DGGS Cook Inlet investigation continues

A team of scientists led by geologists from Alaska’s Division of Geological & Geophysical Surveys, or DGGS, has been continuing a multiyear research program in Alaska’s Cook Inlet basin. The object of the program is to assess the potential for finding new natural gas resources in what geologists call “stratigraphic traps” in the Tertiary rocks of the basin, and to assess the quality of potential oil reservoir rocks in the Mesozoic strata that lie beneath the Tertiary.

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