Atle Rotevatn

From outcrop to reservoir simulation model: Workflow and procedures

Advances in data capture and computer technology have made possible the collection of three-dimensional, high-resolution, digital geological data from outcrop analogs. This paper presents new methodologies for the acquisition and utilization of three-dimensional information generated by ground-based laser scanning (lidar) of outcrops. A complete workflow is documented—from outcrop selection through data collection, processing and building of virtual outcrops—to geological interpretation and the building of geocellular models using an industry-standard, reservoir-modeling software. Data sets from the Roda Sandstone in the Spanish Pyrenees and the Grabens region of Canyon-lands National Park, Utah, USA, are used to illustrate the application of the workflow to sedimentary and structural problems at a reservoir scale. Subsurface reservoir models are limited by available geological data. Outcrop analogs from comparable systems, such as the Roda Sandstone and the Grabens, are commonly used to provide additional input to models of the subsurface. Outcrop geocellular models can be analyzed both statically and dynamically, wherein static examination involves visual inspection and the extraction of quantitative data on body geometry, and dynamic investigation involves the simulation of fluid flow through the analog model. The work presented in this study demonstrates the utility of lidar as a data collection technique for the building of more accurate outcrop-based geocellular models. The aim of this publication is to present the first documentation of a complete workflow that extends from outcrop selection to model investigation through the presentation of two worked data sets.

Overlapping faults and their effect on fluid flow in different reservoir types: A LIDAR-based outcrop modeling and flow simulation study

In this article we focus on the potential of fault-overlap zones as conduits for fluid flow in a variety of reservoir types. Light detection and ranging (LIDAR) technology were applied to collect a three-dimensional, spatially constrained data set from a well-exposed fault-overlap zone that crops out in the Devils Lane area of the Canyonlands National Park in Utah. A virtual outcrop was generated and used to extract structural and stratigraphic data that were taken into a reservoir modeling software and reconstructed. The outcrop-based model was flow simulated and used to test fluid flow through a real-world fault-overlap zone. A structural framework was built based on collected outcrop data and combined with a series of nine different facies models. The different facies models included an eolian model based on the outcrop and a range of synthetic fluvial and shallow marine systems. Results show that, for certain depositional models, cross-fault reservoir communication may be poor despite the geometric connectivity of the relay beds. This was the case for low net/gross fluvial models and shoreface models. Conversely, high net/gross fluvial systems and eolian systems show good communication through the same relay zone. Overall, the results show that, in the presence of a fault-overlap zone, pressure communication across a relay ramp may still be poor depending on the scale of the faults and relay ramp as well as the geometry and volume of the sands.

Fault interaction in porous sandstone and implications for reservoir management; examples from southern Utah

Different types of fault interaction are examined and compared to a single fault situation with respect to density, distribution, and orientation of subseismic structures. Fault branch points are found to be considerably more complex than single faults. The damage zone in these areas shows a wider range in orientation of deformation bands and fractures, and the damaged volume extends far into the fault blocks. Overlapping structures develop wide damage zones at early stages, typically with structures that are oblique to the faults and, thus, represent potential flow barriers. The damage associated with relay structures is inherited by later stages, when the fault segments are coalesced and behave as a single fault. At advanced stages, the damage zones are uncommonly wide in breached relay locations. Such locations can be recognized as places where faults make abrupt steps or bends. The extent to which complications associated with both single-tip and double-tip interactions affect reservoir performance depends on the nature of the minor structures in the damage zone. It is thus crucial that the physical nature of minor structures is investigated so that their influence on reservoir performance can be evaluated.

