Stephen S. Flint

The Geological modelling of hydrocarbon reservoirs and outcrop analogues

Part 1 Quantitative data collection: quantitative clastic reservoir geological modelling - problems and perspectives alluvial architecture in a sequence stratigraphic framework - a case history from the upper cretaceous of southern Utah, USA sedimentary architecture of field analogues for reservoir information (SAFARI): a case study of the fluvial Escanilla formation, Spanish Pyrenees quantitative facies analysis of coal-bearing sequences in the Bowen basin, Australia - applications to reservoir description quantification of turbidite facies in a reservoir analogous submarine-fan channel sandbody, southern-central Pyrenees, Spain outcrop studies of shale smears on fault surfaces predicting reservoir sandbody orientation from dipmeter date - the use of sedimentary dip profiles form outcrops applications of the formation micro-scanner to modelling of Palaeozoic reservoirs in Oman permeability patterns in point bar deposits - Tertiary Loranca basin, central Spain knowledge base development for the estimation of reservoir rock properties in the interwell area - examples from the Texas Gulf coast. Part 2 Modelling methods: sedimentary flow units in hydrocarbon reservoirs - some shortcomings and a case for high-resolution permeability data the use of 3-D seismic in reservoir geological modelling the use of length distributions in geological modelling a theoretical study of fluvial sandstone body dimensions stochastic modelling of fluvial sandstone bodies a 3-D modelling approach for providing a complex reservoir description for reservoir simulations.

U-Pb zircon ages from the southwestern Karoo Basin, South Africa - Implications for the Permian-Triassic boundary

U-Pb ages determined using sensitive high-resolution ion microprobe-reverse geometry on 205 single-grain zircons from 16 ash beds within submarine fan deposits of the Ecca Group provide the first evidence of a marine Permian-Triassic (P-T) boundary in the Karoo Basin of South Africa. These U-Pb ages provide an objective basis for correlating the deep-marine sediments of the southwest Karoo Basin with fluvial-deltaic deposits in the central and eastern parts of the basin where the P-T boundary is recorded in a diverse macrofauna. Furthermore, these new zircon ages and their correlation imply asymmetric subsidence and variable sedimentation rates across the basin.

Sequence Stratigraphy in Lacustrine Basins: A Model for Part of the Green River Formation (Eocene), Southwest Uinta Basin, Utah, U.S.A.

ABSTRACT In the middle Green River Formation of central Nine Mile Canyon, Uinta Basin, Utah, several lacustrine-dominated intervals 10 m thick comprise aggradational carbonate parasequence sets and a progradational clastic parasequence. Maximum flooding surfaces are best identified within profundal oil shale that caps some of the clastic parasequences. These lacustrine transgressive systems tracts therefore exhibit parasequence stacking patterns unlike typical marine sequences. Two types of sequence boundary are identified. Type A sequence boundaries display evidence for a basinward shift in facies across a regionally mappable surface that is an angular or, rarely, parallel unconformity, and they typically juxtapose amalgamated braided fluvial channel sandstone (late lowstand systems tract) onto the profundal oil shale. They also bound depositional sequences that show a distinct asymmetry, being dominated by transgressive systems tracts 5-80 m thick. Highstand systems tracts are less than 4 m thick and may be removed completely, by erosion on overlying sequence boundaries. Other surfaces satisfy only some of the standard criteria of sequence boundaries and are termed type B sequence boundaries. Type A sequence boundaries mark pronounced base-level falls following times when the Uinta Lake had merged with a lake in an adjacent basin to form a much deeper lake. Such merging permitted the establishment of a new threshold at higher elevation following lake-level balancing. Type B sequence boundaries are interpreted as marking base-level falls from a barely merged lake or a lake that had an outflow. Over a 200 m stratigraphic thickness, type A sequence boundaries are more common upsection, indicating that, with time, a pluvial climate became more pronounced or that the adjacent lake was more easily filled. Type A sequence boundaries also become angular rather than parallel unconformities upsection, suggesting increased tilting of the basin margin over time.

Sedimentological parameterization of shallow-marine reservoirs

The key causes of heterogeneity within progradational shallow-marine reservoirs have been defined as: shoreline type (wave vs. fluvial dominated); shoreline trajectory; the presence of permeability contrasts associated with dipping clinoform surfaces within the shoreface or delta front; the presence of cemented barriers between parasequences; and the progradation direction of the shoreline (described with respect to the main waterflood direction in the simulated reservoir). These parameters were recorded from a series of 56 modern and ancient depositional systems from a variety of climatic and tectonic settings. These data were then used to build the 408 synthetic sedimentological models that formed the basis for the SAIGUP study.

