Characterizing Modern Sediments to Better Understand Reservoir Sand Provenance, Taranaki Basin, New Zealand

Doran, Linda
[email protected]

The offshore Taranaki Basin of New Zealand is the country’s most important area of petroleum exploration and production, but many of its known petroleum reservoirs are nearing depletion. Information that facilitates discovery of additional reservoirs is of great interest. The objective of this study is to determine the sediment provenance (source) of existing petroleum reservoirs to enable more targeted exploration. Paleo-river systems draining uplifted basement terranes were the likely source of sandstone reservoirs. Modern equivalents of these terranes crop out in the Nelson region of South Island. They include the variously metamorphosed Paleozoic Buller and Takaka terranes on the west side of the north-south trending Alpine Fault and the Late Paleozoic to Mesozoic Murihiku, Dun Mountain-Maitai, Brook Street, Caples, and Torlesse composite terranes to the east. Intruding these terranes are Paleozoic to Mesozoic granitic batholiths. The testable hypothesis is that basement terranes continue to shed sandy sediment of distinct composition into modern river systems and these modern sands can be used to interpret the provenance of offshore petroleum reservoir sandstones. Characterization is based on detailed petrographic analysis. Samples are sieved into five sand fractions – very coarse, coarse, medium, fine, and very fine – to identify grain size-dependent changes in composition such as the concentration of feldspars and micas as well as compaction and cementation. Detailed point counts (400 per sample) of stained thin sections in 40+ categories enable estimates of the relative abundances of different grains. To trace sandstones to their source, results are compared with published reports describing offshore reservoirs. Preliminary point counts indicate that plutonic terranes shed quartzofeldspathic sand and metamorphosed terranes shed mostly lithic fragments.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013