Alaska Department of Administration

Alaska Oil and Gas Conservation Commission

AOGCC Pool Statistics

Pt Thomson, Thomson Oil Pool


Operator
EXXONMOBIL ALASKA PRODUCTION INC.
Discovery Well
EXXON CORP
PT THOMSON UNIT 1
Permit No.176-085
API No. 50-089-20005-00-00
Sec. 32, T10, R23E, UM
Depth: 13298' MD / 13298' TVD
December 8, 1977
Map of Pt Thomson, Thomson Oil Pool

Summary

The Point Thomson Undefined Oil Pool is located on the Arctic Slope of Alaska, about 60 miles east of Prudhoe Bay. The pool was discovered in 1977 by Exxon Corp's PTU No. 1 exploratory well that flowed 18.5°API gravity “pipeline-quality oil” at an average rate of about 2,300 barrels per day from the Thomson sand. The average gas-oil ratio reported was 5,800 cubic feet per barrel.1 The existence of hydrocarbons at Point Thomson was confirmed during 1978 and 1979 by the PTU No. 2 and No. 3 exploratory wells.2 Five tests of the Thomson sand conducted in four wells flowed between 170 and 2,283 barrels of oil per day and between 3.4 and 13.3 million cubic feet of gas per day, yielding reported gas-oil-ratios (GOR) from 3,890 to 22,705 standard cubic feet per stock tank barrel.3

Well tests conducted on the Thomson Sand reservoir in PTU-area wells yielded gas-oil ratio (GOR) values that range from about 850 to 17,000 standard cubic feet of gas per stock tank barrel of oil (scf/stb).4 According to ExxonMobil, the initial producing GOR for PTU gas expansion project wells is expected to be less than 20,000 scf/stb. Per Regulation 20 AAC 25.990(45), GOR values less than 100,000 scf/stb oblige the AOGCC to classify wells producing from the Thomson Sand as oil wells.5 Accordingly, the AOGCC classifies the hydrocarbon accumulation within the Thomson Sand reservoir as an oil pool.

During 2010, ExxonMobil drilled two new exploratory wells, PTU No. 15 and PTU No. 16, as part of a gas-cycling project6 designed to test the production potential of the reservoir.

On October 15, 2015, the AOGCC issued Conservation Order No. 719, which formally defines the Pt Thomson, Thomson Oil Pool as the accumulation of hydrocarbons underlying the Pt Thomson Affected Area that is common to, and correlates with, the interval between 16,126 and 16,377 feet MD on the VISION/Scope™ Measured Depth Log recorded in reference well PTU No. 15.7 The Pt Thomson, Thomson Oil Pool supersedes the Pt Thomson, Undefined Oil Pool. All activities, production, and injection conducted after September 2015 are recorded and reported under the name Pt Thomson, Thomson Oil Pool. On November 9, 2015, the AOGCC issued a corrected version of Conservation Order No. 719, which currently governs this oil pool.8

During March and April of 2015, ExxonMobil drilled PTU DW-1, a dedicated Class I disposal well for the Pt Thomson gas-cycling project. This disposal well is regulated by the US Environmental Protection Agency, Region 10. In the winter of 2015-2016, ExxonMobil drilled, hydraulically fractured, and completed development well PTU No. 17,9the fourth well in their planned, multi-year gas-cycling project. For this project, natural gas and natural gas-condensate produced from PTU 17 are routed to ExxonMobil’s initial production facilities, which are designed to process up to 10,000 barrels per day of gas-condensate liquids.10 Natural gas liquids extracted from the production stream are transported via the new, 22-mile long Pt Thomson pipeline to the Badami Pipeline, which ties into the Endicott Pipeline, and then on to the Trans-Alaska Pipeline.11 Natural gas recovered from the production stream is then re-injected into the Thomson Oil Pool through wells PTU 15 and PTU 16 to maintain reservoir pressure. The first regular natural gas condensate production from the Thomson Oil Pool occurred in late April, 2016. Production has since increased. For December 2016 and January 2017, the pool averaged 7,266 BOPD with a water cut of 1% and a gas-oil ratio of about 18,900 scf/stb.12

Production

Geology

The Thomson Oil Pool encompasses the early Cretaceous-aged (Neocomian)13 Thomson Sand, a conglomeratic sandstone that measures about 12 miles long, 5 miles wide, and ranges up to 330 - 350 feet thick in the Point Thomson area.14 It is stratigraphically equivalent to the Kemik Sandstone that is located approximately 30 miles to the south.15 The Thomson Sand consists of very fine- to very coarse-grained, dolomite-rich sandstone that contains argillaceous rock fragments. Dolomite-rich pebble-, cobble- and boulder-conglomerates are locally abundant.16 The abundance of coarse, dolomite clasts and close association with an erosional unconformity suggest a local source for the Thomson Sand.17

