Title: Controls on clay distribution at bed-level in deep-water sandstones; implications for permeability and hydrocarbon drainage
Researchers: Arif Hussain and Prof. Peter Haughton
Transport of sediment to deep-marine basins is commonly via turbidity currents and debris flows. Early models for turbidity currents envisaged largely turbulent flows capable of segregating sand from clay, with clay deposited from a trailing cloud at the top of the sand component (forming the Te division of the Bouma sequence i.e. mudcaps. However, many turbidites were known to contain significant interstitial clay and were classified as greywackes. Recent experimental studies have highlighted the role of detrital clays in transforming flow character through turbulence damping, resulting in failure of the flows to efficiently sort their load (Baas et al., 2009).
Turbulence damping occurs first in parts of the flow where the total turbulent kinetic energy is least (e.g. the lateral margins, top and rear of the flow), or sections of the flow overloaded with local erosion products (e.g. ripped up mud clasts). Consequently, a depositing flow may have zones that remain fully turbulent (typically the front and core of the flow), and other areas where the turbulence is damped, particularly where flows are forced to decelerate i.e. on exiting a channel or other constriction. The deposits of such flows do not resemble traditional end-member sediment gravity flow facies, but rather comprise deposits that appear to result from both turbulent and laminar deposition as part of the same event. Such deposits have been termed ‘Hybrid event beds (HEBs)’ (Haughton et al., 2009). The idealised HEB bed comprises of up to five vertically stacked divisions, including clean and often graded structureless or dewatered sandstone (H1), a banded sandstone (H2), an argillaceous sandstone (H3) typically with mudstone clasts, dispersed clay, mica, organic matter and other low density flakes, a thin well-structured parallel and/or ripple laminated fine-grained sandstone (H4) and a mudstone cap (H5, see Fig.1). Recent exploration in the Gulf of Mexico and the North Sea demonstrate HEB facies are widespread in deep-water successions (Davis et al., 2009; Kane & Ponten, 2012) and impact on reservoir properties and well deliverability.
On-going work on the structure and context of hybrid event beds has now highlighted a wide variety of bed types (Talling, 2013) and indicates that flows depositing ‘dirty’ sands (with prominent H2 and H3 divisions) can arise in a number of ways and at different points along the depositional pathway. Observed variability in HEB expression reflects a complex interplay of various autogenic (flow magnitude, substrate character, lobe shifting etc.) and allogenic (sediment supply switching, basin gradients etc.) factors. However, a common theme is evidence for flow transformations driven by the incorporation and/or segregation of dispersed clay and/or mud clasts. Experimental work shows changes in flow character is commonly linked to small changes in clay content (just a few percent) in the case of both sticky bentonite and less cohesive kaolinite clays (Bass et al., 2009). Variable clay and/or clast distribution has important implications for bed-scale heterogeneity and reservoir quality. Even small volumes of clay can have a major impact on permeability (permeability profiles). Typically this impact is detrimental; however, sometimes it is positive where clay coatings inhibit later overgrowth cements (in these cases, the clays are generally diagenetic). Therefore the volume, distribution and habit of clays are important issues in assessing what is effective reservoir and how the net clean sand components of a deep-water succession might be connected vertically and laterally. However, quantifying and accounting for the details of clay distribution at high-resolution within single event beds, determining the likely source of clays incorporated within the flows and constraining processes controlling clay distribution still presents significant challenges. This partly reflects limited sample spatial resolution (clays in the subsurface are normally characterised in conventional core plug offcuts spaced ~ 25 cm vertically) while what is required is continuous vertical sampling to identify vertical trends and steps in the clay abundance.
Key objectives of the current work are:
- Undertake a detailed characterisation of clay type, morphology and distribution vertically through sediment gravity flow deposits, focussing particularly on hybrid event beds, an important heterogeneity in many deep-water reservoirs.
- Develop continuous XRF profiling as a proxy for characterising vertical trends in clay content and mineralogy.
- Consider links between deposit texture, clay mineralogy and flow processes, including better understanding lateral trends.
- Link variable clay distribution with reservoir properties especially porosity, permeability and water saturation.
- Investigate the extent to which packages dominated by HEBs can be distinguished using conventional open-hole logs using an extensive dataset from the Ross Sandstone Fm where an extensive digital bed inventory has been developed in previous projects.