Antarctica as an Exploration Frontier—Hydrocarbon Potential, Geology, and Hazards

Edited by Bill St. John


The 10 papers contained in this publication are oriented toward the hydrocarbon potential of Antarctica. Contents include regional seismic surveys involving tectonic and stratigraphic interpretations extending from the Adelie Coast margin, over the Ross Sea and Bellingshausen Sea, throughthe Bransfield Straight and along the northern Antarctic Peninsula. Mesozoic sedimentary basins are compared in detail, and a tectonic synthesis of Antarctica and the surrounding southern seas is presented.

  1. Page 1

    During the 1987 United States Antarctic Program-Polar Duke Cruise, 3200 kilometers of seismic reflection profiles were collected on the northern Antarctic Peninsula shelf. These data, plus the results of land-based studies (from Polish and U.S. scientists) and ocean drilling (DSDP Leg 35) were used to reconstruct the tectonic and climatic development of the shelf and to assess possible hydrocarbon prospects of the region.

    The study area consists of a foredeepened shelf, typical of the Antarctic continental shelf. The continental margin has evolved from an active margin to a tectonically passive one as the Aluk Ridge was gradually subducted at the Antarctic Plate Boundary. This transition was diachronous, as the timing of ridge subduction proceeded from south to north (oldest to youngest). Thus, the shelf is segmented both tectonically and sedimentologically as the extent of tectonic deformation and post-tectonic sedimentation varies correspondingly.

    Besides the obvious tectonic controls, major changes in shelf sedimentation also took place due to climatic changes during the Ceno- zoic. Antarctica is unique in that as its climate cooled and ice sheets formed, the source of terrigenous organic carbon to the shelf was completely eliminated. Also, streams and rivers were gradually eliminated (by early Neogene) so that running water was no longer contributing to the transport of terrigenous sediments to the shelf. The shelf was later overdeepened by glacial erosion (middle Miocene?). By the late Miocene there was an intensification of oceanic circulation and an increase in the flux of organic-rich, siliceous biogenic sediments to the continental margin.

    Seismic records show four sequences reflecting different episodes of shelf development. Sequence 4, the oldest sequence, consists of folded and faulted pretectonic and syntectonic (subduction) deposits, presumably volcaniclastic material deposited as subduction occurred. Sequence 3 is an accretionary sequence unconformably overlying S4 and reflecting efficient sediment transport across the shelf after subduction ceased. Sequence 2 rests unconformably on Sequence 3 and is interpreted as glacial deposits that are bounded by erosional surfaces. This sequence marks the onset of glaciation sufficient to overdeepen the shelf. Sequence 1 is believed to consist of glacial-marine deposits.

    The hydrocarbon potential for that portion of the continental shelf situated north of the Tula Fracture Zone is low, but is slightly higher for that portion of the shelf situated south of the Tula Fracture Zone. This is because the age and thickness of sedimentary deposits increases to the south, and the time window for formation and burial of suitable source and reservoir rocks increases in that direction. Sequence 4 is believed to include carbonaceous marine shales deposited when the climate was temperate and is considered to be the most likely hydrocarbon source, at least south of the Anvers Fracture Zone. The siliciclastic deposits of Sequence 3 are the most probable reservoir rocks.

  2. Page 13

    Application of sequence stratigraphic concepts to seismic reflection profiles from the Bransfield Basin indicates that this modern backarc basin began to form during the waning stages of subduction at the South Shetland Trench at about 4 Ma. Two distinct systems tracts stack to form depositional sequences; organic-rich hemipelagic sediments drape the basin during highstands/interglacial periods, whereas large volumes of glacially eroded terrigenous sediments prograde into the basin during lowstands/ glacial maxima. Although the juxtaposition of organic- rich diatomaceous muds with the high heat flow of the backarc spreading system is favorable for the generation of hydrocarbons, reservoir quality sands and suitable traps have yet to be identified.

  3. Page 31

    Antarctica's continental shelf averages 500 m in depth and exhibits a landward slope, due to the combined effects of isostatic loading and glacial erosion. These effects are more pronounced near the continent. The highly rugged topography of the shelf typifies high latitude continental shelves.

    Antarctica is the coldest, driest, windiest place on earth and the extremely hostile climate represents a formidable obstacle to the exploration for hydrocarbons. Sea ice covers the entire continental shelf during most of the year and presents another serious threat to the explorationist. The distribution and movement of sea ice on the continental shelf are hard to predict and have historically been responsible for the demise of several research vessels. Even less predictable is iceberg movement. Individual icebergs within the same area may drift at different speeds and in different directions because their size and draft determines to what extent winds and currents affect them. Drift speeds up to 3 km/hr and drafts exceeding 400 m have been reported.