The sediments that comprise the Thomson Sand reservoir were derived from basement rocks that were exposed in the northern and northeastern portions of the Point Thomson Field and bordered to the southwest by a sea. Eroded sediments were transported down-gradient toward the southwest and progressively deposited in alluvial fan, fan-delta, and marine shoreface environments. Wave and current activity extensively reworked these sediments and distributed them in southeast-trending bands arranged subparallel to the shoreline. From northeast (proximal) to southwest (distal), these bands generally consist of alluvial fan breccia, conglomerate, conglomerate with minor sandstone, sandstone, silty sandstone, and siltstone. In general, coarser-grained, proximal lithologies are dominated by carbonate clasts, with quartz and ductile grains becoming increasingly prominent in the more distal areas that lie to the southwest.18

ExxonMobil informally divides the Thomson Sand into an upper member and a lower member based on core descriptions and well log correlations. The lower member is dominantly progradational, whereas the upper member is dominantly retrogradational.19

The Thomson Sand is unconformably overlain by siltstone, mudstone, and shale assigned to the Canning Formation, Hue Shale, and HRZ, in descending stratigraphic order. Erosion thinned the Hue and HRZ shale intervals toward the northeast, and completely removed these intervals from the northern and eastern portions of the PTU.

The structure of the proposed Thomson Oil Pool is a gently dipping, four-way anticlinal closure. Based on well and 3D-seismic control, the top of the pool lies about -12,500 feet TVDSS, and the structure extends to a depth of about -14,500 feet TVDSS within the PTU area. The Thomson anticlinal closure is cut by several, north- and north-northeast-trending, normal faults. The vertical displacement of faults observed within the Thomson Sand interval averages about 65 to 95 feet, with a maximum of about 200’, but none of these faults appear to completely displace the Thomson Sand or create isolated compartments within it. None of the faults are expected to act as flow barriers.

Well log and seismic information indicate that hydrocarbon distribution within the proposed Thomson Oil Pool is influenced by both structural and stratigraphic elements. The broad, east-southeast-trending anticlinal closure provides primary control for the accumulation. Internal facies changes within the Thomson Sand interval strongly influence reservoir quality and the distribution of hydrocarbons, especially in the southern and western portions of the PTU.

The Thomson Sand is overlain by a thick, laterally extensive section of siltstone, mudstone, and shale assigned, in descending stratigraphic order, to the Canning Formation, Hue Shale, and HRZ Shale. These intervals provide the top seal for the proposed Thomson Oil Pool. In the northern and northeastern parts of the PTU, where the Hue and HRZ intervals are either absent or are very thin, mudstone and siltstone assigned to the lower Canning Formation provides a top seal.20

The Thomson Sand is underlain predominantly by thick Pre-Mississippian-aged dolomite, phyllite, and quartzite basement rocks.21 Fractured and/or karsted dolomite appears restricted to the northern part of the field, and this rock may serve as a secondary reservoir in communication with the Thomson Sand.22,23

Within the Point Thomson Field, the accumulation within the Thomson Sand comprises a nearly 500-foot thick, high-pressure, condensate-gas “cap” (gas cap) and an underlying, 37-foot thick rim of viscous oil. Relict oil saturation exists within the gas cap due to multiple oil migration events. Relict oil saturation increases downward, toward the gas-oil contact. Average oil saturation within the gas cap is approximately 10%. Condensate yield for the reservoir is estimated to be about 60 to 65 stock tank barrels (stb) per 1 million standard cubic feet of gas (MMSCF). Flow tests and reservoir pressure measurements indicate that the Thomson Oil Pool is not separated into isolated compartments within the Affected Area.24

The oil rim consists of 10° to 18° API gravity oil that has a viscosity of about two centipoise at reservoir conditions. The lower portion of the oil rim consists of an oil-water transition zone, where both oil and water are partially mobile.25

The Thomson Oil Pool is abnormally geo-pressured: average reservoir pressure is about 10,100 psi at ExxonMobil’s specified pressure datum of -12,700 feet TVDSS (a pore-pressure gradient of about 0.795 psi/ft). Abnormal fluid pressure gradients as high as 0.84 psi per foot, have been reported for the basement rocks, Thomson Sand, pebble shale, Hue Shale, and lower Canning Formation (in ascending order).26,27

