Wednesday, May 30, 2012

Finding a good target; Statoil’s first well in Chukchi Sea will test the company’s Amundsen prospect

Alan Bailey
Petroleum News

Statoil’s first Chukchi Sea exploration well, planned for 2014, will likely target the Amundsen prospect, a large geologic structure that appears to contain rock horizons with oil and gas reservoir potential within what geologists refer to as the “rift sequence,” April Parsons, lead geologist for Statoil Exploration Alaska, told a meeting of the Alaska Geological Society on May 17.

Parsons said that the Amundsen prospect is located in some promising looking geologic structures that Statoil had found from seismic data prior to the 2008 Chukchi Sea lease sale in which the company had purchased its leases. The company has identified two oil prospects within its lease acreage — the Augustine and Amundsen prospects — with the Amundsen prospect, the larger of the two, becoming the primary exploration target.

Statoil operates 16 offshore leases about 100 miles northwest of the Chukchi Sea coastal village of Wainwright.

3-D seismic
During the 2010 Arctic open water season the company conducted a 3-D seismic survey across an area that encompasses its leases, to obtain a more detailed understanding of the subsurface geology and identify drilling targets. Statoil contracted with Fugro GeoServices to use a modern geophysical vessel, the Fugro Geo Celtic, to carry out the survey by towing 12 streamers of seismic recorders, Parsons said. With the 4,000-meter streamers spaced at 100-meter intervals behind the vessel but fanning out to a separation of 125 meters, it was possible to cover the planned survey area very efficiently, she said.

But with only five very widely spaced wells ever having been drilled in the entire Chukchi Sea, there is a severe shortage of geologic data that can be tied into images of the subsurface constructed from seismic data. So, with the closest of the Chukchi Sea wells, the Burger well, lying about 35 miles south of the Statoil leases, in addition to shooting the 3-D survey Statoil elected to shoot a single 2-D seismic line from the Burger well up to the 3-D area. The 2-D line enabled Statoil geoscientists to extrapolate the rock stratigraphy, as determined by well logs and rock samples from the Burger well, to the Statoil prospects.

Kuparuk sandstone
Of particular interest is the fact that a 110-foot sandstone interval in the Burger well demonstrated good reservoir characteristics and is known to contain a major pool of natural gas. The sandstone, equivalent in age to the oil-bearing Kuparuk sandstone in the Kuparuk River field in the central North Slope, lies underneath a regional geologic discontinuity called the Lower Cretaceous unconformity (an unconformity is a geologic feature created during a break in sediment deposition, when strata below the unconformity become eroded and then subsequently covered by younger rock formations).

The Statoil geoscientists were able to identify the Lower Cretaceous unconformity in a seismic cross section constructed from the 2-D seismic line. They then used the unconformity as a marker to trace the reservoir sand from Burger across into the Statoil prospects, Parsons said.

In fact, the detailed 3-D seismic over the Statoil prospects depicts a series of unconformities, thus confirming the fact the region has experienced numerous geologic upheavals during its long history. Particularly prominent, in addition to the Lower Cretaceous unconformity, are a regional unconformity in what is called the Brookian, the youngest and shallowest of the major rock sequences of northern Alaska; an unconformity that marks the boundary between the Tertiary and Cretaceous periods; and another regional unconformity of Jurassic age.

A subsurface image constructed from Statoil’s 3-D seismic data shows some especially spectacular geologic features on the unconformity between the Tertiary and the Cretaceous, with river features such as incised valleys clearly visible on what would have been a land surface about 65 million years ago.

Three-way closure
The seismic shows the Amundsen prospect to be what is called a three-way closure — an elongated dome sloping off in three directions and bounded by a major geologic fault. The detailed 3-D seismic has revealed extensive faulting in the structure.

Within the prospect Statoil wants to drill the complete rift sequence, a package of rocks between the lower Cretaceous unconformity and the older and deeper Jurassic unconformity. The rift sequence package across the Amundsen structure is quite thick and, although the Kuparuk-equivalent sand traced from the Burger well is Statoil’s main target, there is potential for finding oil and gas reservoir rocks at various levels within the sequence, Parsons said. Statoil is primarily seeking what are called structural traps, locations where the folding and faulting of the strata have created situations where oil could have accumulated.

The planned first well would penetrate a flank of the Amundsen structure rather than the structure’s crest, to enable Statoil to evaluate oil volumes using a single well, should the well encounter hydrocarbon resources, Parsons said. The well would need to reach a maximum depth of a little less than 10,000 feet to reach the Jurassic unconformity, she said.

Shublik source
Statoil anticipates the Shublik formation, of Triassic age and a prominent source rock in northern Alaska, to be the likely source of any oil and gas in its Chukchi Sea prospects — assessments of regional thermal maturity indicate that the Shublik would likely generate oil at an appropriate location to feed Statoil’s prospects. Statoil wants to find oil rather than natural gas in its Chukchi Sea exploration, Parsons said.

The Shublik lies in an older and deeper rock sequence known as the Ellesmerian, below the Jurassic unconformity. Strata within the Ellesmerian contain several of the major oil fields in the central North Slope, including the giant Prudhoe Bay field. However, although the geologic structures at the Amundsen prospect extend downwards into the Ellesmerian, Statoil does not plan to penetrate and test the Ellesmerian with its first Chukchi Sea well, Parsons said. After drilling that first well and after having gained a better understanding of factors such as the subsurface pressures, Statoil may drill some subsequent wells into deeper structures, she said.

Although at Burger a thick sequence of Cretaceous rock extends all the way from above the lower Cretaceous unconformity to the seafloor, in the Statoil leases there is a thick sequence of younger, Tertiary strata above the Cretaceous. The Brookian sequence above the lower Cretaceous unconformity in the leased area does not appear to contain any significant geologic structures and does not appear to be particularly prospective for oil and gas, Parsons said.

Site surveys
During the open water season of 2011 Statoil checked out some potential drilling sites in its leases by conducting shallow hazards surveys, primarily by gathering shallow, high resolution seismic data. The company also gathered shallow rock cores, to determine the nature of the seafloor at those sites. The seafloor in the area is really hard and quite level, with an almost constant sea depth of about 120 feet, Parsons said.

The most interesting feature on the seafloor is a series of crisscrossing ice scours. But, especially given the general absence of recent sediment on the seafloor, there is no good way of assigning ages to the scours, Parsons said.

However, the 2011 surveying did not find any potential hazards at any of the planned drill sites, she said.