Reservoir Facies Development in Miocene Glacimarine Successions of the Victoria Land Basin, Antarctica
Fully cored stratigraphic drillholes around the Antarctic continental margin are shedding new light on Cenozoic environmental change, and also are adding new insights to our understanding of reservoir potential in glacimarine stratal cycles. The ANDRILL Southern Mcmurdo Sound core was acquired in late 2007 from a location in the Victoria Land Basin, adjacent to the Transantarctic Mountains. The core provides a 1138.54 m long archive of Neogene stratigraphy, with an expanded section representing the time interval 20.5-14.2 Ma. Analysis of the entire core reveals that it can be resolved to 70 sequences, each recording a cycle of glacial advance and retreat, with attendant changes in eustatic sea level, isostatic adjustments and water volume. Within each sequence, a basal sequence boundary is overlain by diamictites representing Lowstand (LST) and early Transgressive Systems Tract (TST.) Diamictites fine upward into muddy sandstones and ultimately into fine-grained mudrocks with marine body fossils representing the Maximum Flooding Stage (MFS) of the cycle. This is in turn overlain by a coarsening-upward Highstand Systems Tract (HST) that culminates in clean, stratified sandstones in many cycles truncated by the next sequence boundary. A granulometric analysis was carried out on 700 samples of different facies throughout the core, and a detailed study of an additional 198 samples focused on one particular interval, 604-655 mbsf, in order to assess the different lithologies for reservoir properties. The granulometric analysis concluded that optimal reservoir conditions are found in sand bodies occupying the HST, which are generally coarser grained and less lithified than other sands in the section. These sands also have a narrow range of grain sizes, suggesting better sorting and thus higher initial porosity. Sand bodies in the HSTs are likely the result of deltaic lobes prograding into the basin during sea level highs prior to the next glacial advance. Siltstones and claystones of the MFS and HST constitute effective seals and some are potential source rocks. Diamictites are generally well cemented and very poorly sorted. TST sands are generally fine-grained and display broader ranges of grain sizes, resulting in lower initial porosity.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009