Reservoir temperature ranges from about 220° to 230° F.28

Other potential secondary reservoirs in the Point Thomson area are marine sandstones29 of the Cretaceous- to Oligocene-aged Canning Formation, which have produced oil and gas during well tests.30


Last Revised
6 Mar 2017 sfdavies

1Exxon Company, USA, 1977, Exxon Announces Test Results on Wildcat Well in AK; Press Release, Nov 1, 1977; PI, 1977, Exxon's Pt. Thomson Wildcat Flows 2,300 BOPD, 13 MMCFD on Test; PI AK Report, V. 23, No. 44, Nov 2, 1977; AOGCC File 176-085, p. 146
2Anchorage Times, 1978, Exxon Unhappy with Test Results; Anchorage Times, August 14, 1978; Alaska Oil and Gas Conservation Commission, Well History File 178-115, p. 120
3Bird, K.J. and others, 1987, Petroleum Reservoir Rocks; in Bird, K.J. & Magoon, L.B., eds, 1987, Petroleum Geology of the Northern Part of the Arctic National Wildlife Refuge, Northeastern Alaska; USGS Bull 1778, p. 79-100
4AOGCC, 1984, Statistical Report, p. 103; PTU 15 Completion Report, in AOGCC File 209-014, p. 17; Fluid Analysis on Bottomhole Samples, Well: PTU 15, p. 12, in AOGCC File 209-014, p. 1225; PTU 16 Completion Report in AOGCC File 209-015, p. 7.
5Regulation 20 AAC 25.990(45): "oil well" means a well that produces predominantly oil at a gas-oil ratio of 100,000 scf/stb or lower, unless on a pool-by-pool basis the AOGCC establishes another ratio.
6Loy, W., 2010, First Point Thomson Well Reaches Target; Petroleum News, Vol. 15, No. 7, Feb 14, 2010
7Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 11
8Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected
9Alaska Oil and Gas Conservation Commission, 2016, Well Information Database
10ExxonMobil, 2016, About Point Thomson, ExxonMobil Corporate website, accessed April 25, 2016
11Nelson, K., 2015, PTE Pipeline, Exxon Apply for Connection, Petroleum News, V. 20, No. 33, August 16, 2015
12 World Oil, 2016, Exxon Mobil Starts Production at Point Thomson on Alaska’s North Slope, World Oil website at www.worldoil.com, accessed April 25, 2016
13Bird, K.J. & Molenaar, C.M., 1987, in Bird, & Magoon, eds., 1987, USGS Bull 1778, p. 48-51; Gautier, D.L., 1987, in Bird, K.J. & Magoon, L.B., eds., 1987, USGS Bull 1778, p. 118
14Bird, K.J. and others, 1987, Petroleum Reservoir Rocks; in Bird, K.J. & Magoon, L.B., eds, 1987, Petroleum Geology of the Northern Part of the Arctic National Wildlife Refuge, Northeastern Alaska; USGS Bull 1778, p. 86
15Bird, K.J. and others, 1987, Petroleum Reservoir Rocks; in Bird, K.J. & Magoon, L.B., eds, 1987, Petroleum Geology of the Northern Part of the Arctic National Wildlife Refuge, Northeastern Alaska; USGS Bull 1778, p. 86
16Gautier, D.L., 1987, cited above, p. 118
17Gautier, D.L., 1987, cited above, p. 119
18Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 4, 5
19Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 5
20Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 5
21Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 5
22Bird, K.J. and Molenaar, C.M., 1987, Stratigraphy; in Bird, K.J. and Magoon, L.B., eds., 1987, Petroleum Geology of the Northern Part of the Arctic National Wildlife Refuge, Northeastern Alaska; USGS Bull 1778, p. 37
23Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 4
24Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 5, 6
25Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 6
26Bird, K.J. and others, 1987, cited above, p. 80, 89; Gauthier, D.L., Bird, K.J., and Colten-Bradley, V.A., 1987, in Bird, K.J. & Magoon, L.B., eds, 1987, USGS Bull 1778, Table 13.1, p.202
27Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected, p. 7
28Alaska Oil and Gas Conservation Commission, 2015, Conservation Order No. 719-Corrected. p. 7
29Gautier, D.L., 1987, cited above, p. 120
30Bird, K.J. and others, 1987, Petroleum Reservoir Rocks; in Bird, K.J. & Magoon, L.B., eds, 1987, Petroleum Geology of the Northern Part of the Arctic National Wildlife Refuge, Northeastern Alaska; USGS Bull 1778, Table 7.3, p. 85