    Rugged topography and interstratification of stiff glacial deposits with water-saturated glacial-marine deposits combine to make the sea floor of the Antarctic continental shelf and slope unstable. Evidence for this exists in the form of abundant sediment gravity flow deposits on the shelf and slope. To date, shallow gas has been observed only in the Bransfield Basin. Significant earthquake activity is virtually nonexistent.

  4. Page 47

    The Ross Sea contains three major depocenters, each underlain by a sediment-filled rift graben and an overlying glacial sedimentary sequence. The sedimentary sections are up to 14 km thick with up to 8 km in the rift grabens and up to 6 km in the presumed-glacial sequences. The rift grabens were downfaulted and filled probably during the late Mesozoic to early Cenozoic continental breakup of Gondwana; their early history may be analogous to coeval rift basins of southeast Australia, Tasmania, and New Zealand. The rift-graben sediments are unconformably overlain by glacial-marine sequences deposited since middle Eocene(?) to early Oligocene time. Renewed down-faulting has occurred along the west and east margins of the Ross Sea probably since Eocene time.

    The hydrocarbon potential of the Ross Sea is poorly known because only post-Eocene glacial rocks have been sampled offshore. The age and type of rocks filling the rift grabens, below the glacial sequence, is unknown. Source beds do not occur in the glacial sequence, but may exist in the rift grabens. Structural and stratigraphic traps are likely near basement structures and unconformities, which are common, and near large sedimentary structures found only in the Victoria Land Basin and along margins of the Eastern Basin. Reservoir rocks are unknown but sands could occur throughout the glacial and rift sequences. Lopatin-Waples models indicate that hydrocarbons could be generated presently at depths of 2.5 to 4.0 km, if source beds exist. Migration is likely in dipping strata along rift-graben flanks and in late-rift fault zones of the Terror Rift.

    Hydrocarbon seeps and accumulations are unknown in the Ross Sea. The preglacial strata that are deeply buried within the early- rift grabens have the best hydrocarbon potential; however, a definitive assessment awaits sampling of these deep,rift deposits.

  5. Page 69

    More than 100,000 km of marine multichannel seismic profiles have been acquired over the continental margin of Antarctica since 1976 by scientific research programs of Australia, Brazil, France, Italy, Japan, Norway, Poland, United Kingdom, United States, U.S.S.R. and West Germany. Although scientific results are reported for most of these data, they also are relevant to petroleum resource assessment. Because of the one or two orders of magnitude greater cost of standard land survey techniques in Antarctica compared with marine techniques in areas of open water, there will likely be no great amount of coverage on the interior of the Antarctic ice sheet. Despite this, several countries are experimenting in a research mode using land systems, and deep crustal reflection surveys at carefully selected interior sites will probably be made soon.

  6. Page 77

    On the Adelie Coast continental margin, a multichannel seismic survey has revealed the presence of a thick sedimentary basin, beneath the outer continental shelf and upper slope, that may exceed 6000 m. This basin results from the creation and evolution of a continental margin, initiated about 100 million years ago from the separation of Australia and Antarctica. Beneath the outer shelf, which is 400-500 m deep, the sedimentary series consists of four units separated by three major unconformities:

    -A prerift unit including a Precambrian basement, possible Paleozoic and early Mesozoic sediments and a Mesozoic synrift sequence;

    -An early postrift unit, ranging in age from Cenomanian to Eocene, assumed to consist mainly of fluviatile to deltaic clastics;

    -An Upper Eocene to Oligocene unit in a shallow marine environment;

    -A Neogene glacial prograding unit.

    The early postrift unit is considered to be a promising petroleum target based on comparison to other passive margins.

  7. Page 89

    We present a new tectonic fabric map of the Southern Ocean south of 45°S derived from Geosat altimeter profiles and published bathymetric charts and magnetic anomaly picks. The interpretation of the Geosat data is based on an analysis of the first derivative of the geoid profiles (i.e., vertical deflection profiles). To improve the accuracy and resolution of the vertical deflection profiles, 22 repeat cycles from the first year of the Geosat/Exact Repeat Mission (Geosat/ERM) were averaged. At wavelengths less than about 200 km, the vertical deflection is highly correlated with sea-floor topography and thus reveals major features in areas that were previously unsurveyed. The density of the Geosat data is greatest in the high latitudes where lineated bathymetric features such as fracture zones, spreading ridges, trenches, and rifted margins stand out. To construct the tectonic fabric chart, the Geosat data are analyzed in combination with available shipboard bathymetric data and magnetic anomaly identifications.

