Accretionary Complexes as an Exploration Frontier: An Example from New Zealand

Gerilyn S. Soreghan, John J. Farrelly, Julie M. Wainscott

Early assessments of accretionary complexes indicated limited petroleum potential based on expectations of inadequate source rock, low geothermal gradients, structural complexity, and poor (volcaniclastic-rich) reservoir rock More recent studies, however, have countered these early projections, documenting the presence of adequate source potential and maturity combined with attractive trapping configurations, and suggest the possibility of clean reservoir rock in appropriate settings. Nevertheless, accretionary complexes remain an under-explored setting.

Tertiary subduction along the Hikurangi Margin of North Island, New Zealand has produced a 150 km wide accretionary borderland consisting of shore-parallel anticlinal ridges and intervening basins. Studies of the Tertiary section exposed onshore established potential source, reservoir, and seal lithofacies; seismic data of the adjacent offshore region image the imbricate-thrust controlled accretionary complex and preserved trench-slope basins. Data quality and reflection continuity within the section involved in the compressional deformation are poor, suggesting complex internal structure typical of an accretionary complex. In contrast, the Miocene section filling the fossil trench-slope basins exhibits clearly definable seismic facies including high-amplitude mounds that reflect pote tial sand-rich turbidite deposition. Deposition during this interval was syntectonic: active anticlinal growth effectively directed flow axially along the narrow trench-slope basins. Structural confinement likely forced submarine fan systems to aggrade, rather than avulse, producing potentially thick, yet essentially undeformed reservoir sections.

Trench-slope basins within accretionary complexes are common components of modern and ancient active margins of the circum-Pacific. Those that developed relatively near a continental provenance potentially ponded thick and relatively clean sandy strata, and may constitute a viable albeit challenging exploration frontier.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995