Are relay ramps conduits for fluid flow? Structural analysis of a relay ramp in Arches National Park, Utah

Abstract Relay ramps associated with overlapping faults are commonly regarded as efficient conduits for fluid flow across potentially sealing intra-reservoir fault zones. The current study demonstrates that structural heterogeneity in the often anomalously wide damage zone of relay ramps may represent potential baffles to intra-ramp fluid flow. A network of ramp-parallel, ramp-diagonal and curved cataclastic deformation bands causes compartmentalization of the ramp studied in Arches National Park, Utah. Harmonic average calculations demonstrate that, although single deformation bands have little or no effect on effective permeability, the presence of even a very small number of low-permeable deformation band clusters could reduce along-ramp effective permeability by more than three orders of magnitude. Thus, although relay zones may maintain large-scale geometric communication, the results of this study demonstrate that caution must be exercised when considering relay ramps as fluid conduits across sealing faults in a production situation. Although relay ramps clearly represent effective migration pathways for hydrocarbons over geological time, the extent to which they conduct fluids in a production situation is more uncertain. Quantitative approaches include adjusting the transmissibility multipliers for faults in reservoir models to allow for increased cross-fault flow. If, however, the effect of internal structural heterogeneity is not taken into consideration, this type of adjustment may lead to gross overestimation of the effect of relay ramps. Sedimentology, stratigraphy, burial history and deformation mechanisms are some of the controlling factors for the formation of such structural heterogeneities.

Fault linkage and graben stepovers in the Canyonlands (Utah) and the North Sea Viking Graben, with implications for hydrocarbon migration and accumulation

Segmented graben systems develop stepovers that have important implications in the exploration of oil and gas in extensional tectonic basins. We have compared and modeled a representative stepover between grabens in Canyonlands, Utah, and the North Sea Viking Graben and, despite their different structural settings, found striking similarities that pertain to other graben systems. In both cases, the stepovers represent relatively high parts within the graben systems that are likely to be among the first to be filled with hydrocarbons generated in deeper parts of the grabens. Furthermore, the relay ramps and smaller fault offsets in stepovers ease hydrocarbon migration and allow stepovers to act as preferred migration routes from deep graben kitchens to structurally higher traps in the basin. Graben stepovers and their related structures should be paid special attention during exploration because they may represent hydrocarbon accumulations complementary to larger traps along the graben flanks. These observations explain the location of the Kvitebjorn, Valemon, and Huldra fields in a stepover structure of the Viking Graben and encourage increased focus on similar graben stepovers in the Viking Graben and other graben systems.

Dynamic investigation of the effect of a relay ramp on simulated fluid flow: geocellular modelling of the Delicate Arch Ramp, Utah

ABSTRACT A fluid flow simulation study was performed to investigate potential contrasts in reservoir performance between models displaying a soft-linked relay ramp vs. models with a continuous fault. The relay ramp model is based on a well-exposed outcrop analogue – the Delicate Arch Ramp, Arches National Park, Utah. In outcrop, the relay ramp exhibits a pervasive system of cataclastic deformation bands, which were mapped and incorporated into the reservoir simulation models. Several models were simulated, using deformation-band permeability as the main variable tested. Results show that when compared to flow across continuous faults, the presence of soft-linked relay ramps enhances net flow across the fault zone in all tested scenarios. Comparing models with deformation bands to the model that includes the ramp but no deformation bands illustrates that, although having an impact on flow tortuosity and sweep efficiency, deformation bands must have a very low permeability and be numerous before having a negative impact on recovery. Deformation bands with midrange permeabilities were shown to have a positive effect on recovery in the relay models, as they increase flow tortuosity and enhance sweep, causing later water breakthrough and prolonging production. Using very low-permeable deformation bands in the same models caused extremely poor pressure communication between the fault-breached compartments, despite the geometric connectivity provided by the sub-continuous relay beds.

Fault linkage and damage zone architecture in tight carbonate rocks in the Suez Rift (Egypt): implications for permeability structure along segmented normal faults

Abstract A field study focusing on fracture systems in a fault linkage zone from the Suez Rift, Egypt, is presented to elucidate the role of fault linkage zones in the permeability structure of segmented normal faults in tight carbonate rocks. Fracture systems in the linking damage zone show significantly increased structural complexity compared to that typical of isolated faults. The linkage zone is characterized by high fracture frequencies and multiple fracture sets of different orientations. Notably, pervasive fracture corridors strike at high angles to the fault trend and are interpreted to have formed during the latest evolutionary stages of what is interpreted as a breached relay. The structural observations indicate that along segmented normal faults in carbonate rocks, fault linkage zones represents locations of progressively increased cross- and along-fault permeability through the stages of relay growth and breaching. Our findings, in combination with previously published work, indicate that fault linkage zones represent localized conduits not only for increased fluid flow across faults, but also (vertically) within fault zones. Appreciating this has wide-ranging implications for understanding fluid transport in carbonate rocks and other naturally fractured lithologies.