  8. Page 101

    The Antarctic Peninsula is a relatively accessible area of the continent, a fact which has stimulated interest in its hydrocarbon potential. This chapter uses known stratigraphic information to provide general constraints on the hydrocarbon potential of the Mesozoic basins of the Antarctic Peninsula.

    The peninsula lies on a medium-sized block of continental crust. It is one of a mosaic of crustal blocks forming West Antarctica which underwent a complex tectonic evolution during Gondwana breakup. It was the site of an active volcanic arc above easterly subducting proto-Pacific ocean floor throughout the Mesozoic and part of the Cenozoic.

    As a result the exposed Mesozoic basins display a complex stratigraphy, reflecting local tectonic and volcanic events. No units can be correlated between any two basins, but there are a few general trends. Almost all basins are post-Oxfordian; their fill is entirely clastic, and largely derived from the Antarctic Peninsula volcanic arc. Most basins were affected by a period of arc expansion in Late Jurassic or Early Cretaceous times, which manifests itself as inputs of lava or coarse volcaniclastic sediment. Berriasian and older mudstones are generally finer-grained and darker than mudstones from post-Berriasian strata. Deformation is variable, but rarely penetrative.

    This stratigraphic information provides the basis for general constraints on the hydrocarbon potential. Organic geochemistry shows that Berriasian and older mudstones from the backarc region are the best potential oil source rocks; all other mudstones tend to be lean and gas-prone. Reservoir and seal facies tend to be better in deep marine (generally older) strata. Reservoir quality is generally poor due to breakdown of labile volcanic grains, but younger sandstones tend to be more quartz-rich.

    The only significant prospective basin is the Larsen Basin, east of the Antarctic Peninsula. Geological evidence suggests that the best plays would involve deep strata. The major problem in this basin is the very difficult access, even by Antarctic standards.

  9. Page 127

    Marine geophysical investigations off the northern tip of the Antarctic Peninsula by the Brazilian Antarctic Program in 1987 and 1988 have revealed the complex geologic evolution and structure of the Bransfield Basin, and the Bellingshausen continental margin. The Bransfield Basin, within the Bransfield Strait, has an asymmetrical profile with a steeper slope along its northern margin and a conspicuous spreading center closer to the South Shetland Islands. A sedimentary wedge deposited along the southern margin of the basin forms the northern continental margin of the Antarctic Peninsula. Structural features and sedimentary sequences in this wedge show an Atlantic-type margin setting with an older rift sequence and a younger drift sequence.

    The Bellingshausen continental margin shows a well-developed continental rise, including a deep-sea fan to the north of Adelaide Island, a steep continental slope and a broad continental shelf. At the outer shelf, clinoforms indicate a prograding shelf to slope environment similar to that of the continental shelf of an Atlantic- type margin. These sediments have prograded above an erosional unconformity, below which tilted and faulted layers are observed and appear to represent an earlier "active" margin setting. A basement high occurs at the eastern limit of the younger passive margin sedimentary wedge, and a closed and buried basin has been discovered to the east of the basement high. The basement high and the closed basin could represent an eroded island arc and a fossil backarc basin, respectively.

  10. Page 143

    CIROS-1, the most recent scientific drill hole in Victoria Land Basin, Antarctica, cored to 702 m below sea floor (mbsf) near the basin's western margin. The cored sequence is largely marine sandy mudstone and diamictite, with lesser sandstone and conglomerate, and shows a glacial influence throughout. The strata range in age from early Miocene near the sea floor to the Eocene/Oligocene boundary at 702 mbsf, some 800 m above acoustic basement.

    The strata cored by CIROS-1 have a uniformly low organic carbon content (average total organic carbon 0.34%), little hydrocarbon generating potential (maximum 0.34 mg hydrocarbons per gram of rock), and a low level of thermal alteration (vitrinite reflectance of 0.36% at 670 m). Kerogen is mainly highly oxidized terrestrial organic matter reworked from older strata.

    Residual asphaltic oil was found in minute quantities in sandstone at 632 mbsf. Geochemical studies indicate that it formed at greater depth, probably in nearshore clastic sediments with both marine and terrestrially derived organic matter. Similar coarse- to fine-grained feldspathic sandstones are common in the mudstone that forms the lower half of the core, and offer reservoir potential. Calcite and zeolite cements are common but in-situ porosities range up to 22%, with good dissolution porosity in some intervals.

    In balance, the possible prospects of Oligocene strata on the margin of the Victoria Land Basin must be considered low. Source characteristics are unfavorable and only minute amounts of hydrocarbons have been encountered. In addition, the up-dip location of CIROS-1 with respect to the basin center suggests that the possible prospects of the western Victoria Land Basin should also be regarded as low.

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