Jonathan Stewart

Characterization of stratigraphic architecture and its impact on fluid flow in a fluvial-dominated deltaic reservoir analog: Upper Cretaceous Ferron Sandstone Member, Utah

Fluviodeltaic stratigraphic architecture and its impact on fluid flow have been characterized using a high-resolution, three-dimensional, reservoir-scale model of an outcrop analog from the Upper Cretaceous Ferron Sandstone Member of central Utah. The model contains two parasequence sets (delta complexes), each with five or six parasequences, separated by an interval of coastal plain strata. Each parasequence contains one or two laterally offset teardrop-shaped delta lobes that are 6 to 12 km (4–7 mi) long, 3 to 9 km (2–6 mi) wide, 5 to 29 m (16–95 ft) thick, and have aspect ratios (width/length) of 0.4 to 0.8. Delta lobes have a wide range of azimuthal orientations (120) around an overall east-northeastward progradation direction. In plan view, delta lobes in successive parasequences exhibit large (as much as 91) clockwise and counterclockwise rotations in progradation direction, which are attributed to autogenic lobe switching. In cross-sectional view, parasequence stacking is strongly progradational, but a small component of aggradation or downstepping between parasequences reflects relative sea level fluctuations. We use flow simulations to characterize the impact of this heterogeneity on production in terms of the sweep efficiency, which is controlled by (1) the continuity, orientation, and permeability of channel-fill sand bodies; (2) the vertical permeability of distal delta-front heteroliths; (3) the direction of sweep relative to the orientation of channel-fill and delta-lobe sand bodies; and (4) well spacing. Distributary channel-fill sand bodies terminate at the apex of genetically related delta lobes and provide limited sand body connectivity. In contrast, fluvial channel-fill sand bodies cut into, and connect, multiple delta-lobe sand bodies. Low, but non-zero, vertical permeability within distal delta-front heteroliths also provides connectivity between successive delta-lobe sand bodies.

A comparative study of reservoir modeling techniques and their impact on predicted performance of fluvial-dominated deltaic reservoirs

Multiple techniques are available to construct three-dimensional reservoir models. This study uses comparative analysis to test the impact of applying four commonly used stochastic modeling techniques to capture geologic heterogeneity and fluid-flow behavior in fluvial-dominated deltaic reservoirs of complex facies architecture: (1) sequential indicator simulation; (2) object-based modeling; (3) multiple-point statistics (MPS); and (4) spectral component geologic modeling. A reference for comparison is provided by a high-resolution model of an outcrop analog that captures facies architecture at the scale of parasequences, delta lobes, and facies-association belts. A sparse, pseudosubsurface data set extracted from the reference model is used to condition models constructed using each stochastic reservoir modeling technique. Models constructed using all four algorithms fail to match the facies-association proportions of the reference model because they are conditioned to well data that sample a small, unrepresentative volume of the reservoir. Simulated sweep efficiency is determined by the degree to which the modeling algorithms reproduce two aspects of facies architecture that control sand-body connectivity: (1) the abundance, continuity, and orientation of channelized fluvial sand bodies; and (2) the lateral continuity of barriers to vertical flow associated with flooding surfaces. The MPS algorithm performs best in this regard. However, the static and dynamic performance of the models (as measured against facies-association proportions, facies architecture, and recovery factor of the reference model) is more dependent on the quality and quantity of conditioning data and on the interpreted geologic scenario(s) implicit in the models than on the choice of modeling technique.

Geologic and seismic modeling of a coarse-grained deep-water channel reservoir analog (Black’s Beach, La Jolla, California)

The Ardath Shale and Scripps Formation exposed along Blacks Beach north of La Jolla, California, record a deep-water channelized slope system of an Eocene forearc basin. The outcrop exposure, which is approximately 100 m (330 ft) high by 1.7 km (1 mi) long, offers insight into reservoir distribution and connectivity within coarse-grained, confined, deep-water channel systems. To use this outcrop as a quantitative subsurface analog, a detailed two-dimensional lithologic model was constructed from measured sections and interpreted photopanels. Elastic rock properties, including compressional-wave velocity, shear-wave velocity, and density typical of shallow offshore west African reservoirs were used to construct an impedance model. This model was convolved with 15-, 25-, and 50-Hz quadrature-phase Ricker wavelets to generate near- and far-angle stack one-dimensional and two-dimensional synthetic seismic reflection models. Because deep-water lithofacies have distinct amplitude-variation-with-offset behaviors and the interpretation of surfaces is intimately coupled with predicting lithofacies, simple bed interface models of conglomerate, sandstone, interbedded sandstone and mudstone, and muddy sandy debrite were used to build a template for successful interpretation. Interpretation of these forward seismic models demonstrates (1) the limits of and uncertainty associated with the interpretation of seismic data at different frequencies commonly encountered in the exploration, development, and production of deep-water reservoirs; and (2) how the combination of near- and far-angle seismic data can be used to interpret channel-fill lithofacies and improve seismic interpretation. Large-scale channel complex set surfaces with significant impedance contrast (e.g., conglomerate overlying interbedded sandstone and mudstone) are readily interpretable at all frequencies with an increasing vertical error of 5 to 30 m (16 to 98 ft) from 50 to 15 Hz, respectively. Channel and channel complex surfaces can only be accurately mapped on the 50-Hz data, albeit with significant uncertainty. Near- to far-angle stack changes enable the identification of upward-fining, amalgamated, and fine-grained channel-fill lithofacies. Far-angle seismic reflections can provide a more detailed image of boundaries defining channel architecture and reservoir facies distribution.

A comparison of streamer and OBC seismic data at Beryl Alpha field, U. K. North Sea

Compressional P-wave ocean-bottom-cable (OBC) seismic data from the Beryl Alpha field in the U. K. North Sea provide a superior image of the subsurface compared to heritage streamer seismic data. To determine the reason for the superiority of OBC data, the results of a detailed comparison of these OBC and streamer data sets are compared. The streamer and OBC data sets are reprocessed using a strategy that attempts to isolate the roles of processing, fold, azimuth, PZ combination, and hydrophone and geophone data have on the improved OBC image. The vertical component of the geophone (OBC Z) provides the major contribution to the improved OBC image. The imaged OBC Z datacontain fewer multiples and have a higher signal-to-noise ratio than the streamer. The OBC data have a lower level of multiple contamination because of the contribution from the OBC Z component, together with an effective suppression of receiver-side water-column reverberations as a result of the combination of the OBC hydrophone and geophon...