3D structure and evolution of folds during normal fault dip linkage

Understanding the 3D geometry and evolution of extension-related folds is important because they may document the geometry and evolution of the associated faults, influence sediment routing and accommodation development, and may represent targets for hydrocarbon exploration or CO2 storage. Previous work on extension-related folds has largely been restricted to a 2D plane of observation; in this study we use 3D seismic reflection data from the Gulf of Suez, Egypt to determine the 3D geometry and evolution of fault-parallel folds during dip linkage of a vertically segmented extensional fault array that is locally decoupled across a salt-bearing interval. The 3D geometry of individual faults in the array and adjacent hanging-wall folds varies along strike; rollover structures occur above listric faults, whereas fault-bend folds occur above faults that have a ramp–flat–ramp geometry. Quantitative analysis of fault–fold attributes (e.g. fold amplitude) and the growth history of the fault array indicate that fault shape is controlled by the style of dip linkage, which in turn is controlled by the lateral separation of sub- and supra-salt segments prior to linkage. Small lateral separation yields a relatively subtle change in the overall convexity of the listric fault, whereas larger lateral separation results in a ramp–flat–ramp fault geometry, with the layer-parallel detachment lying within the salt. This study provides a link between fault spacing, style of dip linkage, final fault shape and, ultimately, the style of hanging-wall folding in mechanically layered stratigraphy. Our study indicates that 3D seismic reflection data have the ability to provide us with new 3D insights into the variability of, and controls on, the geometry and evolution of fault-related folds.

Evolution and structural style of relay zones in layered limestone–shale sequences: insights from the Hammam Faraun Fault Block, Suez rift, Egypt

A fully breached relay zone was investigated to gain insight into relay growth and breaching in layered limestone–shale sequences. Associated fracture patterns were analysed to gain a qualitative understanding of fault- and damage-zone evolution and fracture-related fluid conduits associated with fault overlap zones. Internally, the relay zone is characterized by dilatant brittle deformation in mechanically strong limestone of the stratigraphically upper part of the ramp, and down-dip shear along extensional detachments in stratigraphically lower and mechanically weak shale layers at the base. The linking damage zone is characterized by multi-directional fracture patterns, including fracture corridors at high angles to the main (bounding) faults. The causal relationship between fault growth and the complex fracture patterns lies in the interaction and progressive rotation of the local stress fields of overlapping or linking fault segments. Increasingly complex fracture patterns may therefore be expected during growth and linkage of fault segments. The observed fracture patterns, in concert with complex juxtaposition relations generated by dipping relay beds, indicate that fault linkage points in carbonate rocks represent localized conduits for cross-fault as well as vertical along-fault fluid flow. These have implications for hydrocarbon migration pathways and for reservoir connectivity during production from carbonate reservoirs.

Techniques to determine the kinematics of synsedimentary normal faults and implications for fault growth models

Abstract Normal faults grow via a sympathetic increase in their displacement and length (‘isolated model’) or by rapid establishment of their near-final length prior to significant displacement accumulation (‘constant-length model’). The isolated model has dominated the structural geology literature for >30 years, although some 3D seismic data-based studies support the constant-length model. Because they make different predictions regarding rift development, and earthquake size and recurrence intervals in areas of continental extension, it is critical to test these models with data from natural examples. Here we outline a range of techniques that constrain the kinematics of synsedimentary normal faults and thus test competing fault growth models. We then apply these techniques to three seismically imaged faults, showing that, in general, they grew in accordance with the constant-length model, although periods of relatively minor tip propagation and coeval displacement accumulation, characteristics more consistent with the isolated model, also occurred. We argue that analysis of growth strata represents the best way to test competing fault growth models; most studies utilizing this approach support the constant-length fault model, suggesting it may be more widely applicable than is currently assumed. It is plausible that the very early development of large faults is, however, characterized by the development of faults that, pre-linkage, grow in accordance with the isolated model; we may simply lack the data resolution, especially in the subsurface, to resolve this very early stage of fault growth.