Submarine slope degradation and aggradation and the stratigraphic evolution of channel–levee systems

Abstract: Two seismic-scale submarine channel–levee systems exposed in the Karoo Basin, South Africa provide insights into slope conduit evolution. Component channel fills in a levee-confined channel system (Unit C) and an entrenched channel system (Unit D) follow common stacking patterns; initial horizontal stacking (lateral migration) is followed by vertical stacking (aggradation). This architecture is a response to an equilibrium profile shift from low accommodation (slope degradation, composite erosion surface formation, external levee development, sediment bypass) through at-grade conditions (horizontal stacking and widening) to high accommodation (slope aggradation, vertical stacking, internal levee development). This architecture is likely common to other channel–levee systems. Supplementary material: A detailed correlation panel (presented schematically in Figure 2) is available at www.geolsoc.org.uk/SUP18456.

Sedimentological response of an alluvial system to Neogene thrust tectonics, Atacama Desert, northern Chile

The Llano de la Paciencia is a thrust sheet top basin in which the sedimentological and topographic evolution can be linked to thrust tip propagation. It is an elongate gravel plain which borders the Salar de Atacama, a major intermontane basin in the Andean forearc of northern Chile. The Llano is bounded to the west by the Cerros de Purilactis a Cretaceous–Paleocene sequence uplifted by the Frontal Domeyko Thrust. The eastern margin of the Llano is formed by the Cordillera de la Sal which was uplifted by a linked back thrust-frontal thrust system. The Salar de Atacama is thus divided into a number of discrete sub-basins: the Llano de la Paciencia, the Pampa Visachita and the western sub-Llano which from east-west are bounded by the Cordillera de la Sal, the northern imbricates and the ignimbrite back thrust. Two phases of sedimentological evolution can be distinguished within the Llano on the basis of Quaternary to Recent sediment dispersal patterns. Initially, Phase 1 alluvial fan lobes prograded eastwards into the main Salar de Atacama basin. Subsequently, uplift of the Cordillera de la Sal deflected drainage systems southwards parallel to the structural strike. These Phase 2 alluvial deposits drain into the Salar de Atacama at the lateral termination of the Cordillera de la Sal frontal thrust. In places where thrust tip ramps are emergent within the Llano, gullies have been incised into the drainage pathways. This has resulted in the reworking of the early Phase 1 gravels and progradation of the Phase 2 fan lobes. The evolution of the Llano de la Paciencia illustrates the close link between topography and syntectonic alluvial drainage patterns in an arid intermontane basin.

Seismic responses to ridge-transform subduction:Chile triple junction

The first local seismic network established in the remote region bordering the Chile triple junction monitored earthquakes at a rate of three per day with magnitudes M = 0 to M = 4, far smaller than any recorded previously from that region. Focal mechanisms were obtained by the adaptation of the relative amplitude method to local earthquakes. Epicentral locations define a lineament corresponding to the active segment of a subducted transform fault, and events corresponding to the predicted position of a subducted ridge show extensional mechanisms, with the exclusion of thrusting and trench-parallel mechanisms. These data suggest that oceanic spreading and seismic activity continue in the subducted ridge-transform system. Extension in the slab influences the overlying crust, giving rise to dextral transtension. Over the past 10 m.y., the locus of ridge subduction has migrated northward; we postulate that the pattern through time of extension and basin development in the overlying crust has mirrored this.

Extensional tectonics in convergent margin basins: An example from the Salar de Atacama, Chilean Andes

