tertiary strata.

“We see these thick packages that we think are channel sands deposited on the westerly vergent side of the fold,” Clifford said of an example fold between Aurora’s Kaloa and Moquawkie fields — the fold is vergent or “tips” to one side.

The use of modern well logging also pays dividends — as with the seismic, the old data tends to be very limiting.

“Often all we’ve got in Cook Inlet is sonic logs for porosity,” Clifford said.

But state-of-the-art logging techniques enable sophisticated analysis, such as determining the flow direction of the water that deposited the Tertiary sediments. And that all helps in tracing out the subsurface reservoir sands, especially the subtle gas sands. For example, a log interpretation at Nicolai Creek indicates a north-south channel in that reservoir.

Clifford stressed the importance of working out the sequence of folding and faulting in a particular area — geologists can piece together evidence about which of the structures formed first and that can provide insights into the formation of potential reservoir sands.

“Structural models are important to determine whether structures will be sand poor or sand rich,” Clifford said.

For example, some early work by ARCO at the Nicolai Creek field had shown a relationship between faulting and the deposition of sand, perhaps on the side of a fault that had dropped down.

Challenges

Seismic data acquisition remains difficult, Clifford said.

“Impediments to future success are the poor seismic imaging with the steep dips, in the deeper plays particularly,” he said. “… (There is) poor acoustic impedance contrast in the Hemlock formation specifically.”

And in some areas faults are not clearly defined in seismic section, perhaps because the sediments are soft and unconsolidated. Stratigraphic traps are particularly difficult to locate; and the thin, discontinuous sands that typify the Tertiary are difficult to image.

Land access is also an issue, with many prospective areas held inside existing units, Clifford said. Large areas of the Kenai Peninsula within the Kenai National Wildlife Refuge can’t be accessed for oil and gas exploration.

And offshore exploration will require a jack-up rig, which hopefully will be resolved soon by partners Escopeta Oil and Centurion Gold.

But Aurora has demonstrated exploration success by acquiring new seismic data, reprocessing old data, using a toolkit of seismic processes and using a suite of well logs. The seismic processing is expensive but pays dividends when it comes to developing the challenging Cook Inlet reservoirs, Clifford said.

“It’s not cheap … but we think it helps us down the road with saving development dollars,” he said.