Compositional Limitations on Rock Types Contributing to Zones of Sealing Phyllosilicate-Rich Fault Gouge: ABSTRACT

Abstract

A common approach to fault-zone seal analysis in siliciclastic strata involves analyzing the lithologic composition of the displaced stratigraphic interval along a fault as a means of predicting the spatial extent of sealing phylloilicate-rich gouge zones. It is often assumed that only the mudstones within the stratigraphic section contribute to the formation of this phyllosilicate-rich gouge. This study documents composition-dependent contrasts in natural deformational style of sandstones and mudstones that challenge this assumption. Faulting of phyllosilicate-poor (<20%), porous sandstones results in the formation of deformation bands. In contrast, the deformation of mineralogically less-mature sandstones generates gouge with a fine-grained, phyllosilicate-dominated matrix. Capillary-pressure tests indicate that this phyllosilicate-rich gouge is capable of sealing large hydrocarbon columns, whereas the deformation bands generally do not have small enough pore throats to serve as good seals. Most mudstones; deform by intergranular sliding and other grain-scale mechanisms to form [open quotes]scaly[close quotes] gouge typical of sealing [open quotes]shale smear[close quotes] fault zones. Other mudstones, however, deform by transgranular fracturing and brecciation, potentially leading to the formation of fluid conduits rather than seals. A transition in deformation mechanism from fracturing to intergranular sliding typically occurs when phyllosilicate content of the original mudstone exceeds 20-30%. At naturalmore » conditions suitable for petroleum preservation, faulting of sandstones and mudstones containing in excess of 20-30% phyllosilicates generally produces phyllosilicate-rich fault gouge. Thus, during fault-seal analysis in siliciclastic sequences, this compositional criterion can be used to distinguish between lithologies that do and do not contribute to the formation of high-quality sealing fault gouge.« less