The Salar de Atacama basin of northern Chile preserves stratigraphic evidence for the evolution of the Andean cycle. It has evolved from a non-arc-related rift, through back-arc and inter-arc stages, to a Neogene fore-arc basin. Accumulation of the sedimentary succession was mainly due to extensional faulting. Important but short-duration contractional episodes do link to known fast-order plate-margin changes, but their stratigraphic effect appears to be restricted to uplift/erosion rather than creation of significant flexural subsidence. The Salar de Atacama basin originated as part of a regional rift system during Permian time and contains 2 km of Permo-Triassic continental detritus and volcanic rocks. The area remained above depositional base level throughout Late Triassic to Late Cretaceous time. Syn-rift continental red beds were deposited to a thickness of at least 2 km on a parallel rift segment in the Domeyko area to the west. Continued Triassic-Jurassic extension in the Domeyko basin resulted in a classic continental to marine transition, with deposition of a 2 km+ Jurassic mixed carbonate/clastic sequence. In latest Cretaceous-Eocene time, the Salar basin was an arc-related basin and accommo-dated some 4 km+ of continental detritus (Purilactis Group) due to back-arc extension, sediment being derived from the Domeyko Cordillera and arc rocks to the west. Late Eocene right-lateral strike-slip faulting and associated restraining bend uplift were driven by a high rate of oblique convergence between the Farallon and South American plates. This deformation complicated the stratigraphy of the Purilactis Group and inverted the western basin margin, which formed the dominant provenance area for a 2-km-thick Oligocene continental basin-fill component (Paciencia Group).The Oligocene basin was an extensional to trans-tensional basin. The Miocene-Holocene Salar basin is a continental fore-arc basin. This latest segment of the basin fill comprises pyroclastic and continental sedimentary rocks thrust over Quaternary gravels in many places. The Cordillera de la Sal is an intrabasinal uplift, initiated as a thin-skinned contractional feature. Thus, the superposed basin-fill components represent responses to distinctly different geodynamic settings.

Sensitivity of the impact of geological uncertainty on production from faulted and unfaulted shallow-marine oil reservoirs: objectives and methods

Estimates of recovery from oil fields are often found to be significantly in error, and the multidisciplinary SAIGUP modelling project has focused on the problem by assessing the influence of geological factors on production in a large suite of synthetic shallow-marine reservoir models. Over 400 progradational shallow-marine reservoirs, ranging from comparatively simple, parallel, wave-dominated shorelines through to laterally heterogeneous, lobate, river-dominated systems with abundant low-angle clinoforms, were generated as a function of sedimentological input conditioned to natural data. These sedimentological models were combined with structural models sharing a common overall form but consisting of three different fault systems with variable fault density and fault permeability characteristics and a common unfaulted end-member. Different sets of relative permeability functions applied on a facies-by-facies basis were calculated as a function of different lamina-scale properties and upscaling algorithms to establish the uncertainty in production introduced through the upscaling process. Different fault-related upscaling assumptions were also included in some models. A waterflood production mechanism was simulated using up to five different sets of well locations, resulting in simulated production behaviour for over 35 000 full-field reservoir models. The model reservoirs are typical of many North Sea examples, with total production ranging from c. 15×106 m3 to 35×106 m3, and recovery factors of between 30% and 55%. A variety of analytical methods were applied. Formal statistical methods quantified the relative influences of individual input parameters and parameter combinations on production measures. Various measures of reservoir heterogeneity were tested for their ability to discriminate reservoir performance. This paper gives a summary of the modelling and analyses described in more detail in the remainder of this thematic set of papers.

U-PB zircon tuff geochronology from the Karoo Basin, South Africa: implications of zircon recycling on stratigraphic age controls

Along the >650 km long southern margin of the Karoo Basin in South Africa, we traversed four evenly spaced stratigraphic transects and collected 22 samples of volcanic, air-fall tuffs thought to be distal deposits derived from the Permian–Triassic Southern Gondwanan volcanic arc. We present 469 new U-Pb zircon ages determined by sensitive high-resolution ion microprobe reverse geometry (SHRIMP-RG) at the Stanford–USGS Microanalytical Center in order to constrain the maximum depositional ages for the southern Karoo Basin strata. Weighted means of these youngest coherent zircon populations were selected to maximize the number of analyses while minimizing the mean square weighted deviation (MSWD) to increase the robustness and decrease the influence of Pb-loss and inheritance in determining the maximum depositional age. Maximum depositional ages for the marine Ecca Group range from 250 to 274 Ma, whereas in the conformably overlying terrestrial Beaufort Group maximum depositional ages ranged from 257 to 452 Ma. Across the southern Karoo Basin, the Ecca Group tuffs produce maximum depositional ages that young upward; however, the Beaufort Group tuffs yield maximum depositional ages that are geochronologically out of sequence. Furthermore, maximum depositional ages of the Beaufort Group tuffs are consistently older than ash ages within the underlying marine strata. Our results are supported by previously published U-Pb tuff zircon geochronology in the Karoo Basin and demonstrate that the presence of out-of-sequence, older tuff ages are repeatable in Beaufort Group tuffs along the southern margin of the basin. We propose that tuffs in the Karoo Basin are correlative with tuffs in southern South America, and that the age spectra of these tuffs were influenced by magmatic crustal recycling. We use these data to highlight the complexity of U-Pb zircon datasets from tuffs, address the use of U-Pb zircon ages to provide absolute age controls, and discuss the implications of these new age controls on the Permian-Triassic Karoo strata.