Depositional Reservoir Quality and Its Distribution in Deep-Marine Sandstones, with Emphasis on Transitional and Composite Flow Deposits

Depositional reservoir quality is strongly controlled by both detrital clay content and grain size distribution, which equally control, and are controlled by, flow processes during transport and deposition. In many deepwater reservoirs, sands which are interpreted to have been deposited by flows with transitional rheologies, fluctuating between turbulent and laminar, and flows with discrete rheological boundaries (composite flows and/or co-genetic flows), are common. The resultant deposits (banded sandstones and hybrid event beds respectively) have complex spatial and stratigraphic distributions and highly variable reservoir quality and are thus crucial to understand in order to develop predictive sedimentological concepts for deep-marine reservoir quality distribution.

Here we present an analysis of the variability of reservoir quality within deepmarine sandstones deposited by a range of sediment gravity flow types, using case studies from the Vøring Basin (Norwegian Sea), the Wilcox Fm.
(GoM), and outcrop data from the Oligocene Grès d’Annot (France) and Carboniferous Ross Fm.
(Ireland). Reservoir quality distribution is typically predictable within relatively simple settings, however, whilst simplistic proximal to distal trends and stratigraphic stacking patterns may be applicable for some basin configurations, the addition of static, and in particular dynamic topography, adds significant complexity. In areas affected by dynamic topography, slope grade evolution may result in continual, or episodic erosion and entrainment of clay which modifies rheological properties; in turn, the rheologically differing flows, and divisions within individual flow events, have different runout lengths, which may be amplified against confining or partially confining slopes. Whilst the addition of small amounts of clay enables turbidity currents to travel further and overcome significant topography, heavily clay-laden flows, and in particular laminar flows do not have the same ability and tend to be more strongly controlled by topography. Accordingly, the internal divisions of hybrid event beds, i.e., net vs. non-net sand, may be variably affected by topography, which has implications for both reservoir quality prediction and stratigraphic trapping potential.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California