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Exploring the upper Jurassic There’s plenty of oil in these North Slope sands but the challenge is to hit just the right combination of oil quality and reservoir characteristics Alan Bailey Petroleum News Contributing Writer
The upper Jurassic sands of Alaska’s North Slope pose some tantalizing questions for oil exploration — people know that the sands contain lots of oil but you need to hit just the right combination of oil quality and reservoir characteristics to find an oil accumulation that’s commercially viable. The 1990s discovery of the North Slope’s Alpine field by ARCO Alaska proved that these intriguing rocks can carry a valuable payload.
The Alpine sandstone that forms the reservoir for the Alpine field, the major field in the ConocoPhillips-operated Colville River unit, is the youngest of three major upper Jurassic sandstone rock bodies on the North Slope.
The ways in which these sandstones formed help explorers understand their viability as oil reservoirs.
A shallow ocean During the upper Jurassic period, at a time when dinosaurs ruled the earth, the geography of the North Slope region was very different from today. Sand and other detritus from a long since disappeared landmass to the north or northwest flowed out into a shallow sea. The seabed sloped gently south from the shoreline. A so-called rifting event, in which the earth’s crust extended and cracked apart, was just starting. As a result of this upheaval, some sections of the crust dropped between flanking fault lines.
“The ocean shelf at that time was very flat, with low relief, so that there was very little space to deposit sediments,” Mark Myers, director of the Alaska Division of Oil and Gas, told Petroleum News.
But periods of extension of the crust created localized holes or spaces that sand and other sediments could fill, Myers said. In addition, changes in sea level resulted in the erosion of valleys that could accommodate more sediment. Periods of higher sea level might also give rise to more extensive areas of sand.
“You have localized areas where you may get much thicker accumulations. Then you have much broader spread areas where, if you get enough space, you’ll preserve the whole sand package,” Myers said.
Many of these accumulations remain today beneath the earth’s surface, both as isolated sandstone pockets and as more widely spread sandstone layers.
Very sophisticated toolbox needed Anchorage-based petroleum geologist Doug Hastings, considered by Myers to be one of the most knowledgeable geologists on the subject of the North Slope’s upper Jurassic sands, told Petroleum News that locating the sands requires high-quality seismic data.
“A key to all that is getting good three-dimensional seismic data,” he said.
Hastings, who currently consults with Petrotechnical Resources of Alaska but spent 22 years in Alaska with ARCO Alaska and Phillips Alaska, said “Alpine is up to 100 feet thick, but there’s a lot of (the sands) 50 feet thick and that’s below (seismic) tuning.” There are quite a few wells in the region, so that the seismic can be tied back to well data, Myers said. However, with oil traps resulting from the rock stratigraphy rather than folding or faulting of the rocks, tracing the sands can prove challenging.
“It takes a very sophisticated toolbox to try to determine the individual sands,” Myers said. Poor reservoir rocks During the upper Jurassic, below the surface of the shallow sea, wave action and a multitude of burrowing critters churned the sediments and mixed fine quartz sand with muddy silt. As a consequence, many of the sands make rather poor oil reservoirs — the silt and mud reduce both the porosity (the ability of the rock to store oil) and the permeability (ease with which oil flows through the rock).
“They weren’t particularly great sands in the first place and then (the animals) churned them until there’s virtually no permeability,” Hastings said.
However, the sands tend to become coarser towards their top surfaces and in proximity to the ancient shoreline. These coarser sands exhibit better reservoir characteristics than the finer sediments. So an understanding of the geological setting provides a key to locating the presence of a usable reservoir. Because the ancient shoreline lay in the north of the region, geologists expect to find the best upper Jurassic oil reservoirs towards the north of the western North Slope.
“If you understand where the old shorelines were ... then you understand the broader-based ... sands, then if you can go to the next stage and understand where some of these (valley) incisions or fault relief areas are ... you might pick up even more sand,” Myers said.
To the south, towards the Brooks Range foothills, the rocks should become finer grained because the sands that formed them had carried some distance from the shoreline. The rocks to the south should also suffer from compression and compaction by an increasing depth of younger sediments above them. The joint “strikes” of finer grain and stronger compaction may limit the ability of the rocks to hold economic oil deposits. Two strikes you’re out? “It may be that two strikes and you’re out,” Hastings said, “but we don’t know because we haven’t shot the data.”
However, Myers said that there are indications of Jurassic sands to the south.
“The overall extent of the (sea) shelf was very broad, so subtle changes in sea level can make a big difference (to the distribution of the sands),” he said.
Although geologists expect better quality sands to the north, exploration in the north runs into another problem — upheavals in the earth’s crust during ancient times resulted in erosion of the sandstone bodies. This erosion becomes progressively worse the further north you go, until the rocks disappear completely.
“You play a game between having the best quality sands and them actually being preserved,” Myers said. Abundant oil Despite the poor reservoir quality of the sandstones they often contain oil.
“Some of these Jurassic sands have very large volumes of in-place oil,” Myers said. “There are stratigraphic traps in most every place you look — the sands are all charged in the public wells.”
The oil has probably come from a variety of source rocks through different migration paths. So, the composition of the oil at a particular location will depend on several factors.
“You’ve got to know the timing issues, you’ve got to look at your structural plays, you’ve got to know the relation to source rocks,” Myers said.
Hastings said that the Jurassic sands themselves have provided a source of oil.
“There’s not a single, say 50 foot, highly concentrated interval like the Shublik or like the HRZ (formation), “ Hastings said, “... (but) as much as 2,500 feet of rock can generate a lot of oil just through the sheer volume of rock, not necessarily through the concentration of organics.”
But, although there’s a high probability of finding oil in the sands, the oil has generally proved too heavy for economic production, given the poor reservoir characteristics of the rocks. For example, Texaco and Amerada Hess drilled in the northern Colville River Delta area in the mid-1980s and encountered oil in the middle of the three upper Jurassic sands. However, the flow rates proved too low for viable field development — a viable field requires either lighter oil or a better reservoir.
Alpine and where else? With the Alpine field, however, ConocoPhillips struck a combination of both a good reservoir rock and a relatively light oil. For some reason an oil migration path from a suitable source rock coincided with an unusually good sandstone reservoir — the Alpine reservoir rock has a higher permeability than is normally found in the upper Jurassic sands, Hastings said.
Could these conditions occur elsewhere? The Alpine field and a couple of satellites in the National Petroleum Reserve-Alaska are the only upper Jurassic fields currently known in the area, Myers said.
Hastings said that people still don’t fully understand why the Alpine field is where it is.
“The key questions for all explorationists are ‘Why is that sand better? What’s the depositional environment?’“ Hastings said.
However, the potential for other productive Alpine deposits, especially to the east of the Alpine field, intrigues Hastings. The small extent of some of the pockets of Alpine sand means that another deposit could exist between some of the wells that have been drilled in the area.
“There’s a lot of well information, but the real question is ‘Can you have hidden Alpine sands between ... the wells?’“ Hastings said.
And could some of the other upper Jurassic sands contain areas with a favorable combination of reservoir characteristics and oil quality?
“I think Alpine’s shown just how prolific these sands can be and how well they can produce,” Myers said.
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