Title: Reservoir significance of thin-bedded successions in deep-water settings
Researchers: Dr Emma Morris, Prof. Peter Haughton
Typically, it is the sand-prone intervals within deep-water depositional systems that are the focus of both academic and industry attention, these deposits are commonly attributed to either the axial parts of terminal lobes that pass distally into sheet-like deposits, or the axial fills of submarine channels. The finer grained material within sediment gravity flows are more commonly partitioned into levee/overbanks or distal lobe fringe settings. This grain-size segregation is attributed to the internal stratification of turbidity currents where the coarser fraction is found at the base and front of the flow, becoming finer grained upwards and towards the tail of the flow, this allows the finer fraction to overspill or be stripped onto overbank settings resulting in a narrower grain-size range further down slope. Much of the depositional history of these systems is recorded in the thinner bedded, finer-grained components as they typically form a more complete depositional record relative to the axial sandbodies as sand-on-sand erosion in the case of the latter limits our ability to reconstruct the flow history. Significantly, thin-bedded deposits in deep-water settings are rarely investigated and are thus relatively poorly understood, despite being important baffles and barriers in many hydrocarbon reservoirs. Thin-bedded sediment gravity deposits form significant components of many deep-water systems and are associated with both channel and lobe settings. Thin-bedded deposits associated with channels are highly heterogeneous and can occur in a range of different sub-environments: frontal lobes, crevasse/avulsion splays, internal levees, external levees and channel margins. These sub-environments record the evolution of the channel system from its inception (frontal lobes), through to channel extension and growth (crevasse/avulsion splays, internal levees, external levees and channel margins) to its abandonment (abandonment drapes). Thin-bedded deposits associated with depositional lobes are also found in a range of sub-environments: channel-lobe transition zone, lobe off-axis, frontal and lateral lobe fringes, and distal lobe fringes. Thus, they record the transition from proximal to distal settings, and they are often used to subdivide lobes stratigraphically to recognise hierarchical stacking arrangements. Criteria to distinguish the range of thin-bed settings in subsurface data are required as the correct diagnosis can potentially provide information on the relative position of thicker higher net sand bodies (lithology prediction). It can help gauge the correlation value of different types of thin bed stack and constrain the geometry of stratigraphic traps (e.g. confined lobe pinch-outs against slopes). A more comprehensive understanding of these deposits can allow for a better grasp of reservoir performance where the thin beds themselves acts as reservoirs or explain pressure support in hydrocarbon reservoirs where there is hydraulic connectivity between levees and the adjacent axial channel sands. This project focuses on the analysis of thin beds (stacked cm- scale sandy or mixed sand-mud event beds) that form a key heterogeneity in many deep-water channel and lobe settings, to develop a criteria to interpret the disparate origin of these deposits. It will combine insights from the Clare Basin behind-outcrop drilling project with subsurface examples and the Karoo Basin behind-outcrop drilling projects to focus on the distinction between channel margins, internal levees, external levees, frontal lobes, crevasse lobes/splays, lobe fringe and distal lobe fringes. The work will dovetail with parallel iCRAG work on deep-water systems including developing of training resources in Clare and targeted projects focussing on turbidite reservoir characterisation and modelling.
• Peer-reviewed articles. Currently, there are no published diagnostic criteria that can be used across different datasets to aid the characterization and prediction of thin-beds in deep-water settings, therefore one paper related to thin-bedded deposits within channelised deposits from the Karoo Basin, one paper related to the thin-bedded deposits within the Ross Formation at outcrop and in core, and another paper comparing subsurface thin-bedded deposits.
• A table designed using the Clare Basin dataset, detailing a suite of criteria to differentiate between these thin-beds and to develop an understanding of how, when and where these deposits were developed. This will then be tested and improved upon using the Karoo and other subsurface datasets to provide a robust set of criteria upon which to define sub-environments using thin-bedded facies in core.
• Collaboration with researchers working in the Karoo Basin, and potential collaboration with oil majors exploring in the Wilcox Formation, Gulf of Mexico, or the Forties System of the North Sea.
• A facies atlas detailing the variety of thin-beds from the Clare Basin and their sub-environments of deposition. This can be further developed to include thin-beds from the Karoo Basin and subsurface analogues.
• Conference papers detailing findings from the outcrop and core datasets from the Clare and Karoo Basins.
• Training exercises designed using the core database that can be ground truthed at outcrop – these could be used internally within UCD (undergraduate and masters teaching) or for outside agencies/sponsors.
