Finding the pieces of a Susitna jigsaw

DGGS-led team carries out successful first field season, looking for natural gas potential in the Tertiary rocks of Susitna basin

Alan Bailey

Petroleum News

A team of scientists led by Alaska’s Division of Geological & Geophysical Surveys, or DGGS, has been moving ahead with an investigation of the hydrocarbon geology of Alaska’s Susitna basin, the geologic basin under the huge area of lowland lakes and muskeg of the Susitna Valley, north of Anchorage. With known coal seams and a geology which appears somewhat similar to that of the prolific Cook Inlet basin to the immediate southwest, geologists think that the Susitna basin may have potential for natural gas development, an enticing prospect given the proximity of Alaska Railbelt population centers and the tightening of Cook Inlet gas supplies.

In addition to DGGS geologists, the team includes scientists from Alaska’s Division of Oil and Gas, from the U.S. Geological Survey and from Purdue University. Funding comes from a couple of state grants authorized by the state Legislature.

10-day program

The vast majority of the gas produced from the neighboring Cook Inlet basin is what geologists refer to as “biogenic gas,” produced from the bacterial decomposition of organic material in rocks. This is distinct from “thermogenic gas” that forms when organic material is heated to appropriate temperatures. And in the Cook Inlet basin the biogenic gas has flowed into gas field reservoir rocks from coal seams, as forces within the Earth’s crust lift the coal seams up, lowering the pressure in the coal.

The fundamental question is whether the Cook Inlet biogenic gas model works in the Susitna basin, LePain said.

Cook Inlet comparisons

Among other things, the DGGS team wants to assess how closely the Susitna Tertiary rocks resemble and correlate with those in the Cook Inlet basin; to assess the ways in which the Susitna rocks were deposited; and to seek evidence for the history of how the basin formed and evolved, LePain explained. The Tertiary strata were deposited from ancient river systems, with sand-filled river channels having the potential to form gas reservoirs, and with areas of mud deposited from flooding between rivers possibly sealing gas in those reservoirs, where mudstones overlie abandoned river channel-fill deposits. Lush vegetation in swamps would have later transformed into coal seams that potentially source gas.

For the field program, the team split into two groups, one of which investigated the nature of the Tertiary strata, developing an understanding of the types of river system that operated in the basin and collating data about the Tertiary stratigraphy, LePain said. The other group examined exposures of Mesozoic rocks at the basin margins, looking for evidence that would shed light on the basin’s history, he said.

Isolated exposures

And the field geologists documented detailed measured stratigraphic sections along the rock exposures.

In general, the group investigating the nature of the Tertiary rocks confirmed the earlier finding of the USGS that the rocks of the Susitna basin bear striking resemblances to rocks of equivalent age in the Cook Inlet basin, LePain said. However, there are also some rocks that appear distinctly different. For example, one exposure on Lake Creek in the northwestern part of the basin exhibited conglomerates with massive granitic boulders lying directly over more typical sandstones and mudstones, similar to those of the Cook Inlet Tyonek formation, LePain said. The granitic material in the conglomerate does not match any rocks now observed nearby, and the nature of the rocks indicates the past existence of a geologic structure that would have created a steep surface gradient down which streams would have carried the boulders and other detritus.

To gain detailed insights into the ages of the various Tertiary rocks, the geologists collected plant fossils and rock samples containing plant pollen. A consultant firm in Anchorage is now evaluating the fossil and pollen samples.

Basin margins

The DGGS led team is now waiting for the results from the laboratory testing of the various rock samples collected during the field program. However, the team is also in the process of incorporating some subsurface data into its basin analysis, tying that data into its field findings. The subsurface data consists of publicly available data from the handful of wells that have been drilled in the basin, and data from a couple of Chevron seismic lines that the team has been able to use with permission.

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By getting relevant geological data into the public domain, the Alaska Division of Geological & Geophysical Surveys’ work helps spur exploration investment in basins such as this and provides government policy makers with useful information to aid in making land use decisions, LePain said.

DGGS Cook Inlet investigation continues

A stratigraphic trap is an underground hydrocarbon trap formed as a result of the way in which rocks have been deposited — a reservoir sand body in an ancient river channel sandwiched between mudstone layers, for example. All operational oil and gas fields in the Cook Inlet basin have reservoirs in Tertiary sands, but these reservoirs are in structural traps, formed by the folding and faulting of the strata.

This year the DGGS-led team did three weeks of fieldwork in July, particularly focusing on some excellent exposures of some of the older Tertiary stratigraphy between the Beluga and Capps glaciers on the west side of the inlet, DGGS geologist Dave LePain told Petroleum News Nov. 14. This wrapped up four or five years of fieldwork in that area, with a map of the area due to be published in about a year, he said.

Detailed mapping of rocks in surface outcrop provides insights into the geology of potential gas-bearing rocks in the subsurface. And DGGS, in conjunction with Alaska’s Division of Oil and Gas, is in the process of tying its surface observations into subsurface data from wells and a couple of seismic lines that the group has permission to use.

By then making its findings publicly available, DGGS can help encourage exploration of the Cook Inlet basin, particularly for companies that are new to the region, LePain said. In fact, during this summer’s field program DGGS organized a day-and-a-half trip during which geologists from Cook Inlet newcomer Apache Corp. visited some key Tertiary outcrops, he said.

As part of its investigation of Mesozoic reservoir rocks, the DGGS-led team has been examining Mesozoic rock exposures along the west side of the lower Cook Inlet, near Iliamna Volcano. One of the more exciting finds has been sand containing residual oil in upper Cretaceous strata at Saddle Mountain, a few miles east of the volcano. This demonstrates that there are source rocks generating oil at depth in the area, LePain said.

The Mesozoic of the Cook Inlet region has a reputation for poor reservoir quality because of the prevalence of volcanic material in potential reservoir rocks — the volcanic material tends to decompose into pore clogging minerals. However, there are rocks in the Cretaceous, particularly in the upper Cretaceous, which have the characteristics of effective reservoirs, LePain said. And even in the Jurassic, where volcanic debris tends to be particularly prevalent, it is possible to conceive of situations where reservoir quality could be preserved, he said.

The upper Cretaceous sand at Saddle Mountain has excellent reservoir properties and seems analogous to oil-bearing sands of similar age penetrated by ARCO’s lower Cook Inlet Raven well in 1980, LePain said. In fact the Raven well discovered an oil field in the upper Cretaceous but, because of the shallow depth of the reservoir, the oil could not be produced efficiently, he said.

USGS geochemist Paul Lillis has analyzed an oil sample from a Saddle Mountain exposure and found that its chemical composition matches that of oil recovered from the Raven well, LePain said. And the chemical “signature” of the Saddle Mountain oil indicates that it was sourced from the middle Jurassic Tuxedni group, the oil source for the oil fields in the upper Cook Inlet region. DGGS anticipates publishing a paper on this work in the next couple of months, LePain said.

Under the waters of the upper Cook Inlet, Mesozoic strata lie beneath a thick sequence of Tertiary strata — there is currently a race to earn a $25 million state tax credit by being the first company to test for Mesozoic reservoired oil in the upper inlet by drilling from a jack-up rig.

—Alan Bailey