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Pinpointing the traps Three dimensional seismic and geologic modeling in Northeast NPR-A have proved key factors in locating Jurassic oil and gas plays west of Alpine Alan Bailey Petroleum News
When exploring for oil and gas, assumptions often turn out to be way off the mark. And it may take only one new find to turn conventional thinking on its head.
Take, for example, the National Petroleum Reserve-Alaska and the Jurassic rocks of that area.
ConocoPhillips geologist Greg Wilson described to a Geological Society of Alaska audience on Jan. 5 how several years ago some people had written off NPR-A as an area for oil and gas exploration — more than 100 wells had been drilled in the area without a major find. And despite the discovery of a significant North Slope oil field in almost every part of the geologic column, explorers had drawn a complete blank in the Jurassic of the region.
Indeed, from what was known of the Jurassic strata, geologists saw little point in searching for Jurassic plays.
“Geologists wouldn’t have recommended the Jurassic as a target,” Wilson said. “… The Jurassic was a big pile of shale out there.” The discovery of Alpine Then came the discovery of the 429 million barrel Alpine field in 1994, when ARCO (now ConocoPhillips) was drilling in the Jurassic beneath the Colville Delta, just east of NPR-A.
The Alpine reservoir is in a stratigraphic trap in Jurassic sandstones that straddle the upper Jurassic unconformity, Wilson explained. The upper sand, known as Alpine C, is fine-grained, with a porosity of 20 percent and a 15 millidarcy permeability. The lower sand, known as Alpine A, is very fine grained and not quite as clean. Although these fine-grained sandstones don’t make particularly good reservoirs, the light 40 degrees API oil in the field flows easily. And following first production in November 2000 the Alpine field continues to exceed expectations, Wilson said.
“We believe that the Alpine oils are Jurassic in source,” Wilson said.
The Alpine discovery quickly triggered interest both in Jurassic exploration plays and in exploring the nearby NPR-A.
So, in 1999 the Bureau of Land Management held a lease sale in the northeast planning area of NPR-A. That was followed by another sale in 2002 and a sale in the northwest planning area in 2004. ConocoPhillips, partnering with Anadarko and Pioneer Natural Resources, bought interests in a large number of leases in these sales.
The lease purchases have led to a series of exploration wells across a swathe of territory, southwest from Alpine and out into NPR-A: Clover A, Spark 1 and Rendezvous A in 2000; Lookout 1 and Moose Tooth C in 2001; and Carbon 1 in 2004. Most of these wells encountered significant hydrocarbon accumulations in Jurassic analogues to the Alpine field. Subtle traps Locating these NPR-A discoveries has required state-of-the-art technology.
Using information from wells and seismic data at the Alpine field, ConocoPhillips was able to trace the Jurassic stratigraphy into seismic images of the subsurface in NPR-A. But pinpointing slivers of productive sand in thousands of feet of strata presents some formidable difficulties.
“The challenge out here was that we were looking at very subtle stratigraphic traps,” Wilson said.
Three dimensional seismic proved to be a key technology. This type of seismic involves combining data from a grid of seismic lines, rather from just single lines as in a 2-D survey. The greater resolution of subsurface detail revealed in a 3-D survey, as compared with a 2-D survey, enables small stratigraphic features to be identified in the subsurface.
But even with high-quality seismic data, drilling can yield surprises. For example, an exploration well in one part of northeast NPR-A encountered what appeared to be 1,000 feet of the gamma ray zone, a distinctive North Slope horizon that is typically 100 to 200 feet thick. Geologists then realized that the well had encountered what people refer to as the “Fish Creek Slumps”, an area west of the Colville Delta in which some of the rock strata have become broken and jumbled. The gamma ray zone had become tipped on end, thus creating the illusion of a thicker rock horizon than actually existed.
One important tool in interpreting the seismic data and locating Jurassic oil and gas prospects is a depth structure map, Wilson explained. By using a prominent marker horizon known as the Nuiqsut as a datum level it is possible to reconstruct a map showing the depth of the erosion surface associated with the upper Jurassic unconformity. The Jurassic reservoir sands lie on that erosion surface. Ancient incisions The map reveals a series of undulations in the upper Jurassic landforms, with deep incisions where ancient rivers had eroded into the land surface. Those incisions form complex patterns towards the Colville Delta but open into a simpler pattern of broad ridges and crests towards the west in NPR-A.
This detailed mapping from 3-D seismic has changed geologists’ understanding of the subsurface structures. For example, Husky Oil drilled the North Inigok well in 1981 into a broad ridge in the Jurassic in NPR-A, to the west-southwest of the recent area of exploration activity. In 1981, using 2-D seismic data, geologists interpreted the Jurassic ridge as an offshore bar in the ancient sea, Wilson said.
“What we know now, and with the 3D (seismic) … this is an erosional remnant south of a deep incision,” he said.
And that’s a critical point when it comes to identifying oil and gas prospects — the ancient incisions formed spaces that accommodated sand accumulations that later formed stratigraphic traps.
“(Locating traps) requires detailed interpretation of erosional surfaces,” Wilson said. “… (There is) a complex set of stratigraphic traps in the area, but a very effective set of stratigraphic traps.”
Wilson showed a cross-section of northeastern NPR-A, with wells such as Lookout A penetrating sand in the cores of the deep upper Jurassic incisions.
And a well core revealed marine reservoir sandstone containing the mineral glauconite above the sharply defined Jurassic erosion surface. Pebbles and organism burrows on that surface may indicate subaerial erosion, while siltstones and shales below the surface indicate an earlier marine environment. New opportunities So, the discovery of Alpine truly opened some new opportunities that have resulted in some significant discoveries.
Wilson said that the discovery at Spark tested 1,550 barrels of oil per day and 26 million cubic feet of gas per day; Rendezvous A tested 360 barrels of oil per day and 6.6 million cubic feet of gas per day and Lookout 2 (the neighboring well to Lookout 1) tested 4,000 barrels of oil per day and 8 million cubic feet of gas per day; and Carbon 1 tested 1,250 barrels of condensate per day and 24 million cubic feet of gas per day. The low numbers for Rendezvous reflected the fact the testing was not stimulated, he said.
ConocoPhillips has not released reserves data for any of these discoveries. It’s clear, however, that the Jurassic of NPR-A has considerably more potential than people had thought not too many years ago.
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