Stuart Clarke

Modelling the effects of stratigraphical uncertainty on fault seal and trap-fill in faulted structures

A three-dimensional approach to migration modelling through faulted structures is described that allows the effects of stratigraphical uncertainty on potential hydrocarbon accumulations to be assessed. Deterministic, three-dimensional fault seal analysis typically produces results that are extremely sensitive to input parameters such as structural architecture and stratigraphical variation. These parameters can be amongst the most poorly defined of modelling inputs because subsurface structural detail and stratigraphical variation are often below the limit of seismic resolution and are not sampled by well data. The new technique is fully integrated with three-dimensional fault seal analysis and hydrocarbon flow pathway modelling to give a workflow that predicts the likelihood of fault-controlled hydrocarbon accumulations, given these uncertainties. Multiple deterministic realizations of the model are used to highlight specific uncertainties in stratigraphy to which predicted hydrocarbon accumulations are sensitive. The results of these realizations are incorporated into, and used to condition, a stochastic model to risk predicted accumulations based on their likelihood of occurrence and volume. This technique has many advantages over either purely deterministic or purely stochastic approaches. A single deterministic realization places over-optimistic faith in the accuracy of the geological model because of the high sensitivity of fault seal analysis to input parameters. Multiple realizations allow specific input parameter uncertainties to be investigated, and the resulting common traps can be considered low risk, but accumulations exclusive to individual realizations cannot be risked. Moreover, a purely stochastic simulation based on all uncertainties will, at best, reduce efficiency by modelling uncertainties to which the result is insensitive or, at worst, may bias results with geologically implausible, stochastically-generated trials.

Dynamic fault seal analysis and flow pathway modelling in three-dimensional basin models

Faults are important elements of petroleum systems that influence the migration and trapping of hydrocarbons in sedimentary basins. When faults undergo displacement, their fluid transmissibility properties change as a result of juxtaposing lithologies across the fault, by smearing semi-permeable or impermeable argillaceous rocks within fault zones and by pumping or valving aqueous fluids. We describe here a 4D hydrocarbon migration model that incorporates juxtaposition and argillaceous smear processes. The technique uses the geometries of strata that are cut by faults and their physical properties to construct 3D models in which the evolution of cross-fault relationships can be calculated and the development of fault-zone argillaceous smear predicted. Hydrocarbon migration pathways through faulted structures are then investigated with a 4D migration model based on invasion percolation (IP) techniques. The controls on hydrocarbon migration have been investigated for fieldwork-derived models of rock volumes from the Moab Fault, Utah, USA to test the modelling techniques against reality. As a result predominantly of argillaceous smear, hydrocarbons are shown to accumulate in the hanging wall of parts of the modelled section of the Moab Fault, but leak across juxtaposed sandstones elsewhere on the fault to produce footwall accumulations. The techniques are then applied to a seismically derived model of the Artemis Field, UK Southern North Sea, to demonstrate how hydrocarbon charging history and pathways are influenced by fault geometries. Multiple model realizations enable the risk associated with charging of individual fault compartments to be assessed.

Evidence for pre-Caledonian discontinuities in the Achnashellach Culmination, Moine Thrust Zone: the importance of a pre-thrust template in influencing fold-and-thrust belt development

South of the Loch Maree Fault in the Northwest Highlands of Scotland, an unexplained step-wise thickening of Torridon Group strata, from c. 200–1000 m, occurs towards the Loch Maree Fault, within the trailing edges of the stacked thrust sheets of the Achnashellach Culmination in the Caledonian Moine Thrust Zone. This thickening cannot be explained readily by Caledonian thrust tectonics alone, and suggests that thrusting was superimposed upon a pre-Caledonian non-layer-cake template giving rise to a thrust-parallel thickness change of Torridon Group strata in the thrust belt. Cross-section constructions within the culmination constrain discrete abrupt thickness changes of the Torridon Group succession preserved within the Coire nan Clach and Toll Ban thrust sheets. We infer the existence of a pre-existing discontinuity in the form of either a set of pre-Caledonian faults striking parallel or sub-parallel to the long-lived Loch Maree Fault in its southwestern wall, or palaeovalleys creating locally greater thicknesses of Torridon Group sediments in the pre-thrust template. Such palaeovalleys may have been eroded along pre-existing discontinuities. In either case, these discontinuities will have contributed to generating step-wise thickness changes in preserved Torridon Group strata prior to Cambro-Ordovician overstep and then contributed to controls on the observed (lateral) variations in thrust architecture and the northwards step-wise thinning of the Achnashellach Culmination towards the Loch Maree Fault. This northern termination of the Achnashellach Culmination demonstrates the importance of the pre-thrust template in constraining the three-dimensional architecture of lateral changes within fold-and-thrust belts.

Sedimentology and the facies architecture of the Ghaggar-Hakra Formation, Barmer Basin, India: Implications for early Cretaceous deposition on the north-western Indian Plate margin

Fluvial strata of the Lower Cretaceous Ghaggar‐Hakra Formation are exposed in fault blocks on the central‐eastern margin of the Barmer Basin, Rajasthan. The sedimentology of these outcrops are described from 114 logs (thicknesses up to 100 m) and 53 two‐dimensional correlation panels. The formation comprises three distinct channel belt sandstone packages defined as the Darjaniyon‐ki Dhani, Sarnoo and Nosar sandstones separated by thick siltstone‐dominated floodplain successions. The sediments were deposited in a sub‐tropical, low sinuosity fluvial system that matures into a highly sinuous fluvial system. The Nosar Sandstone, the youngest of the three packages, exhibits a significant increase in energy and erosive power compared to those underlying it. This distinct change in fluvial style is interpreted as being rejuvenation due to an actively developing rift network forming accommodation space, rather than climatic controls acting on part of the depositional system. Consequently, the Ghaggar‐Hakra Formation at outcrop represents Lower Cretaceous syn‐rift deposition within the Barmer Basin with active localized fault movement from Nosar Sandstone times onward. These findings provide sedimentological evidence in support of pre‐Palaeogene northwest–southeast extension in the Barmer Basin. Moreover, they imply Cretaceous extension took place widely along the northern extremity of the West Indian Rift System consistent with plate tectonic models of the break‐up of Gondwana and evolution of the Indian Ocean. Outcrops of Lower Cretaceous strata are patchy across India and Pakistan. This study provides valuable material which, when combined with the available published data, facilitates a re‐evaluation of Lower Cretaceous palaeogeography for the north‐west Indian Plate. The reconstruction demonstrates a complex fluvial system, where the sediments are preserved sporadically as early syn‐rift strata. The findings imply a high preservation potential for early Cretaceous fluvial successions within rifted fault blocks near Saraswati and Aishwarya of the Barmer Basin beneath the Palaeogene fill that likely have significant potential for further hydrocarbon exploration.