- Conference Title – Core Values: the Role of Core in 21st Century Reservoir Characterisation
- Topics – reservoir geology, seismic surveys, borehole coring, petrophysics, reservoir mapping
- Organiser – Geological Society of London
- Dates – 4th-7th May 2021
- Location – Virtual Event
We are pleased to announce PetroStrat’s John Cater (Reservoir Geologist) will present at the Geological Survey virtual event “Core Values: the Role of Core in 21st Century Reservoir Characterisation”. This conference addresses the role of coring in the characterisation of hydrocarbon reservoirs.
“I’ve always enjoyed the detective work involved in the description and interpretation of cores, and deviated wells cored through the slumped, injected and faulted margins of deep marine channel fills certainly made us all think!”
John Cater (Reservoir Geologist, PetroStrat)
John continues; “Interpretation and development planning would have been impossible without core, which is why I was keen to contribute to this Geological Society symposium on the Value of Core – you could say that core is invaluable, really; but that might be too brief a talk!”
Abstract of John Cater’s Talk
Turbidite or injectite? The importance of core for correct interpretation of reservoir geometries
Seismic-scale injectites commonly comprise large volumes of clean, high quality sandstone sealed within and/or charged from shales. Spectacular examples include Alba (UK) and Volund (Norway), together with more recent discoveries at Catcher (UK) and Agar/Plantain (UK/Norway). This presentation reviews the characteristics of injectites and shows that core is vital to correctly interpret their origin and predict reservoir geometry and performance.
Many seismic scale injectites have a distinctive ‘Loch Ness Monster’ shape in cross section and ‘inverted shepherd’s hat’ 3D geometry. As a result they cross-cut the host rock at a variety of angles. Oblique injectites (clastic dykes) are readily identified on seismic and in core, due to disconformable contacts with the host lithology. Volund is a classic Loch Ness type that illustrates this. Conformable injectites (clastic sills) are less easily distinguished from in situ sandstones. This is seen in Catcher, where sands were injected across a fault. Cores from both types of injectite are illustrated and discussed in this presentation.
Sandy turbidites, deposited from suspension, commonly display a loosely-packed grain fabric, whereas the flow of injected sands in the subsurface generates a more compact fabric with generally lower reservoir quality (Duranti & Hurst, 2004: Sedimentology 51: 503-529). Distinguishing between turbidites and injectites is therefore key to predicting primary reservoir quality (before diagenetic overprinting). Observations from core are the most effective way to do this; image logs generally lack the resolution needed to identify diagnostic features.
Identifying and distinguishing turbidites and injectites from core observation is not straightforward. For example, sandy turbidites deposited from suspension commonly record fluid escape as the sediment/water suspension collapses and fluid elutriates. This process forms coarse-tail graded beds, consolidation laminae, dish structures and fluid-escape pipes, which are diagnostic characteristics of turbidites. Similar features are seen in cored oblique injectites (e.g. Volund), showing that injection can deform entrained sand and silt mixtures, forming stratification that records the transfer of fluid through the injectite. Whether these features record true suspension fall-out is a matter of some controversy (e.g. Hurst & Cronin, 2001: JSR 71: 136-143).
The scale of investigation can hinder the differentiation of sands injected into intact host rocks from large clasts of host rock enclosed within a sand matrix (e.g. bank collapse deposits). Examples of such bank-collapse deposits are shown from core (Norwegian data) and outcrop (Hubbard et al., 2007: AAPG Mem. 87:199-207). The correct identification of bank collapse features in gravity-flow systems (as opposed to injectites) can have profound implications for depositional modelling. For example, they are common in the channel lobe transition zone in turbidite fans. This zone is a source of hybrid event beds (HEBs), which are commonly transferred down range and deposited on the fringes of depositional lobes where they form baffles to flow.
Petrographic characterization of injected and in situ sands, including XRF- and XRD-based chemical and mineralogical typing, can be used to link injected sands to their origin. In the Catcher discovery in situ Eocene Tay and Palaeocene Cromarty sands have been differentiated based on Ti/Zr ratios (quantified using XRF) and feldspar content (semi-quantified with XRD). The provenance of injectites in off-set wells has been traced using the same techniques. Whilst ditch cuttings can be used to characterize very thick (10m scale) units, this approach is best used for cored intervals, where discrete injected sands can be identified and sampled.
Injectites form an economically and volumetrically significant reservoir component, which is sometimes visible on seismic. They commonly form the uppermost part of a reservoir, hence their irregular geometries add uncertainty to the development of predictive reservoir models. This is particularly important when trying to exploit attic oil. Even late in a field’s life cycle it can be valuable to identify and quantify injectites within reservoir sands. This is best achieved using core.
Suggested further reading
Hurst A., Cartwright J. A., Duranti D. 2003b. Fluidisation structures in sandstone produced by upward injection through a sealing lithology. In: van Rensbergen P., Hillis R. R., Maltman A. J., Morley C. K. (eds) Subsurface Sediment Mobilization, Geological Society, London, Special Publications, 216, 123–137
