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Fractures as clues for oil potential
DGGS reports on patterns of rock fracturing in the Mesozoic of the lower Cook Inlet as a potential factor in oil and gas exploration Alan Bailey Petroleum News
Alaska’s Division of Geological and Geophysical Surveys has published a report on an investigation into the fracturing of Mesozoic rock in the lower Cook Inlet region, near the Iniskin Peninsula. The rock sequence contains known oil source rocks and potential oil reservoirs. However, because of a combination of rock compaction as a result of deep burial, and of the clogging of rock pores by the degradation of volcanic material into clays, there are questions over the quality of those possible reservoirs. But fractures are commonly observed in the rocks, and these fractures could act as both conduits for the subsurface flow of hydrocarbons, and as spaces within which hydrocarbons could accumulate.
Mesozoic potential The producing oil and gas fields of the upper Cook Inlet have reservoirs in rock strata of Tertiary age, younger and shallower than the Mesozoic strata. But the Mesozoic strata are thought to have significant petroleum potential - a better understanding of the fracturing of these rocks may shed light on potential exploration strategies.
In general the folding and faulting of rocks in the Cook Inlet basin has resulted from the interaction between plates of the Earth’s crust over an extended period of time through to recent history, in a tectonically active region. And the DGGS team discovered that the fracturing in the Mesozoic is consistent across all of the rock units in the stratigraphic sequence, with the fracturing appearing to relate to movement of geologic faults and the opening of large fractures rather than to the rock types.
Four fracture sets The team identified four distinct sets of fractures with different orientations along compass bearings of 310, 210, 250 and 360 degrees north. Fractures in each set occurred at broadly similar frequencies, although the 310-degree set showed a wider overall range of frequencies and the highest recorded frequencies. It appears that fractures in this set have the greatest potential to open or re-open when subject to modern geologic stresses and may, therefore, have the greatest influence over fluid migration in the Cook Inlet basin, the team concluded.
The team also investigated the potential to use the frequency of micro-fractures observed in microscope slides of the rocks to project the likely frequency of larger fractures in the rocks, thus enabling an assessment of subsurface fracture frequency that cannot be directly observed. There is a mathematical relationship between the frequency of micro-fractures of a given fracture set and the frequency of larger fractures in that same set.
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