Nature and distribution of clay minerals in high net-to-gross deepwater fans: Taranaki Basin outcrops, New Zealand case study

Abstract

Submarine fans can be highly complex in their internal stratigraphic architecture and the way sand and mud are spatially distributed. Previous works have focused on predicting the spatial distribution of sand in channel and lobe complexes of high net-to-gross systems (HNG) for exploration purposes (e.g. King and Browne, 2001; Baas et al., 2005; Hempton et al., 2005; Lynds and Hajek, 2006; Mayall et al., 2006). In contrast, addressing the spatial distribution of clay content in these types of systems has received little attention, even though clay content can be the most significant factor to flow behavior in these HNG reservoirs. Similarly, the type of clay minerals and their overall fabric can increase reservoir heterogeneity. In order to address this question, research of well-established deepwater outcrops is integrated with microscopic methodologies (petrography, XRD, SEM) and spectroscopy. The Late-Miocene aged Mount Messenger Formation in the Taranaki Basin, New Zealand provides well-exposed and previously studied outcrops that allow the examination of the distribution and nature of clay minerals in proximal to distal deposits associated within a HNG fan system. A new, non-destructive, hand-held near-infrared spectroscopy (NIRS) approach was used to identify clay minerals from four distinct facies architectures (i.e. slope channel-axis, channel margin, frontal lobe, distal frontal lobe); from upper slope-channel to basin-floor. The use of the spectrometer allowed for a rapid identification of clay minerals in the field by sacrificing the accuracy of other methodologies such as XRD. The main minerals identified by the spectrometer, from the most to least common respectively, were chabazite, gmelinite, and illite. Chabazite is a calcium-bearing zeolite which XRD methods did not identify. Instead, XRD analyses concluded that zeophyllite—a different type of calcium-bearing zeolite—was only present in the distal frontal lobe and channel margin deposits. This brings into question the accuracy of spectroscopy versus XRD, yet we acknowledge the potential value of spectroscopy in a project-specific basis. Clay mineral segregation was difficult to observe in the studied outcrops due to the chemically-unstable nature of the volcanic-rich Mount Messenger Formation. Study of key samples under SEM indicates that diagenesis has overprinted original detrital minerals, and therefore the observed clay minerals are likely authigenic in origin. Nonetheless, we were able to hypothesize on the distribution of detrital clay minerals based on the location of their precursor counterparts. Hydrodynamic fractionation in theses deposits is based primarily on grain shape and size. Micaceous minerals are found in the most distal deposits and can provide the necessary cations to form the authigenic phases of chlorite, illite and smectite. In summary, understanding the spatial distribution of clay minerals as well as precursors of authigenic clay phases can improve predictive reservoir-quality models in HNG submarine fans.